Abstract

In most Palaeozoic temnospondyls, thin round-oval scales covering the flanks and the back of the trunk can be distinguished from ventral, elongate gastral scales arranged in a chevron pattern. The extensive growth series of the temnospondyl Sclerocephal reveals that the morphology of the gastral scales in small larvae corresponds to the round-oval scales of the rest of the body. During subsequent ontogeny, the gastral scales differentiate and attain a spindle-shaped morphology. The tapering end of each gastral scale fits into a dorsal groove on the medial adjacent scale. This arrangement allowed telescoping of the scales and thus provided a high degree of flexibility. In the ontogenetically most advanced specimens of Sclerocephal the gastral scales attain a rhomboid outline, and the articulation by well-defined facets has reduced the flexibility between them. In most temnospondyls, the gastral scales retain the ‘juvenile’ spindle-shape or the ‘larval’ round-oval shape, which can be interpreted as a paedomorphic trait. This suggests that the different types of gastral scales in temnospondyls, as well as the scales of the back and the flanks, can be traced back to the same Anla of round-oval scales that differentiated early in ontogeny. In the Mesozoic, a complete reduction of dermal scalation occurred independently in distinct dissorophoid, capitosauroid, and trematosauroid temnospondyls. This reduction was probably the result of several factors unique to each group, such as cutaneous respiration, the demand for greater mobility, and the decreased importance of belly protection in fully aquatic temnospondyls.

The presence of ossified dermal scales covering the body is widespread in temnospondyls and other basal tetrapods of the Palaeozoic, and is likely to be a heritage from their fish ancestors (Dias & Richter, 2002; Castanet et a, 2003). In contrast to Palaeozoic sarcopterygian fish, the earliest known tetrapods have eliminated the enamel and dental components of their scales, in convergence with most extant fish (Colbert, 1955; Dias & Richter, 2002; Castanet et a, 2003).

The first in-depth study of dermal scalation in a temnospondyl (and Palaeozoic basal tetrapod) was provided by von Meyer (1858) in his monograph on Archegosaurus decheGoldfuss, 1847. Von Meyer recognized two different types of dermal scales: spindle-shaped scales that form rows arranged in a chevron pattern (a repeated Λ pattern) on the ventral side of the trunk between the pectoral girdle and the pelvis, and thin cycloid or oval scales that cover the flanks and the dorsal portion of the trunk as well as the limbs. Further descriptions of basal tetrapod scales were given by Fritsch (1883, 1889) in his work on the fauna of the gas coal in Nýřany and Credner (1881, 1886, 1893) involving the scalation of ‘branchiosaurs’ of the Döhlen Basin in Saxony. Dermal scales have been subsequently reported in a variety of different temnospondyl taxa. Broili (1926, 1927), Boy (1988), and recently Schoch (2003) commented on the scalation in the actinodontid Sclerocephal, and scale morphology and their arrangement in different dissorophoids were described by Steen (1931), Gregory (1950), Boy (1972, 1978, 1986, 1987), Werneburg (1987, 1994), and Daly (1994). Case (1935) described dermal scales in Trimerorhach, as did Romer & Witter (1941) in Eryo. In 1955 Colbert published a study on scale morphology of Trimerorhach, including a comparison with other temnospondyls as well as with fish and gymnophionans. His description of the scalation in Trimerorhach was basically confirmed by Chase (1965) and Berman (1973) in their works on the trimerorhachids Neldasaur and Lafoni, respectively. Studies dealing exclusively with the ventral scalation of stereospondylomorph temnospondyls were published by Findlay (1968) on Uranocentrod and by Dias & Richter (2002) on Australerpet. Further descriptions of dermal scales in stereospondylomorphs are included in the works of Van Hoepen (1915), Broili & Schröder (1937), Nilsson (1946), Konzhukova (1955), Watson (1958), Warren & Hutchinson (1987), Janvier (1992), Hellrung (2003), and Pawley & Warren (2004, 2005). Carroll (1967) and Milner & Sequeira (1994) demonstrated dermal scalation in the basal temnospondyls Dendrerpet and Balanerpet, respectively. Among extant amphibians, the occurrence of dermal scales is restricted to representatives of gymnophionans. Although Colbert (1955) regarded the scales in gymnophionans as heritage from their Palaeozoic ancestors, Zylberberg & Wake (1990) emphasized that it is equally parsimonious to assume that the gymnophionan scales are de no acquisitions.

The purpose of the present paper is to analyse the evolution of scalation in temnospondyls. First, the morphology of the scales and their arrangement are described and compared in the different temnospondyl lineages. A further focus will be the documentation of the ontogeny of the scalation. Then, the plesiomorphic condition of the scalation pattern in temnospondyls will be ascertained, whereby the stem-tetrapods Ichthyoste, Tulerpet, and Greererpet are taken as the primary source for polarity determination. Finally, the functional significance of the ventral and dorsal scales in temnospondyls will be assessed in the light of the new morphological and ontogenetic data.

Definition of anatomical terms

Ruibal & Shoemaker (1984) used the term ‘dermal scale’ for the dermal ossifications of osteichthyans and gymnophionans, whereas they referred to the ossified structures in the dermis of anurans and squamates as ‘osteoderms’. Zylberberg & Wake (1990) adopted this terminology and listed differences between scales and osteoderms relative to their association with soft parts (e.g. scales are located in pockets in contrast to osteoderms). They emphasized, however, that this distinction does not imply common ancestry of either dermal scales or osteoderms. Because soft parts of the dermis are mostly not preserved in basal tetrapods, the structural distinction between dermal scales and osteoderms of Castanet et a (2003) is followed in the present study: osteoderms are plates of dermal bone that often bear a pitted outer surface; dermal scales, in contrast, are thinner than osteoderms, often round or elongate oval in outline, and may overlap. The present paper deals with ossifications of temnospondyls that can be interpreted as dermal scales.

The term ‘gastralia’ is often used to refer to the elongate ventral ossifications of basal tetrapods because their Λ-shaped arrangement closely resembles the gastralia or ‘abdominal ribs’ of several amniotes. Indeed, the gastralia of amniotes are probably derived from the ventral scales of basal tetrapods (Baur, 1889; Voeltzkow & Döderlein, 1901; Romer, 1956). In extant crocodilians and Sphenod, the gastralia develop in the dermis and become secondarily embedded in the rectus abdominis muscle later in ontogeny (Voeltzkow & Döderlein, 1901; Howes & Swinnerton, 1901; Claessens, 2004). In contrast, the ventral scales of the temnospondyl Australerpet were located in the dermis both in juveniles and adults, as Dias & Richter (2002) were able to demonstrate, and this must also be assumed for other basal tetrapods. To avoid confusion between the more rod-like gastralia of amniotes, and their probable precursors in basal tetrapods, the term ‘gastralia’ is avoided in the present study, and the term ‘gastral scales’ is used here to refer to the ventral ossifications in basal tetrapods. Similarly orientated gastral scales overlapping each other along their long axis are referred to as ‘rows’ (Fig. 1). Two associated rows from the same metameric segment articulating in the ventral midline are called a ‘chevron’. Two posterolaterally aligned rows from opposite sides of the trunk form an anteriorly directed chevron, and two anterolaterally directed rows form a posteriorly directed chevron. The remaining round-oval scales that cover the flanks and back of the trunk, the limbs, and the tail are referred to as ‘dorsal scales’.

Figure 1

Schematic drawing of gastral scales in the anterior region of the belly in a temnospondyl illustrating the terms ‘row’, ‘anteriorly directed chevron’, and ‘posteriorly directed chevron’.

Figure 1

Schematic drawing of gastral scales in the anterior region of the belly in a temnospondyl illustrating the terms ‘row’, ‘anteriorly directed chevron’, and ‘posteriorly directed chevron’.

Institutional abbreviations

CMNH, Cleveland Museum of Natural History, Cleveland, OH, USA; IGS, Institut de Géologie Strasbourg, Université Louis Pasteur, Strasbourg, France; MB, Museum für Naturkunde, Humboldt Universität, Berlin, Germany; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA; MMG, Staatliches Museum für Mineralogie und Geologie, Dresden, Germany; PIN, Palaeontological Institute of the Russian Academy of Sciences, Moscow, Russia; ROM, Royal Ontario Museum, Toronto, ON, Canada; SMNK, Staatliches Museum für Naturkunde, Karlsruhe, Germany; SMNS, Staatliches Museum für Naturkunde, Stuttgart, Germany.

MATERIAL EXAMINED

The skull length (sl) is used as a measure of size, herein defined as the distance from the tip of the snout to the posterior margin of the postparietals.

Archegosaurus decheGoldfuss, 1847 from the Lower Rotliegend (Autunian) of Lebach, Saar-Nahe Basin (Saarland, south-western Germany): MB.Am.227 (sl 84 mm, complete skeleton), MB.Am.229 (sl 79 mm, complete skeleton), MB.Am.252 (sl c. 65 mm, almost complete skeleton including tail), MB.Am.273 (sl estimated 200 mm, pelvic region plus tail), MB.Am.289 (sl c. 110 mm, with anterior half of the trunk); IGS U II 3/1 (postcranial remains of large specimen).

Branchierpeton amblystom (Credner, 1881) from the Lower Rotliegend (Autunian) of Niederhäslich, Döhlen Basin, Saxony, Germany: MMG SaP 154 (sl 15 mm, complete skeleton), MMG SaP 700 (sl 17 mm, complete skeleton).

Cheliderpeton latirost (Jordan, 1849) from the Lower Rotliegend (Autunian) of the Saar-Nahe Basin (Saarland and Rheinland-Pfalz, south-western Germany): MB.Am.1271 (sl 58 mm, complete skeleton), MB.Am.1275 (sl 39 mm, complete skeleton), MB.Am.1293 (sl 54 mm, skull and anterior part of the trunk), MB.Am.1312 (sl 61 mm, skull and anterior part of the trunk); ROM 5735 (sl 58 mm, skull with anterior half of the trunk).

Dendrerpet sp. from the Westphalian D of Florence, Nova Scotia: MCZ 8779 (disarticulated skull, pectoral girdle, gastral and dorsal scales).

Eryops megacephalCope, 1877 from the Belle Plains Formation, Wichita Group, Lower Permian, Texas, USA: MCZ 1539 (caudal vertebrae with dorsal scales), MCZ 1738 (part and counterpart of gastral scales).

Greererpeton burkemoraRomer, 1969 from Deckers Creek, West Virginia, Upper Mississippian, USA: CMNH 11073 (sl 150 mm, with anterior part of the trunk), CMNH 11219 (complete skeleton), CMNH 11233 (sl 120 mm, with complete postcranial skeleton), CMNH 11236 (trunk plus pelvic region).

Plagiosuchus pustulifer (Fraas, 1896) from the Middle Triassic (Ladinian) of Baden Württemberg, Germany: SMNS 84794 (complete skeleton).

Platyoposaurus stuckenber (Trautschold, 1884) from the Upper Permian (Kazanian) of Belebey, Bashkortostan (Southern Ural), Russia: PIN 164/1–9 (interclavicle, scapulocoracoid, articulated and isolated gastral scales).

Sclerocephalus haeuseGoldfuss, 1847 from the Lower Rotliegend (Autunian) of the Saar-Nahe Basin, Rheinland-Pfalz, south-western Germany: MB.Am.1233 (sl c. 80 mm, with anterior trunk), MB.Am.1298 (sl 72 mm, complete skeleton), MB.Am.1314 (sl 22 mm, complete skeleton), SMNK uncatalogued (sl 183 mm, complete skeleton); SMNS 90507 (sl 205 mm, almost complete skeleton, plaster cast of the neotype housed in the Palaeontological Institute of the University of Mainz, Germany; Boy, 1988), SMNS 90055 (sl 198 mm, complete skeleton), SMNS 54065 (sl 156 mm, complete skeleton).

Trimerorhachis insignCope, 1878 from the Lower Permian, Clyde Formation, Clear Fork Group, Baylor County, Texas, USA: MCZ 1080 (pectoral girdle, vertebrae, dorsal and gastral scales).

DESCRIPTION

The arrangement of gastral scales

In the vast majority of temnospondyls the gastral scales are arranged in a chevron pattern, i.e. a repeated Λ pattern. Anteriorly directed chevrons are present in the middle and posterior trunk region, whereas the chevrons are directed posteriorly in the anterior trunk region immediately posterior to the pectoral girdle (Fig. 2). A region termed the ‘nodal point’ by von Meyer (1858) lies between the anteriorly and posteriorly directed chevrons.

Figure 2

Archegosaurus deche. A, MB.Am.229. Anteriorly directed chevrons with round-oval scales of the flanks, the cloacal region, and the hindlimb. The angle of the chevrons in the ventral midline was enlarged after the death of the animal by a slight displacement of the rows. B, MB.Am.289. Gastral scales articulating in the ventral midline of the trunk, ventral view. C, MB.Am.289. Nodal point of gastral scales in dorsal view between anteriorly and posteriorly directed chevrons.

Figure 2

Archegosaurus deche. A, MB.Am.229. Anteriorly directed chevrons with round-oval scales of the flanks, the cloacal region, and the hindlimb. The angle of the chevrons in the ventral midline was enlarged after the death of the animal by a slight displacement of the rows. B, MB.Am.289. Gastral scales articulating in the ventral midline of the trunk, ventral view. C, MB.Am.289. Nodal point of gastral scales in dorsal view between anteriorly and posteriorly directed chevrons.

Anteriorly directed chevrons

In most temnospondyls approximately 70–90 anteriorly directed chevrons are present, and this number does not change during ontogeny in taxa with preserved growth series, e.g. A. deche and S. haeuse. The posterolaterally running rows of gastral scales follow the shape of a shallow ‘S’ (Fig. 2A). In the first anteriorly directed chevrons the rows of gastral scales articulate in the ventral midline of the trunk, at approximately a right angle. This angle decreases gradually to approximately 60° in the posterior-most chevrons. The last chevron is located a distance of approximately two vertebrae anterior to the ilium in the temnospondyls examined. In the ventral midline of the trunk, the rows articulate in the following way (Fig. 2B): each medial-most gastral scale has a rounded medial end, the posterior portion of which is overlapped by the medial-most scale from the opposite row. The rounded medial end is expanded in some temnospondyl taxa. The overlap in the ventral midline of the trunk takes place alternating from the left and right side of the trunk, and each medial-most gastral scale articulates with two medial-most ones from opposite rows. Thus the medial-most gastral scales imbricate in the anteroposterior direction in the midline of the trunk in ventral view (Fig. 2B).

The ‘nodal point’

The ‘nodal point’ is the point of inversion from the anteriorly to the posteriorly directed chevrons, located a short distance posterior to the end of the interclavicle. This region is rarely preserved in temnospondyls, but its rather complicated arrangement is clearly visible in specimens of Archegosaur, Sclerocephal, and Branchierpet (Figs 1, 2C). Undisturbed rows of spindle-shaped scales run diagonally between the opposing sets of chevrons. The number of undisturbed rows in individual specimens varies from one to eight in the Archegosaur and Sclerocephal specimens studied. At the nodal point, a number of rows are present that run parallel to the anteriorly and posteriorly directed chevrons, respectively, but do not meet in the ventral midline. Instead, they contact the opposing rows at an angle of approximately 90° (Figs 1, 2C).

Posteriorly directed chevrons

The gastral scales of the posteriorly directed chevrons run parallel to the posterolateral margins of the interclavicle (Fig. 2C). The anterolaterally directed rows meet in the ventral midline in the same manner as described above for the anteriorly directed chevrons, so that the medial-most scales imbricate from posterior to anterior in ventral view. In most specimens of Archegosaur, Sclerocephal, and Branchierpet, the number of posteriorly directed chevrons varies individually between eight and 12. In some individuals they may be absent altogether. Fritsch (1889: pl. 56, fig. 2) illustrated the ventral scalation of an individual of Cheliderpeton vranFritsch, 1877 in which the first anteriorly directed chevron contacts the interclavicle, so that no undisturbed rows and no posteriorly directed chevrons are present. Instead, anterolaterally directed rows contact the anteriorly directed chevrons at an angle of about 90°. Another specimen of this species possesses posteriorly directed chevrons (Werneburg & Steyer, 2002: fig. 4a). The same pattern, as illustrated by Fritsch (1889), is visible in the rhinesuchid postcranium described by Pawley & Warren (2004: fig. 3), in Platyoposaur (PIN 164/1), in C. latirost (ROM 5735), and in the Sclerocephal specimen MB.Am.1203, although in the latter two specimens three undisturbed rows of gastral scales are present.

Morphology of the gastral scales

The morphology of the gastral scales differs by taxon and ontogenetic stage. The gastral scales can be divided into three morphological groups with transitions between each: ovoid, spindle-shaped, and rhombic gastral scales.

Ovoid gastral scales

Ovoid gastral scales are present in the basal temnospondyl Balanerpet, and among the dissorophoids in micromelerpetontids, branchiosaurids, and amphibamids, and in a derived condition in trimerorhachids. These gastral scales are broadly oval with bluntly rounded medial and lateral ends. Each scale bears concentric growth rings and is ventrally (externally) convex and dorsally (internally) concave. Within the same row of gastral scales, the medial end of each scale overlaps the dorsal surface of its medial neighbour. The degree of overlap is extensive (up to more than 50% of the scale length). The posterior margin of each gastral scale slightly overlaps the ventral surface of the neighbouring scale of the posteriorly located row.

The most plesiomorphic micromelerpetontid, Limnogyrin, has thick gastral scales that are composed of two layers and bear concentric rings and radial striae (Werneburg, 1994). In contrast, the scales of the micromelerpetontids Branchierpet and Micromelerpet are thinner and have attained a derived morphology with anastomosing longitudinal striae on the ventral surface and tubercles on the posterior margin (Boy, 1972; Werneburg, 1991). The chevron arrangement of scales with a nodal point as described above is especially conspicuous in Branchierpet specimens (Credner, 1886: pl. 19, figs 3, 8 and 9; Werneburg, 1991: figs 11 and 15; F. Witzmann, pers. observ.: MMG SaP 154, MMG SaP 700) (Fig. 3A). Among branchiosaurids, the plesiomorphic Branchiosaur has gastral scales that are similar to Limnogyrin (Werneburg, 1987), whereas in other branchiosaurids the scales have become much thinner (Boy, 1993). Branchiosaurid scales are composed of a basal layer with concentric rings and a thinner upper layer with longitudinal striae (Boy, 1972). Within amphibamids, Amphibam has ovoid gastral scales arranged in a chevron pattern (Gregory, 1950). Large specimens of Platyrhino have poorly ossified gastral scales (Clack & Milner, 1993). The gastral scales are possibly arranged in transverse rows and not in the chevron pattern in the amphibamid Eoscop (Daly, 1994).

Figure 3

Ovoid gastral scales. A, Branchierpeton amblystom. Gastral scales articulating in the ventral midline, redrawn after Werneburg (1991). B, Sclerocephalus haeuse, MB.Am.1302. Gastral scale. C, Trimerorhachis insign, MCZ 1080. Posterior part of interclavicle with gastral scales in ventral view.

Figure 3

Ovoid gastral scales. A, Branchierpeton amblystom. Gastral scales articulating in the ventral midline, redrawn after Werneburg (1991). B, Sclerocephalus haeuse, MB.Am.1302. Gastral scale. C, Trimerorhachis insign, MCZ 1080. Posterior part of interclavicle with gastral scales in ventral view.

Small larvae of the basal stereospondylomorph S. haeuse also have ovoid gastral scales, e.g. MB.Am.1302 (sl 18 mm) (Fig. 3B). In the rhytidosteoid stereospondyl Mahavisaur, broadly oval gastral scales are present on the ventral trunk immediately posterior to the pectoral girdle (Lehman, 1966; Janvier, 1992). The capitosauroids Paracyclotosaur and Wellesaur may also have ovoid gastral scales, although the situation is not unequivocal because of inadequate preservation. Paracyclotosaur has irregularly set oval scales at least on the posterior part of the belly (Watson, 1958), whereas spindle-shaped gastral scales are not preserved (see below). Also in Wellesaur, no spindle-shaped scales are present, but round-oval scales are visible on the trunk between the ribs (R. R. Schoch, pers. comm.) and might have covered the belly.

Trimerorhachids also possess ovoid gastral scales, but they differ from those of the other species in being anteroposteriorly elongate. In MCZ 1080, the anterior-most gastral scales are visible posterior to the interclavicle (Fig. 3C). They appear to be arranged in posteriorly directed chevrons, but this cannot be stated with certainty because of poor preservation. As in micromelerpetontids and branchiosaurids, the gastral scales of trimerorhachids consist of two layers, a thin upper layer with longitudinal striae, and a thicker lower layer with concentric rings (Colbert, 1955; Chase, 1965).

Spindle-shaped gastral scales

The majority of temnospondyls have slender, spindle-shaped gastral scales. In this morphology, one end of the scale is bluntly rounded, whereas the other end tapers to a long point. Their outline is rather conservative in the different groups of temnospondyls. The following description is based on A. deche, the spindle-shaped gastral scales of which possess a generalized morphology. The ventral side of each gastral scale bears a pronounced posterior ridge separated by a slight concavity from a less pronounced anterior ridge (Fig. 4A). Both ridges become more flattened towards the tapering end. In dorsal view, each gastral scale bears a deep groove and is asymmetric anteroposteriorly: the posterior margin of the dorsal groove is formed by a steep, thickened, bulge-like crest, whereas a sharp, anterodorsally directed crest delimits the groove anteriorly (Fig. 4B). The groove shallows gradually towards the tapering (medial) end, but terminates rather abruptly towards the rounded (lateral) end of the gastral scale. In the anteriorly directed chevrons, the tapering end is directed anteromedially towards the ventral midline of the trunk, and fits into the dorsal concavity of the medial neighbour (Figs 2C, 4B), whereby the degree of dorsal overlap generally amounts to more than 50% of scale length. Therefore, the gastral scales of the same row imbricate in a posterolateral direction in ventral view. In contrast, the tapering ends of the gastral scales of the posteriorly directed chevrons point anterolaterally (in the direction to the flanks) (Fig. 2C); thus the gastral scales of each row imbricate in posteromedial direction in ventral view. The anterior-most rows consist of less elongate, mostly nonoverlapping scales that are situated on the smooth posteroventral surface of the interclavicle. Both the concave dorsal surface and the ventral side of the scales are smooth, with the exception of the aforementioned ventral ridges and short striae that are aligned with the long axis of the scale. The lateral-most gastral scale of each row is thinner than the more medially situated ones. Similar to the ovoid gastral scales and the dorsal scales (see below), these scales bear concentric growth rings. In contrast to the scalation pattern in micromelerpetontids and branchiosaurids the chevrons do not overlap each other, but the anterior margin of each gastral scale abuts against the thickened posterior margin of the adjacent gastral scale of the anterior chevron.

Figure 4

Spindle-shaped gastral scales. A, B, Archegosaurus deche, MB.Am.289. A, gastral scales in ventral view. B, gastral scales in dorsal view. C, Dendrerpet sp., MCZ 8779. Two gastral scales in dorsal view plus isolated dorsal scale. D, Eryops megacephal, MCZ 1738. Gastral scales in dorsal view. E, Plagiosuchus pustulifer, SMNS 84794. Gastral scales with small, globular osteoderms in dorsal view.

Figure 4

Spindle-shaped gastral scales. A, B, Archegosaurus deche, MB.Am.289. A, gastral scales in ventral view. B, gastral scales in dorsal view. C, Dendrerpet sp., MCZ 8779. Two gastral scales in dorsal view plus isolated dorsal scale. D, Eryops megacephal, MCZ 1738. Gastral scales in dorsal view. E, Plagiosuchus pustulifer, SMNS 84794. Gastral scales with small, globular osteoderms in dorsal view.

Boy (1988: 127, fig. 11) depicted the complete ventral trunk region of S. haeuse covered with gastral scales, and considered their morphology to be highly variable and of irregular outline. However, the outline and arrangement of these scales are in fact quite regular, being virtually identical to those of Archegosaur in late larval to subadult specimens of Sclerocephal (skull length c. 20–150 mm). Spindle-shaped gastral scales of basically the same morphology are present in the basal temnospondyl Dendrerpet (MCZ 8779) (Fig. 4C), in the eryopids Eryo (MCZ 1738) (Fig. 4D) and Onchiod (Credner, 1893), in the basal stereospondylomorphs C. vran (Werneburg & Steyer, 2002) and C. latirost (MB.Am.1271, MB.Am.1275, MB.Am.1293, MB.Am.1312, ROM 5735), and the stereospondyls Uranocentrod (Findlay, 1968), Lydekkeri (Broili & Schröder, 1937; Pawley & Warren, 2005), and Edingerel (Lehman, 1966; Janvier, 1992: pl. 2, figs 2a, b). In the rhytidosteoid Acerast, the gastral scales are proportionally more elongate than in the aforementioned taxa (Warren & Hutchinson, 1987). Plagiosaurids have proportionally the most elongate and slender gastral scales (Fig. 4E). In Gerrothor, they are located on the dorsal side of plate-like ventral osteoderms. The rows are reduced in number and well separated from each other, and no articulation in the ventral midline of the trunk takes place (Nilsson, 1946; Hellrung, 2003). The gastral scales of Plagiosuch are likewise very long and slender (Fig. 4E). However, the osteoderms were reduced to tiny globules that lay within the dermis (SMNS 84794). The enigmatic temnospondyl Peltobatrach has small, needle-shaped gastral scales, whereas the back of the trunk and the tail are covered by plate-like osteoderms (Panchen, 1959).

Rhombic gastral scales

The stereospondylomorph P. stuckenber has well-ossified gastral scales of rhombic outline (PIN 164/1–9) (Fig. 5 A, B). In contrast to the spindle-shaped gastral scales, the anterior margins of the rhombic scales of one row overlap the posterior parts of the gastral scales of the preceding row dorsally, so that the chevrons overlap from anterior to posterior in ventral view. The gastral scales in the anterior half of the trunk are heavily ossified and thick, whereas they become thinner in the posterior half of the trunk. In ventral view, each gastral scale has a pronounced sculpture of ridges and tubercles, and several foramina penetrate the bone (Fig. 5A). Each scale possesses a ventromedial facet that overlaps the dorsal surface of the neighbouring medial gastral scale of the same row, so that the scales imbricate in posterolateral direction within each row in ventral view. In its medial portion, this ventromedial facet is convex and bears a prominent, sharp crest, whereas the lateral end of the facet is deeply concave. The dorsal (internal) side of each rhombic gastral scale bears a well-defined dorsolateral facet that constitutes approximately 60–70% of the length of the scale (Fig. 5A, right). The anterior border of this facet is formed by a deep and sharp anterodorsally directed crest, whereas the posterior border of the facet is formed by a less deep, but thick bulge. The lateral portion of this facet is convex; it becomes increasingly concave medially and forms a distinct depression. This pattern is more pronounced in the more strongly ossified gastral scales of the anterior trunk region. Medial to the facet, the dorsal surface of the scale is flat and becomes slightly convex at the medial end.

Figure 5

Rhombic gastral scales. A, B, Platyoposaurus stuckenber, PIN 164/1–9. A, isolated gastral scale, probably from the anterior part of the trunk, in anteroventral (left) and anterodorsal view (right). B, isolated gastral scale, probably from the posterior part of the trunk, in anteroventral (left) and anterodorsal view (right). C, D, Sclerocephalus haeuse, SMNS 90507. C, gastral scales articulating in the ventral midline, ventral view. D, articulating gastral scales in ventral view.

Figure 5

Rhombic gastral scales. A, B, Platyoposaurus stuckenber, PIN 164/1–9. A, isolated gastral scale, probably from the anterior part of the trunk, in anteroventral (left) and anterodorsal view (right). B, isolated gastral scale, probably from the posterior part of the trunk, in anteroventral (left) and anterodorsal view (right). C, D, Sclerocephalus haeuse, SMNS 90507. C, gastral scales articulating in the ventral midline, ventral view. D, articulating gastral scales in ventral view.

The same morphology and arrangement of gastral scales as in Platyoposaur is present in the largest known specimens of S. haeuse (SMNS 90507, SMNS 90055; SMNK uncatalogued) (Fig. 5C, D) and the stereospondyl Australerpeton cosgrifBarberena, 1998(Dias & Richter, 2002). The postcranial skeleton of a rhinesuchid also bears rhombic gastral scales (Pawley & Warren, 2004: fig. 3).

Morphology and arrangement of the dorsal scales

In contrast to the well-ossified gastral scales, a comparison of the dorsal scales in temnospondyls is hampered by the fact that these more delicate ossifications are often not preserved or only poorly preserved, or were destroyed during preparation. As a morphological basis, the dorsal scalation of the basal stereospondylomorphs Archegosaur and Sclerocephal will be described first. The morphology and arrangement of the scales in these taxa is well preserved in almost all parts of the body. Subsequently, the dorsal scales of other temnospondyls are addressed.

Archegosaurus decheni

As already reported by von Meyer (1858), Jaekel (1896), and Broili (1927), the round-oval dorsal scales next to the gastral scales possess concentric growth rings and are cup-shaped, i.e. internally concave and externally convex. In contrast, the dorsal scales of the flanks and the back of the trunk are distinctly thinner and are preserved two-dimensionally. This indicates a decrease in ossification from the gastral scales of the belly to the cup-shaped scales of the ventral part of the flanks, and then to the thin scales of the flanks and the back. The dorsal scales of the trunk do not mutually overlap (Fig. 2A, C); at best, there is point contact such that there is always a space between them. This pattern does not change during ontogeny. Scales covering the limbs are smaller than those of the trunk and have a cup-like morphology with a distinct medial elevation in large specimens (e.g. IGS U II 3/1). The scales are tightly arranged but do not overlap each other. In MB.Am.229, enlarged round-oval scales are located on the ventral side immediately behind the posterior-most chevron and anterior to the base of the ilium (Fig. 2A). The long axis of such a ‘cloacal scale’ is approximately two times longer than that of a dorsal scale of the lateral trunk region, and four times longer than a limb scale. As illustrated by von Meyer (1858: pl. 18, figs 1, 2), similar enlarged scales are also located in the region anterior to the clavicles and the cleithra. Dorsal scales that are anteroposteriorly elongate are preserved in the lateral region of the ilium in MB.Am.273 (Fig. 6 A). They are aligned in transverse rows in such a way that each scale of one row is located between two scales of the preceding row. The dorsal scales in the tail are cup-shaped, tightly set, and possibly overlap each other slightly (MB.Am.252, MB.Am.273) (Fig. 6B), and so are more closely spaced than those of the lateral and dorsal parts of the trunk and the sacral region.

Figure 6

Dorsal scales. A, B, Archegosaurus deche, MB.Am.273. A, scales of the flanks in the region of the ilium. B, scales of the tail, showing concentric rings and radial striae. C, Sclerocephalus haeuse, MB.Am.1314. Scales of the tail. D, schematic reconstruction of the arrangement of dorsal scales in Sclerocephalus haeuse (left), schematic reconstruction of the arrangement of dorsal scales in the sacral region of Archegosaurus deche (right). E, Eryops megacephal, MCZ 1539 (cast). Scales of the tail. F, H, Greererpeton burkemora. F, CMNH 11233. Dorsal scales in external (above) and internal (below) view. G, CMNH 11219. Dorsal scales showing an imbricating pattern.

Figure 6

Dorsal scales. A, B, Archegosaurus deche, MB.Am.273. A, scales of the flanks in the region of the ilium. B, scales of the tail, showing concentric rings and radial striae. C, Sclerocephalus haeuse, MB.Am.1314. Scales of the tail. D, schematic reconstruction of the arrangement of dorsal scales in Sclerocephalus haeuse (left), schematic reconstruction of the arrangement of dorsal scales in the sacral region of Archegosaurus deche (right). E, Eryops megacephal, MCZ 1539 (cast). Scales of the tail. F, H, Greererpeton burkemora. F, CMNH 11233. Dorsal scales in external (above) and internal (below) view. G, CMNH 11219. Dorsal scales showing an imbricating pattern.

A pattern of fine undulating radial striae is visible on the scales of the tail in MB.Am.273 and is superimposed on the above-described, more pronounced concentric rings (Fig. 6B). Some very thin parts of scales found in MB.Am.273 display only the radial striations, but no concentric rings are visible. This shows that the dorsal scales of Archegosaur are composed of a lower layer with concentric rings, and an upper, thinner layer that bears the radial striae.

Sclerocephalus haeuseri

The dorsal scales of Sclerocephal possess the same morphology as described for Archegosaur, with fine radial striae that are superimposed on the concentric rings (Schoch, 2003). As in Archegosaur the dorsal scales are cup-shaped lateral to the gastral scales, and they are thin and two-dimensionally preserved on the flanks and on the back (Boy, 1988). However, in larval and juvenile Sclerocephal specimens (at least until a skull length of 72 mm; MB.Am.1298), the dorsal scales overlap regularly in the manner of a shingled roof (Fig. 6C). This overlap takes place from anterior to posterior on the trunk and the tail, and from proximal to distal on the limbs. Each scale of one transverse row is positioned between two scales of the anterior transverse row. The arrangement of these scales is reconstructed schematically in Fig. 6 (D, left) as compared with the arrangement in Archegosaur (Fig. 6D, right). In MB.Am.1298, dorsal scales that are smaller than those of the back and the tail are preserved on the stem and the head of the cleithrum. These scales are loosely set and have no point contact. In larger specimens of Sclerocephal, the dorsal scales are mostly not perserved or only poorly preserved, so that it is difficult to determine their arrangement. In SMNK uncatalogued (sl 183 mm), dorsal scales with concentric growth rings are preserved in several regions of the trunk and the tail. The scales on the surface of the cleithrum and the scapulocoracoid are loosely set without point contact. This applies also for the scales that are preserved on the broad ribs in the anterior trunk region. In the posterior half of the trunk, the scales are more tightly set but do not overlap. At least on the flanks, the scales are aligned in rows that represent the continuation of the gastral scale rows. Each scale of one row is located posteriorly between two scales of the anterior row, as described above in Archegosaur. Only the anterior part of the tail is preserved in this specimen, on which the dorsal scales are densely arranged and possibly overlap. On the hindlimb, scales half the size of those of the trunk are closely set but do not overlap. In SMNS 90055 (sl 198 mm), only the dorsal scales of the tail are preserved. They exhibit the same pattern of overlap as in the small specimens.

Dendrerpeton sp.

The basal temnospondyl Dendrerpet sp. has round-oval dorsal scales that are distinctly broader than the gastral scales (Carroll, 1967; F. Witzmann, pers. observ.) and possess concentric growth rings (Fig. 4C). According to Colbert (1955) the dorsal scales of Dendrerpet sp. overlap regularly.

Trimerorhachids

The dorsal scales of trimerorhachids have basically the same morphology as their gastral scales described above: they are anteroposteriorly elongate and show regular overlap from anterior to posterior (Colbert, 1955; Chase, 1965; Berman, 1973). In contrast to these authors, Olson (1979) came to different conclusions concerning the nature of dermal ossifications in Trimerorhach. He observed a thick layer of up to more than 20 thin scales (designated by him as osteoderms) within the dermis of an individual. Personal observations of the Trimerorhach specimens stored in MCZ, however, corroborate the findings of Colbert, Case, and Berman, and thus their results will be acknowledged here. Nevertheless, Olson’s findings remain puzzling; as he wrote, it cannot be ruled out that they might be the result of shrinking of the integument by desiccation.

Dissorophoids

In micromelerpetontids and branchiosaurids, the dorsal scales exhibit a very similar morphology to the gastral scales (see above), but they are slightly less elongate. They show extensive overlap (Credner, 1886; Boy, 1972). In amphibamids dorsal scales are preserved only in Eoscop (Daly, 1994), and they show regular overlap.

Eryopids

In Eryo, trunk and tail are covered by tightly set, but not overlapping, round-oval scales (MCZ 1539; Romer & Witter, 1941). Some of these scales are rounded polygonal rather than oval (Fig. 6E). The scales are arranged in the same way as on the flanks of Archegosaur and the large specimens of Sclerocephal. However, the long axes of the scales are inclined posterodorsally at an angle of approximately 30°. In the closely related Onchiod, dorsal scales are known only from larval specimens. They are round-oval and densely cover the flanks and the back, but do not overlap (Werneburg, 1988); as in Archegosaur and Sclerocephal, comparatively thick cup-shaped dorsal scales are located lateral to the gastral scales (Credner, 1893).

Stereospondylomorpha

Among basal stereospondylomorphs, dorsal scales are present in C. latirost in addition to A. deche and S. haeuse (see above). As in Archegosaur the round-oval scales do not overlap in C. latirost, and there is no point contact between them (ROM 5735, MB.Am.1271, MB.Am.1275).

Dorsal scales are also present in different stereospondyls. Van Hoepen (1915) and Findlay (1968) reported very thin dorsal scales in the rhinesuchid Uranocentrod. Their arrangement is unknown. The capitosauroid Edingerel has very small, cup-shaped dorsal scales that are loosely scattered with large spaces between them (Lehman, 1966; Janvier, 1992). In Wellesaur the morphology and arrangement of the dorsal scales probably correspond to its gastral scales described above (R. R. Schoch, pers. comm.), as is also the case in Paracyclotosaurus davi, which bears dorsal, round-oval scales that are irregularly set and are not in contact with each other (Watson, 1958). Among trematosaurids, only Tertremoid has nonoverlapping, round-oval scales on the dorsal surface of the anterior trunk region (Janvier, 1992) that are arranged in rows similar to Archegosaur.

Temnospondyls without scalation

Reduced (thin) scales are present in the amphibamids Amphibam and Platyrhino (Clack & Milner, 1993), and in the branchiosaurids (Boy, 1987). Among dissorophoids, scales are absent in the Lower Triassic amphibamid Microphol (Schoch & Rubidge, 2005). The excellent state of preservation of the articulated specimens strongly suggests that the lack of scales is not an artifact of preservation, but that this temnospondyl had reduced its scalation completely. The Permo-Carboniferous dissorophids (DeMar, 1968) possess plate-like osteoderms on the dorsal surface of the trunk, but no scales have been found. It is certainly possible that the scales are simply not preserved; however, even articulated specimens of Dissoroph show no traces of scales.

Within stereospondylomorph temnospondyls, scales are absent in trematosaurids (Steyer, 2002; Schoch, 2006) and metoposaurids (Dutuit, 1976; Schoch, in press). Because many representatives of these taxa are known from well-preserved articulated skeletons that are devoid of any scales, one can suggest that trematosaurids and metoposaurids had a naked skin. An exception is the trematosaurid Tertremoid (see above). Furthermore, no scalation is preserved in the capitosauroid Mastodonsaur (Schoch, 1999). It cannot be stated with certainty that it had reduced scalation, because no articulated specimens are known, and the same applies to the basal stereospondyl Benthosuch, from which many postcranial fragments but no scales have been reported (Bystrow & Efremov, 1940). Also, no scalation is visible in the articulated skeleton of the rhytidosteoid Sidero (Warren & Hutchinson, 1983).

Phylogeny of scalation in temnospondyls

To ascertain the plesiomorphic situation in temnospondyls, and to trace the development of scalation in different temnospondyl lineages, the scalation pattern for each taxon is mapped on an existing cladogram of temnospondyls based on Witzmann & Schoch (2006) and Schoch & Milner (2000) (Fig. 7). The numbers represent the following characters: 1, presence of gastral scales arranged in a chevron pattern; 1a, rhombic gastral scales; 1b, spindle-shaped gastral scales; 1c, ovoid gastral scales; 2, dorsal scales overlapping on the trunk; 3, dorsal scales not overlapping on the trunk; 4, complete reduction of scales.

Figure 7

Scalation pattern in temnospondyls mapped on an existing cladogram based on the results of Witzmann & Schoch (2006) and Schoch & Milner (2000). Characters: 1, presence of gastral scales arranged in a chevron pattern; 1a, rhombic gastral scales; 1b, spindle-shaped gastral scales; 1c, ovoid gastral scales; 2, dorsal scales overlapping on the trunk; 3, dorsal scales not overlapping on the trunk; 4, complete reduction of scales. *Taxa in which no dorsal scales are known; **taxa in which no gastral scales are known.

Figure 7

Scalation pattern in temnospondyls mapped on an existing cladogram based on the results of Witzmann & Schoch (2006) and Schoch & Milner (2000). Characters: 1, presence of gastral scales arranged in a chevron pattern; 1a, rhombic gastral scales; 1b, spindle-shaped gastral scales; 1c, ovoid gastral scales; 2, dorsal scales overlapping on the trunk; 3, dorsal scales not overlapping on the trunk; 4, complete reduction of scales. *Taxa in which no dorsal scales are known; **taxa in which no gastral scales are known.

Ingroup

The temnospondyls described above with well-preserved scalation were assigned to the ingroup, and the ontogenetically most advanced stages are considered in each case. In the case of temnospondyls without scales, only those taxa in which the excellent state of preservation of the skeleton suggests that the absence of scales represents a real reduction, and is no artifact of preservation (Microphol, trematosaurids, and metoposaurids), were considered. The comparison of the dorsal scales is admittedly impaired by their poor preservation in many taxa.

Outgroup

The Mississippian stem tetrapod Greererpet is considered as an outgroup, and is supplemented by data derived from the Devonian stem tetrapods Acanthoste, Ichthyoste, and Tulerpet. Greererpet has rhombic gastral scales arranged in a chevron pattern (Godfrey, 1989; F. Witzmann, pers. observ.), the morphology and articulation of which corresponds to Platyoposaur, Sclerocephal, and Australerpet (Fig. 8). As described by Godfrey (1989), the trunk of Greererpet is covered laterally and on the back with round-oval dorsal scales that overlap regularly from anterior to posterior like a shingled roof (Fig. 6F, G). The dorsal scales are well ossified, internally concave, and externally convex, and some of them bear a medial elevation on the external side as well as concentric growth rings. Scales of this type are also visible on the belly posterior to the last chevron of gastral scales and on the tail (CMNH 11236). The shape of the dorsal scales ranges from almost circular to elongate oval. The resemblance between the thick dorsal scales of Greererpet and the ventrolateral cup-shaped scales of Archegosaur, Sclerocephal, and Onchiod is striking. Also, Edingerel and Tertremoid have cup-shaped scales, which are, however, smaller and less thick than in Greererpet. It is entirely possible that the thin dorsal scales on the flanks and the back in Archegosaur and Sclerocephal were externally convex and internally concave in the living animal, but were compressed two-dimensionally in the fossil. Acanthoste has spindle-shaped gastral scales arranged in a chevron pattern, but round-oval scales are not preserved (Coates, 1996). Thin round-oval scales are known in Ichthyoste, which densely covered the tail and the posterior trunk region (Jarvik, 1952). In Tulerpet, thin, round-oval overlapping scales covered the body completely, including the belly (Lebedev & Coates, 1995). There is no clear evidence that the scales on the belly were arranged in a chevron pattern; however, this pattern might be obscured by multiple folding of the skin (O. Lebedev, pers. comm.).

Figure 8

Ontogeny of gastral scales in Sclerocephalus haeuse, and interpretation of the gastral scale morphologies of other temnospondyls and the stem-tetrapod Greererpeton burkemora as ‘larval’, ‘juvenile’, and ‘adult’, respectively. Except for Sclerocephal, the ontogenetically most advanced stages of gastral scales are shown in each case. Acerast redrawn after Warren & Hutchinson (1987), Australerpet redrawn after Dias & Richter (2002), Balanerpet redrawn after Milner & Sequeira (1994), Branchierpet redrawn after Werneburg (1991), Gerrothor redrawn after Nilsson (1946), Greererpet based on CMNH 11073, Mahavisaur redrawn after Janvier (1992), Onchiod redrawn after Credner (1893), Platyoposaur based on PIN 164/1–9, Sclerocephal based on MB.Am.1302, MB.Am.1233, and SMNS 90507, Uranocentrod redrawn after Findlay (1968).

Figure 8

Ontogeny of gastral scales in Sclerocephalus haeuse, and interpretation of the gastral scale morphologies of other temnospondyls and the stem-tetrapod Greererpeton burkemora as ‘larval’, ‘juvenile’, and ‘adult’, respectively. Except for Sclerocephal, the ontogenetically most advanced stages of gastral scales are shown in each case. Acerast redrawn after Warren & Hutchinson (1987), Australerpet redrawn after Dias & Richter (2002), Balanerpet redrawn after Milner & Sequeira (1994), Branchierpet redrawn after Werneburg (1991), Gerrothor redrawn after Nilsson (1946), Greererpet based on CMNH 11073, Mahavisaur redrawn after Janvier (1992), Onchiod redrawn after Credner (1893), Platyoposaur based on PIN 164/1–9, Sclerocephal based on MB.Am.1302, MB.Am.1233, and SMNS 90507, Uranocentrod redrawn after Findlay (1968).

Results

The cladogram illustrates that the presence of gastral scales arranged in a chevron pattern is plesiomorphic for basal tetrapods, and this pattern is highly conservative and retained in different temnospondyl lineages. The only exceptions might be trimerorhachids and the amphibamid Eoscop, in which it cannot be ascertained if the gastral scales are aligned in a chevron pattern or in transverse rows. In most temnospondyls, the gastral scales are spindle shaped. Only adult Sclerocephal, Platyoposaur, Australerpet, and the rhinesuchid described by Pawley & Warren (2004) have rhombic gastral scales. Ovoid gastral scales are exhibited by Balanerpet, trimerorhachids, and among dissorophoids at least by micromelerpetontids, branchiosaurids, and amphibamids. Furthermore, among stereospondyls, the rhytidosteoid Mahavisaur has ovoid gastral scales, and this is possibly also the case in Paracyclotosaur and Wellesaur.

Regularly overlapping dorsal scales are present in the outgroup (with the exception of Acanthoste, in which no dorsal scales are preserved). This is also the case in Dendrerpet, trimerorhachids and – where the dorsal scales are preserved – in dissorophoids. This pattern can be regarded as the plesiomorphic arrangement of dorsal scales in temnospondyls. A reduction of the dorsal scalation took place in different lineages of temnospondyls. Scales appear to be absent in Microphol (Schoch & Rubidge, 2005). Although the dorsal scales are well ossified in eryopids, they are not overlapping. In the basal stereospondylomorph Sclerocephal, the dorsal scales overlap only in the larval and juvenile phase, whereas they are nonoverlapping in the trunk of the adult specimens. From Cheliderpet crownward, the dorsal scales are nonoverlapping in all growth stages. The spindle-shaped or rhombic gastral scales in basal stereospondylomorphs and in rhinesuchid stereospondyls are well ossified, and spindle-shaped gastral scales occur in lydekkerinids, plagiosaurids, and in the rhytidosteoid Acerast. In capitosauroids, spindle-shaped gastral scales can only be demonstrated in Edingerel (Janvier, 1992), whereas exclusively round-oval scales have been found in articulated skeletons of Paracyclotosaur and Wellesaur. Scalation was completely lost in trematosaurids and metoposaurids (see above). The reduction probably occurred independently in both groups, as the trematosaurid Tertremoid has round-oval dorsal scales at least in the neck region (Janvier, 1992).

DISCUSSION

Ontogeny of dermal scales and paedomorphosis

The ontogeny of dermal scales can best be traced in Sclerocephal (Fig. 8), because this taxon is represented by an unparalleled, extensive growth series from small larvae to large adults, with skulls ranging from 10 mm to more than 200 mm in length. In small larvae, the gastral scales are arranged in a chevron pattern as they are in large specimens. However, they are distinctly less well ossified and less elongate, so that they resemble the dorsal scales of the flanks and the back. The gastral scales of all temnospondyls –rhombic, spindle-shaped, or ovoid – can probably be traced back to the same type of ventral scales. Furthermore, it can be assumed that dorsal and gastral scales also developed from the same Anlag. This is also supported by the very similar morphology of gastral and dorsal scales in many micromelerpetontids and branchiosaurids (see above). The ovoid gastral scales of larvae become proportionally more elongate in further ontogeny, and attain a spindle-shaped outline in Sclerocephal (Fig. 8). The rhombic morphology of gastral scales in the largest Sclerocephal represents the ontogenetically most advanced condition (i.e. the highest degree of ossification) and can be designated as ‘adult’. Accordingly, the very similar rhombic gastral scales of Platyoposaur and Australerpet must have developed ontogenetically from the spindle-shaped ‘juvenile’ morphology. The rhombic outline is in particular the result of an expansion of the sharp anterodorsal crest that borders the dorsal groove anteriorly (Figs 4B, 5A, B). This crest underwent accelerated growth in an anterodorsal direction and became deeper than the broader, bulge-like crest that limits the dorsal groove posteriorly. The expanded anterior crest overlaps the internal surface of the posterior part of the adjacent scale of the preceding chevron, so that the chevrons overlap anteroposteriorly in ventral view (Fig. 5D). The dorsal groove, which was almost smooth in the spindle-shaped scales, becomes a well-defined dorsolateral facet, the lateral portion of which is slightly convex (most pronounced in Platyoposaur), and the medial portion of which is distinctly concave. As described for Archegosaur above, the ventromedial side of each spindle-shaped gastral scale bears a slightly concave area between an anterior and a posterior ridge (Fig. 4A). During subsequent ontogeny this area differentiated into the well-defined ventromedial facet of the rhombic scale, the contours of which are complementary to the above-described dorsomedial facet. Thus, the convex part of the ventromedial facet fits closely into the concave part of the dorsolateral facet. In Platyoposaur, a sharp crest is developed on the convex part of the ventromedial facet that fits into a corresponding depression on the dorsolateral facet of the adjacent scale. However, most temnospondyls retain the ‘juvenile’ gastral scale morphology of the slender, spindle-shaped outline, and do not attain the ‘adult’ rhombic condition, which is interpreted as a paedomorphic trait (Figs 7, 8). This also holds true for the large-growing, well-ossified Eryo (MCZ 1738) and the giant rhinesuchid Uranocentrod (Findlay, 1968), the gastral scales of which have the largest absolute size of all temnospondyls (30–40 mm in length), but nevertheless retain a spindle-shaped morphology. The basal temnospondyl Balanerpet (Milner & Sequeira, 1994) and many dissorophoids maintain the less ossified, ovoid ‘larval’ morphology of gastral scales, as do the gastral scales of Mahavisaur (Janvier, 1992) (Figs 7, 8).

Functional aspects

Protection and strengthening of the trunk

Romer (1956), Findlay (1968), and Claessens (2004) suggested that the well-ossified gastral scales of early tetrapods and temnospondyls served as protection while crawling on the ground. However, apart from one Devonian trackway (Warren & Wakefield, 1972), no tetrapod trackway from the Palaeozoic and Mesozoic shows clear evidence of a body trace, except for tail drags (Haubold, 1971; Fichter, 1983), indicating that basal tetrapods walked with their bodies supported clear from the ground. Therefore, the gastral scalation was less likely to have served as protection against abrasion during locomotion (moreover the scales were deeply embedded in the dermis, see above), but rather as protection of the viscera from compression while lying on the substrate (R. Holmes, pers. comm.).

The mode of articulation of the spindle-shaped gastral scales allowed telescoping of the scales within the rows during sidewards flexion of the body, and would also accommodate inwards movement of the abominal wall if basal tetrapods exhaled by contraction of the transverse abdominis muscle. Comparison with extant amphibians suggests that this mode of exhalation was already used by basal tetrapods (Brainerd & Monroy, 1998). The combination of protection and flexibility, like that of a ‘chain-mail shirt’, is a possible explanation for the retention of the ‘juvenile’ spindle-shaped morphology in most temnospondyls.

In contrast to the spindle-shaped gastral scales, the mode of articulation by indentation between the dorsolateral and ventromedial facets reduced the flexibility between the rhombic gastral scales. This applies especially to the anterior half of the trunk where the scales are more highly ossified. The development of rhombic gastral scales in large specimens of Sclerocephal might be regarded in the context of the mode of swimming. Sclerocephal was a superficially crocodile-like aquatic predator with a long and deep swimming tail. In extant crocodiles, the trunk is stiffened by the osteoderms, the gastralia, and the hypaxial musculature (von Wettstein, 1937). In combination with the long, powerful swimming tail, the stiffened trunk enables crocodiles to accelerate rapidly in water towards prey (Troxell, 1925). Similarly the rhombic scalation might have stabilized the trunk in large individuals of Sclerocephal, supported by the conspicuous uncinate processes on the ribs in the anterior trunk region. The corresponding gastral scalation and the hook-like uncinate processes in Platyoposaur (Konzhukova, 1955) and Australerpet (Dias & Schultz, 2003) probably had a similar effect in these piscivorous predators. In the smaller individuals (known only from Sclerocephal) with a more flexible scalation of spindle-shaped scales, uncinate processes were smaller or absent, and these animals might have performed a more axial mode of swimming.

Phylogenetic reduction of scalation

As outlined above, a reduction of scalation – from an overlapping to a nonoverlapping pattern, a decrease in scale thickness, or even a complete loss of the scales – took place independently in different groups of temnospondyls. Among dissorophoids, the scales became thinner in derived representatives of micromelerpetontids and branchiosaurids, as well as in amphibamids, and they were completely reduced in Microphol. This might be connected with the increasing importance of cutaneous respiration in these small forms (Boy, 1993).

A reduction of scalation also took place among stereospondylomorphs. All basal stereospondylomorphs and rhinesuchid stereospondyls bore well-ossified gastral scales, whereas the scales are often smaller and less ossified or even completely lost in Mesozoic steresopondyls. The Mesozoic stereospondyls were mainly restricted to aquatic habitats, as indicated by the lateral line sulci and the feebly ossified limbs (Schoch & Milner, 2000). Well-ossified gastral scales for protection of the belly (see above) can be expected in temnospondyls that touched the substrate frequently, either in water or on land. Metoposaurid and trematosaurid stereospondyls, which were probably active swimmers and preyed in the open water (Hunt, 1993; Schoch & Milner, 2000; Steyer, 2002), were devoid of gastral scales. The loss of scales provided more freedom of movement for an axial type of locomotion, similar to many fish (Böss, 1982). The need for greater mobility was obviously more important in pelagic swimmers than the protection of scales. In contrast, capitosauroid stereospondyls were benthic animals rather than active swimmers. Nevertheless, the gastral scalation in at least some capitosauroids consists only of thin ovoid scales or might even be absent. The anatomy of most capitosauroids suggests that they could hardly leave the water (Schoch & Milner, 2000), therefore a well-ossified gastral scalation for protection of the belly and support of the internal organs may not have been required.

On the other hand, it is puzzling that no gastral scales are known in the terrestrial stereospondyl Sclerothor. Instead, nodular osteoderms are preserved on the top of the neural spines (von Huene, 1932), and it cannot be ruled out that other parts of the body were also covered by these dermal ossifications. The primarily terrestrial stereospondyl Laidler has no scales, but was densely covered by irregularly shaped osteoderms on the back and on the belly (Kitching, 1957). The only aquatic Triassic stereospondyls with dermal armour of osteoderms are the highly derived plagiosaurids (Nilsson, 1946; Hellrung, 2003; F. Witzmann, pers. observ.) and the rhytidosteoid Acerast (Warren & Hutchinson, 1987), which possess plate-like or nodular osteoderms on the ventral side (and on the back in plagiosaurids). Although gastral scalation is present, it is feebly developed as the gastral scales are needle-like, and, at least in the case of plagiosaurids, there is a large interspace between the chevrons. In these Mesozoic taxa, the osteoderms had probably taken over the function of protection of the body. This could also apply to the Late Palaeozoic osteoderm-bearing forms, Peltobatrach and the dissorophids, which were supposedly terrestrial. Although the gastral scales are small and needle-like in Peltobatrach (Panchen, 1959), no scales are preserved in dissorophids (see above).

Inferences for the integument

The spindle-shaped, and especially the well-ossified rhombic, gastral scales indicate that the integument was originally very thick in basal tetrapods and temnospondyls, because the gastral scales were deeply embedded in the dermis, as histological sections have shown (Dias & Richter, 2002). This is also supported by the thick, cup-shaped dorsal scales in Greererpet and several Permo-Carboniferous temnospondyls. The fact that many Triassic temnospondyls have less ossified, thinner gastral and dorsal scales could indicate that the integument has become proportionally thinner in these forms (exceptions are the above mentioned, osteoderm-bearing stereospondyls). This is supported by the fact that aquatic sterespondyls have conspicious, deep lateral line sulci on their skull roofing bones. In contrast, the lateral line sulci of most Palaeozoic temnospondyls are comparatively weakly impressed, and consist of long oval depressions that are sometimes difficult to distinguish from the furrows of the dermal sculpture. For example, the piscivorous Upper Permian stereospondylomorphs Platyopsaur and Australerpet, which possess thick rhombic gastral scales, have poorly developed lateral lines on the dermal skull bones (Bystrow, 1935; Barberena, 1998). One may hypothesize that the lateral lines left only weak traces on the bones because the integument was proportionally thicker, whereas in the forms with the possibly thinner integument the lateral lines came to lie on the surface of the bones.

The possibly thinner skin in many Mesozoic temnospondyls might be related to cutaneous respiration and changes of the oxygen content in the atmosphere. Temnospondyls used the buccal pump mechanism for lung breathing, like extant amphibians, whereas their rib morphology precluded the more effective costal aspiration performed by amniotes (Janis & Keller, 2001). Therefore, it is also possible that large temnospondyls like eryopids and stereospondylomorphs, in spite of their disadvantageous surface area to volume ratio, might have relied on cutaneous respiration as an accessory mode of breathing, especially after the gills had been resorbed. A naked skin devoid of scales is surely more favourable for cutaneous gas exchange. Nevertheless, the presence of scales still allows skin breathing to a notable degree. For example, more than 30% of the oxygen intake is processed via the skin in the scaly Reed-Fish (Erpetoichthys calabaric) and the sea-snake Pelamis platur (Feder & Burggren, 1985). Because the bony scales of temnospondyls were located in the dermis, blood capillaries could have spread in the dermal layers and in the epidermis above the scales. Furthermore, it is important to consider that different geochemical models indicate a distinctly higher atmospheric oxygen level during the Carboniferous until the Mid-Permian than is present today, accompanied by a low carbon dioxide level (Graham et a, 1997). This surely considerably facilitated oxygen uptake and carbon dioxide release via the skin in Palaeozoic temnospondyls. In the Upper Permian, the oxygen level of the atmosphere decreased and reached a minimum in the Lower Triassic (Graham et a, 1997). The possibly proportionately thinner integument of many Mesozoic temnospondyls (and the reduction of scales in some forms) might have enhanced the effectiveness of cutaneous respiration in the oxygen-poorer atmosphere, as compared with the Palaeozoic forms with heavily ossified scales within a possibly thicker integument.

CONCLUSIONS

Most temnospondyls exhibit a paedomorphic morphology with respect to the gastral scales, probably driven by the need for a greater flexibility of the trunk. The coupling with the somatic development was decelerated compared with the ancestral situation in many temnospondyls, resulting in the retention of the ‘juvenile’ spindle-shaped morphology in large adults. In miniaturized forms, like small dissorophoids, the development of the scales was truncated at an early stage so that the scales remained thin and ovoid in outline.

The scalation was reduced independently in different temnospondyl lineages, especially in Mesozoic clades. The thinner scales in Mesozoic stereospondyls could indicate a relatively thinner integument than in many Palaeozoic temnospondyls, which agrees with their much more conspicuous lateral line sulci. This might have resulted in an increased ability for cutaneous respiration.

ACKNOWLEDGEMENTS

I am grateful to Rainer Schoch, Hans-Peter Schultze, Johannes Müller, Jürgen Kriwet, and Oleg Lebedev for many helpful comments. Robert Holmes and one anonymous reviewer are thanked for thoughtfully reviewing the manuscript. Many thanks also to Linda Tsuji who kindly corrected the English grammar of the text. Yuri Gubin (PIN), Oliver Hampe (MB), Jean-Claude Horrenberger (IGS), Wolfgang Munk (SMNK), Michael Ryan (CMNH), Charles Schaff (MCZ), Rainer Schoch (SMNS), Kevin Seymour (ROM), and Ronald Winkler (MMG) kindly gave access to the collections in their care. The Deutsche Forschungsgemeinschaft (DFG) is thanked for financial support.

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