Abstract

The neurocranial osteology of the giant monitor lizard Varan (Megalan) pris Owen, 1859 is described in detail for the first time. Optimization of neurocranial characters onto phylogenetic topologies for varanoids, including Lanthanot, Heloderm and Varan species nests V. pris within an Indo-Australian clade of Varan on the basis of characters of the otic capsule. A sister-taxon relationship between V. pris and Varanus komodoens Ouwens, 1912 is proposed based on apomorphies of the crista prootica, fenestra vestibuli, occipital recess, and supraoccipital. These results support a monophyletic clade of giant monitors among Indo-Australian species, and unambiguously synonymize Megalan with Varan at both generic and subgeneric levels.

INTRODUCTION

Megalania prisOwen, 1859 is the largest known terrestrial ‘lizard’, with maximum estimated body lengths approaching 5–7 m, and with masses of up to 2200 kg (e.g. Hecht, 1975; Rich & Hall, 1979; Auffenberg, 1981; Molnar, 2004). The taxon is restricted to the Pleistocene of eastern Australia, and is known from associated elements and fragmentary remains, including elements from all regions of the axial skeleton, partial girdles, and limbs, and cranial elements (Hecht, 1975; Molnar, 2004).

The systematic history of Megalan is complicated, and includes initial comparison with varanids, subsequent conflation with meiolaniid turtle elements, assignment to Agamidae, separation of turtle elements, reassignment to Varanidae, generic and specific inflation of known elements, and subsumation into a single taxon, with Megalan taking precedence. The syntype series of Megalan consists of three dorsal vertebrae from the Darling Downs of Queensland (Owen, 1859: BMNH 32908a–c). Initial comparisons led Owen to suggest that these specimens represented a gigantic lizard, and he noted a strong resemblance between the vertebrae and those of extant varanids. Additional specimens found in the same area, including the occipital region of one skull (BMNH 39965) and an almost complete skull with marked horn-like protuberences (BMHH R391), were described subsequently and referred to the same taxon (Owen, 1880, 1886a). The extensive ornamentation present on BMNH R391 caused Owen to abandon his earlier comparisons with varanids, and led him to suggest that Megalan was closely allied with agamids such as Molo(Owen, 1880, 1886a). New material from Lord Howe Island, consisting of skull and limb material, as well as a ‘caudal sheath’ of fused vertebrae and osteoderms, was referred to a second ‘megalanian’ reptile, Meiolan(Owen, 1886b): purported similarities between the tail of Meiolan and that of Molo bolstered Owen's views on the agamid affinities of Megalan. However, Huxley (1887), Lydekker (1888, 1889), and Woodward (1888) noted that Meiolan was actually a chelonian; moreover, one of the skulls attributed to Megalan(BMNH R391) by Owen (1880) was referable to a new species of Meiolan, Meiolania owe(see Woodward, 1888). (The latter specimen has now been removed from Meiolan, and is now the holotype of the meiolaniid turtle Ninjemys owe; see Gaffney, 1992). Owen (1888) later conceded that the ‘megalanians’ (Megalan and Meiolan), which he continued to regard as close relatives, shared some similarities with both squamates and chelonians, and erected a new suborder (Ceratosauria) to accommodate these taxa. However, the name Ceratosauria was preoccupied (for a group of theropod dinosaurs; Marsh, 1884), and subsequent authors (e.g. Lydekker, 1888; de Vis, 1889; Zietz, 1899) demonstrated that Megalan was a varanid lizard, as Owen (1859) had originally proposed. The majority of twentieth century authors have regarded Megalan as a varanid (e.g. Nopcsa, 1908; Etheridge, 1917; Anderson, 1930; Hecht, 1975; Estes, 1983; Molnar, 2004), with the exception of de Féjérváry (1918), who supported agamid affinities. Three additional varanid taxa from the Pleistocene of eastern Australia, Notiosaurus dentatOwen, 1884, Varanus dirde Vis, 1889, and Varanus warburtonensZietz, 1899, are regarded as junior subjective synonyms of M. pris(Etheridge, 1917; de Féjérváry, 1935; Hecht, 1975; Estes, 1983).

Megalan was first synonymized with Varan on the basis of comparisons between its vertebrae and those of Varanus sivalens Falconer, 1868 from the Miocene Siwalik Group of Pakistan (Lydekker, 1888). Several other authors also regarded Megalan as a junior subjective synonym of Varan, including Nopcsa (1908), Anderson (1930), and McDowell & Bogart (1954). Estes (1983) suggested that Megalan should be regarded as a subgenus of Varan. However, the name has persisted in the literature, but with frequent allusions to future studies necessitating official subsumation into the extant genus (e.g. Hecht, 1975; Pianka, 1995; Lee, 1996; Molnar, 2004). Megalan has also formed the basis for two suprageneric taxa: Megalaninae Camp, 1923(a subfamily within Varanidae) and Megalanidae de Féjérváry, 1918. However, these names are now regarded as junior synonyms of Varanidae Gray, 1827(e.g. Estes, 1983).

Despite the early recognition of morphological similarities with Varan, phylogenetic relationships of Megalan with extant species and other varanoids are effectively unknown. Lee (1996) provided the only hypothesis of inter-relationship based on cranial osteology, proposing that Megalan is closely related to extant Varanus gigante Gray, 1845, a hypothesis that would nest the taxon within the Indo-Australian ‘goul group’ (Ast, 2001), the basal members of which achieve large body sizes (Gould & McFadden, 2004). Lee's hypothesis was tentative, as it was based on only two characters of the frontal.

Thus, whereas a preliminary phylogenetic hypothesis has been proposed, our understanding of the evolutionary history of Megalan is very limited. Tests of evolutionary relationships and a thorough understanding of the taxon require analysis of the most complex and potentially diagnostic available morphology.

Here, we describe the neurocranium of Megalan(BMNH 39965) in detail for the first time, compare it with that of other varanoids, use its morphology to provide an emended diagnosis for the taxon, and discuss its implications for understanding the systematic relationships of the largest known terrestrial lizard. We also demonstrate that Megalan is firmly nested within the clade Varan, and refer to this taxon as V. pris henceforth (see below for further discussion). Anatomical terminology follows Oelrich (1956), Rieppel & Zaher (2000), Conrad (2004), Bever, Bell & Maisano (2005), and Conrad & Norell (2006), unless otherwise noted.

SYSTEMATIC PALAEONTOLOGY

SquamataOppel 1811 AnguimorphaFürbringer 1900 VaranidaeGray 1827,VaranusMerrem 1820,Varanus priscaOwen 1859

1859 Megalania pris Owen: 43, pl. 7 (figs 1A, 2A, 3A, 4A), pl. 8 (figs 1A, 2A, 3A, 4A) (original description).

Figure 1

Neurocranium of Varanus pris(BMNH 39965) in dorsal (A), ventral (B), anterior (C), and left lateral (D) views. Scale bar: 5 cm. Anatomical abbreviations are defined in Appendix 2.

Figure 1

Neurocranium of Varanus pris(BMNH 39965) in dorsal (A), ventral (B), anterior (C), and left lateral (D) views. Scale bar: 5 cm. Anatomical abbreviations are defined in Appendix 2.

Figure 2

Neurocranium of Varanus pris(A, BMNH 39965) compared with Varanus komodoens(B, USNM 228163) and Varanus salvat(C, ROMV-R 1040) in posterior view. Scale bars: 2 cm for (A) and (B); 1 cm for (C). Anatomical abbreviations are defined in Appendix 2.

Figure 2

Neurocranium of Varanus pris(A, BMNH 39965) compared with Varanus komodoens(B, USNM 228163) and Varanus salvat(C, ROMV-R 1040) in posterior view. Scale bars: 2 cm for (A) and (B); 1 cm for (C). Anatomical abbreviations are defined in Appendix 2.

Figure 3

Stereopairs of the otic region of (A) Varanus pris(BMNH 39965), (B) Varanus komodoens(USNM 228163), and (C) Varanus salvat(ROMV-R 1040) in right lateral views. Scale bar: 1 cm. Anatomical abbreviations are defined in Appendix 2.

Figure 3

Stereopairs of the otic region of (A) Varanus pris(BMNH 39965), (B) Varanus komodoens(USNM 228163), and (C) Varanus salvat(ROMV-R 1040) in right lateral views. Scale bar: 1 cm. Anatomical abbreviations are defined in Appendix 2.

Figure 4

Stereopairs of the otic region of (A) Varanus pris(BMNH 39965) in left ventrolateral view, (B) Varanus komodoens(USNM 228163), and (C) Varanus salvat(ROMV-R 1040) in right ventrolateral views. Scale bar: 1 cm. Anatomical abbreviations are defined in Appendix 2.

Figure 4

Stereopairs of the otic region of (A) Varanus pris(BMNH 39965) in left ventrolateral view, (B) Varanus komodoens(USNM 228163), and (C) Varanus salvat(ROMV-R 1040) in right ventrolateral views. Scale bar: 1 cm. Anatomical abbreviations are defined in Appendix 2.

1884 Notiosaurus dentat Owen: 249, pl. 12 (figs 1–9).

1888 Varanus prisc Lydekker: 284, fig. 66 (incorrect spelling, new combination).

1889 Varanus dir de Vis: 98.

1899 Varanus warburtonens Zeitz: 210.

Designated lectoty:

BMNH 32908c, a dorsal vertebra.

Paralectotyp:

BMNH 32908a and BMNH 32908b, partial dorsal vertebrae.

Emended diagnos:

Extremely large body size; dorsal vertebrae with low, flattened neural arch; incipient zygosphene-zygantral articulation present; processus alaris of prootic narrow and short, not extending dorsally to level of supraoccipital processus ascendens; prootic inferior process strongly anteroventrally angled, with convex dorsal margin; crista prootica with broadly convex posterior process; fenestra vestibuli and occipital recess incompletely separated medial to crista interfenestralis. See Hecht (1975) and Estes (1983) for additional apomorphies.

Referred materi:

Lydekker (1888), de Féjérváry (1935), Hecht (1975), Estes (1983), and Molnar (2004) each provided comprehensive lists of referred specimens, which will not be duplicated here. Known material includes isolated cranial elements [maxillae, a frontal, parietal, neurocranium (described herein), dentaries, and isolated teeth], numerous vertebrae (representing all regions of the axial column), fore- and hindlimb material, and girdle elements.

Distributi:

Pleistocene of eastern Australia (New South Wales, Queensland, and South Australia). A comprehensive list of localities is available in Hecht (1975).

Commen:

The correct species name is Varanus pris, although several authors have adopted an alternative, incorrect spelling (prisc: e.g. Lydekker, 1888). Varanus pris was erected on the basis of three syntypes (BMNH 32908a–c: Owen, 1859), and no holotype specimen was designated. In accordance with Article 74 of the International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature, 1999), one of these syntypes (BMNH 32908c, a dorsal vertebra) has been designated the lectotype, and the remaining syntypes (BMNH 32908a–b) have been designated as paralectotypes. BMNH 32908c was chosen as the lectotype, as it had been illustrated previously (Owen, 1859: pl. 7, figs 1–4), in accordance with ICZN Recommendation 74B, and as it is almost complete, and is therefore the most informative specimen from the original syntype series.

DESCRIPTION

General comments

The neurocranium (BMNH 39965) was first described and illustrated by Owen (1880); Hecht (1975) provided some additional details. It was collected by M. St. Jean from Gowrie, near Drayton, on the Darling Downs, Queensland, in 1866, and was presented to the British Museum by Sir Daniel Cooper in the same year. Referral of this specimen to V. pris is based on anatomical grounds (it differs from the braincases of all other known varanids) and its size, which is consistent with assignment to V. pris. Moreover, several caudal vertebrae referable to V. pris were discovered at the same locality (Owen, 1880). The neurocranium is well preserved and nearly complete (Figs 1–4), but is missing the right paroccipital process and distal portion of the left process, the basipterygoid processes, and the parasphenoid rostrum and cultriform process. The periosteal surface is well preserved, but some physical damage, probably resulting from early preparation attempts, is present along the posteroventral margin of the basioccipital region. Few sutural contacts are preserved, even in regions of good periosteal preservation. Obliteration of contacts suggests that the specimen represents a somatically mature, and potentially aged, individual.

Osteology

Supraoccipital

In dorsal view (Fig. 1A), the supraoccipital is a broadly rectangular element, the midline of which is marked by a well-defined, narrow ridge that is continuous with a prominent, ossified processus ascendens (Fig. 2A). This ridge is bordered laterally by deep embayments that enter the main body of the element (Fig. 2A). Both the ridge and the acute angle between the supraoccipital and parietal, evidenced by the steep lateral margins of the processus ascendens, were considered apomorphic for V. pris by Hecht (1975).

Otooccipital

The otooccipital (senConrad, 2004; Bever et a, 2005) is a composite element derived from a fusion of the exoccipital and opisthotic in most squamates, and forms the posterior and lateral walls of the braincase (Fig. 1A, D). In lateral view (Fig. 1D), the element articulates with the prootic along the posterior margin of the crista interfenestralis, just anterior to a deep, elongate stapedial groove (Rieppel & Zaher, 2000). The otooccipital forms the medial wall of both a narrow foramen vestibuli and slit-like occipital recess (Figs 3A, 4A). In other species of Varan, the fenestra vestibuli and occipital recess are separated by the crista interfenestralis (Rieppel & Zaher, 2000). However, in V. pris the more dorsoposteriorly positioned foramen vestibuli is only partially separated from the anteroventral occipital recess by an anteriorly directed flange that originates from the surface of the crista tuberalis (Fig. 4A). Separation of the fenestra vestibuli and occipital recess is incomplete anteriorly, a condition that appears to be unique to V. pris among varanid taxa. Posteriorly, the otooccipital expands in a wide, posterolaterally expanded crista tuberalis that forms the posterior margin of the otic capsule. The crista tuberalis possesses a broadly convex margin, similar to the condition in extant Varan(Fig. 3). Ventrally, the otooccipital contributes to a prominent spheno-occipital tubercle at the confluence of the crista interfenestralis and crista tuberalis (Fig. 4A).

In dorsal view (Fig. 1A), the otooccipital contacts the prootic along the dorsomedian margin of the paroccipital process in a straight-line articulation that extends medially to the lateral margins of the supraoccipital. The paroccipital processes are elongate and robust, and extend posterolaterally at an angle of approximately 45° to the transverse axis of the occiput. In posterior view (Fig. 2A), each paroccipital process is deeply excavated. This forms an elongate fossa that encloses the opening of a large jugular foramen, which is situated at the medial apex of the fossa. The otooccipital forms the lateral margins of a large occipital condyle, and also contributes to the sidewalls of the subcircular foramen magnum. The otooccipital contributions to the condyle are widely separated from each other by the dorsal surface of the basioccipital.

Prootic

The prootic forms the anterolateral walls of the braincase (Fig. 1C, D) and the anterodorsal margin of the paroccipital process. Anteriorly, the trigeminal notch is well developed. The dorsal margin of the notch is capped by a narrow, blunt, anterodorsally directed processus alaris (the crista alaris of Rieppel & Zaher, 2000) that would have overlapped the lateroventral margins of the parietal. The processus is relatively narrower and shorter than in other Varan species, and does not extend dorsally to the level of the supraoccipital processus ascendens. Medial to each processus alaris, a deep fossa is present at the prootic-supraoccipital contact. Varanus pris lacks the prominent, horizontal inferior process (senRieppel & Zaher, 2000) of the prootic that forms the anteroventral margin of the trigeminal notch in most other taxa. Instead, the inferior process is short, slopes anteroventrally, and is confluent with the crista sellaris (clinoid process of Rieppel & Zaher, 2000) of the parabasisphenoid, as in Varanus komodoens Ouwens, 1912 (Figs 1D, 3A, B). The dorsal margin of the process is convex, unlike other Varan, where it is excavated to form a concave ventral margin of the trigeminal notch (Fig. 3). Laterally, the prootic is dominated by an elongate crista prootica that extends from the anterior margin of the element, and continues along the entire length of the lateral margin of the paroccipital process (as preserved). The crista prootica forms the roof and lateral margin of a deep, broad recessus venae jugularis for the passage of the internal jugular vein (Oelrich, 1956) (Fig. 4A). The anteroventral part of the crista prootica partially obscures the posterior opening of the Vidian canal (which lies within the parabasisphenoid). The margin of the crista prootica is partially incomplete, but preserved regions indicate that the edge of the crista was smoothly convex posteriorly, and did not include a well-defined, elongate, or tabular process at the level of the fenestra vestibuli, in contrast with the situation in most Varan species. Instead, the crista prootica decreases in transverse width towards the posterior margin of the prootic.

In V. pris, the facialis foramen is divided into two widely separated foramina (Fig. 4A). The foramen for the palatine branch is small, and is located on the medial surface of the recessus venae jugularis, at a point level with the fenestra vestibuli, in lateral view. The foramen for the hyomandibular branch is positioned posterodorsal to the fenestra vestibuli, and is situated within the lateral edge of the crista interfenestralis, at the point where this crest merges with the main body of the prootic. Both the palatine and hyomandibular foramina are relatively smaller and more widely separated in V. pris than in other varanid taxa. The angle at which the passage for the hyomandibular branch enters the prootic suggests that there was extensive bone growth dorsal to the facial nerve.

In lateral view, the posterior margin of the prootic forms the crista interfenestralis. The crista extends dorsally from a point on the lateral surface of the prootic just dorsal to the spheno-occipital tubercle to the ventral margin of the paroccipital process, where it becomes a low ridge that forms the anterior margin of the stapedial groove (Rieppel & Zaher, 2000). For most of its length, the crista interfenestralis forms the anterior margin of the external openings of the fenestra vestibuli and occipital recess (Fig. 4A). The crista extends approximately vertically in its ventral portion, but exhibits a distinct break in slope at a point level with the otic foramina, and its posterior portion extends posterodorsally. The crista interfenestralis is relatively larger than in other varanid taxa, and its posterior extension results in a narrowing of the fenestra vestibuli and occipital recess, as also occurs in V. komodoens(Figs 3A, B, 4A, B).

The medial margin of the prootic is formed by the broadly concave margins of the auditory vestibule, and contributes to a poorly defined auditory recess at the ventrolateral margins of the anterior braincase. The recess includes an anterior facial nerve foramen, a smaller dorsal acoustic nerve foramen, and a large, recessed median opening of the recessus scala tympani, present at what is inferred to be the prootic–otooccipital contact. The presence of medial openings of the endolymphatic foramina cannot be identified.

Basioccipital

The basioccipital forms the floor of the foramen magnum, and the ventral surface of the posterior endocranial cavity (Fig. 1B). It also forms the majority of the subcresentic occipital condyle. The dorsal margin of the occipital condyle is deeply excavated (Fig. 2A). Ventrally, the spheno-occipital tubercles are prominent and anteriorly positioned relative to the occipital region in lateral view (Fig. 1D). The tubercles are asymmetrical: the right tubercle is similar in size and shape to the condition in other varanid taxa, but the left is positioned more dorsally, and extends posteriorly (rather than posteroventrally). The bone comprising the ventral surface of the left tubercle is unfinished and has an irregular texture: this in combination with the unusual shape of the tubercle indicates that it is either pathologically or teratologically deformed.

Parabasisphenoid

The parabasisphenoid forms the anteroventral portion of the braincase (Fig. 1B, C). The right basipterygoid process extends lateroventrally, and is ovoid in cross section (Fig. 3A). The posterior opening of the Vidian canal is well developed, and pierces the body of the element at the base of the recessus venae jugularis, just posterodorsal to the basipterygoid process (Fig. 3A). The anterior opening of the Vidian canal lies at the dorsomedian margin of the basipterygoid process, just ventral to the sella turcica (Fig. 1A). The Vidian canal openings are comparatively larger than in other species of Varan.

The sella turcica is large and bears a narrow, ovoid, and deep hypapophyseal fossa (Fig. 1C). Foramina for the cerebral arteries open in the posterolateral margins of the fossa. These foramina are large, and reach approximately half of the diameter of the fossa. The dorsum sellae is tall, and slopes anterodorsally, and is capped by a prominent crista sellaris (Figs 1D, 3A). The lateral margins of the crista project anteroventrally, and are continuous with the inferior process of the prootic. The foramina for cranial nerve VI pierces the dorsum sellae from the ventral floor of the braincase to enter the sella turcica, as in other squamates (e.g. Säve-Söderbergh, 1946).

DISCUSSION

Systematic inter -relationships and taxonomic priority ofVaranus

Varan is a diverse and speciose lineage with a long evolutionary history (e.g. Mertens, 1942; Baverstock et a, 1993; Pianka, 1995; Pianka & Vitt, 2003), but a comprehensive species-level phylogenetic hypothesis based on morphology has yet to be constructed. Such an analysis is beyond the scope of this study, so to determine the inter-relationships of V. pris with other varanoids, we optimized ten morphological characters of the neurocranium and skull roof (derived from observations in this study: Appendices 1 and 2) onto the tree topology derived from the most comprehensive molecular dataset (Ast, 2001), using MacClade 4.05 (Maddison & Maddison, 2002).

Character polarities are based on comparisons with Lanthanot(McDowell & Bogart, 1954; Rieppel, 1983; http://digimorph.org/specimens/Lanthanotus_borneensis/), Heloder(specimens listed in Appendix 5), and Estes(Norell & Gao, 1997). Inclusion of Estes is necessitated in part by the condition in Heloder, which possesses apomorphic and extremely reduced otic cristae (e.g. Oelrich, 1956). Among Varan species, we sampled representatives from all major clades (the specimens are listed in Appendix 5; Fig. 5), and collapsed the speciose clade of Australian dwarf monitors (subgenus Odatr) into a single terminal taxon, based on character observations for Varanus acanthur Boulenger, 1885, Varanus gille Lucas & Frost, 1895, and Varanus timorens Gray, 1831. Character states for taxa not represented by examined specimens are based on Mertens (1942).

Figure 5

Phylogenetic inter-relationships of Varanus pris within Varanoidea. The position is based on the optimization of morphological characters onto the molecular tree of Ast (2001). The position of Estes is based on Norell & Gao (1997). Data for Varanus rudicol, Varanus keithhorn, Varanus melin, Varanus dorean, Varanus yuwon, Varanus spence, Varanus merten, and Varanus panopt were not available. These taxa were added post hoc. The position of pris within the Indo-Australian clade unambiguously assigns the taxon to Varan.

Figure 5

Phylogenetic inter-relationships of Varanus pris within Varanoidea. The position is based on the optimization of morphological characters onto the molecular tree of Ast (2001). The position of Estes is based on Norell & Gao (1997). Data for Varanus rudicol, Varanus keithhorn, Varanus melin, Varanus dorean, Varanus yuwon, Varanus spence, Varanus merten, and Varanus panopt were not available. These taxa were added post hoc. The position of pris within the Indo-Australian clade unambiguously assigns the taxon to Varan.

Character optimization on the topology of Ast (2001) results in a sister-taxon relationship between pris and komodoens within the Indo-Australian radiation (Fig. 5). This topology (tree length, TL = 50; consistency index, CI = 0.60; rescaled consistency index, RC = 0.22) is three steps shorter than the next most parsimonious tree (pris+ vari), four steps shorter than the topology of Lee (1996), and between six and nine steps shorter than all other possible topological arrangements for V. pris, including a topology that would place the taxon as basal to all other species of Varan.

This analysis demonstrates that ‘Megalanpris is unambiguously nested within the genus Varan, and that the genus MegalanOwen, 1859 should be synonymized with VaranMerrem, 1820, which has taxonomic priority. We therefore follow Lydekker (1888), and officially subsume Megalan within Varan, thereby validating numerous taxonomic predictions (e.g. Pianka, 1995; Lee, 1996; Molnar, 2004).

Comparisons with other taxa

Varanus pris shares numerous morphological characteristics with other Varan species (e.g. Hecht, 1975; Molnar, 1990). However, a morphological diagnosis of V. pris that does not include features arising as a consequence of large body size has previously been absent. Moreover, until now the relationships of this taxon with other varanids have been poorly constrained. Neurocranial osteology provides new insights into both of these issues.

Neurocranial osteology of V. pris provides diagnostic characters. The prootic margins of the trigeminal foramen posses a short, convex, inferior process, and a relatively narrow and short processus alaris that does not extend dorsally to the level of the base of the supraoccipital processes ascendens. The fenestra vestibuli and occipital recess are incompletely separated medial to the crista interfenestralis (Fig. 4A), unlike most species where the fenestra and the recess are either broadly separated by a thick horizontal lamina of the otoocipital posterior to the crista interfenestralis (Fig. 3C), or are separated by a thin lamina medial to the crista (Fig. 4B). The posterior process of the crista prootica is distinct. Most Varan species possess a wide tabular or spinous process of the crista prootica that projects posteriorly and laterally overlaps the stapes as it exits the fenestra vestibuli. The morphology of the process is variable, and it is prominent and slightly recurved (Figs 3C, 4C), is a reduced, broad tabular crest (Figs 3B, 4B), or is absent [correlated with a reduction of the entire crista prootica, as in Varanus gouldi Gr, 1838]. Varanus pris has an elongate, reduced process formed ventrally by an anterior concavity of the crista prootica at the level of the occipital recess, similar to V. komodoens(Figs 1D, 3A, B, 4A, B), but the preserved posterior margins of the process in V. pris indicate that it is broadly convex as opposed to the approximately rectangular shape in V. komodoens(Fig. 3B).

Ossification of the supraoccipital processus ascendens, presence of a well-developed median ridge, and an acute angle between the supraoccipital and parietal were considered to be apomorphic for V. pris by Hecht (1975). In comparison with extant taxa, the dorsal process in V. pris is relatively narrower; however, the processus ascendens is variably ossified in V. komodoens(Fig. 2) and V. gigante. Additionally, a tall median ridge occurs in multiple varanid taxa including V. komodoens(Fig. 2A, B), and somatically old individuals of both Varanus nilotic Linnaeus, 1766 and Varanus exanthematic Bosc, 1792. The wide range of morphologies in the latter taxa suggests that aspects of supraoccipital morphology may be size, specifically ontogenetically, dependant.

Within the genus, V. pris shares several features of the otic region with V. komodoens. In both taxa, the crista interfenestralis extends posteriorly to laterally overlap both the foramen vestibuli and occipital recess (Figs 3A, B, 4A, B). Development of the crista interfenestralis also results in an elongate, slit-like occipital recess, unlike the condition in most taxa, where the recess is rounded or ovoid (Fig. 3C). The crista interfenestralis also extends posteriorly in Varanus salvat Laurenti, 1768, but only ventrally at the level of the recess, and is reduced dorsally, exposing the division between the recess and the fenestra vestibuli, as in most other Varan species.

The configuration of the foramen facialis is also shared by V. pris and V. komodoens. As noted by Rieppel & Zaher (2000), separate exits for the palatine and hyomandibular branches of cranial nerve VII in Varan results from bone growth over the extracranial geniculate ganglion (Watkinson, 1906; Barbas-Henry, 1988) on the medial margin of the recessus venae jugularis. Separation of the two exits is variable in extant Varan, with African taxa [Varanus albigular Daudin, 1802, V. exanthematic, and V. nilotic] possessing either a single opening or individual (and intracranial) polymorphism, and most Indo-Asian-Australian taxa possessing at least weak division into palatine and hyomandibular foramina. In many taxa, the separation is incomplete or narrow (Fig. 4C; see also Conrad, 2004; Bever et a, 2005; for similar conditions in Shinisaur).

In V. pris and V. komodoens, the foramina are more widely spaced than in other Varan species, with the hyomandibular canal exiting at or near the base of the crista interfenestralis. In V. pris, the canal extends through the crista, exiting at its lateral margin along the prootic–otooccipital contact (Fig. 4A). This condition is polymorphic in V. komodoens, where the canal exits at the base of the crista, as in V. pris(Fig. 4B), or on the medial surface of the recessus jugularis, as in all other extant species.

The characters of the skull roof used by Lee (1996) to support the monophyly of the clade (pris+ gigante) are corroborated here for both taxa, but we note that similar morphologies are also present in other Varan species. A portion of the interfrontal suture is raised in specimens of V. komodoens, as well as in older individuals of V. exanthematic and V. nilotic, perhaps suggesting that this character state may be size related. However, this condition is also present in small-bodied taxa, including Varanus flavesce Hardwicke & Gray, 1827 and Varanus olivace Hallowell, 1857. A rugose dorsal surface of the frontals, including laterally oriented ridges, occurs in multiple varanid taxa.

It is possible that many of the characters we employ to both diagnose V. pris and determine its inter-relationships may be artifacts of large body size evolution, as postulated here for ossification of the supraoccipital processus ascendens. However, there does not appear to be a correlation between large body size and morphology of the otic capsule, as evidenced by comparisons between large-bodied species such as V. salvat(weak or no division of foramen facialis, wide, ovoid exposure of fenestra vestibuli, and occipital recess) with V. komodoens(strong division of foramen facialis, slit-like fenestra vestibuli, and occipital recess). As a result, we infer the neurocranial osteology of V. pris as diagnostic, independent of body size.

Evolutionary implications

The extreme body size of V. komodoens has previously been considered to be the result of insular gigantism. However, Gould & McFadden (2004) have argued that this taxon forms part of a radiation of large-bodied species, in which the evolution of body size was independent of geographic isolation. The placement of V. pris within this clade supports this hypothesis, and indicates that gigantism has evolved on three separate occasions within Varan: in Australian V. gigante; in a monophyletic African clade (Pianka, 1995; Fig. 5); and in the Indo-Australasian clade that includes V. salvat, Varanus vari White, 1790, V. komodoens, and V. pris. The inter-relationships of V. pris also have implications for the biogeographical history of the genus. The clade within which V. pris is nested is sister taxon to a predominately Australian radiation of Varan(Fig. 5). The salvat+ (vari+ (komodoens+ pris)) clade includes Indian–Indonesian taxa (V. salvat), Indonesian endemics (V. komodoens), and Australian taxa (V. vari and V. pris). The monophyly of (salvat+ (vari+ (komodoens+ pris))) and the clade including Australian goannas and dwarf monitors indicates multiple dispersal events between Meganesia and Indonesia, including the dispersal of giants.

ACKNOWLEDGEMENTS

We thank C. McCarthy (BMNH), K. Seymour (ROM), C. J. Bell (TMM), and K. de Queiroz, G. Zug, and R. McDiarmid (USNM) for access to comparative material, C. J. Bell for obscure literature, S. Moore-Fay for further preparation of BMNH 39965, and P. Crabb (NHM) for photography. This research was supported by a grant from the Royal Society to PMB and a NSERC Discovery Grant to JJH.

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Appendices

APPENDIX 1

Institutional abbreviations: BMNH, The Natural History Museum, London; ROMV-R, Royal Ontario Museum, Recent Collections; UCMZ, Museum of Zoology, University of Cambridge; USNM, United States National Museum, Smithsonian Institution.

APPENDIX 2

Anatomical abbreviations: av, anterior opening of the Vidian canal; Bo, basioccipital; bp, basipterygoid process; ci, crista interfenestralis; cp, crista prootica; cs, crista sellaris; ct, crista tuberalis; ff, foramen facialis; fh, foramen facialis hyomandibular branch; fp, foramen facialis palatine branch; fv, fenestra vestibuli; if, inferior process of prootic; jf, jugular foramen; or, occipital recess; Ot, otooccipital; pl, processus alaris; pa, processus ascedens; Pb, parabasisphenoid; po, paroccipital process of otoociptial; Pr, prootic; pv, posterior opening of Vidian canal; se, sella turcica; So, supraoccipital; st, spheno-occipital tubercle; tr, trigeminal notch.

APPENDIX 3

Character descriptions: character polarities are based on Lanthanot, Heloder, and Estes; states for Heloder and Varan are based on specimen observation and Mertens (1942).

  1. Crista prootica posterior process: reduced (0); tall tabular, or curved rhomboidal process (1). A discrete, elongate or tabular posterior process of the crista prootica is present in most Varan species (e.g. Bahl, 1937) (Figs 3, 4).

  2. Fenestra vestibuli and occipital recess: widely separated, visible in lateral view (0); narrowly separated, overlapped by crista interfenestralis in lateral view (1) (Figs 3, 4).

  3. Fenestra vestibuli: anterior to spheno-occipital tubercle (0); dorsal or posterior (1) (Norell & Gao, 1997).

  4. Occipital recess: round or ovoid (0); slit (1) (Figs 3, 4).

  5. Foramen facialis: not divided (0); short or incomplete separation (1); wide separation, separation fully ossified (2) (Fig. 4).

  6. Interfrontal suture: smooth (0); raised sagittal crest (1) (Lee, 1996).

  7. Frontal ornamentation: smooth (0); rugose with lateral ridges (1) (Lee, 1996).

  8. Median ridge on supraoccipital: absent or poorly defined (0); present, tall (1) (Fig. 2). A distinct median ridge with vertical lateral margins is present on the posterior margin of the supraoccipital in multiple taxa.

  9. Supraoccipital dorsal process: broad (0); narrow, defined by deep lateral recesses (1). The lateral margins of the supraoccipital dorsal process are defined by deep recesses in multiple taxa (Fig. 2).

  10. Supraoccipital processus ascendens: not ossified (0); ossified (1) (Hecht, 1975) (Fig. 2).

APPENDIX 4

Character–taxon matrix: A = 0/1; B = 1/2.

 Characters 
Taxon 10 
Estes 00000 00000 
Heloder 00000 00000 
Lanthanot 0010? 00100 
V. beccar 10100 00000 
V. bengalens 10101 A0000 
V. dumeri 00100 0A000 
V. exanthematic 1010A 1A0A0 
V. flavesce 10101 11000 
V. gigante 11101 1110A 
V. goul 00100 10000 
V. grise 0010A 10000 
V. indic 101?1 1?000 
V. jobiens 10101 00A00 
V. komodoens A111B 11111 
V. nilotic A010A A0AA0 
Odatr subgenus 0010? 00A00 
V. olivace 10101 1???? 
V. prasin 1010? 00100 
V. pris 01112 11111 
V. salvado 00100 A0AA0 
V. salvat 10110 00000 
V. vari 0?1?? ?0010 
 Characters 
Taxon 10 
Estes 00000 00000 
Heloder 00000 00000 
Lanthanot 0010? 00100 
V. beccar 10100 00000 
V. bengalens 10101 A0000 
V. dumeri 00100 0A000 
V. exanthematic 1010A 1A0A0 
V. flavesce 10101 11000 
V. gigante 11101 1110A 
V. goul 00100 10000 
V. grise 0010A 10000 
V. indic 101?1 1?000 
V. jobiens 10101 00A00 
V. komodoens A111B 11111 
V. nilotic A010A A0AA0 
Odatr subgenus 0010? 00A00 
V. olivace 10101 1???? 
V. prasin 1010? 00100 
V. pris 01112 11111 
V. salvado 00100 A0AA0 
V. salvat 10110 00000 
V. vari 0?1?? ?0010 

APPENDIX 5

Examined specimens
Heloder:

horrid, ROMV-R 278; suspect, ROMV-R 279, ROMV-R 556.

Varan:

acanthur, ROMV-R 0082; albigular, BMNH 1908.4.24.35; beccar, ROMV-R 7483; bengalens, BMNH 1931.1.10.2, BMNH 1931.1.10.4, BMNH 1974.2479, UCMZ R.9616, UCMZ R.9617; dumeri, ROMV-R 6417, ROMV-R 7937, ROMV-R 7949; exanthematic, BMNH 1920.1.20.3755, BMNH 1932.6.1.12, BMNH 1964.744, BMNH 1974–2480, ROMV-R 7475, ROMV-R 7715, ROMV-R 7870; flavesce, ROMV-R 0071, ROMV-R 0758; gigante, UCMZ R.9586, UCMZ R.9587; gille, BMNH 1910.5.28.13; goul, BMNH 1983.1132; grise, BMNH unregistered ‘78, Algeria’, BMNH 71.6.6.2, BMNH 1920.1.20.722, BMNH 1974.2482, ROMV-R 0075, ROMV-R 0374; indic, BMNH 1932.4.19.2, ROMV-R 0078; jobiens, ROMV-R 7938, ROMV-R 7950; komodoens BMNH 1934.9.2.1, ROMV-R 7565, USNM 228163; nilotic, BMNH 1900.9.22, BMNH 1900.9.22.9, BMNH 1931.1.12.1, BMNH 1934.9.2.1, BMNH 1970–1983, BMNH 97.5.31.1, ROMV-R 0062, ROMV-R 0422, ROMV-R 1882, ROMV-R 7287, ROMV-R 7303, UCMZ R.9551, UCMZ R.9555, UCMZ R.9558; (nilotic?), UCMZ R.9560; olivace, BMNH 1946.8.30.98, USNM 222400; prasin, BMNH ‘88’; pris, BMNH 39965; salvado, ROMV-R 6793, ROMV-R 7926; salvat BMNH 64.9.2.77, BMNH 1961.1761, BMNH 1972.2160, BMNH 1972.2161, ROMV-R 136, ROMV-R 7161; timorens, BMNH 90.1.14.12.