The evolutionary diversity of barnacles, with an updated classification of fossil and living forms

1Biodiversity Research Center, Academia Sinica, Taipei 115, Taiwan 2Department of Life Science, National Taiwan Normal University, Taipei, Taiwan 3Biodiversity Program, Taiwan International Graduate Program, Academia Sinica, Taipei, Taiwan 4Natural History Museum of Denmark, Invertebrate Zoology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark 5School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK 6Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW75BD, UK 7Marine Biodiversity Group, Department of Biology, University of Bergen, Bergen, Norway 8Center for Macroecology, Evolution and Climate, GLOBE Institute, University of Copenhagen, DK-2100, Copenhagen, Denmark 9Zoologisches Museum, Christian-Albrechts Universität zu Kiel, Kiel, Germany 10Computational Biology Institute, Department of Biostatistics and Bioinformatics, George Washington University, Washington, DC 20052, USA 11CIBIO-InBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, Universidade do Porto, Campus Agrário de Vairão, Vairão 4485-661, Portugal 12White Sea Biological Station, Biological Faculty of Moscow State University, Moscow 119899, Russia 13Department of Invertebrate Zoology, US National Museum of Natural History, Smithsonian Institution, Washington, DC 20013, USA 14Marine Biology Section, Department of Biology, University of Copenhagen, Universitetsparken 4, DK2100, Copenhagen, Denmark


INTRODUCTION
Barnacles are traditionally known as Cirripedia, which comprise an extremely diverse array of sessile crustaceans that are either acorn, stalked or parasitic . From the classical taxonomy summarized by Martin & Davis (2001), Cirripedia forms an important subgroup with Facetoteca ( Fig. 1) and Ascothoracida ( Fig. 2) under Thecostraca. From molecular phylogenetic analyses (Pérez-Losada et al., 2008, 2012a, b, 2014, Thecostraca forms a monophyletic group, with Facetoteca being the sister group to a clade comprising Ascothoracida and Cirripedia (Figs 3-6). In the present study, we consider the evolutionary history and classification of Thecostraca and treat the group as comprising all barnacles in the widest sense.
The class Thecostraca comprises ~65 families, 367 genera and 2116 species distributed in three subclasses: Facetotecta (12 species; Fig. 1), Ascothoracida (114 species; Fig. 2) and Cirripedia (1990 species; Figs 3-6; Supporting Information, Table S1). The Cirripedia includes the Thoracica, the Acrothoracica and the Rhizocephala (Fig. 7). The Thoracica are a highly specialized group of suspension feeders. Their external body armor includes a system of mineralized plates. These shell plates are mineralized parts of the cuticle and are not shed at moults, but increase gradually in both thickness and area. In the variably-sized zones between these plates, the cuticle is moulted regularly. But this occurs in a highly modified fashion that produces a system of parallel growth lines, also called 'cuticular slips' (Anderson, 1994;Blomsterberg et al., 2004). The Acrothoracica, although also suspension feeders, lack mineralized plates (Kolbasov & Høeg, 2000). Instead, they are symbiotic and inhabit self-excavated borings in a wide array of calcareous substrata, including corals, coralline red algae, gastropod shells occupied by hermit crabs, live gastropods, bivalves, limestone and foraminiferal chalk, thoracican barnacles and bryozoans (Kolbasov, 2009;Botha et al., 2020). The Rhizocephala are all highly specialized parasites on crustaceans (mainly Decapoda). The adults lack all organs and structures normally found in Crustacea (Høeg, 1995a;Walker, Figure 1. Facetotecta. A-F, selected live images of y-nauplii representing some of the morphological diversity: lecitotrophic ones in A, C, D, F; planktotrophic ones in B, E. Note distinct differences in spination and body shapes. G, H, detailed morphology of y-larvae using scanning electron microscopy: G, ventral view; H, dorsal view, showing cuticular facets on the shield. I-K, the drastic metamorphosis from early y-nauplius (I) to last nauplius instar (J) and y-cyprid (K). These three larvae are conspecific. L, higher magnification view of the big rectangular box in K, showing an scanning electron micrograph of the cyprid antennules, with distal hooks (white circles). M, scanning electron micrograph of small square box in K, showing the lateral side of the y-cypris with a spiny labrum. N-P, metamorphosis from y-cypris to free ypsigon. This is most likely to be a larval or juvenile stage of an endoparasitic adult stage in hosts that have yet to be found. The specimens in A, D, F, I-J were sampled in coastal waters in Keelung, Northern Taiwan. The specimens in B, C, E, G, H were sampled on Green Island, Taiwan. The specimens in N-P originate from Sesoko Island, Japan. Abbreviations: a1, antennule; a2, second antennule; ce, compound eye; lb, labrum; mdb, mandible. Specimens J and K provided by Dr. Mark J. Grygier. Specimen J re-photographed from Grygier et al., (2019). 2001; Høeg et al., 2005) and show a highly complex life cycle, including their mode of infestation (Glenner et al., 2000;Glenner, 2001;Dreyer et al., 2020) (Fig. 4). The subclass Ascothoracida is also parasitic and infests either echinoderms or cnidarians (Fig. 2). The most plesiomorphic ascothoracidans are little modified, but the more derived taxa, such as the Dendrogastridae, can have strongly modified endoparasitic adults, although they always retain body segmentation and appendages (Grygier, 1996a). Finally, the subclass Facetotecta is the most elusive group within the Thecostraca. First recorded > 100 years ago (Hansen, 1899), they occur as y-larvae in the marine plankton throughout the world, but the adult stage has never been found in the wild (Grygier, 1996b). Hormonally stimulated y-cypris larvae metamorphosed in vitro into a slug-shaped, highly reduced 'ypsigon' stage, which exhibits many similarities with the rhizocephalan vermigon, such as lacking segments, eyes and a gut (Glenner et al., 2008). This strongly suggests that adult facetotectans might also be endoparasites (Pérez-Losada et al., 2009) (Fig. 1).
Studies on phylogenetic relationships of the Thecostraca started with the benchmark paper by Newman et al. (1969). Since then, a number of morphology-based studies have addressed the evolution and phylogeny of larger groups of thecostracan taxa, such as all Cirripedia or all or major parts of the Thoracica, often including both extant and fossil forms (e.g. Newman & Ross, 1976;Newman, 1987Newman, , 1996Pitombo, 2004;Buckeridge & Newman, 2006, and papers cited therein). The concept of cladistic analysis came relatively late to Crustacea; therefore, partly for historical reasons, few of the early studies offered a chacter state matrix amenable for numerical analysis, whether or not this method was the one used. Exceptions were the studies by , Buckeridge (1995), Pitombo (1999Pitombo ( , 2004 and , the last of which was the first study using larval characters across all thoracican barnacles. The last 15-20 years have seen the publication of molecular phylogenetic studies on all Thecostraca (Pérez-Losada et al., 2002, 2009 or major parts of the clade (e.g. Pérez-Losada et al., 2004Glenner & Hebsgaard, 2006;Yusa et al., 2010;Rees et al., 2014;Herrera et al., 2015;Lin et al., 2016;Tsang et al., 2017;Høeg et al., 2019Høeg et al., , 2020. These molecularly based phylogenies are in surprising mutual agreement, except for minor but interesting problems (Figs 7,8). Moreover, they confirm the monophyly of many thecostracan taxa and relationships derived from morphological studies, e.g. the mutual relationship of the subclasses, the monophyly of the Rhizocephala, the the early branching of the Iblidae in Thoracica, the sister-group relationship of the Verrucomorpha and Balanomorpha, and the monophyly and overall relationship of the superfamilies in the latter taxon.
Altogether, this yields high confidence that we are approaching a deeper understanding of thecostracan phylogeny (Figs 7,8). Yet, these molecular studies have also challenged some important concepts, especially in thoracican character evolution and phylogeny, such as the pattern of shell plate acquisition and the relationship of vent-and seep-inhabiting and asymmetrical barnacle forms. Lately, there have appeared several cladistically framed morphological studies, primarily on fossil forms, that again challenge earlier concepts, but in many respects concur with the molecular phylogenies (e.g. Gale, 2014bGale, , 2019Gale & Sørensen, 2014).
The molecular analyses have demonstrated that all the thecostracan subclasses (as understood here) are monophyletic, but a recent molecular phylogeny by Petrunina et al. (2014) suggested that also the Tantulocarida, which are parasites on copepods and other small-sized crustaceans, might be included in the Thecostraca. This analysis used only nuclear 18S ribosomal DNA (rDNA) and is not supported by morphological data; therefore, we choose not to consider tantulocarids as within Thecostraca. Molecularly based trees anchored with fossil and/ or geological calibration points have also provided a time frame for barnacle radiations (Pérez-Losada et al., 2008, allowing for the reconstruction of the evolutionary history of key phenotypic traits (Pérez-Losada et al., 2012a) and generating valuable insights into their diversification at the order and family levels (Pérez-Losada et al., 2012b. Most phylogenetic studies have focused on the Thoracica and the Rhizocephala (Ewers-Saucedo et al., 2019). Less attention has been given to the enigmatic Facetotecta, the parasitic Ascothoracida, and the shelland rock-boring Acrothoracica. Within Thoracica, morphology-based studies have consistently depicted the position and diversification of some assemblages (i.e. the basal split of the Iblomorpha and the diversification of acorn barnacles) but have not concurrently solved the radiation of other groups (e.g. stalked barnacles). Our current understanding of barnacle relationships based on recent molecular phylogenies (Figs 7,8) has led to some important changes in phylogenetic hypotheses and thus interpretations of character evolution (e.g. Pérez-Losada et al., 2004Gale, 2014b;Høeg et al., 2019Høeg et al., , 2020. We use these molecular phylogenies as the basis for our revised classification, although we emphasize that there is still much work to be done in order to achieve a fully resolved phylogeny of barnacles as a taxon, because species sampling remains poor in some groups and many polytomies still exist. It will also be interesting to see future studies that use a large morphological character matrix and wide taxon sampling for numerically based phylogenetic analyses that could either support and extend or falsify the claims based on the molecular data, such as those in the studies by Pitombo (1999Pitombo ( , 2004 and Gale (2015b).
In contrast to phylogenetic studies, barnacle taxonomy has not been revised comprehensively since the work of Martin & Davis (2001), and phylogenetic analyses since then have shown that half of the thecostracan families listed are polyphyletic. Central to most barnacle studies is a robust phylogenetically based taxonomic framework for appropriate group comparisons and biological inferences (e.g. Yusa et al., 2012;Tsang et al., 2014Tsang et al., , 2015Tsang et al., , 2017Lin et al., 2016). Likewise, a solid taxonomy is pivotal to the arrangement of organismal diversity and genomic information in databases (e.g. National Center for Biotechnology Information (NCBI) and Global Biodiversity Information Facility (GBIF)) (Hinchliff et al., 2015). Unfortunately, there is not yet a single taxonomic summary of the Thecostraca, despite a highly active research community describing barnacle species for > 200 years. A few important and helpful rearrangements have been published recently, involving Rhizocephala  and Acrothoracica (Kolbasov, 2009;Lin et al., 2016). But the extensive molecular phylogenetic data now available for Thoracica (stalked and acorn) barnacles have not yet been integrated into a formal phylogenetically based taxonomy . Therefore, we are still missing a comprehensive resource that brings together all the barnacle taxonomic information in an evolutionary context.
Here, we attempt to integrate, update and revise barnacle (Thecostraca) taxonomy with insights gained through previous molecular phylogenetic, morphological and taxonomic studies. We identify a number of areas of controversy with previous phylogenetic interpretations, identify what we consider to be promising areas for future study and provide our conclusions in the sections below. We review the fossil record of acorn and stalked barnacles and incorporate all fossil taxa into the new classification. Finally, we present the first comprehensive taxonomic summary for all barnacles, including all recent and extinct forms down to the genus level. We will integrate these results into the World Register of Marine Species (WoRMS) (Horton et al., 2020) for continued update and revision.

MATERIAL AND METHODS
Despite the several molecularly based studies addressing Thecostraca phylogeny (Fig. 8), there is as yet no analysis that investigates this taxon down to at least the family level across all subgroups. To arrive at a phylogenetically based taxonomy, we therefore decided to adopt the approach used by Høeg et al. (2020) for the Rhizocephala. Here, the trees from several molecularly based studies were used to arrive at a composite phylogeny for the entire taxon.
There is no disagreement about the monophyly of the Thecostraca or of its three principal subgroups, here recognized as subclasses Ascothoracida, Cirripedia and Facetotecta (Figs 7,8). For the intrinsic phylogeny of these subclasses, we still lack detailed molecular information for the Ascothoracida and Facetotecta, although such data are emerging (Pérez-Losada et al., 2009) (Fig. 7). In the Facetotecta, we have as yet no family-or genus-level taxonomy for what appears to be many but, in most cases, still undescribed species. For the Ascothoracida, we have relied on the morphologybased taxonomy of Grygier (1987a). Unpublished molecular phylogenetic analyses of Ascothoraicda suggest that clades are monophyletic at family levels. Within the Cirripedia, there are now detailed and comprehensive studies on all three infraclasses. For the Rhizocephala, we have relied entirely on the recently published study by Høeg et al. (2020). For the Acrothoracica, we use the study by Lin et al. (2016). For the Thoracica, there are three studies that address all or most of the taxon (Pérez-Losada et al., 2008;Herrera et al., 2015;Lin et al., 2015). In addition, there are several studies addressing problems within particular parts of the taxon (Pérez-Losada et al., 2014;Rees et al., 2014;Tsang et al., 2017;Chan et al., 2017b). With few, although notable, exceptions, these studies all agree both on the gross structure of thoracican phylogeny and within the major subgroups, but some taxa of great interest vary in position among these studies (Fig. 8). For extant Thoracica, we therefore decided to take a conservative approach and recognize only taxa that were returned in all the molecular analyses. Our final taxonomy is reflected in the composite family-level tree (Fig. 7). It confers confidence in this scheme that most of it is also in agreement with the most recent analyses based on morphology (Gale, 2014a(Gale, , b, 2015a(Gale, , b, 2019. This is especially true for our Scalpellidae, Neolepadoidea, Verrucomorpha and Balanomopha. Our main aim has been to base the new taxonomy on monophyletic groups that are supported by the most recent molecularly based analyses of the Thecostraca. The reason for using primarily molecularly based analyses is that the extreme morphological variation across the Thecostraca makes it virtually impossible to use morphology-based characters alone . Taxa with only extinct species (fossil taxa) have been placed using morphological arguments in the taxonomy constructed largely from molecular phylogenies. For some of the fossil-only taxa, paraphyly was the only practical solution. Wherever possible, we also use morphological characters to support our arguments, and this confers the great advantage of integrating the many extinct taxa into the taxonomy to render a comprehensive taxonomy (recent and fossil taxa) based on established evolutionary history.
In naming taxa above the superfamily level, we decided, for reasons of consistency, to use common name suffixes as far as possible. Thus, all ordinal level taxa within the Cirripedia Thoracica now end with '-morpha', but the taxonomic authorities have been retained in cases where there is identity to the same group under a different name suffix. Thus, Buckeridge & Newman (2006) remain the taxonomic authorities for the Iblomorpha, which they called Ibliformes. Applying specific Linnean ranks to taxa identified in a phylogeny is always a compromise. We have generally tried to use groups and names that are already recognized and to use the time of origin, number of species or morphological distinctness as guidelines to absolute rank level, but such decisions must remain an art more than a science (see Kallal et al., 2020). We are generally opposed to erecting empty absolute ranks, where not strictly required by the Zoological Code. Nevertheless, for consistency, we have done this in a few cases, such as the Calanticomorpha and Scalpelloidea, which in our taxonomy contain only the Calanticidae and the Scalpellidae, respectively.
We have made an effort to assure that families are monophyletic, and this has entailed some revisionary steps. Several non-monophyletic families have been abandoned and their species subsumed in other families. Therefore, there are few, if any, cases where monophyly of families is contradicted by any published molecularly based analyses. In several cases, we were also able to abandon some small monogeneric or monotypic families, when the analyses showed that the species concerned were nested within existing families. There are several cases where genera, often small ones or even monotypic, are nested within other genera. This obviously calls for revision, but we have decided not to take any taxonomic steps at generic and species levels here. Below the family level, we recognize subfamilies or even lower categories, where there is either good phylogenetic evidence for their monophyly or where it seems the best conservative approach to retain them until more data are available. This is especially true within the Balanoidea. We emphasize that the few differences between the molecularly based phylogenies of the Thoracica and their general agreement with recent morphological interpretations provide confidence that we are approaching a robust scheme and that interpretations of character evolution will be largely the same, despite the few deviations.

THECOSTRACA TAXONOMY AND LARVAL CHARACTERS
The Facetotecta (Fig. 1) are known only as larvae, and the Rhizocephala (Fig. 4) can be compared with other taxa solely by means of their nauplii and cyprids (Høeg et al., 2009b). It is therefore not surprising that larvae have featured prominently in the history of thecostracan taxonomy Pérez-Losada et al., 2009). It was larval characters that first allowed the Cirripedia to be recognized as crustaceans, the inclusion of the Rhizocephala in that taxon and, much later, the recognition of the Thecostraca as a monophyletic taxon (see details provided by Grygier, 1987c;Høeg et al., 2009a;Martin et al., 2014). Ultrastructural details of the lattice organs in cypridoid larvae were instrumental in resolving the relationships among both the thecostracan subclasses and the infraclasses of the Cirripedia . These results were subsequently supported by molecular data (Pérez-Losada et al., 2009). Within Rhizocephala, scanning electron microscopy of cypris larvae could pinpoint family-level relationships (Høeg & Rybakov, 1992). These results were again fully supported by molecularly based phylogenetic studies (Glenner et al., 2010). This bodes well for the use of such morphological data for rhizocephalan taxonomy in the many situations where molecular evidence is lacking. Cypris characters also seem to be useful in the Ascothoracida and Acrothoracica (Kolbasov & Høeg, 2007;Kolbasov et al., 2008) and will, in the future, be important for a morphology-based taxonomy of the Facetotecta. Høeg et al. (2004Høeg et al. ( , 2009a introduced the use of the 'a-cyprid' for the settlement stage (ascothoracid larva) of the Ascothoracida to emphasize its potential homology with the facetotectan y-cypris and cirripede cyprid (Figs 1K, 2C, D, 3B, 4B, E, 5B). The similarities include a range of structures, such as lattice organs on the carapace, prehensile antennules and frontal filaments. A-cyprids differ markedly in some characters, such as not undergoing a profound metamorphosis and in having more antennular segments.
Attempts at using larval characters for taxonomy have met with much less success within the Thoracica. The Lepadidae is clearly characterized by apomorphies in both nauplii (e.g. exceedingly long spines and frontal horns) and cyprids (many details of the sensory and attachment sturctures; Fig. 5B), and cyprids might also be useful in characterizing groupings within the Scalpellidae. But within Thoracica, larval characters have not yet been used extensively for taxonomic purposes. The attempt by  to use naupliar appendage setation to resolve thoracican systematics met with little success, possibly because the morphologies are closely associated with swimming and feeding and might well have been subject to extensive convergent evolution. Several recent studies have also highlighted that the morphology of the cypris antennules might also have been the result of convergent evolution (Al-Yahya et al., 2016;Chan et al., 2017b;Dreyer et al., 2020;Yu et al., 2020). Whatever their taxonomic use, the larvae  connected to the ramified internal root system. I-P, diversity of selected Rhizocephala externae. I, Peltogasterella sulcata on constitute a crucial part of thecostracan biology during both the free-swimming and the settlement phases. It is therefore an exciting and informative enterprise to map characters associated with this part of the life cycle onto a phylogeny to evaluate how larval morphology evolves in concert with complex biological traits, such as swimming, feeding and reproductive strategies (Ewers-Saucedo & Pappalardo, 2019;Dreyer et al., 2020;Wong et al., 2020).

SubclaSS Facetotecta GryGier, 1985
Facetotecta or 'y-larvae' (Fig. 1) were discovered in the North Sea plankton in 1887 by C. A .V. Hensen. Although he assigned them to the copepod family Coryaeidae, H. J. Hansen coined the term 'y-larvae' in 1889 as unidentified larvae of barnacles. There is only a fragmented and scattered literature on y-larvae, and many species have been described based on single, plankton-collected larval specimens.  erected the superorder Facetotecta. Y-larvae occur world-wide and have been reported from different parts of the Atlantic Ocean (McMurrich, 1917;Schram, 1972;Grygier, 1987d;Belmonte, 2005), the Arctic Ocean (Mileikovsky, 1968;Kolbasov & Høeg, 2003), the western Pacific (Itô, 1984(Itô, , 1985(Itô, , 1986(Itô, , 1987(Itô, , 1989Itô & Ohtsuka, 1984;Grygier et al., 2019) and a few areas of the Indian Ocean (Swathi & Mohan, 2019). Y-larvae are minute, planktonic crustacean larvae ranging from 250 to 700 μm in length (Høeg et al., 2014a). Their nauplii are either lecithotrophic or planktotrophic and terminate development as a nonfeeding y-cyprid instar ( Fig. 1A-K). Few studies have described both nauplii and cyprids of the same 'type' or species (Itô, 1986;Kolbasov & Høeg, 2003), and there are few accounts of nauplius-to-cyprid metamorphic details (Grygier et al., 2019). The nauplii sport a highly ornamented cephalic shield of complex honeycombed patterns (Fig. 1H), which gave rise to their name. In the y-cyprid, these patterns transform into elongated ridges that run in the anterior-posterior plane (Itô, 1985(Itô, , 1986Høeg et al., 2014a) (Fig. 1K). These ridges have been used in species descriptions, although the nomenclature of the naupliar plate regions remains a challenge (Schram, 1972;Itô, 1987). The nauplii occur in a range of transparency levels ( Fig. 1A-F). The planktotrophic species are almost entirely transparent, whereas the lecithotrophic forms exhibit a range of colours owing to nutrient-laden cells (Itô, 1985(Itô, , 1986Høeg et al., 2014a;Grygier et al., 2019;Fig. 1). The single nauplius eye is pigmented. The naupliar body consists of a cephalic anterior part, covered by the head shield, and a posteriorly projecting hind body, which invariably terminates in one or several conspicuous cuticular spines and, usually, two smaller, ventrally located furcal rami or spines. The larval labra are highly variable in external morphology, and during metamorphosis to the y-cyprid they are transformed into a spinous structure (Høeg et al., 2014a;Grygier et al., 2019). The naupliar limbs resemble those in other crustacean nauplius larvae, with uniramous antennules and biramous antennae and mandibles. These may carry spinous gnathobases in feeding species (Martin et al., 2014). Bresciani (1965) was the first to describe the cypridoid larva, or y-cyprid, from the Sound in Denmark. The y-cyprid (Fig. 1K) has a univalved carapace, with five pairs of lattice organs. They also have clawed antennules specialized for putative mechanical attachment (Fig. 1L), a spinous labrum (Fig. 1M), so-called paraoccular processes and postocular filamentary tufts, and paired compound eyes (Fig. 1J, K). The six pairs of thoracopods are biramous (Fig. 1K). The abdomen consists of two or four segments, including a long telson, which carries pores and sensory setae. It terminates in two unsegmented furcal rami, which hold setae (Høeg et al., 2014a).
Exposure of y-cyprids to a crustacean moulting hormone succeeded in inducing them into a slug-like stage called the ypsigon. This stage lacks eyes, a gut, appendages or any other external structures (Glenner et al., 2008). The ypsigon is unsegmented ( Fig. 1N-P), with the body surrounded by a < 5-nm-thick cuticle and an epithelium, enclosing a variety of cells, including a relatively large neuropile (presumed to be derived from the y-cyprid central nervous system) and a series of vesicles, apparently containing stored lipids. The ypsigon therefore resembles the rhizocephalan vermigon, which is the first internally parasitic stage in the parasitic barnacles. The y-cyprid and the ensuing ypsigon, therefore, strongly suggest that juvenile and adult facetotectans are endoparasites in hosts that are yet to be found.
It is already clear from published work (Pérez-Losada et al., 2009)   . The cirri are rudimentary, and the base of the peduncle has a root-like structure embedded inside the tissue of the shark to absorb nutrients. D, Fistulobalanus albicostatus, common on mangrove tree trunks. E, the family Pyrgomatidae are associated with scleratinian corals, and the bases are embedded in the coral skeleton. Cantellius sp. extending the cirri. F, Ibla cumingi, which is located at the most basal position on the molecular phylogenetic tree of thoracican barnacles. G, Megabalanus, Amphibalanus and Lepas inhabiting the capitulum can exist even at the same localities ( Fig. 1A-I). We recommend that species descriptions in Facetotecta be based on molecular data backed by detailed morphological description of comparable larval instars. For this, we suggest using the last naupliar instar (Fig. 1J), which can be documented in vivo and then collected as an exuvium after the metamorphosis to the y-cyprid (Fig. 1J, K). From this, data can potentially be added for prior and succeeding instars from the same individual when available (Fig. 1I). The surprising morphological variation among facetotectans at even a local scale also extends to the ypsigon stages; hence, potentially, to the process of host infection. Glenner et al. (2008) depicted both long, slender and short, plump types of ypsigons, and this recalls the variation in the invasive stage (kentrogons and vermigons) found among the rhizocephalan parasitic barnacles Høeg et al., 2012). Future studies must investigate how these variations in morphology of larval and parasitically invasive stages relate to differences in biology among this enigmatic group of thecostracans.
The larvae and adults of Ascothoracida have been studied extensively since the seminal monograph of Wagin (1976), but there are but few accounts on laboratory rearing of larval series and none on their settlement and metamorphosis into parasites. Important accounts of larval development and features were given by Ito & Grygier (1990). Høeg et al. (2014b) summarized the available information on ascothoracidan larval biology, which is highly variable because the larvae can be released as either planktotrophic or lecithotrophic nauplii or directly as the cypridoid stage. The latter should now be called the 'a-cyprid' (Fig. 2C) for comparison with the homologous facetotectan y-cyprid (Fig. 1K) and the cirripede 'true' cyprid (Figs 3B,4B,E,5B) and is equivalent to the term 'ascothoracid larva' used until recently. Grygier ( , 1987a performed comprehensive taxonomic and morphological reviews of ascothoracidans, the majority of which forms the basis of the following text. Generally, the body is covered by a laterally compressed, smooth and bivalve-like carapace ( Fig. 2E-N), under which the embryos or larvae (= nauplii and a-cyprids) are carried (brooded). Some species have fused valves, sometimes with bizarre extensions. Generally, the females and males have a mid-gut diverticulum, and their gonads are placed in the carapace. They carry four pairs of appendages followed by 11 free trunk somites in total. The prehensile antennules are uniramous, consisting of four to six segments. On the distal segment, they carry a moveable or unmoveable claw, a claw guard and sensory setae, some of which are aesthetascs (Fig. 2C, D). Antennae are absent in adults. The labrum forms a conical oral cone that surrounds piercing mouthparts, which comprise paired mandibles, maxillules and maxillae and unpaired paragnaths, although some of these appendages can be reduced. The penis can be both uni-and biramous and is always located at the seventh trunk somite. The last segment (telson) has moveable, unsegmented furcal rami. The thorax and abdomen can be differentiated or undifferentiated, with three to six pairs of thoracopods of both uniramous and biramous nature. The thoracopods are plesiomorphically setose and biramous, with two-segmented exopods and three-segmented endopods (two-segmented in pairs 1 and 6); advanced forms can have uniramous unsegmented thoracopods. The female gonopores are situated at the base of the first pair of thoracopods, and the coxae of thoracopods 2-5 in females usually carry seminal receptacles for spermatozoa. Compound of a penduculated barnacle. H, Capitulum mitella, a stalked barnacle inhabiting rocky shores. In molecular phylogenetic analyses, it is the sister to Anelasma (see C). I, Conchoderma, a stalked barnacle with reduced shell plates, epibiotic on many marine taxa, mostly crabs and whales. J, An early dwarf male settled on the surface of an adult Heteralepas. K, Octolasmis cor lives exclusively on the surface of crab gills. L, Scalpellum scalpellum, a stalked barnacle epibiotic with hydroids. The inserts in LM (upper) and SEM (lower) shows the hermaphrodite receptacles, which can house dwarf males. Inserts show a dwarf male with extended penis (upper) and an SEM micrograph of the receptacle structure. M, Waikalasma is a deep-sea inhabitant, having a row of imbricating plates on the base of shells. N, Chthamalus malayensis is common on intertidal shores. O, Tetraclita, with four shell plates, common on rocky and tropical shores.  eyes are present only in larval stages, although their rudiments are fused with the frontal filaments into a sensory organ.
T h e r e a r e f e w m o l e c u l a r a n a l y s e s o f t h e Ascothoracida, and there is no detailed study of the molecular systematics of the group. The systematics based on anatomical characters have been studied in detail (Wagin, 1976;Grygier, 1983Grygier, , 1987a. We follow the taxonomy of Grygier (1987a), which recognizes two orders (here ranked suborders), each with three families. Although not formally based on apomorphies, this taxonomy nevertheless attempted to recognize monophyletic groups, and our molecular data seem to confirm the monophyly of most of the six families. Members of the order Laurida clearly exhibit many plesiomorphies, whereas those of the Dendrogastrida are mostly more advanced.

SubclaSS cirripedia burmeiSter, 1834
The Cirripedia comprise the infraclasses Acrothoracica (burrowing barnacles), Rhizocephala (parasitic barnacles) and Thoracica (stalked and acorn barnacles). Owing to the morphological divergence among these taxa, the only characters that can be compared are those that concern the larvae (Figs 3-5). Thompson (1830Thompson ( , 1836 was the first to recognize that all cirripedes are united by the apomorphy of having nauplii equipped with so-called frontolateral horns (Høeg et al., 2009b;Martin et al., 2014) (Figs 3A, 4A, 5A). To this has since been added the possession of a cypris larva. The cyprid possesses a host of apomorphic traits that are adaptations to its role of locating a settlement site and cementing itself before initiation of metamorphosis Høeg & Møller, 2006). Molecular analyses all agree that each of the three infraclasses is monophyletic, with Comparison of four recent hypotheses on the phylogenetic relationships among recent Thoracica. Note the overall similarity in topology. Major discordances are highlighted by colour. Note especially the difference in the precise position of the genera Capitulum, Lithotrya and Pollicipes, but also that these taxa always appear 'close together' (highlighted in yellow) in the trees. In the analysis by Herrera et al. (2015), these genera are nested inside the Balanomorpha (dotted square). See detailed explanation in the main text.
the Acrothoracica diverging first (Pérez-Losada et al., 2009;Lin et al., 2016), being sister group to a clade comprising Rhizocephala and Thoracica (Fig. 7). The reduced morphology of rhizocephalans entails that only a few morphological apomorphies in the cypris larvae are shared between these parasites and thoracican barnacles (Høeg et al., 2009b). Moreover, for the same reason it cannot be assured whether early fossils with a thoracican-type morphology should be situated above or below the split between these two taxa. For this reason, there is little purpose in creating a new name and rank for the Rhizocephala-Thoracica clade.
The Acrothoracica differ from the stalked and acorn barnacles (Thoracica) in lacking calcareous capitular and opercular shell plates altogether   (Fig. 3). A calcified element exists in some species of Lithoglyptida, but it is highly debatable whether this structure has any homology with thoracican elements (putatively with thoracican calcareous basis). Such homology is also dubious because stem cirripedes would undoubtedly have had phosphatized shell plates when the Acrothoracica diverged (Newman, 1971(Newman, , 1974Grygier & Newman, 1985;Kolbasov, 2009). During the voyage of HMS Beagle, Darwin discovered his first barnacle, an acrothoracican in a gastropod shell from Chile (Tomlinson, 1987). He had such difficulty in assigning this species to a taxonomic group that he named it 'Mr. Arthrobalanus' and only later described it as Cryptophialus minutus.
Acrothoracican barnacles are dioecious, with largesized, suspension-feeding females and dwarf males attached on the exterior of the female mantle sac (Kolbasov, 2009;Chan et al., 2014a). The aperture of the female mantle sac is surrounded by a pair of chitinous opercular bars at the upper part and a comb collar around the posterior margin of the aperture, which is reduced in a few specialized species. The basal (morphologically anterior) part of the mantle forms an attachment disc, serving the purpose of fixing the animal to the burrow. The boring apparatus comprises ctenoid multifid scales, with several spines and papillae. The adult females retain the larval post-oral adductor muscle. The thorax is highly elongate and is equipped with four to six pairs of segmented and generally biramous cirri, of which three to five terminal pairs form a tuft or basket for food capture. The terminal cirri are widely separated from a pair of mouth cirri, situated adjacent to the oral cone. Unlike the situation in Thoracica, the acrothoracican mandibles retain the plesiomorphic connection with the mandibular palps. The dwarf males range from being pear shaped to being elongated sometimes with lateral projections; Fig. 3H) and they are attached on the mantle sac of the females or on the burrow wall close to the opening. The males are non-feeding, and their morphology varies considerably among the different taxa. Some morphological features are undoubtedly autapomorphies for the Acrothoracica (opercular bars, orificial knob, the entire burrowing apparatus with multifid ctenoid scales and the separation of mouth cirri from the terminal feeding basket). In contrast, the postoral position of the adductor muscle, exactly as in cypris larvae, is clearly a plesiomorphic character, because the same condition is found in the Ascothoracica.
Based on combined morphological and larval characters, Kolbasov (2009) divided the Acrothoracica into two orders, Lithoglyptida and Cryptophialida, and three families, Lithoglyptidae, Trypetesidae and Cryptophialidae. Before this, Berndt (1907) had originally divided the burrowing barnacles into two orders, the Pygophora and the Apygophora, based on the morphology of terminal cirri and the presence or absence of an anus. This division was based on few morphological characters, with some now understood as being symplesiomorphies, e.g. biramous terminal cirri and presence of an anus in Pygophora. In contrast, Apygophora have three pairs of uniramous terminal cirri, and they lack an anus (Berndt, 1907;Tomlinson, 1969). The Pygophora were divided into two families, the Lithoglyptidae and Crytophialidae, whereas Apygophora comprised only the Trypetesidae. Grygier & Newman (1985) considered the Lithoglyptidae as being paraphyletic owing to their several symplesiomorphies, and this view also gains some support from molecular analyses (Lin et al., 2016). Kolbasov (2002) found that dwarf males of Cryptophialida and Lithoglyptida differ in both cuticular structure and body form, and Kolbasov & Høeg (2007) described significant differences in cypris ultrastructure among the three families. The detailed study by Kolbasov (2009) offered the most comprehensive study of the Acrothoracica and presented a revision of the taxon that contradicts the systematics of Berndt (1907). Based on all this information from larvae and adults, Kolbasov (2009) erected two new orders, the Lithoglyptida (with families Lithoglyptidae and Trypetesidae) and the Cryptophialida (with only the Cryptophialidae). Thus, two former orders, Pygophora and Apygophora, were here considered as the families Lithoglyptidae and Trypetesidae under Lithoglyptida.
Several characters of the Lithoglyptida are de facto symplesiomorphies, such as bag-shaped mantle sacs, the presence of well-developed mouth cirri in females, the saddle-shaped labrum, elongated opercular bars, biramous natatory thoracopods in the cypris larvae and the presence of terminal pores in cyprid lattice organs. Several autapomorphies characterize the Trypetesidae, e.g. the reduced, uniramous terminal cirri, the absence of a comb collar, reduced mouthparts, the absence of an anus in females, and a complex external morphology of the dwarf males. The Lithoglyptidae and Trypetesidae share several characters not found in the Cryptophialidae, such as the orificial knob in females, lateral 'wings', a long attachment stalk in the dwarf males, and more setulated and externally complex setae on the fourth antennular segment in cyprids. The cryptophialid females have flask-like mantle sacs with an elongated neck area and lateral bars, crown-like opercular bars, an elongated labrum, reduced mouth cirri and a gastric mill; furthermore, the cryptophialid cyprids are characterized by the union of the subterminal and terminal setae on the fourth segment antennular segment and thoracopods reduced to the extent whereby they cannot swim but move only by walking on their antennules.
Here, we follow Kolbasov (2009) and Lin et al. (2016) in dividing the Acrothoracica into Lithoglyptida and Cryptophialida. This is at present the best available scheme, although Lithoglyptidae has an unresolved relationship with the Trypetesidae, whence the monophyly of the Lithoglyptida is not assured. Cryptophialida is monophyletic.
Females have a big, sac-like mantle, a wide aperture, an operculum without a neck, long opercular bars and a big labrum. The mouth cirri are well developed. The thorax lacks long dorsal processes, and gastric mill is absent. The dwarf males have numerous cuticular projections and lack mantle teeth. The burrow aperture is elongated and slit-like (Fig. 3F). The cyprids have developed natatory thoracopods and an unperforated, smooth carapace with frontolateral pores. The five pairs of lattice organs, situated on the dorsal part of the carapace, are narrow and elongated. The terminal pores are sited in an anterior position in lattice organ pair 2, but posterior in pairs 1, 3-5. The surface of each organ is perforated by small, round pores. On the antennules, the setae of the fourth segment are clearly separated into subterminal and terminal clusters. Several of the lattice organ details are plesiomorphic compared with the Rhizocephala and Thoracica and therefore support the molecular evidence for a basal divergence of the Acrothoracica. The Lithoglyptidae is distinguished by the number of terminal cirri in females. Kolbasov & Newman (2005) revised the largest and perhaps most plesiomorphic genus, Lithoglyptes Aurivillius, 1892 s.l., and divided it into three genera. One (Kolbasov & Newman, 2005) turned out to be a junior synonym; therefore, the correct generic names are now Lithoglyptes (s.s.), Auritoglyptes Kolbasov & Newman, 2005and Balanodytes Utinomi, 1950(Chan et al., 2013. Lithoglyptid females have opercular bars and comb collars. The operculum carries rows of multifid scales, which are often associated with pores and papillae. The protopod of the mouth cirri are two-segmented and the mouth appendages are well developed. The terminal cirri are multisegmented, biramous and widely separated from the mouth cirri. Caudal appendages can be present or absent depending on species. Thoracic lappets and an anus are present. Several species have an anterior (functionally basal) calcareous plate, which is covered by the cuticle of the attachment disc. Dwarf males lack lateral lobes at the base of their attachment antennules or stalk, and their posterior end has an apertural slit, which may be covered by a semicircular fold.
The species from the Trypetesidae live in the columella of gastropod shells occupied by hermit crabs. The females have thin opercular bars, and the comb collar is reduced. The operculum lacks pores, papillae and multifid ctenoid scales. The mouth appendages are reduced in size, and the protopod of the mouth cirri is unisegmented. There are three pairs of foursegmented, uniramous terminal cirri but no caudal appendages. Thoracic lappets are reduced in number and size, and there is no anus. Trypetesid dwarf males have a complex shape and form, with a pair of lateral lobes at the base of the attachment stalk, but they have no apertural slit. A recent study provided new data on reproduction and the relationship to the hermit crab host in Trypetesa lampas (Hancock, 1849) (Larsen et al., 2016).
The species of the Cryptophialida live on the surface of gastropod shells, corals and calcareous rocks. The females have a flask-like mantle. The neck of the operculum is elongated, with well-developed rod-like lateral bars. The aperture is small, with crown-like opercular bars along its total length. The labrum elongated and tongue-like, but the mouth cirri are rudimentary. The thorax has one or two whip-like, long dorsal processes and thoracic lappets with a clear cuticular border. Internally, the intestine has a gastric mill or gizzard. Caudal appendages are lacking. The dwarf males lack external cuticular, papilliform projections, but their posterior end has circular ribs and often also conspicuous mantle teeth. The cyprid carapace has a perforated sculpture, but there are no frontolateral pores. Frontolateral pores are almost universally present in cirripede cypris larvae, but they are also absent in some specialized species within the Rhizocephala. The elongated lattice organs have no large terminal pores, but numerous small pores.

inFraclaSS rhizocephala müller, 1862
A new taxonomy of the Rhizocephala (Fig. 4) based on molecular data was recently published  and is followed here. That study and Høeg et al. (2019) also provided an in-depth account of rhizocephalan morphology and biology across the several families, whence we only summarize the main points. Owing to the highly reduced morphology of the rhizocephalan parasites, the taxa recognized here are defined by means of molecular characters. Høeg et al. (2020) also offer morphologically based diagnoses, but they are only rarely based on apomorphies for the taxa concerned.
General biology and life cycle: Rhizocephalans are highly advanced parasites on other crustaceans, principally decapods, and the adult stages are so reduced that they offer virtually no clue to the phylogenetic position as crustaceans or even as arthropods (Høeg, 1995a;Høeg & Lützen, 1995;Walker, 2001;Høeg et al., 2015Høeg et al., , 2019Høeg et al., , 2020. The freeswimming larval stages are, in contrasts, similar to those of other cirripedes, with which they share several apomorphies. Therefore, since Thompson's (1836) benchmark study of the larvae, the relegation of these parasites to the Cirripedia has remained virtually unchallenged. In contrast, their position within the Cirripedia, whether or not they constitute a monophyletic taxon and their intrinsic relationships had largely to wait for results from molecularly based studies (Glenner & Hebsgaard, 2006). The reason is that the parasitic stages have an extremely simplified gross morphology, lacking segmentation, appendages and most other structures apart from the reproductive organs ( Fig. 4I-N). Therefore, systematics of Rhizocephala had to be based on simple similarities in the few characters available, such as the shape of the external parasite (externa) or the gross structure of the reproductive organs (Øksnebjerg, 2000). The parasitic phase is initiated after a cypris larva settles on the potential host. This occurs either by means of a so-called kentrogon stage that can follow the settled cyprid (Fig. 4E, F) or by action of the cyprid itself . Then follows an entirely endoparasitic phase until the parasite emerges on the surface of the host as an external reproductive sac (externa), connected to an internal and nutrient-absorbing root system by a stalk (Glenner, 2001) (Fig. 4F-H). All rhizocephalans have separate sexes, with extremely reduced dwarf males hosted within the female externa (Fig. 4C, D). The males can either reside in special organs within the female (receptacles or spermatogenic islets) or be situated directly in the female tissues (Yanagimachi, 1961;Høeg, 1987Høeg, , 1991Høeg & Lützen, 1995). Owing to this specialized morphology, there is no option for an outgroup comparison with other cirripedes, whence any morphological character matrix will be virtually impossible to polarize. In contrast, characters in the larvae can be compared directly with other cirripedes and are therefore useful in the few cases where they are available in detail (Glenner et al., 2010).
Phylogeny and taxonomy: The molecular phylogenies have now shown that Rhizocephala is a monophyletic taxon and is placed consistently as sister to Thoracica (Fig. 7). A number of detailed studies on species from across the taxa provided the basis for the complete family-level revision by Høeg et al. (2020), and this resulted in a taxonomy with 13 monophyletic families that could be placed in a fairly well-resolved phylogeny.
The new taxonomy entailed several important revisionary steps. One was the division of the largest family into two, a revised Sacculinidae and the new family Polyascidae. Species within these two families are impossible to separate by morphology, but the molecular data clearly indicate that they form two taxa that are not closely related. The revision also entailed splitting off two new families from a re-diagnosed Peltogastridae. Finally, a major change was the abandonment of the former taxonomic subdivision into the suborders Kentrogonida and Akentrogonida (Høeg & Rybakov, 1992). These suborders were based on the host invasion proceeding either by a kentrogon stage (Fig. 4F) following the cyprid ('Kentrogonida') or by the cyprid itself using one of its antennules to penetrate the host integument ('Akentrogonida'). The molecular data showed that the kentrogonid type represents a plesiomorphy and that the akentrogonid type of metamorphosis has evolved at least twice homoplastically. The position of the 'akentronid' Mycetomorphidae close to the Peltogastridae and separate from the remaining 'akentrogonid' families ( Fig. 4) argues for the lack of utility or the absence of a kentrogon as a unique synapomorphy to characterize a taxon. In contrast, the molecular data also suggest that the remaining 'akentrogonid families', other than the Mycetomorphidae, form a monophyletic unit (Fig. 4). Moreover, this taxon can also be characterized by morphological apomorphies, such as the absence of paired receptacles exiting through a cuticular duct (Høeg, 1991) and a reduced pattern of setae on the cypris antennules (Glenner et al., 2010). Therefore, we might in the future again recognize these taxa as a monophyletic group with a formal rank and new name. At present, however, there are no taxonomic ranks above the family level within the Rhizocephala, although the 13 families can be placed in a fairly well-resolved phylogeny.
Character evolution: It is now clear that it is plesiomorphic to infest the host by means of a kentrogon and for the female externa to have paired male receptacles communicating with the mantle cavity through a cuticular duct. These receptacles serve to receive the trichogon larva (Fig. 4C, D) that follows settlement of male cyprids (Fig. 4B, C) on the virginal parasite and to host and nourish the resulting dwarf male through several reproductive cycles (Ritchie & Høeg, 1981;Høeg, 1987Høeg, , 1991. The original host taxon was Anomura, such as in the families Triangulidae, Peltogastridae and Peltogasterellidae (Fig. 4I). Evolution onto Brachyura and other hosts occurred in other lineages (e.g. Sacculinidae and Polyascidae), and in some this was accompanied by both loss of the kentrogon stage in host infection (e.g. Clistosaccidae and Thompsoniidae) and advanced modes of implanting and hosting the males in the female body without trichogons or receptacles (Høeg, 1991;Høeg & Lützen, 1995). A unique parasitic host specialization was reached in species of the Chthamalophilidae, which infest balanomorphan barnacles and where males are hosted in so-called spermatogenic islets that float freely in the mantle cavity of the female (Høeg, 1991;Yabuta et al., 2020). Uniquely among all cirripedes, except the Acrothoracica Cryptophialidae, the chthamalophilid cyprids lack thoracic appendages and can therefore disperse only by walking on the substratum using their two antennules. An evolutionary bridge to this advanced family may be seen in the Duplorbidae, which have a similar sexual system and infest various Peracarida, but where the cyprids retain a more plesiomorphic morphology, with thoracopods for swimming.

inFraclaSS thoracica darwin, 1854
The Thoracica (Fig. 5) comprises the pedunculated (with a stalk) and sessile (no stalk) barnacles. In the ground pattern (Anderson, 1994), the body of thoracicans consists of a flexible peduncle and a capitulum, the latter being armed with a number of mineralized plates. Secondary loss of the peduncle has occurred convergently at least twice, viz. in the Neoverrucidae and in the 'sessilian' lineage leading to the Verrucomorpha and Balanomorpha (Fig. 5). The original number of shell plates was four; the paired scuta-terga that guards the opening into the mantle cavity. Later forms acquired increasing numbers of plates, but reductions and even complete loss of plates also occurred in several lineages (Chan et al., 2009) (Figs 5I-K, 9, 10). Closing of the mantle cavity is by the scutal adductor muscles, the position of which has phylogenetic value. Inside the capitulum, the soma is suspended in the mantle cavity. The soma carries the mouth cone composed of the labrum, with mandibles, maxilla and maxillules (these appendages are often called trophi) (Chan et al., 2009). Unlike the Acrothoracica, the mandibular palp has separated from the mandible itself. The trophi are followed by six pairs of biramous cirri (Fig. 5E) and sometimes paired caudal appendages. The specialization of anterior cirri into mouth appendages is important in both classification and feeding biology. The soma represents both parts of the cephalon and the thorax, and an abdomen is lacking. It is uncertain which, if any, of these characters constitute autapomorphies for the Thoracica, especially compared with the Rhizocephala, for which these characters cannot be scored (Høeg et al., 2009b). It is likely that stem-line cirripedes below the Thoracica-Rhizocephala node also had a thoracican-like morphology, sporting both a peduncle and shell plates. Nevertheless, this does not change the monophyly of the extant forms, which is supported by all molecular analyses published until now (Pérez-Losada et al., 2002, 2004Rees et al., 2014;Lin et al., 2015).
Shell plates and growth: Within the mantle cavity, moulting occurs by complete shedding of the old cuticle, as in any other crustacean, but on the external surface the process is highly modified (Fig. 11). The shell plates are mineralized parts of the original cuticle, and their presence has profoundly changed the mode of growth in thoracicans (Bourget & Crisp, 1975;Bourget, 1987;Blomsterberg et al., 2004). The plates themselves increase in area by progressive mineralization around their edges. Regular moulting occurs in the variably sized cuticle areas between the shell plates, and here new cuticle is produced in infolded areas of the epidermis that form a system of growth zones between the plates. At moulting, the old cuticle is shed only as narrow band above the new cuticle and splits apart on either side (Mahmoud, 1959(Mahmoud, , 1960. The two edges of the old cuticle therefore form a pair of 'growth lines' running in parallel on either side of the growth zone area. In balanomorphans, these lines have also been called 'cuticular slips' and are often hard to see (Bourget & Crisp, 1975), but in species with broad cuticle areas between smaller sized plates, e.g. Octolasmis Gray, 1825 and some scalpellids, the bands of multiple, parallel growth lines form characteristic patterns on the external surface that testify to the number of moulting events experienced by the specimen (Figs 5M, 9) (Dreyer et al., 2018c).
Completely separate from the cuticular slips or growth lines, the shell plates also show a pattern of semiconcentric lines resulting from their progressive increase in size by stepwise mineralization along the edges. Importantly, these lines do not correspond in number to the cuticular growth lines that result from moulting. On increasing in size, the older organic cuticle areas, including the growth lines, come to lie on the surface of the plates, where they are gradually abraded. Such a system of retaining the old cuticle and producing growth lines is known also from the Branchiopoda 'Conchostraca' (Bourget & Crisp, 1975;Bourget, 1987;Blomsterberg et al., 2004). Multiple times, especially in the Lepadoidea, shell plates have been secondarily reduced to rudiments or disappeared altogether. The plates are obviously secreted by the epidermis and are thus part of the body cuticle. Unfortunately, little attention has been given to the plate microstructure and the process by which the mineralization proceeds, but recently, state-of-the-art techniques offer hope for renewed insight (Checa et al., 2019;Mitchell et al., 2019). It is clear that the presence of mineralized shell plates in the integument involves multiple interrelated apomorphies with no parallel elsewhere in the Arthropoda. It complicates the issue that the shell plates were originally phosphatic and only later changed to calcitic mineralization. Unfortunately, we cannot decide precisely where in the phylogeny shell plates and their mineralization first appeared, because it is open to speculation whether the ancestors to the Rhizocephala and Acrothoracica were completely unmineralized or whether they evolved from a cirripede with some form of armature that was secondarily lost. The only solution to this important question would be a detailed analysis that places crucial fossils within the part of the cirripede tree below the last common ancestor to all recent Thoracica, but no such species has been identified with any certainty (Høeg et al., 2009b). In contrast, as detailed below, the change from phosphatic to calcitic plates and their gradual increase in number can be followed in the fossil record (Figs 9, 10).

Plate numbers in thoracican evolution:
The number of plates and their disposition have always featured prominently in studies on thoracican evolution. The traditional hypothesis suggested that, commencing with four plates (paired scuta and terga) in the Iblomorpha, the number of capitular plates increased progressively to five (Lepas Linnaeus, 1758 and Praelepas), six (Eolepas Withers, 1928) and then to eight with the addition of paired upper latera (Neolepas Newman, 1979) (Fig. 9). Subsequent evolution led to an increase in the number of lateral plates, resulting in a multi-plated condition (Broch, 1922;Newman, 1987;Anderson, 1994;Buckeridge & Newman, 2006). This interpretation was based on the ontogeny of the juvenile barnacles, especially in pollicipedids (Broch, 1922), calanticids and scalpellids, where growth starts with a near-simultaneous development of five primordial chitinous plates, followed by increase in a 6-8-12+ pattern (Fig. 9). The interpretation is also supported, in part, by the fossil evidence, because in phosphate-shelled forms there was a progressive increase in plates through a five-plated Praelepas (with carina added; Carboniferous) to a six-plated Eolepas (also with a rostrum; Triassic-Jurassic). It is nevertheless important to state that none of the extant forms with five plates (Lepadoidea) or eight plates (Neolepadomorpha) has retained an ancestral condition but that they owe the number of their plates to secondary loss from multi-plated ancestors (Figs 7,9). Both the molecularly based phylogenies and new interpretations of fossil data agree that such numbers of plates resulted from several separate plate-loss events from multi-plated ancestors (Pérez-Losada et al., 2008, 2012aGale, 2015aGale, , 2019Gale & Schweigert, 2015). In fact, the supposedly 'primitive' Neolepas and Lepas (Newman, 1987; did not appear until the Eocene and have calcite shells (Ullman et al., 2018;Gale et al., 2020).
The mineral transition to calcite took place in a shell that had all six primary plates present, followed by a gradual addition of lateral plates and the addition of peduncular scales. The Jurassic Archaeolepadomorpha (Archaeolepadidae, Myolepadidae and Stramentidae) started with few or no lateral plates, but had large, robust and imbricating peduncular plates arranged in ten vertical columns. The Stramentidae had two pairs of laterals, and a multi-plated situation was reached in the Jurassic Zeugmatolepadidae, which have eight or more laterals in addition to the six primary plates (Gale, 2019).
A few extant thoracicalcareans, such as Capitulum Gray, 1825, are still armed with numerous plates, and fossil evidence points to this as a plesiomorphic condition for the more derived thoracicalcareans (Gale, 2019). Calanticids can carry a highly variable Figure 9. Timeline of Thoracican evolution. Generic diversity of the Thoracica from the latest Triassic to the present. Early forms are entirely phosphatic, followed by calcitic pedunculated forms. Balanomorphans (acorn barnacles) appeared in the Cretaceous, but they radiated only after a low-diversity period following the Cretaceous-Palaeogene (K-Pg) extinction. The diversification starting in the Neogene is still ongoing. Living genera shown include only forms with shell plates, in order to compare with the fossil record. Further explanation is given in the main text. (1) Eolepadomorpha, Eolepadidae, Eolepas; (2) Eolepadomorpha, Eolepadidae, Toarcolepas; (3) Pollicipedomorpha, Zeugmatolepadidae, Concinnalepas; (4) Archaeolepadomorpha, Archaeolepadidae, Archaeolepas; number of numerous, small lateral plates (Jones & Hosie, 2009), whereas in the Scalpellidae the maximum is reduced to 14, and in various fossil taxa it is reduced even further, down to six plates in some members of the Titanolepadidae, Virgiscalpellinae and Zeugmatolepadidae (Gale, 2020a;Gale et al., 2020). In Neolepadidae and some Brachylepadidae, only eight plates are present, again owing to secondary loss (Fig. 9).
All this shows that the armature of shell plates has been subject to complicated patterns of gain and loss throughout thoracicalcarean evolution. The reason for the five-plated condition in extant Lepadoidea might be that they simply arrest development at what corresponds to the earliest ontogenetic stage in multiplated forms. Most lepadoid species are adapted either to an epibiotic life, where protection is less important, or to living on ephemeral substrata, where rapid attainment of sexual maturity is at a premium. The apex of this trend is seen in lepadoideans that have lost all plates, such as all members of the Heteralepadidae and some species of the Poecilasmatidae (Anderson, 1994;Yusa et al., 2001;Hosie, 2014;Buhl-Mortensen & Mifsud, 2017).
Phylogeny and taxonomy: Traditionally, Thoracica were often divided into the 'Pedunculata' and the 'Sessilia'. The diagnosis of Pedunculata (stalked barnacles) was the possession of a body divided into a peduncle (stalk) and a capitulum, but this is clearly a plesiomorphic condition, and the taxon is accordingly not monophyletic. In the study by Buckeridge & Newman (2006), the Thoracica were divided into the orders Cyprilepadiformes (fossil only), Ibliformes, Lepadiformes, Scalpelliformes and Sessilia (nonstalked barnacles), but they did not base this division on morphological apomorphies, and molecular data were not yet available. Here, we also recognize some of these taxa, although sometimes with different names, definitions and composition of species.
As already mentioned, we do not recognize Priscancermarinus and Cyprilepas as cirripedes. Priscancermarinus from the Burgess Shale formation (Collins & Rudkin, 1981) does not possess any convincing cirripede features, and the dating of lineages provided by Pérez-Losada et al. (2004) would probably have been different if this fossil had been omitted. Cyprilepas is a small, bi-valved, organically walled fossil found attached to Silurian eurypterids (Wills, 1963) and with limited clear morphological features revealed (without any signs of the presence of appendages). Although having some similarity with a newly settled barnacle, the validity of Cyprilepas as a cirripede is still controversial. Even the conclusion by Wills (1963) suggests that Cyprilepas might also be an adult entomostracan, and we might add that it could even be a giant shelled ciliate. To decide on this will require re-analysis by modern analytical methods that can reveal internal structures, as was done for another alleged cirripede, Rhamphoverritor Briggs et al., 2005, from the Silurian. This was claimed to be a crown group cirripede, but the presence of a distinct and segmented abdomen puts it at best as a stem-line representative of that taxon (Høeg et al., 2009b). Therefore, we consider the oldest certain record of a thoracican cirripede to be Praelepas from the Carboniferous (320-330 Mya) (Figs 9, 10).
Initially, plates covering the body might have consisted of cuticle only, and all palaeontological evidence indicates that the original mineralization was by phosphatization, with calcification being apomorphic (Fig. 9). Based on this, we follow Gale (2015aGale ( , 2019 and Gale & Schweigert (2015) in dividing the Thoracica into Phosphatothoracica and Thoracicalcarea (Figs 7,9). The Phosphatothoracica is clearly paraphyletic when extinct taxa are included, but it is a convenient concept to host phosphatic thoracicans, many of which cannot easily be put into a resolved phylogeny. The Thoracicalcarea comprise all extant Thoracica except the Iblomorpha (Fig. 7) and is monophyletic in all molecularly based analyses.
Among extant species, the morphological difference between Phosphatothoracica and Thoracicalcarea is extensive, but this largely disappears when fossil forms are included. In the hard parts, the difference involves the transition from a plesiomorphic condition with only four phosphatic plates (paired scuta and terga; Iblidae), with the addition of a carina (Praelepadidae), a rostrum (Eolepadidae) and through to forms with a calcitic plate armature with numerous plates, such as in extant Capitulum and Pollicipes Leach, 1817. In soft body anatomy, there are numerous additional differences between the two groups (Anderson, 1994), a spectacular one being the plesiomorphic retention in Phosphatothoracica of post-oral scutal adductor muscles as opposed to the pre-oral position in Thoracicalcarea. A post-oral position is also found in the Acrothoracica, the Ascothoracica and the cypris larva of all cirripedes. The two adductor muscles are clearly not homologous, because ontogenetic studies have shown that both can exist at the same time during the metamorphosis from cyprid to juvenile, and probably also did so during evolution from Ibla Leach, 1825 to the extant Thoracicalcarea (Glenner . Phylogram of thoracican phylogeny, based on a combination of morphology (fossils and extant taxa) and guided by molecular data. The successively later first appearance of more crownward (derived) taxa supports the overall story, although there are some 'ghost' ranges within individual clades, in which fossil taxa appear anonymously late in the record. Note that based on the morphological interpretation, the Pollicipedomorpha are here placed basalmost among extant Thoracicalcarea. Compare with the tree derived from molecular analysis of extant forms (Fig. 7) and thoracican diversity through time (Fig. 9).  & Høeg, 1998). This shows that the accumulation of shell plates towards extant Thoracicalcarea must also have been related intimately to changes in soft body characters, such as muscles.

Superorder phoSphatothoracica Gale, 2019
The Phosphatothoracica is a paraphyletic taxon that comprises all extinct and extant thoracicans with phosphatized shell plates. Among extant forms, it comprises only the Iblomorpha, which is probably monophyletic. Iblomorphans have only four shell plates (Fig. 5F), viz. the paired scuta and terga, and all evidence points to this as an original condition.  . Moulting and growth in Thoracica. Moulting occurs in the cuticular zones separating shell plates. A, new cuticle is formed by infolded epidermis in a narrow growth zone between two adjacent shell plates. B, old cuticle is shed only above the growth zone. The broken-off cuticle leaves distinct growth lines on either side of the growth zone. C, the new cuticle expands by outfolding; the growth zone is not distinct during intermoults. D, a new growth zone forms for the next moult; the shell plate can now expand laterally under the cuticle; older growth lines can thereby be included on top of the plate. Constructed from descriptions by Mahmoud (1959Mahmoud ( , 1960, Bourget & Crisp (1975) and Blomsterberg et al. (2004Blomsterberg et al. ( ). et al., 2015, and they form the sister group to the Thoracicalcera, i.e. all remaining extant thoracicans (Fig. 7). This agrees with morphology, because most, if not all, morphological characters distinguishing extant iblomorphans from other Thoracica are putative plesiomorphies Buckeridge & Newman, 2006). These characters include the position of the adductor muscle, chitinous spines on the peduncle, the so-called comb collar around the mantle aperture, and some characteristics in the nauplii. Buckeridge & Newman (2006) offered a review of the only eight described species of Iblomorpha and divided them into the families Iblidae and Idioiblidae.
Although not based on a cladistic analysis, we retain these families and the subfamilies contained in them.

Order eOlepadOmOrpha Ord. nOv.
These extinct phosphatothoracicans include the Carboniferous Praelepadidae (Praelepas and Illilepas Schram, 1986), with a carinal plate, and the Triassicearly Cretaceous Eolepadidae (Eolepas), which also had a rostrum, thus showing a progression in the number of plates towards the Thoracicalcarea (Figs 9, 10). The Eolepadidae Buckeridge, 1983 was redefined by Gale et al. (2020) to house Eolepas and Toarcolepas Gale & Schweigert, 2015, while excluding the calcified eight-plated members in the Neolepadinae, which were raised to family status and are presently placed much higher in the phylogeny within the Thoracicalcarea.

Superorder thoracicalcarea Gale, 2015
All molecular and morphological analyses agree that the Thoracicalcarea is a monophyletic unit (Fig. 7). There is also general agreement among the molecular studies about the monophyly of the major lineages within the group, and again, this largely agrees with the recent morphological study by Gale (2019). Nevertheless, the precise relationships of these major lineages differ somewhat between studies (Fig. 8). There is general agreement among the molecular studies on the monophyly of our Balanomorpha, Calanticomorpha, Scalpellomorpha and Verrucomorpha (Fig. 8). The sister relationship between our Lepadoidea and Scalpelloidea (= Scalpellomorpha) is also favoured by morphology, but Herrera et al. (2015) put the scalpellomorphs closer to the Sessilia (Fig. 8). The molecular studies always show the Calanticomorpha diverging below the Lepadoidea and Scalpelloidea (= our Scalpellomorpha), but Gale et al. (2019) argues by morphology that these three taxa together might form a monophyletic clade. Most uncertainty revolves around the position of our Pollicipedomorpha, comprising Capitulum, Lithotrya Gruvel, 1905 and Pollicipes (highlighted in yellow in Fig. 8). The position of these three genera is discussed in detail below. In our taxonomy, we have chosen to reflect the uncertainties by having the orders Balanomorpha, Calanticomorpha, Pollicipedomorpha, Scalpellomorpha and Verrucomorpha put as equally ranked taxa (orders) within the Thoracicalcarea. The remaining nodes in our composite family tree (Fig. 7) remain unnamed and unranked. A more resolved phylogenetic hypothesis based on morphology is available in the study by Gale (2019).

Order pOllicipedOmOrpha Ord. nOv.
Capitulum, Lithotrya and Pollicipes are pedunculated barnacles from the rocky intertidal zone, and by molecular evidence these three genera move around in different phylogenetic analyses (highlighted in yellow rectangular areas in Fig. 8). Only Herrera et al. (2015) recovered these genera as a monophyletic group. Recent mitochondrial genome-based phylogenetic analyses showed that Pollicipes and Capitulum are sister to each other Kim et al., 2018Kim et al., , 2019. Pending future analyses, we have chosen to unite the three genera into a single and, most probably, paraphyletic taxon, the Pollicipedomorpha. Most studies have placed them close to the Balanomorpha. In the studies by Pérez-Losada et al. (2008) and Rees et al. (2014), they diverged immediately below a node uniting the Balanomorpha and Verrucomorpha. Herrera et al. (2015) have them situated within the balanomorphan tree, but morphology argues strongly against this position. A recent full mitochondrial genome analysis placed Pollicipes + Capitulum as sister to the Balanomorpha, with the Verrucomorpha diverging from this node, but that study did not include Lithotrya (Kim et al., 2018). In contrast, Lin et al. (2015) placed Capitulum as sister to the remaining Thoracicalcarea (Fig. 8), with Pollicipes + Lithotrya being sisters to the Calanticomorpha as a poorly supported clade; in our Figure 10, based on fossil data, we suggest the same position. Such a basal position of pollicipedomorphs is in agreement with morphological data, where a multi-plated condition with several latera is seen as ancestral within all extant thoracican orders listed here (Gale, 2019). Both Capitulum and Pollicipes have been recorded from the Cretaceous; Capitulum and Pollicipes from the Late Cretaceous (Campanian, 80 Mya) in rocky shoreline deposits of Sweden (Gale & Sørensen, 2015); and Capitulum also from similar (94 Mya) deposits in the Czech Republic (Kocova Veselska et al., 2015).
In the study by Pérez-Losada et al. (2008), Capitulum and Pollicipes are sister groups, although without high support, and we maintain these two genera within the Pollicipedidae. Based on mitogenome phylogenetic analyses, Capitulum and Pollicipes are sister groups (Tian et al., 2020). Rees et al. (2014) showed with high confidence that Anelasma squalicola (Lovén, 1844), a parasite of lantern sharks, is a sister group to Capitulum; therefore, we also place this parasite in the Pollicipedidae and abandon the former Anelasmatidae. Keeping these diverse genera in the same family will serve exactly to emphasize extreme divergence within monophyletic lineages, something that would be obscured if Anelasma Darwin, 1852 was kept at the family rank. This surprising relationship shows that a pedunculated and suspension-feeding barnacle evolved into a parasite that gains nutrition through a system of roots penetrating into the host, as also hypothesized for the origin of the Rhizocephala (Glenner & Høeg, 2002). The divergence between Anelasma and extant Capitulum might have taken place late in the Cretaceous, although its origin of parasitism could have been much later. Lithotrya has only eight plates and is morphologically specialized for boring into calcareous substrata. At present, its uncertain position argues for maintaining the monogeneric family Lithotryida.

Order calanticOmOrpha Ord. nOv.
Both Lin et al. (2015) and Pérez-Losada et al. (2008) agree that the Calanticidae is a monophyletic assemblage and that it diverged before the Lepadoidea + Scalpelloidea (= our Scalpellomorpha), with the two studies disagreeing only in the position of the Pollicipedomorpha (see above). Gale (2019) and Gale et al. (2019) argued, from fossil and morphological evidence, that the Calanticidae diverged before the Scalpellomorpha (Fig. 9) but that calanticids are the first branch on a monophyletic lineage comprising also our Lepadoidea and Scalpelloidea. There is, in fact, substantial morphological similarity between calanaticids and scalpellids, but this could be a plesiomorphy at this level in the phylogeny. Calanticids can have numerous lateral plates, and the number is highly variable (Jones & Hosie, 2009). Following the molecular analyses, we have chosen to keep the Calanticidae within a separate order, situated at present in a polytomy that also comprises the Pollicipedomorpha, Scalpellomorpha and Sessilia (Fig. 7). The Calanticomorpha have dwarf males, in which the body consists of a peduncle and a capitulum with shell plates. These males can be situated in different positions on their female or hermaphrodite partner (inside the capitulum, on the scutal edge, on the subrostrum or on the stalk).

order Scalpellomorpha bucKeridGe & newman, 2006
Most molecular analyses agree on the monophyly of the superfamilies Lepadoidea, Neolepadoidea and Scalpelloidea, as comprised here, and that together they form a larger monophyletic clade, our Scalpellomorpha, that diverges between the Calanticomorpha and Sessilia (Figs 6, 7). Both the Neolepadoidea (eight plates) and Lepadoidea (five plates) are believed to have secondarily reduced the number of shell plates from the 14 found in the Scalpellidae. In slight disagreement, Herrera et al. (2015) have the Scalpelloidea + Neolepadoidea situated closer to the Sessilia than the Lepadoidea (Fig. 8). Within Scalpellomorpha, the Scalpelloidea and Neolepadoidea are sister groups in all molecular studies, but we do not provide a formal rank or name for that taxon. It is difficult to give a diagnosis that conforms to all members of the Scalpellomorpha, because most species have 12 plates but other forms are wholly or virtually naked (Fig. 5). Thus, the recognition of the Scalpellomorpha clade is based on it being returned in all recent molecular phylogenetic analyses (e.g. Perez-Losada et al., 2008;Rees et al., 2014;Lin et al., 2015). This does not mean that morphological apomorphies did not originally exist in the stem-line of the taxon, but they have been secondarily lost or obscured in the Lepadoidea clade and even in some Scalpellidae. A purely morphological analysis yields a slightly different phylogeny (Gale, 2019). Here, the Neolepadoidea are placed as sister to the two sessilian lineages under the name Unilatera, the apomorphy being the loss of all lateral plates except the paired upper latera. The advantage of this scenario is that the plate reduction to only a single pair of latera occurred once, viz. in the stem-line to the Unilatera, rather than convergently in lines leading to the neolepadoideans and sessilians. Topologically, the difference really concerns only a single node in the tree.
In our taxonomy, we follow the molecular studies, but we also emphasize that these disagree slightly among themselves and that every hypothetical cladogram presents its own problems in explaining character evolution. Dwarf males of Scalpellomopha are sac like, with much reduced plates, and are located in receptacles on the inner surface of the female or hermaphrodite scuta (Klepal, 1987;Buhl-Mortensen & Høeg, 2006Spremberg et al., 2012;Dreyer et al., 2018a, b, c).

Superfamily lepadOidea Superfam. nOv.
The monophyly of this taxon is supported by all molecular and morphological analyses. All members either sport five shell plates or display an even further reduced armament, with some species being completely naked (Figs 5,7,8,9). A number of lepadoid species are minute in size, specialized to epibiosis on a variety of organisms (Fig. 5G, I, K), and some were, until now, contained in families with only one or two species. Rhizolepas Day, 1939 (two species) are poorly known parasites of polychaetes. They have highly reduced cirri, a non-functional alimentary canal and have a branching root system inside the host that emanates from the peduncle. Pagurolepas Stubbings, 1940 (two species) is symbiotic with sea anemones that sit on gastropod shells inhabited by hermit crabs (Keeley & Newman, 1974). Koleolepas Stebbing, 1900 (three species) is epibiotic on sea anemones, whence they feed and may therefore be considered a true parasite (Yusa & Yamamoto, 1999;Hosie, 2014). Malacolepas Hiro, 1933 (one species) lives symbiotically with bivalves. Microlepas Hoek, 1907 andRugilepas Grygier &Newman, 1991 are two similar genera, which live on the test of diadametid sea urchins, where they induce gall formation (Yamamori & Kato, 2020). Their nutrition was recently shown to be particulate organic matter and not, as previously believed, tissue from the host. Molecular evidence now enables us to abandon the Koleolepadidae and Microlepadidae because their species are nested within other genera in existing families (Yamamori & Kato, 2020). Pagurolepas and members of the Rhizolepadidae and Malacolepadidae have not been analysed molecularly, and morphology offers virtually no clue to their position other than being naked or near naked. We therefore provisionally let Paguroplepas remain within the Poecilasmatidae and retain the two monogeneric families as incertae within the Lepadoidea. The recently demonstrated sister relationship between Anelasma and Capitulum (Figs 6, 7) hints that such specialized forms might be situated almost anywhere within pedunculated thoracicans.
Family Lepadidae: This taxon comprises both fiveplated species and naked or near-naked ones. Dosima Gray, 1825 and Lepas are five-plated forms that are highly specialized to a life floating at the surface of oceanic waters, and they form a well-supported group together with the naked or near-naked species of Conchoderma von Olfers, 1814, all of which are epibiotic (Fig. 5I). The taxonomy within the genus Lepas is not well understood, and there is a need for a molecularly based study of all the species.
Family Heteralepadidae: The members of this large family either lack any armament of shell plates completely or retain only traces of them (Fig. 5J). This is not a unique situation, because 'naked' forms are also found elsewhere in the thoracican phylogeny. Until this study, the Heteralepadidae comprised Heteralepas Pilsbry, 1907(42 species), Paralepas Pilsbry, 1907 and Alepas Rang, 1829 (five species), all of which are epibiotic forms on a variety of host animals. The analysis by Chan et al. (2009) shows a close relationship between Paralepas and Heteralepas, giving support to this family. In addition, Yamamori & Kato (2020) found that Koleolepas is, in fact, nested within Paralepas and therefore also belongs in the Heteralepadidae. We therefore abandon the former monogeneric Koleolepadidae, and our redefined Heteralepadidae now comprises Paralepas, Heteralepas, Alepas and Koleolepas, and we place it as sister to Lepadidae (Fig. 7). Some uncertainty concerns Alepas, which is epibiotic and likely to be parasitic on jellyfish (Yusa & Yamamoto, 1999), because no species have yet been analysed by molecular methods. Numerous taxonomic changes have previously taken place with the position of the multiple species of Heteralepas and Paralepas, probably because these naked forms offer few useful morphological characters. Although the monophyly of the Heteralepadidae is well supported, we might yet see some rearrangements as molecular evidence accumulates.
Family Poecilasmatidae: This family comprises both five-plated species and some with fewer plates or none at all. All species are epibiotic on a wide array of substrata, including the branchial chamber of crabs, such as in some Octolasmis species (Fig. 5K), and surfaces of decapods or sea snakes. The former Microlepadidae comprised two similar genera, Rugilepas (one species) and Microlepas (one species), both epibiotic on spines of diadematid sea urchins. Yamamori & Kato (2020) used molecular characters to place Rugilepas among species of Octolasmis. We therefore abandon the Microlepadidae and place both Microlepas and Rugilepas in the Poecilasmatidae. The family includes several genera, but species from only a few of these genera have been analysed molecularly. We therefore see no reason at present to uphold a subfamilial subdivision, especially given that the former Oxynaspinae contains only the genus Oxynaspis Darwin, 1852. Both mitochondrial and nuclear DNA sequences place Poecilamatidae as sister to Lepadidae (Pérez-Losada et al., 2008;Kim et al., 2019). At present, there is no molecular information on a special group of naked, stalked barnacles living in the inner shell surface of bivalves, Arcalepas Jones & Morton, 2009 and Malacolepas. We retain the family Malacolepadidae in the present study until further molecular phylogenetic studies can examine the phylogenetic position of this family.

Superfamily ScalpellOidea Superfam. nOv.
Our Scalpelloidea is a convenience to host only the Scalpellidae. All molecular analyses agree that the Scalpellidae (s.s., i.e. excluding the Pollicipedidae and Lithotryidae) is a monophyletic entity, and this is supported by the detailed morphological cladistic analysis of Gale (2015b), which also included fossil forms.
Family Scalpellidae: With 250+ species, the Scalpellidae is the largest family within both Thoracica and Cirripedia in general, and the taxon is well delimited by both molecular and morphological characters. In addition to the extant species diversity, the family also holds a wealth of extinct forms (Fig. 6). The number of shell plates is a maximum of 14, but secondary loss has occurred in Scalpellopsis Broch, 1921. The family is of particular interest for the study of the evolution of sexual systems Lin et al., 2015;Høeg et al., 2016;Dreyer et al., 2018a, b, c), and this calls for a detailed study of its intrinsic phylogeny-based taxonomy. The revision by Zevina (1981) included many changes, but in our opinion the diagnoses for generic and species-level taxa suffers from lack of detail. We recommend that future morphological analyses should use scanning electron microscopy of shell plates, which provide a host of details (Gale, 2015b). Until now, only a minority of the species have been subjected to analyses, but both molecular (Lin et al., 2015) and morphological (Gale, 2015b) studies agree on the existence of two species clusters within the family. We recognize these as the subfamilies Scalpellinae and Amigdoscalpellinae. In the study by Lin et al. (2015) both clusters are monophyletic, but Gale (2015b) included species not analysed molecularly, and his Scalpellinae is a paraphyletic 'ladder'.

S e x u a l s y s t e m s i n C a l a n t i c o m o r p h a a n d Scalpellomorpha:
One interesting similarity between the Calanticidae and Scalpellidae is that all or almost all species have a sexual system in which dwarf males are associated with either a hermaphrodite (androdioecy) or a female (dioecy) partner (Klepal, 1987;Yusa et al., 2012;Lin et al., 2015;Dreyer et al., 2018a). Dwarf males are also found in many species of Heteralepadidae and Poecilasmatidae within the Lepadoidea, where morphologically they are simply hermaphrodites arrested in development and might be able to mature into the latter condition (Yusa et al., 2010. In contrast, males of the calanticids and scalpellids have a reduced morphology not seen in hermaphrodite or female development (Klepal, 1987;Dreyer et al., 2018a). Dwarf males of calanticids are covered by shell plates, and in some species they retain cirri and seem to be able to feed (Klepal, 1987). The males of scalpellids are always much further specialized (Klepal, 1987;Buhl-Mortensen & Høeg, 2006Spremberg et al., 2012;Dreyer et al., 2018a). They are either naked or have only four minute opercular plates and no cirri, whence they are non-feeding and persist only by the resources provided by the egg. Locations of dwarf males in calanticids range from outside of the capitulum to alongside the mantle cavity. In the Scalpellidae, the dwarf males are always small (Fig. 5L) and situated in shell-plate receptacle areas at or inside the edge of the mantle, and in many species the site is morphologically preformed as a pocket, the size of which controls the number of resident males (Dreyer et al., 2018c) (Fig. 5L).
Aside from calanticids, scalpellids and lepadoids, sexual systems with dwarf males are omnipresent in the Acrothoracica (Fig. 3H), Rhizocephala (Fig. 4D) and Iblomorpha. This highlights the entire Cirripedia and especially the Scalpellomorpha for studying the evolution of reproductive systems and therefore the need to obtain an even more precise understanding of their relationship (Høeg, 1991(Høeg, , 1995bKelly & Sanford, 2010;Lin et al., 2016;Yamaguchi et al., 2012Yamaguchi et al., , 2013.

Superfamily neOlepadOidea Superfam. nOv.
One of the most consistent results from the molecular phylogenetic analyses of Thoracica is the grouping of several barnacles from hydrothermal vents and seeps deep within the Thoracicalcarea as sister to the Scalpellidae (Figs 7, 8). The genera in our Neolepadoidea were previously assigned to two different positions on thoracican phylogeny, also when using a more extensive character analysis . More recent morphological analysis by Gale et al. (2020) using character analysis supports the molecular phylogeny. Formerly, the Neolepadidae had subfamily status, in a different composition, within the Eolepadidae, but, as explained above, that taxonomy must be rejected. Eolepas is a six-plated phosphatothoracican with absolutely no close relationship to the eight-plated Neolepas and other neolepadids. The Eolepadidae sensu Gale et al. (2020) is an exclusively fossil (Triassic-early Cretaceous), phosphate-shelled taxon situated on the stem-line towards the Thoracicalcarea. Neolepadidae did not appear until the Eocene (Gale et al., 2020).

Family Neoverrucidae: This comprises Imbricaverruca
In support of our taxonomy, the ontogenetic stages of neoverrucids and neolepadids are similar, underlining their close phylogenetic affinity within the Neolepadoidea (Gale, 2014b(Gale, , 2019. The neoverrucid asymmetry involves only the upper latus, with this plate being either small or absent on one side of the body. The remaining plates are symmetrically placed, or at best placed in a slightly asymmetrical manner, and the whole is surrounded by a whorl of imbricating plates. The asymmetry is therefore much less pronounced and fundamentally different from the one characterizing the Verrucidae (Verrucomorpha), where it involves a morphological and functional asymmetry of the scutaterga, with latera being absent in the extant species. Until recently, the neoverrucids were claimed to be relatively plesiomorphic relatives to the Verrucidae within the previous concept of the Verrucomorpha (e.g. Buckeridge & Newman, 2006). All molecular analyses contradict this view (Pérez-Losada et al., 2008;Rees et al., 2014;Herrera et al., 2015). Furthermore, the morphological cladistic study of  found no putative synapomorphies between the Neoverrucidae and the Verrucidae, whence they were unable to confirm any close relationship. Gale (2014b) has given a new account on verrucomorphan evolution that does not involve Neoverruca or any such morphological stage.
Family Neobrachylepadidae: The family formerly had subfamily status in the Brachylepadomorpha but is here elevated to family and transferred to the Neolepadoidea. It contains only the monotypic genus Neobrachylepas Newman & Yamaguchi, 1995, which resembles the extinct Brachylepadomorpha. Neobrachylepas is symmetrically shaped, with basal imbricating scales and without a peduncle, but it exhibits an opercular plate morphology similar to that of Neoverruca. In full agreement, the molecular analysis of Herrera et al. (2015) places Neobrachylepas relica Newman & Yamaguchi, 1995 as intercalated between a monophyletic Neoverrucidae and a monophyletic Neolepadidae. Its morphology and phylogenetic position both argue for a separate monotypic family.
Evolution of vent and seep barnacles: All living species of the Neolepadoidea inhabit vent or cold seep habitats, but originally their ancestors might have had a different mode of life. Carriol et al. (2016) recently described the extinct species Stipilepas molerensis Carriol, 2016, from the Danish Eocene Moler formation, which has a clear morphological affinity to the Neolepadoidea but was attached to tree logs, perhaps much like extant Lepas. At present, the most parsimonious interpretation is that some common adaptations enabled the ancestor of extant Neolepadoidea to invade the vent and seep habitats, which might have acted as a refuge that prevented extinction of the clade. The adaptations concern not only the adults but also the larvae, which must disperse over long distances in search of their rare and patchy habitats (Yorisue et al., 2012). In the Thoracica, evolution into these specialized habitats happened convergently in only one other lineage, viz. in the balanomorphan species Eochionelasmus Yamaguchi, 1990, which is situated among the Chthamaloidea.

'SeSSilian' thoracicanS
The name Sessilia is used for the monophyletic taxon comprising the Verrucomorpha and the Balanomorpha. In most analyses, these two orders are sister groups with high support (Figs 7, 8). We recognize this relationship but decided to leave the Sessilia clade without a formal rank, because we can then conveniently have the Verrucomorpha and Balanomorpha as orders. The Sessilia name refers to the lack of a peduncle but is misleading because such loss also occurred convergently in the Neolepadoidea (Neoverrucidae). The verrucomorphans and balanomorphans are otherwise morphologically and biologically distinct. The Verrucomorpha has a strongly asymmetrical disposition of the wall plates and an opercular lid formed by a moveable scutum and tergum (Darwin, 1851;Gale, 2014b). The Balanomorpha, often called acorn barnacles, are perfectly symmetrical, with an operculum that evolved convergently with the verrucomorphans and a wall that includes so-called marginal plates unique to that lineage (Gale & Sørensen, 2014). The 'sessilian' forms lack a peduncle, both as adults and during ontogeny, and this can be seen as a synapomorphy that evolved in their brachylepadid ancestors, such as Pycnolepas Withers, 1914(Gale, 2014b. The two extant lineages have independently lost the upper latus and imbricating peduncular plates.

order brachylepadomorpha witherS, 1923
This extinct order contains only the extinct Brachylepadidae, because we place the extant Neobrachylepas in the Neolepadoidea (see above). It is a paraphyletic taxon that was abundant and diverse in the Upper Cretaceous (Gale & Sørensen, 2014) and forms a stem group to both the Verrucomorpha and the Balanomorpha (Fig. 10). The Brachylepadomorpha includes basal sessile barnacles, which have lost the peduncle, but they do not show the full suite of derived character states of either the Balanomorpha (new wall plates, orifice and sheath) or the Verrucomorpha (box-like asymmetric shell made up of four plates, plus opercular lid). In all Brachylepadomorpha, the carina and rostrum are large and concavo-convex, and an upper latus is present between the tergum and scutum; whorls of imbricating plates surround the capitulum.

order verrucomorpha pilSbry, 1916
The monophyly of Verrucomorpha is strongly supported by the presence of their characteristic asymmetry. Being mostly found in deeper and deepsea habitats, only a handful of verrucid species from four genera have been analysed molecularly, but these studies suggest the monophyly of the family. In extant verrucomorphans, the number of shell plates is limited to carina, rostrum, terga and scuta, all of which are arranged asymmetrically. The wall plates consist of the carina and rostrum that meet and articulate on one side and one set of tergum and scutum that are fixed to the substratum. The remaining scutumtergum form a moveable and asymmetrically oriented operculum. Basal fossil forms (Eoverrucidae) possessed additional imbricating peduncular plates surrounding the wall, and Gale (2014b) recently gave an account of how the Verrucomorpha evolved in the Cretaceous from pedunculate, multi-plated and symmetrical ancestors to the highly derived condition in extant forms. The distinct asymmetry of the Verrucomorpha was displayed by extinct relatives, such as Faxelepas Gale, 2014, and the late Cretaceous Eoverruca Withers, 1935 is considered the most basal member of the Verrucomorpha. The extant fauna includes both shallow-water and deep-sea forms. Gale (2014b) suggests that the asymmetry might have evolved as an adaptation to catch crawling benthic prey in nutrient-poor deep-sea habitats, but this is speculative because extant species, such as Verruca stroemia (O.F.Müller, 1776), use the cirri for normal suspension feeding. The Cretaceous and asymmetric Proverruca Withers, 1914 is not closely related to the Verrucomorpha as defined here, but is now seen as a separate, convergent evolution of asymmetry. Thus, asymmetry evolved at least three times in the Thoracica and in morphologically different ways, viz. in the Neoverrucidae, in the Verrucomorpha and in the Proverruca lineages (Gale, 2014b).

order balanomorpha pilSbry, 1916
The Balanomorpha are morphologically distinct and appear to be monophyletic in almost all molecular analyses. The only exception is the unusual position of the pollicipedomorphan genera basal within balanomorphans in the analysis of Herrera et al. (2015) (Fig. 8). Morphology disagrees with this result, and we have discussed (see above) the uncertain position of pollicipedomorphans across the available molecular analyses (Fig. 8). All balanomorphan species share a common morphology, where a peduncle is lacking and the body is, in all species except the whale barnacle, Xenobalanus Steenstrup, 1852, encased by a wall of articulating shell plates and covered by a distinct operculum (lid) of paired scuta and terga (Newman & Ross, 1976). The operculum articulates with a so-called sheath on the inside of the wall plates and altogether this offers a watertight closure of the mantle cavity. This special morphology enables balanomorphans to inhabit the upper intertidal zone and thus endure long periods (up to weeks) of exposure to air. Undoubtedly, this was the main reason for the immense success of this taxon from the late Cretaceous and, in support of this, almost all members of the plesiomorphic Chthamaloidea inhabit the intertidal habitat, whereas deeper water and epizoic forms began to evolve only later in the phylogeny. Another biologically important innovation is the clear separation of the anterior two or three pairs of thoracic appendages into specialized mouth cirri. This seems to be accompanied by an ability to catch much smaller food items from the plankton in comparison to other cirripedes (Anderson, 1994). The loss of the flexible peduncle also necessitated that the soma is suspended flexibly in the mantle cavity in order that it can be turned to face the current when extended for feeding and can even reverse orientation rapidly with shifting waves (Trager et al., 1994).
Plate evolution in the Balanomorpha: The number of plates in the wall was originally eight, but in most groups the number has been reduced to six or four by secondary losses or plate fusions. Much discussion has taken place concerning the homology of the plates between balanomorphans and pedunculated thoracicalcareans, such as the Pollicipedomorpha and Scalpellidae (e.g. Newman, 1996). Here, we focus on a novel interpretation based on detailed examination of extant and extinct forms, including extensive new fossil material (Gale & Sørensen, 2014). In the view of Gale & Sørensen (2014), the only balanomorphan plates with a homology in pedunculated forms are the scuta-terga, the rostrum and the carina, whereas all lateral plates known from pedunculated forms have been lost. The key to understanding balanomorphan plate evolution lies in the Brachylepadomorpha. Among these stem-line forms (see above), the peduncle was lost, but they had the capitulum still raised above the substratum on an inflexible body part encircled in numerous imbricating scales. The only lateral plates were the pair corresponding to the upper latera in scalpellids, and these had become small, integrated into the operculum, and were also subsequently lost altogether. In the crucial fossil, Brachylepas Woodward, 1901, this left a large gap between the rostrum and carina, filled only in part with small imbricating scales. During later evolution, this gap gradually became filled with entirely new wall plates that probably originated from the said scales and thus have no homology whatsoever among other pedunculated cirripedes. These barnacles first developed a pair of marginal plates, to be followed by two additional pairs, the carino-and rostromarginals. This resulted in the wall of eight plates that is now held to be plesiomorphic for extant (crown) balanomorphans and is found among some extant chthamaloid forms, such as Octomeris Sowerby, 1825 (see Fig. 9). Some chthamaloid species also retain one or several rows of imbricating scales around the base of the wall plates, as in their brachylepadomorphan ancestors.
I t i s n o t e w o r t h y t h a t o n t o g e n y o f e x t a n t balanomorphans reflects the evolutionary addition of the novel median plates. The juvenile barnacle first lays down a wall of the rostrum, carina and two medians, to be followed only later by the other median plates (Broch, 1922;Glenner & Høeg, 1993). From the original eight-plated wall, later evolution among crown balanomorphans implicated either losses or fusions of plates to, for example, four walled forms (e.g. Tetraclita Schumacher, 1817) or fusion of all wall plates into a solid ring, such as seen in the Pyrgomatidae. All these later processes were lucidly analysed by Newman & Ross (1976), with the only necessary change being that the 'laterals' of former descriptions should now be called 'marginals' (Gale & Sørensen, 2014). The operculum also underwent gradual but significant changes in the brachylepadomorphan stem lineage, enabling it to act as a watertight lid, as explained above.
Taxonomy of the Balanomopha: The morphological characters offer a wealth of largely unharvested data for taxonomic purposes. The mineralization of the hard parts has begun to be studied by modern techniques only recently (but see Mitchell et al., 2019). There are also numerous specializations of the hard parts. The base can be membranous or calcified. Both the base and the wall plates can be furnished with tubes, and these can be partly or wholly filled with cuticle. Additional hard part characters concern details of the sheath, which is a structure on the inside of the wall plates that supports the operculum. In the soft body, the Balanomorpha have both the first and the second pair of thoracic cirri specialized into mouth cirri (maxillipeds), as opposed to non-balanomorphan thoracicalcareans, in which only the first pair is specialized in this way. There are probably many additional soft body apomorphies for the Balanomorpha, such as at the ultrastructural level relating to the setation of the mouthparts (Høeg et al., 1994) and the morphology of the cirri (Chan et al., 2017b). Some of these hard and soft part characters have been used to characterize subsets of the Balanomorpha, but there has never been an analysis that studies these hard and soft body characters across the entire order. Such an analysis is needed to assess the extent to which these characters are phylogenetically conservative or subject to ad hoc adaptive evolution.
Until now, only a small number of balanomorphan species have been analysed molecularly, but the available studies nevertheless include good coverage of all presently recognized families, thus enabling a thorough revision. Newman & Ross (1976) gave the classic taxonomic account, and most major lineages recognized in their study are largely recovered by the molecular analyses as monophyletic groups (Pérez-Losada et al., 2012bChan et al., 2017a;Tsang et al., 2017). We presently recognize four superfamilies, with the Chthamaloidea diverging first, then the Elminoidea (new superfamily) and, finally, the Coronuloidea and Balanoidea as sister groups (Fig. 4). These superfamilies and the families we recognize are all monophyletic according to the recent molecular analyses. We accept many of the previous families, but a few poly-or paraphyletic ones are subsumed in others, and there are also some movements of genera between taxa. We do not make any changes at the genus level, but it is clear from the published analyses that much revisionary work will need to be done to arrive at genera that are monophyletic. Therefore, the present affiliation to genus, not least within the Balanidae, should not be taken per se as signifying phylogenetic relationship.

SuperFamily chthamaloidea darwin, 1854
This superfamily is generally agreed to display the most plesiomorphic morphology within balanomorphans, but nevertheless to form a monophyletic taxon (Pérez-Losada et al., 2014;Chan et al., 2017a). They have eight, six or four wall plates. The three other superfamilies have a maximum of four wall plates. In soft body anatomy, chthamaloids also have only two pairs of mouth cirri, whereas coronuloids and balanoids have three pairs, leaving only the posterior three cirri for the feeding basket (Southward, 2008). Epibiosis, a mode of life common in coronuloids and balanoids, is rarely found among the chthamaloids, where most species occupy an intertidal habitat on hard substrata (Fig. 5N). A spectacular exception is the hydrothermal vent-inhabiting Eochionelasmus. An interesting character in several chthamaloids is the presence of one or several whorls of scales, or 'imbricating plates' around the base of the wall plates, e.g. Catomerus Pilsbry, 1916, Catophragmus Sowerby, 1826, Chionelasmus Pilsbry, 1911, Eochionelasmus and Waikalasma Buckeridge, 1983(Buckeridge & Newman, 1992Chan et al., 2020a). Such whorls were found in the extinct Brachylepadomorpha, and their presence is therefore considered to be plesiomorphic. Accordingly, it might have been expected that extant forms with such whorls would be clustered at the base of the balanomorphan tree, but their positions indicate that loss of whorls occurred independently several times (Pérez-Losada et al., 2012b;Chan et al., 2017a). We recognize five families within the superfamily (Chthamalidae, Pachylasmatidae, Catophragmidae, Chionelasmatidae and Waikalasmatidae), all of which are monophyletic in our definition. The fossil form Archaeochionelasmus Kočí, Newman & Buckeridge, 2017 does not appear in our taxonomy, because it is not a cirripede but a fragment of a rudist bivalve (see Gale & Skelton, 2018).

Superfamily elminOidea Superfam. nOv.
This superfamily contains only the Elminiidae, which is here elevated from subfamily to family level. The molecular analyses all agree in putting it as an independent branch, diverging between the Chthamaloidea and the large clade consisting of the Corunuloidea + Balanoidea (Fig. 7). Erection of a new superfamily is therefore warranted for consistency. The elminid species are all rocky intertidal forms.

SuperFamily coronuloidea leach, 1817
The Cornuloidea comprises the monophyletic Tetraclitidae, Bathylasmatidae, Austrobalanidae, Chelonibiidae and Coronulidae from molecular evidence (Chan et al., 2017a) (Fig. 7). Owing to the topology of the molecularly derived trees, we abandon the former Tetraclitoidea (Martin & Davis, 2001), because all six families form a single monophyletic clade, and the members of the former Platylepadidae are here subsumed into the Coronulidae (Hayashi et al., 2013). Members of the Tetraclitidae and Austrobalanidae are found on hard substrata and mostly inhabit the rocky intertidal zone, whereas Bathylasmatidae are deep-sea inhabitants. Members of the Chelonibiidae and Coronulidae are all epibiotic on a wide array of invertebrates and vertebrates, including, for example, whales, sea cows, sea snakes, marine turtles and various crustaceans. Some species are even relaxed in their host preferences and can be found on crabs, turtles and horseshoe crabs (Ewers-Saucedo et al., 2016). Species of the Chelonibiidae are little modified in their morphology compared with their intertidal relatives, whereas the Coronulidae show perhaps the most extensive morphological specialization among balanomorphans; for example, the whale barnacles Tubicinella Lamarck, 1802 and Xenobalanus, where the body is only little covered by shell plates and superficially resembles a pedunculated form (Seilacher, 2005;Dreyer et al., 2020).

SuperFamily balanoidea leach, 1817
Traditionally, this superfamily consists of three families, Balanidae, Archaeobalanidae and Pyrgomatidae. The major morphological difference between Balanidae and Archaeobalanidae is that Balanidae has tubiferous wall plates, whereas most of Archaeobalanidae have solid, non-tubiferous shells representing a plesiomorphic condition. Pyrgomatidae is composed of species living in corals and a few in sponges, and they have either four wall plates or the wall plates are fused into a solid ring. Molecular analyses suggest that the multiple species of the Balanoidea fall into two large clades, the Balanidae + Archaeobalanidae clade and the Pyrgomatidae clade. Given that the balanid and archaeobalanid species are mixed, we recognize only two families, the Balanidae and the Pyrgomatidae. At present, an increasing number of species within these two families are being subjected to molecular analysis, and it is already clear that serious taxonomic changes can be expected at all levels below the family.
Family Balanidae: This large family now combines the former Archaeobalanidae and Balanidae because molecular evidence has confirmed that neither of these was monophyletic in their former composition. It includes free-living species, sponge-associated species and fire coral-associated species. The last half-century has seen many taxonomic rearrangements, but from the emerging molecularly based analyses it seems clear that many additional rearrangements are needed to arrive at monophyletic units also down to genus level. Several of the balanid species are used as 'models' in many contexts, not least larval settlement and antifouling studies. This highlights the need to strive for a robust and phylogenetically based taxonomy in the future, because generalizations from models can depend crucially on phylogeny. Wanella Anderson, 1993, found on fire corals, does not group with the remaining coral barnacles in the Pyrgomatidae but is the sister to all remaining species of the Balanidae. This argues for recognizing Wanella as its own subfamily (Tsang et al., 2014;Yu et al., 2020). At present, we keep all the existing species in Archaeobalanidae and Balanidae under the Balanidae but extensive further molecular studies should be conducted to examine the phylogenetic relationships of these subfamilies. But note that at least the Acastinae is not monophyletic by molecular anlaysis .
Family Pyrgomatidae: All members of this family are highly specialized to epibiosis in either corals or marine sponges (Anderson, 1992;Chan et al., 2013Chan et al., , 2020b. Interestingly, it seems that evolution onto such hosts presents interesting patterns of homoplasy. Molecular phylogenetic studies (Simon-Blecher et al., 2007;Malay & Michonneau, 2014;Tsang et al., 2014) showed that Pyrgomatidae is composed of three monophyletic clades, accepted here as subfamilies. Recent phylogenetic analysis of sponge and coral barnacles  showed that Conopea Say, 1822, which live on gorgonian corals, is situated in the pyrgomatid clade, suggesting that Conopea should be considered a genus within the pyrgomatids. The molecular position of Conopea will therefore require further molecular analyses. Kolbasov et al. (2016) conducted a morphological revision of Conopea.
Subfamily Megatrematinae: The second monophyletic clade is the Megatrematinae (Malay & Michonneau, 2014), sister group to the Ceratoconchinae. Megatrematines have almost world-wide distribution, being absent from the eastern Pacific, and have low diversity (seven living and six extinct species) (Ross & Pitombo, 2002).
Subfamily Pyrgomatinae: This is the third clade and the most species-rich subfamily of Pyrgomatidae (> 20 genera and > 80 species), being present in the Indo-West Pacific, including the Red Sea, but absent from the Atlantic and Mediterranean. Pyrgomatinae comprise three tribes. The Pyrgosellini contains Pyrgopsella Zullo, 1967, inhabiting corals, andPyrgospongia Achituv &Simon-Blecher, 2006, inhabiting sponges. Pyrgospongia has been recorded from the Philippines, Andaman Islands and Hong Kong. The Hoekiini include coral-eating barnacles in the coral genus Hydnophora Fischer von Waldheim, 1807 only. This tribe is presently recorded from the Great Barrier Reef and Japan. The third tribe, Pyrgomatini, is composed of 20 genera and > 80 species and has great diversity in shell forms and opercular plate structures (Ross & Newman, 1995. Fossil records of Pyrgomatini are fragmented, being recorded only in Nobia Sowerby, 1839 and Savignium Leach, 1825 in the Miocene of Fiji and Pleistocene of the Marshall Islands (Ross & Newman, 2000). Cantellius Ross & Newman, 1973, Darwiniella Anderson, 1992and Galkinius Perreault, 2014 are found in the Holocene terraces in Japan (Asami & Yamaguchi, 1997;Ross & Newman, 2000). Although Malay & Michonneau (2014) included these three tribes in the molecular phylogenetic analysis, the phylogenetic relationships for these three tribes appear not to be supported by molecular evidence but confirm that each genus in the Pyrgomatidae is monophyletic.

TIMELINE AND PATTERNS OF THORACICAN EVOLUTION
The generic diversity of the Thoracica since the latest Triassic is depicted in Figure 9. Late Triassic to Early Jurassic forms are exclusively phosphatic (Phosphatothoracica).  Figure 9 include only forms with shell plates, for comparison with the fossil record. The 35+ extant genera from the deep sea are unlikely to be represented in the fossil record; therefore, the large present-day diversity is, in part, an artefact of this fact. However, the rapid increase in diversity through the Neogene indicates that the present-day diversity continues a long trend. As Charles Darwin said, we live in the 'Age of Barnacles'.

SUMMARY AND FUTURE RESEARCH
Our study has led to several fundamental changes in taxonomy of the barnacles (Thecostraca) owing to the evidence from molecular phylogenetic studies published over the last 20 years. These were significant in the Rhizocephala , and this is hardly surprising because there were few phylogenetic studies of this taxon. Our analysis also resulted in major changes within the Thoracica, but we could, in contrast, also demonstrate the validity of some existing phylogenetic hypotheses and confirm the monophyly of a substantial number of existing families and superfamilies. The interpretation of morphological evolution within the thoracicans has recently been revised substantially (Gale, 2014a(Gale, , b, 2015a(Gale, , b, 2019(Gale, , 2020aGale & Sørensen, 2014;Gale & Schweigert, 2015). It is satisfying that these new interpretations of morphology are, in most respects, in agreement with the molecularly based systematic arrangements. This opens the door for a 'total evidence' approach to barnacle phylogeny and is of particular importance owing to the wealth of fossil information available for this taxon. Many taxa still require further study before we can arrive at monophyletic units down to generic and species levels. Such studies are especially needed in species-rich but phylogenetically poorly understood groups, such as the Scalpellidae, Balanidae and Pyrgomatidae. Each of these families is species rich and biologically diverse and presents many problems of general biological interest, such as the evolution of reproductive systems (Scalpellidae), larval settlement studies (Balanidae) and adaptation to epibiotic life on a wide variety of organisms (Pyrgomatidae). A full evolutionarily based analysis of such problems can be undertaken only when many species have been analysed and their relationship is known in a phylogenetic context. Recent studies, such as those by Yusa et al. (2012), Lin et al. (2015) and Yu et al. (2020), effectively demonstrate the power of such analyses.

CLASSIFICATION claSS thecoStraca Gruvel, 1905
Diagnosis: Adapted from Høeg et al. (2009b). The taxon is monophyletic by molecular analyses, but see Petrunina, et al. (2014) on the possible inclusion of the Tantulocarida. No post-maxilular limb buds in nauplii; the larval development terminates with a cypridoid larva; the cypridoid with prehensile antennules is used in attachment; well-developed frontal filaments and compound eyes, whenever present, with three crystalline cones; the cypridoid larval carapace with five pairs of lattice organs.
Comment: Owing to the morphological diversity among the adults, the taxon can be diagnosed morphologically only by larval characters; those listed here are synapomorphic for the class; the adult stages are permanently sessile in the most of Ascothoracida and in all Cirripedia and are suspected to be so in Facetotecta.
Comment: Many additional potential species are becoming known as morphological 'types' (Fig. 1A-K) and are presently also being characterized by molecular methods. A future taxonomy will therefore need new genera and probably also new families (Grygier et al., 2019).

SubclaSS cirripedia burmeiSter, 1834
Diagnosis: Following Høeg et al. (2009b). Permanently sessile crustacean; nauplii fitted with frontolateral horns of complex morphology; settlement by means of adhesive cement; the cypris larvae are without an abdomen or the abdomen is reduced to a tiny rudiment; four-segmented antennules, with the first segment divided into two articulating sclerites (Lagersson & Høeg, 2002); multicellular cement gland with muscular sac and terminating on attachment surface of the third antennular segment; the second pair of lattice organs in the cypris carapace with large terminal pore located anteriorly (this character is shared with other ascothoracidans).
Comment: Owing to the reduced morphology in the parasitic Rhizocephala, the taxon can be diagnosed morphologically only by means of larval characters. All those mentioned are synapomorphic for the three cirripede infraclasses.
inFraclaSS acrothoracica, Gruvel, 1905 Diagnosis: Monophyletic by molecular analyses. Minute, burrowing females found primarily in calcareous substrates (corals, molluscs, barnacle shells, bryozoans etc.), with soft carapace, accompanied by dwarf males. All female and male calcareous shell plates are a single pair of largely chitinous plates (opercular bars) guarding the aperture, three to five pairs of terminal cirri, gathering in the posterior elongated portion of the thorax, single pair of developed or reduced cirri located at the side of the mouth, caudal appendages present or absent, abdomen reduced in adults. Borings probably made by acrothoracicans are known as far back as the Lower Devonian, but do not provide morphological information; these are trace fossils and have no taxonomic validity.
order cryptophialida KolbaSov, newman & høeG, 2009 Diagnosis: Monophyletic group in molecular analyses. Females with bottle-like mantle; operculum with elongated neck; aperture small, crown-shaped opercular bars occupy the complete apertural length, lateral and reinforcing bars developed, long and thin; labrum elongated, tongue-like; mouth cirri rudimentary; thorax with one or two whip-like, long dorsal processes, sets of thoracic lappets with cuticular border; intestine with gizzard (gastric mill); caudal appendages absent. Dwarf males with elongated posterior end having circular cuticular ribs. Cypris larvae with rudimentary thorax and thoracopods. Boring aperture rounded.

Family lithoGlyptidae aurivilliuS, 1892
Diagnosis: Monophyletic in molecular analysis. Females with developed opercular bars and comb collar, operculum with rows of massive multifid scales, often associated with opercular pores and papillae; protopod of mouth cirri two-segmented, mouth appendages developed, terminal cirri multisegmented, biramous; caudal appendages present or absent; thoracic lappets developed; intestine with anus. Dwarf males without a pair of lobes at the base of attachment antennules; posterior end with apertural slit.

SubFamily berndtiinae utinomi, 1950
Berndtia Utinomi, 1950 (six species) Weltneria Berndt, 1907 (12  Family trypeteSidae StebbinG, 1910 Diagnosis: Monophyletic in molecular analyses. Females with thin, unclear opercular bars, comb collar reduced, operculum without rows of massive multifid scales; protopod of mouth cirri unsegmented, mouth appendages reduced in size; three pairs of uniramous terminal cirri; caudal appendages absent; thoracic lappets reduced; intestine without anus. Dwarf males with a pair of lobes at the base of attachment process; posterior end without apertural slit. Inhabit columella of gastropod shells occupied by hermit crabs. Tomlinsonia Turquier, 1985 (two species) Trypetsa Norman, 1903 (five species) inFraclaSS rhizocephala müller, 1862 Diagnosis: Here given in abbreviated form after Høeg et al. (2020). Monophyletic by molecular analyses. All parasitic stages without segmentation, appendages and alimentary canal. Specialized cypris antennules; parasites on Crustacea; with parasitic phase starting with an initial internal phase. The adult parasite consists of an internal ramifying and nutrient-absorbing root system and an external reproductive sac (externa). Separate sexes, with the female externa hosting and nourishing one or several dwarf males reduced to sperm-producing tissue and a few somatic cells.
Comment: Høeg et al. (2020) provided diagnoses of all the families; these can only exceptionally be characterized as monophyletic by morphology alone, but the molecular analyses support the monophyly of all almost presently recognized families.
Comment: Monophyly rests on morphology alone because no species has been sampled for molecular analyses. The family shares several potential apomorphies with the Chthamalophilidae. Comment: None of the morphological characters is necessarily an apomorphy compared with the remaining Cirripedia.
In the subsequent taxonomic ranking, taxa containing both fossils and extant species are indicated with †. Taxa with fossil species alone are indicated with † †.

Superorder phoSphatothoracica Gale, 2019
Diagnosis: Thoracicans with phosphatic shell plates; the number of shell plates formed is between four (paired scuta and terga) and six (rostrum and carina added). The primary mineralogy of living taxa was discussed by Reid et al. (2012), and the plates are composed of a hydrogen phosphate-like calcium phosphate biomineral.

Comment:
The extant forms possess only four plates (paired scuta and terga).

Comment:
We change the name only to have thoracican order-level taxa with the same '-morpha' suffix. The morphological characters are likely to be plesiomorphic, but the two species are sister species and form a sister group to all other extant thoracicans based on molecular analyses.
Family iblidae leach, 1825 † †Martillepas Gale, 2014 (Jurassic-Cretaceous) articulation with scuta. Monophyletic by molecular analyses, including diverse forms that have no plates. †Family lepadidae darwin, 1852, amended Diagnosis: Monophyletic by molecular analyses. The morphological characterization is as for the superfamily. The subfamily Lepadinae Darwin, 1852 is hereby redefined and elevated to family status.
Comment: Species of Conchoderma von Olfers, 1814 are completely naked or near-naked forms and form a clade with a sister relationship to species of Lepas and Dosima. The monotypic Dosima is nested within species of Lepas. The plate arrangement of Hyalolepas Annandale, 1906 is basically the same as in Lepas, with the only difference being the extension of a long fork at the base of the carina, which reaches the basal margin of the scutum. At present, no molecular information is available for Hyalolepas; we tentatively allocated it inside Lepadidae owing to its high morphological affinity with Lepas.

SuperFamily balanoidea leach, 1817
Diagnosis: Wall composed of rostrum, carina and one to two pairs of marginals, or wholly concrescent; parietes solid or tubiferous; radii solid or tubiferous; basis commonly calcareous, solid or permeated by tubes, rarely membranous; when basis calcareous, internal surfaces of compartments commonly with uniform ribs and interdigitations with wall; mandible quadri-or quinquidentate; caudal appendages absent; penis with basidorsal point; first three pairs of cirri are maxillipeds.

Comments:
The species at present analysed molecularly form two distinct clades that are here recognized as the Balanidae and Pyrgomatidae. Relationships of the many species within each of these are much less clear and clearly in need of future analysis by molecular methods.

Family balanidae leach, 1817
Diagnosis: Monophyletic by molecular analysis. Wall of six or four plates; parietes tubiferous or solid; tubiferous shell, with tubes basically in a single uniform row formed between inner and outer laminate, although supplementary tubes may form basally; interlaminate figures complex, arborescent; radii either solid or tubiferous; basis commonly calcareous and tubiferous or membranous.
Comments: There are no clear morphologically based apomorphies for this family compared with the Pyrgomatidae. The genus Wanella is sister group to all remaining species as yet analysed by molecules, and this could argue for a separation within two subfamilies. Many genera are polyphyletic, and more low-level taxonomic revision is therefore needed.