The Postcranial Skeleton of Temnospondyls

Published as: Pawley, K. in press. The postcranial skeleton of Trimerorhachis insignis Cope 1878 81 (Temnospondyli: Trimerorhachidae) a plesiomorphic, secondarily aquatic temnospondyl from the Lower Permian of North America. Journal of Paleontology. Copyright © 2006 The Paleontological Society

CHAPTER 3. THE POSTCRANIAL SKELETON OF TRIMERORHACHIS INSIGNIS COPE, 1878 (TEMNOSPONDYLI: TRIMERORHACHIDAE): A PLESIOMORPHIC TEMNOSPONDYL FROM THE LOWER PERMIAN OF NORTH AMERICA

Abstract. The postcranial skeleton of the Lower Permian temnospondyl Trimerorhachis insignis Cope, 1878 is described and figured in detail. Postcranial adaptations for an aquatic existence in T. insignis include the extensive ventral expansion of the interclavicle and clavicles, and poorly ossified ends of the endochondral bones. The endochondral postcranial skeleton of T. insignis is paedomorphic through the process of neoteny, retaining an osteologically immature condition throughout morphogenesis. The endochondral postcranial elements display progressive morphological changes that do not stabilise in larger specimens, indicating indeterminate growth, with a correlation between size and degree of ossification. Some postcranial characteristics are present only in later morphogenetic stages of T. insignis. Within the Temnospondyli, the postcranial skeleton of T. insignis is most similar to that of other members of the Dvinosauria. The morphology of the postcranial skeleton of T. insignis is consistent with a phylogenetic position more derived than the basal temnospondyls Balanerpeton woodi and Dendrerpeton acadianum, but less derived than the Euskelia plus Stereospondylomorpha. A sister-taxon relationship between the Dvinosauria and brachyopoids is not supported by postcranial characteristics of T. insignis. Characteristics that develop last in morphogenesis in temnospondyls, and are consequently only present in well-ossified, morphogenetically mature temnospondyls, are absent in T. insignis due to paedomorphosis. Otherwise, the postcranial skeletons of T. insignis and other aquatic temnospondyls are similar to that of terrestrial temnospondyls, supporting the hypothesis that aquatic temnospondyls had terrestrial ancestors, and are thus secondarily aquatic.

INTRODUCTION

Trimerorhachis insignis Cope, 1878 is a medium-sized, aquatically adapted temnospondyl from the lower Permian of North America. It is the most abundant temnospondyl in the Lower Permian red beds of north-central Texas (Romer, 1928; Olson, 1956). Temnospondyls are the most numerous and diverse group of the archaic Palaeozoic ‘labyrinthodont’ amphibians. Among temnospondyls the postcranial skeleton varies from heavily ossified, with obvious terrestrial adaptations in the stratigraphically oldest taxa, Balanerpeton woodi (Milner and Sequeira, 1994) and Dendrerpeton acadianum Owen, 1853 (Carroll, 1967; Holmes et al., 1998), to poorly ossified, paedomorphic (sensu McNamara, 1986), and aquatically adapted in some of the Permian taxa, including Trimerorhachis insignis, and most of the Mesozoic taxa (Pawley and Warren, 2004). Trimerorhachis insignis is a member of the basal temnospondyl clade Dvinosauria Yates and Warren, 2000. The Dvinosauria, as defined by Yates and Warren (2000), also includes other trimerorhachoids, dvinosaurs, and tupilakosaurs. Traditionally, the literature neglects the postcranial skeletons of temnospondyls. While most of these have associated postcranial material (when described at all), illustrations of individual elements are rarely adequate for complete inter-taxon comparisons, including cladistic analysis. 82 K. PAWLEY PHD THESIS

The postcranial skeletons of plesiomorphic temnospondyls are those most in need of description, both for understanding the evolution of the temnospondyl postcranial skeleton and assessing the phylogenetic relationships of temnospondyls within other early tetrapod groups. As part of a more inclusive long-term study of the temnospondyl postcranial skeleton, a comprehensive description of the postcranial elements of a member of poorly known temnospondyl clades was undertaken in order to facilitate assessment of the phylogenetic distribution of postcranial characteristics, and to tease out homoplasy associated with both aquatic and terrestrial adaptations. The abundant, well- preserved remains of T. insignis make it an ideal subject for these purposes.

Phylogenetic position

All recent phylogenetic assessments involving T. insignis have relied almost entirely on cranial characteristics. The position of the Dvinosauria within the Temnospondyli is contentious: the analysis of Yates and Warren (2000) placed the Dvinosauria as derived sister taxon to the Stereospondylomorpha, whereas Ruta et al. (2003) found that the Dvinosauria were the most basal clade within the Temnospondyli. Most controversially, an ongoing debate has ensued as to whether the Dvinosauria, or the more derived stereospondyls, is the sister taxon to the Brachyopoidea (summarized in Damiani, 2003). Within the Dvinosauria, the postcranial skeleton of Trimerorhachis insignis is the most suitable for description because abundant material is available for study. The poorly ossified articulation surfaces of the endochondral postcranial elements limit the use of T. insignis for comparative purposes; however, descriptions of plesiomorphic temnospondyls are useful because our knowledge of the postcranial skeletons of these taxa is limited. The aim of this description is to improve our understanding of the postcranial skeleton of T. insignis, document the morphogenetic changes that occur, and to describe potential characteristics for use in future phylogenetic analyses.

Aquatic adaptations in temnospondyls

Trimerorhachis insignis, along with many other temnospondyls, displays adaptations for an obligatorily aquatic lifestyle such as lateral line sulci, ossified ceratobranchials, enlarged, anteriorly expanded interclavicle and clavicles, and a poorly ossified endochondral postcranial skeleton with small limbs. This type of postcranial morphology is also characteristic of all other members of the Dvinosauria and the more derived members of the mainly Mesozoic Stereospondyli. There has been much debate over the last two decades involving temnospondyls, concerning the origin of terrestrial vertebrates (summarized in Clack, 2002a; Long and Gordon, 2004), the phylogenetic relationships of early tetrapods, and the origin of modern lissamphibians and amniotes (summarized in Ruta et al., 2003; Schoch and Milner, 2004). From a phylogenetic and functional point of view, it is important to define the differences between the postcranial skeletons of primarily aquatic basal tetrapods (no terrestrial ancestors) such as Acanthostega gunnari Jarvik, 1952 (Coates and Clack, 1990, 1991; Coates, 1996) and secondarily aquatic temnospondyls (Watson, 1919), including T. insignis.

Previous descriptions of Trimerorhachis insignis

The skull of T. insignis was first described by Cope (1878), further descriptions were undertaken by Cope (1884), Case (1911a; 1935) and Cope and Matthew (1915). Nilsson CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 83

(1944) described the mandible, Lombard and Bolt (1988) the stapes, and Watson (1956) and Schoch (1999b) the braincase. Previous partial descriptions of postcranial material of the genus Trimerorhachis Cope, 1878 include those of T. insignis by Cope (1884), Case (1911a; 1935), Cope and Matthew (1915), Williston (1915a), and Shishkin (2000), and of T. sandovalensis (Berman and Reisz, 1980). Williston (1916) gave a brief account of an almost complete, but only partially prepared, skeleton of T. insignis, which was revised by Olson (1979). Overlapping dorsal scales were described by Colbert (1955) and osteoderms and ossified ceratobranchials by Olson (1979). Cope (1878) first described Trimerorhachis insignis, and erected the Trimerorhachidae (Cope, 1884). Olson (1956) revised Trimerorhachis, synonymising all previous species except T. mesops Cope, 1896 (Clear Fork Formation) with the holotypic species, and created a new species, T. rogersii (early Choza Formation). While not revising the genus in any formal sense, Berman and Reisz (1980) considered the two latter species indistinguishable from the holotypic species, and described a new species, T. sandovalensis, from the Abo Formation of New Mexico.

Taxa within the Dvinosauria with associated postcranial material

The only other Trimerorhachis species with associated postcranial material is T. sandovalensis (Berman and Reisz, 1980). Most other taxa within the Dvinosauria have associated postcranial material, including the trimerorhachoids Neldasaurus wrightae (Chase, 1965), Acroplous vorax (Hotton, 1959; Coldiron, 1978; Foreman, 1990) and Isodectes (Eobrachyops, Saurerpeton) obtusus Sequeira, 1998 (Watson, 1956); the dvinosaur Dvinosaurus spp. Amalitzky, 1921 (Sushkin, 1936; Bystrow, 1938; Nikitin, 1995, 1997; Gubin, 2004); and the tupilakosaurs Thabanchuia oomie (Warren, 1998b), Tupilakosaurus spp. (Nielsen, 1954; Shishkin, 1961, 1973), and Kourerpeton bradyi (Olson and Lammers, 1976). While most of these have associated postcranial material, the descriptions lack the detail necessary for comprehensive comparison.

Stratigraphic distribution

Trimerorhachis insignis is found throughout the nonmarine Wolfcampian Lower Permian, from the Wichita to the Clear Fork formations (stratigraphy revised after Hentz and Brown, 1987), to the middle of the San Angelo Formation of the Pease River Group (Olson and Vaughn, 1970; Parrish, 1978). Remains of T. insignis are most abundant in the Nocona and Petrolia formations (Romer, 1928; Olson, 1956). Olson (1956) considered that all specimens found in the Wichita Group belong to T. insignis; whereas the other specimens, found in the later Clear Fork Formation belong to T. mesops and T. rogersii.

Paleoenvironment

Remains of T. insignis are often found associated with fish and other aquatically adapted tetrapods such as the embolomere Archeria crassidisca Cope, 1884; the nectridian Diplocaulus magnicornis Cope, 1882; and semi-aquatic taxa such as the temnospondyl Eryops megacephalus Cope, 1877 and the pelycosaur Ophiacodon uniformis Cope, 1878 (Romer, 1928, 1935; Dalquest and Mamay, 1963; Parrish, 1978; Sander, 1989). The paleoenvironment of some of the T. insignis localities was warm and humid, with many ponds and larger bodies of standing water amongst swamps and lush forests (Olson and Vaughn, 1970; Olson, 1979; Sander, 1987, 1989). Parish (1978) found that the palaeoenvironment of the Thrift bone bed was episodically arid, and that the 84 K. PAWLEY PHD THESIS

FIGURE 21. Figured specimens of Trimerorhachis insignis Cope, 1878. 1, MCZ 8128 articulated pectoral girdle and axial skeleton in ventral view; 2, TMM 40998-39 articulated hind limb in flexor view. CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 85 animals inhabited mudflat ponds. As Trimerorhachis insignis is common in the Early Permian deposits of North America, the evidence suggests it was adaptable, preferred habitats with standing water.

Taphonomy

Despite the extensive collections of Trimerorhachis insignis postcranial material, the vast majority of the specimens consists of ornamented cranial and pectoral girdle bones, intercentra, and larger elements of the appendicular skeleton such as the scapula, humerus, pelvis and femur fragments. Neural arches, pleurocentra, ribs, and distal limb elements are rare, and most of the specimens belong to larger size individuals. Parish (Parrish) found that a wide range of age classes were present as semi articulated specimens in the Thrift bone bed, indicating that the bias in collections is not due to a bias in the original population, at least in this instance. Much of the fossil vertebrate material of the Lower Permian deposits of Texas is fragmentary, size sorted and displays various degrees of abrasion, indicating transportation (Case, 1935; Parrish, 1978; Olson, 1979; Behrensmeyer, 1988; Sander, 1989). There are two possible explanations for the size discrepancy in collected material of T. insignis, collector bias, and taphonomic processes such as size sorting. Collector bias is obvious, particularly with fragmented material, the larger bones and fragments are easier to see than the smaller ones, and so are more likely to be collected. The aquatic environment inhabited by T. insignis indicates the probable influence of size sorting. Water currents, even gentle ones, transport lighter or smaller elements at faster rates than heavier ones. Lighter bones are also less likely to be buried and preserved because of their relatively low densities. This favours longer periods of burial for heavier elements, as well as longer periods of exposure and destruction for lighter elements, resulting in size sorting with the larger, heavier elements over represented (Voorhies, 1969; Behrensmeyer, 1975, 1991). The size sorting observed in the distribution of collected postcranial elements of T. insignis is also easily explained by this process.

Heterochronic effects on the postcranial skeleton of temnospondyls

Previous studies of heterochronic processes within temnospondyls (McNamara, 1988; Schoch, 1995) have focused exclusively on cranial characteristics. While no comprehensive account of heterochrony of the postcranial skeleton in temnospondyls is available, numerous workers have documented morphogenetic changes of the postcranial skeleton, including those of the basal temnospondyl Balanerpeton woodi (Milner and Sequeira, 1994); the dvinosaur Dvinosaurus primus (Sushkin, 1936; Bystrow, 1938; Nikitin, 1995, 1997); the euskelians Eryops megacephalus (Case, 1911a; Bakker, 1982; Pawley and Warren, 2006), and Acheloma cumminsi (Trematops milleri) Cope, 1882 (Olson, 1941); the archegosaur Sclerocephalus spp. Goldfuss, 1847 (Meckert, 1993; Lohmann and Sachs, 2001; Schoch, 2003); and the stereospondyls Benthosuchus sushkini Efremov, 1937 (Bystrow and Efremov, 1940) and Mastodonsaurus giganteus Jaeger, 1828 (Schoch, 1999a). Paedomorphosis through the process of neoteny (s. McNamara, 1986) is widespread among temnospondyls (Schoch, 2002d). Paedomorphosis of the endochondral postcranial skeleton in stereospondyls was discussed by Pawley and Warren (2004); and peramorphosis is observed in terrestrial temnospondyls (Pawley and Warren, 2006). The femur of the stereospondyl Dutuitosaurus (Metoposaurus) ouazzoui Hunt, 1993 displays indeterminate growth (Steyer et al., 2004), in that after an initially rapid growth phase, the femur continues to increase in size and degree of ossification during the life span of the 86 K. PAWLEY PHD THESIS

FIGURE 22. Figured specimens of Trimerorhachis insignis, cont.’d. 1, neural arch AMNH 4763; 2, pleurocentrum MCZ 8312; 3, intercentrum AMNH 4763; 4, intercentrum AMNH 4763; 5, intercentrum AMNH 4763; 6, clavicle UMMP 15995; 7, radius MCZ 8244; 8, ulna MCZ 8413; 9, ulna MCZ 8413; 10, scapula UMCP 174899; 11, scapula UMCP 174898; 12, scapula AMNH 23283; 13, humerus UMCP 174891; 14, humerus UMCP 174886; 15, humerus UMCP 174888; 16, humerus UMCP 174885; 17, humerus AMNH 4720; 18, humerus TMM 40031-81; 19, humerus TMM 40031-80; 20, distal humerus AMNH 233319; 21, ilium AMNH 4270; 22, ilium TMM 40031-61; 23, ilium AMNH 4578; 24, femur TMM 40031-64; 25, femur TMM 40031-71; 26, proximal femur AMNH 23321; 27, distal femur AMNH 23302; 28, ischium UMCP 174894; 29, tibia MCZ 8413; 30, fibula MCZ 8164; 31, metacarpals/phalanges AMNH 4763. CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 87 animal, however in this paedomorphic taxon a high degree of ossification is never achieved, even in the largest specimens. The dramatic effects of the morphogenetic stage of cranial material on cladistic phylogenies were discussed by Steyer (2000), and Pawley and Warren (2004; 2006) discussed the implications of the effects of the morphogenetic stage of postcranial material on phylogenetic analysis. These studies indicate that confusion between morphogenetic and phylogenetic variation may be a significant complicating factor in phylogenetic analysis of temnospondyls. Description of morphogenetic changes in the postcranial skeleton will aid in clarification of the sources of morphological variation, thus enabling identification of true phylogenetic variation in the postcranial skeleton.

MATERIALS STUDIED

Locality and repository information

All specimens examined are listed in Appendix 4, with repository, location, and stratigraphic information.

Institutional abbreviations

AMNH, American Museum of Natural History, New York, New York; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts; NMQR, National Museum at Bloemfontein, Bloemfontein, South Africa; TMM, Texas Memorial Museum, Austin; UMCP, Museum of Paleontology, University of California, Los Angeles; UMMP, University of Michigan Museum of Paleontology, Ann Arbor; USMN, National Museum of Natural History, Washington. Anatomical abbreviations are included in the figure captions.

Specific designation

All specimens in this study were located in the Wolfcampian Nocona and Petrolia formations of the Wichita Group of north-central Texas (formerly Admiral and Belle Plains formations, Hentz and Brown, 1987) and are assigned to the holotypic species, Trimerorhachis insignis, following the latest revision of the genus by Olson (1956).

Specimens described

Illustrations of individual elements were based primarily on the specimens shown in Figure 21 and Figure 22. Of the postcranial elements described here, only the interclavicle was reconstructed from partial specimens, as relatively complete, undamaged specimens of most postcranial elements were available (Appendix 4). Specimens examined included AMNH 4720 (part), a left humerus originally described by Case (1911a), refigured here. Identification and orientation of disarticulated elements was facilitated by comparison with articulated specimens of Eryops megacephalus Cope, 1877 AMNH 4186 (forelimb) and MCZ 7555 (hind limb), and Uranocentrodon (Myriodon) senekalensis Van Hoepen, 1911, NMQR 1438 (complete skeleton). Terminology follows Romer (1922), Bystrow and Efremov (1940), Coates (1996), and Pawley and Warren (2004; 2006) unless otherwise noted. 88 K. PAWLEY PHD THESIS

MORPHOLOGICAL DESCRIPTION

TEMNOSPONDYLI (Zittel, 1888) Yates and Warren, 2000 Dvinosauria Yates and Warren, 2000 Trimerorhachidae Cope, 1884 TRIMERORHACHIS Cope, 1878 TRIMERORHACHIS INSIGNIS Cope, 1878

FIGURE 23. Hind limb of Trimerorhachis insignis. 1, TMM 40998-39 partial articulated hind limb in flexor view. Abbreviations: dig, digit; fem, femur; fib, fibula; tib, tibia. CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 89

General observations

The almost complete skeleton described by Williston (1916) has a large pectoral girdle with the anterior end located between the jaws, and comparatively small limbs. The dermal pectoral elements (clavicle and interclavicle, cleithrum) are well developed; in contrast, the ends of the endochondral bones have imperfectly ossified articulation surfaces, and the coracoid, pubis, carpus and tarsus remain unossified even in the largest specimens. The articulated specimen of the hind limb, TMM 40998-39 (Figure 23), has large gaps between the ends of the femur and the tibia and fibula, indicating a large amount of cartilage existed in life, consistent with the poorly ossified condition of the rest of the postcranial skeleton. In all specimens, the articulation surfaces are rough with long columns of bone that penetrate deep into the cartilaginous epiphysis, indicating that ossification was far from complete (Haines, 1934, 1969). Morphogenetic changes are observable in all the endochondral postcranial elements; they are least apparent in the scapulocoracoid and pelvis, which are strongly paedomorphic.

FIGURE 24. Vertebrae and ribs of Trimerorhachis insignis. 1, MCZ 8128 articulated pectoral girdle and axial skeleton in ventral view. Abbreviations: cl, clavicle; cth, cleithrum; icl, interclavicle; na, neural arch; pc, pleurocentrum; pozy, postzygapophysis; pzy, prezygapophysis; scap, scapula; sit, sternal interclavicular trabecula; tp, transverse process. 90 K. PAWLEY PHD THESIS

Vertebrae

There are 32 typically rhachitomous precaudal vertebrae (Williston, 1916), composed of a neural arch, large paired pleurocentra and crescentric intercentra. In no specimen is the entire length of the tail known. Unfortunately, most articulated series of the vertebrae of Trimerorhachis insignis are not prepared from the ventral surface, due to the presence of articulated pectoral and pelvic girdle elements, so identification of the positions of particular types of pleurocentra and intercentra associated with regional variation in the vertebral column was not possible. Few articulated vertebral columns of temnospondyls are described in sufficient detail to allow identification and assignment of disarticulated vertebral elements. Those of Eryops megacephalus Cope, 1877 (Moulton, 1974) and Mastodonsaurus giganteus Jaeger, 1828 (Schoch, 1999a) do not have the ventrolaterally directed tubercles observed in some of the intercentra of Trimerorhachis insignis. These are present in the intercentra of the plesiomorphic tetrapod Greererpeton burkemorani Romer, 1969 (Godfrey, 1989a) and the embolomeres Archeria crassidisca Case, 1915 (Holmes, 1989a) and Gephyrostegus bohemicus Jaekel, 1902 (Godfrey and Reisz, 1991), which are more completely prepared. Godfrey and Reisz (1991) postulated that these tubercles are plesiomorphic for tetrapods.

Neural arch

There is little regional variation among the neural spines (Figure 24); the anterior spines are upright, and the precaudal and caudal series slope posteriorly (Figure 25.1). The height of the neural spines is subequal to the distance between the pre- and post- zygapophyses. The anterior trunk neural spines (Figure 24) are rectangular in lateral view, whereas the posterior neural arch (Figure 25.1) has a rounded outline in lateral view. Well-defined pre- and post-zygapophyses are present on all neural spines, including the smaller caudal vertebrae close to the tip of the tail. Short but well-developed transverse processes terminate in oval diapophyses. Although the triangular area above both the pre- and postzygapophyses is deeply indented, there is no supraneural canal above the ventrally open neural canal. This feature is present in plesiomorphic tetrapods such as Greererpeton burkemorani, but remains unknown in temnospondyls. In ventral view (Figure 25.1.3), the cancellous surface of the facets for the pleurocentra occupies most of the ventral surface of the diapophyses. In the posterior trunk and caudal neural arches, the rearward slant of the neural arch (Figure 25.1.1) obscures the postzygapophyses.

Pleurocentra

The pleurocentra (Figure 25.2), as is typical for temnospondyls, are paired elements that do not have articulation surfaces for the ribs. All observed pleurocentra lack facets for the opposing pleurocentra, indicating that they do not contact on the dorsal or ventral midline. The lateral surface of the pleurocentra (Figure 25.2.1) consists of an anterodorsal rectangular facet for the neural arch and a lateral periosteal surface that is anteroposteriorly corrugated with grooves, similar to those on the intercentra. The flattened anterior edge (Figure 25.2.3) has a narrow junction between the lateral periosteal and inner cancellous bone, ventral to the neural arch facet. The medial surface is rounded and entirely finished in cancellous bone. The posterior edge (Figure 25.2.2) is thin, and narrowest at the junction of the periosteal bone and cancellous inner surface.

CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 91

FIGURE 25. Vertebrae of Trimerorhachis insignis. 1, posterior trunk neural arch AMNH 4763 (reversed) in 1.1, lateral; 1.2, anterior; 1.3, medial; and 1.4, posterior views; 2, left pleurocentrum MCZ 8312 in 2.1, lateral; 2.2, anterior; and 2.3, posterior views; 3, intercentrum AMNH 4763 in 3.1, lateral; 3.2, anterior; 3.3, posterior; and 3.4, ventral views; 4, intercentrum AMNH 4763 in 4.1, lateral; 4.2, anterior; 4.3, posterior; and 4.4, ventral views; 5, intercentrum AMNH 4763 in 5.1, lateral; 5.2, anterior; 5.3, posterior; and 5.4, ventral views. In 3.4, 4.4, and 5.4, the anterior edge is uppermost. Abbreviations: dia, diapophysis; it, intercentral tubercle; lc, lateral carina; naf, neural arch facet; nec, neural canal; par; parapophysis; pozy, postzygapophysis; pzy, prezygapophysis; tp, transverse process; vc, ventral carina. 92 K. PAWLEY PHD THESIS

Intercentra

Individual variation is present among the disarticulated intercentra (Figure 25.3-Figure 25.5). They are thin, with a large notochordal space; the lateral surfaces do not converge dorsally. The parapophysis for rib articulation is located on the dorsolateral posterior edge. Periosteal bone covers the lateral and ventral surfaces, and a network of pits and ridges covers the ventral surface of some of them (Figure 25.4.4). The lateral and ventral surfaces of the larger intercentra have pronounced carinae, which becoming more pronounced posteriorly.

Cervical vertebrae

The atlas and axis were depicted in lateral, anterior and ventral views by Cope (1884)and Cope and Mathew (Cope and Matthew, 1915), and in lateral view by Case (1911a) and Shishkin (2000) and are not refigured here, however those descriptions are expanded as follows. None of the elements of the atlas or axis were fused to another. The atlantal neural arch lacks a rib facet, and the atlantal neural spine is narrow, cylindrical, not fused to its counterpart, and is nearly as tall as the axial neural spine. The atlantal neural arch has prezygapophyses for articulation with the presumably cartilaginous proatlas, and postzygapophyses for articulation with the axis. The morphology of the axis is similar to that of the trunk vertebrae. One of the smaller intercentra (Figure 25.3) bears small ventrolaterally directed tubercles on the lateral surfaces. Similar processes are present on the cervical intercentra of Greererpeton burkemorani (Godfrey, 1989a) and Archeria crassidisca (Holmes, 1989a), so this intercentrum of Trimerorhachis insignis is likely to be a cervical intercentrum. Flat projections are present on the anterior and posterior ventral borders of one of the larger intercentra (Figure 25.5), but its position in the vertebral column is indeterminate.

Sacral vertebrae/ ribs

Sacral vertebrae and ribs must be present, but none were found during preparation or as partially disarticulated elements.

Haemal arches

The haemal arches are unremarkable, and fused to the intercentra, as is typical for temnospondyls. The haemal spines are long and taper distally.

Ribs

The ribs (Figure 24) are bicipital, gently curved ventrally, the cervical and anterior trunk ribs are distally expanded. The morphology of the ribs is otherwise unremarkable; none of the observed ribs possessed an uncinate process or any other distinctive feature. The atlantal rib is absent, as is typical for temnospondyls.

CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 93

FIGURE 26. Interclavicle and left clavicle of Trimerorhachis insignis. 1, Reconstruction of interclavicle of Trimerorhachis insignis, based on MCZ 8128 and specimen photographs in Case (1935). 2, Clavicle UMMP 15995 (reversed) in 2.1, ventral; 2.2, dorsal; 2.3, lateral; 2.4, medial; 2.5, anterior; 2.6, posterior views. 3, Clavicle MCZ 8128 in left lateral view. In 1, 2.1 and 2.3 the anterior surface is uppermost. Abbreviations: acf, anterior clavicular flange; co, center of ossification; dcr, dorsal clavicular rod; fcl, clavicular facet; pcs, posterior clavicular shelf; pf, pectinate fringe; pis, posterior interclavicular shelf; pl, posterior lamina. 94 K. PAWLEY PHD THESIS

Interclavicle

The interclavicle (Figure 26.1) is diamond-shaped, and flattened dorsoventrally. The anterior portion, anterior to the posterior border of the clavicular facets, is substantially longer and steeper sided than the posterior portion. Deeply cut into the anterior borde of the interclavicle, the finely tapered prongs of the pectinate fringe fan out anteriorly beyond the anterior margin of the clavicular facets. The ventral surface of the interclavicle (Figure 26.1) is flat and covered with finely ridged anastomosing ornament, as is typical for temnospondyls. This ornament radiates from the center of ossification, which is located anterior to the posterior border of the clavicular facets. The ornament is sharply defined where it meets the unornamented, recessed areas of the clavicular facets and the posterior interclavicular border. The thin edges of the clavicular facets and the posterior interclavicular border extend considerably beyond the ornamented main body; in the specimens examined, this border was usually broken. The ornamented edge of the posterior interclavicular shelf (new term, abbreviation = pis) overhangs the unornamented posterior interclavicular border; in combination, this forms a deep recess for articulation with the ventral scutes. In dorsal view (Figure 24), the interclavicle is featureless except for the ridges of the lateral and sternal interclavicular trabeculae (s. Howie, 1970), which run laterally and posteriorly, respectively, from the center of ossification. The broad, flattened, and smoothly rounded lateral interclavicular trabeculae expand in width as they extend laterally. The sternal interclavicular trabecula is a steep sided ridge, which increases in height, but not width, as it extends posteriorly.

Clavicle

The clavicle (Figure 26.2) has a subtriangular ventral blade that is anteroposteriorly longer than wide. While the clavicles extend beyond the ornamented area of the interclavicle when articulated, the anterior edges do not meet across the anterior end of the interclavicle (Figure 21.1). The medial edge of the clavicle is almost straight in the smaller specimens (Figure 26.2.1), but becomes indented in the larger specimens (Figure 21.1). The dorsal clavicular process, composed of the thickened anterior dorsal clavicular rod, and a thin posterior lamina, is much shorter than the clavicular blade, and is located on the posterolateral corner of the blade. The ventral surface of the blade (Figure 26.2.1) is covered in finely ridged anastomosing ornament, similar to that of the interclavicle. The ridges of the ornament radiate out from the center of ossification at the base of the dorsal clavicular process, decreasing somewhat in size and depth towards the medial edge of the blade. The ridges on the clavicle align with those of the interclavicle, so that the ridges of ornament appear to be continuous across the clavicle and interclavicle when they are articulated (Figure 21.1). In lateral view (Figure 26.2.3), the ornament bulges around the center of ossification at the base of the dorsal clavicular process, forming a lip where it meets the base of the unornamented dorsal clavicular rod. The tip of the dorsal clavicular rod is posteriorly recurved, so that the anterior edge of the dorsal clavicular process is sigmoid in lateral view. The anterior clavicular flange is almost non existent in small specimens (Figure 26.2.3), but is well developed in the largest specimens (Figure 26.3), and straddles the upper curve of the anterior edge of the dorsal process ventral to the tip. The posterior of the dorsal clavicular process, expands broadly below the tip, constricting at the level of the center of ossification. In dorsal view (Figure 26.2.2), the dorsal clavicular rod joins the anterolateral edge of the ventral blade smoothly. Fine ridges, that are deepest near the medial edge, radiate from the center of ossification on the dorsal surface of the ventral blade. The ventral most CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 95

FIGURE 27. Morphogenetic series of left scapula of Trimerorhachis insignis. 1, UMCP 174899 in 1.1, lateral; and 1.2, medial views. 2, UMCP 174898 in 2.1, lateral; and 2.2 medial views. 3, AMNH 23283 (reversed), in 3.1, lateral; 3.2, anterior 3.3, dorsal; 3.4, medial; 3.5, posterior; and 3.6, ventral views. Abbreviations: fsgl, supraglenoid foramen; lsr, lateral supraglenoid ridge; sct, scapular tubercle; sgb, supraglenoid buttress; sgc, supraglenoid crest; ssf, subscapular fossa; sctor, scapular torus. 96 K. PAWLEY PHD THESIS part of the posterior lamina continues medially along the posterior dorsal lamina, which forms the posterior border border of the clavicular blade (Figure 26.2.4). This continuation of the posterior lamina rises into a low, sharp edged ridge that is located parallel to the posterior border of the ventral blade (Figure 26.2.2), (posterior clavicular shelf, new term, abbreviation = pcs). A recess is present posterior to the posterior clavicular shelf. This recess is of even width along its length, and is probably for articulation with the ventral scutes. In anterior view (Figure 26.2.5), the anterior edge of the dorsal clavicular process inflects medially, so that when the clavicles are in articulation with the interclavicle, the dorsal clavicular flange is dorsomedially oriented. In posterior view (Figure 26.2.6), the dorsal clavicular process is dorsomedially directed, and offset medially from the lateral edge.

Cleithrum

Unfortunately, extensive efforts failed to locate more than a small portion of the cleithral shaft. In one specimen (Figure 24) the midportion of the shaft is present. In cross section, the shaft is of the typical temnospondyl teardrop shape. The anterior surface of the shaft, which articulates with the dorsal process of the clavicle, is smoothly rounded. The cleithral shaft narrows posteriorly, forming the scapular flange of the cleithral shaft, which is recessed medially to articulate with the scapular blade.

Scapula

Only the scapular portion of the scapulocoracoid is present (Figure 27); an ossified scapula blade, glenoid, and coracoid are all absent, consistent with the low degree of ossification in the rest of the endochondral postcranial skeleton. The scapula is lunate in lateral view in all observed morphogenetic stages (Figure 27); of the perimeter, only the lateral and medial surfaces and the posterior edge are finished in periosteal bone. Due to the highly paedomorphic condition of the scapula, only minor morphogenetic changes are observable. The posterior margin of the supraglenoid buttress (Figure 27.3.5) is smoothly rounded, the ventral portion, that encloses the supraglenoid fossa in well ossified temnospondyls, remains unossified. The large external opening of the anteroposteriorly oriented, ventrally enclosed supraglenoid foramen aligns with the posterior border of the scapula blade in lateral view (Figure 27.3.1). The anterior edge of the supraglenoid foramen is confluent with the almost imperceptible lateral supraglenoid ridge. The medial portion of the anterior border of the scapula curves laterally, so that the leading edge is convex. In medial view (Figure 27.3.4), the thickened ridge of the scapular torus passes from the anterodorsal corner of the scapula to the supraglenoid buttress. Broadest and flattest dorsally, where it merges into the thickened anterodorsal edge of the scapula, the scapular torus narrows ventrally, and is narrowest where it joins the supraglenoid buttress. The area between the supraglenoid buttress and the scapular torus appears recessed, bounded by the raised edges of the scapula torus and the supraglenoid crest (new term, abbreviation = sgc). The broadly flattened supraglenoid buttress is uniformly wide along its entire length. Originating just below the dorsal edge of the scapula, the supraglenoid crest remains constant in height and width until it terminates at the unossified ventral border of the scapula. The supraglenoid crest forms the anterior edge of the supraglenoid buttress, and delineates the boundary between the subscapular fossa and the supraglenoid buttress. The supraglenoid buttress develops with morphogenesis; in smaller specimens, it CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 97

FIGURE 28. Morphogenetic series of left humerus of Trimerorhachis insignis. 1, Extensor view; 2, anterior view; 3, proximal views. .1, UMCP 174891; .2, UMCP 174886 (reversed); .3, UMCP 174888; .4, UMCP 174885 (reversed); .5, AMNH 4720; .6, TMM 40031-81; .7, AMNH 23319 (reversed). Abbreviations: ahk, anterior humeral keel; delt, deltoid crest; ect, ectepicondyle; ent, entepicondyle; ldp, latissimus dorsi process; pect, attachment area for pectoralis muscle; phr, proximal humeral ridge; rac, radial condyle; sup, supinator process. 98 K. PAWLEY PHD THESIS does not hide the internal opening of the supraglenoid foramen (Figure 27.1), in the larger specimens, the enlarged buttress conceals the opening for thesupraglenoid foramen within the deep subscapular fossa. The large internal opening of the supraglenoid foramen occupies most of the internal area of the subscapular fossa (Figure 27.3.2); the coracoid foramen is absent because the coracoid portion is unossified. Deep pits are present in many specimens at the center of ossification, which is located at the dorsal border of the subscapular fossa, above the supraglenoid foramen (Figure 27.3.2). In posterior view (Figure 27.3.5), the external opening of the ventrally enclosed and anteroposteriorly oriented supraglenoid foramen is offset lateral to the midline of the supraglenoid buttress. A small tubercle (scapular tubercle, new term, abbreviation = sct), only observable in later morphogenetic stages, is present on the midline of the posterior flank of the supraglenoid buttress.

Humerus

The humerus has a typical tetrahedral shape, such as was described by Romer (1922), with expanded proximal and distal ends. The shape of the humerus changes dramatically throughout the observed morphogenetic stages (Figure 28). The increasing degree of ossification of the proximal and distal ends of the humerus leads to the emergence of characteristics not present in earlier morphogenetic stages. The degree of torsion of the proximal and distal ends varies between specimens, from approximately a right angle to approximately 45º (Figure 28); it is apparently unrelated to morphogenetic stage, but is difficult to quantify due to the changes in shape. The proximal end of the humerus always exhibits a low degree of ossification, even in the largest specimens. It is entirely concave in the smaller specimens, becoming more convex in the larger specimens. In the smallest specimens, the latissimus dorsi process is absent because it was located in the unossified proximal area of the humerus. With increasing size it becomes more distant from the proximal articulation surface, in the larger specimens, the robust latissimus dorsi process is a sharp, proximodorsally oriented spike of bone, in line with the ectepicondyle. Most prominent on the extensor surface (Figure 29.1) are the large ectepicondyle and entepicondyle. On the anterior side (Figure 29.2), the rounded ectepicondylar ridge originates distal to the head of the humerus, merging imperceptibly into the ectepicondyle distal to the shaft. In the smaller specimens, the ectepicondyle projects dorsally, in the later morphogenetic stages, the distal end of the ectepicondyle is enlarged, and projects anterodorsally (Figure 28.1.7). On the posterior side of the humerus (Figure 29.4), the low, rounded entepicondylar ridge passes from below the proximal articulation surface on the entepicondyle. A shallow groove, most obvious in the morphologically most mature complete humerus (Figure 29), is present between the ectepicondylar and entepicondylar ridges, originating distal to the latissimus dorsi process, and expanding distally into the hollowed area between the ectepicondyle and entepicondyle. The proximal edge of the imperforate entepicondyle is finished in periosteal bone. The observed morphogenetic series displays dramatic changes to the outline of the entepicondyle. In the smallest specimens, the entepicondyle is absent or barely developed (Figure 28.1.1). In successive morphogenetic stages, the outline of the becomes triangular (Figure 28.1.1), as the ossifaction of the distal border proceeds, the width of the entepicondyle increases, and the outline of the entepicondyle becomes squared off (Figure 28.1.7), meeting the cancellous posterior border at approximately a right angle.

CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 99

FIGURE 29. Morphogenetic series of left humerus of Trimerorhachis insignis, cont.’d, largest complete specimen. TMM 40031-80, in 1, extensor; 2, anterior; 3, flexor; 4, posterior; 5, proximal; and 6, distal views. In 5 and 6 the extensor surface is uppermost. Abbreviations: ahk, anterior humeral keel; delt, deltoid crest; ect, ectepicondyle; ectr, ectepicondylar ridge; ent, entepicondyle; entr, entepicondylar ridge; hur, ventral humeral ridge; ldp, latissimus dorsi process; pect, attachment area for pectoralis muscle; phr, proximal humeral ridge; sup, supinator process. 100 K. PAWLEY PHD THESIS

The proximal humeral ridge, located on the anterior surface is unossified in smaller specimens, but ossifies completely and separates the proximal articulationsurface from the deltoid and pectoral crests in the later morphogenetic stages (Figure 28.2). Small areas of the anterior edge of the proximal humeral ridge may remain unossified in larger specimens (Figure 28.2.5). Flattened onto the anterior face of the humerus, the well-developed deltoid and pectoral crests are rugose with prominent muscle scars; a recessed median strip of cancellous bone separates them. The deltoid and pectoral crests are obvious even in small specimens, increasing in depth and rugosity during morphogenesis. The degree of rugosity is phenotypically variable, with specimens of the same degree of ossification displaying differing degrees of development. The sharp-edged anterior humeral keel inflects distally toward the extensor surface; it passes from the distal edge of the deltopectoral crest to the supinator process. In smaller specimens, the supinator process is confluent with the distal end of the anterior humeral keel, and is not a separate process (Figure 28.1.5). With increasing size and ossification of the distal articulation surface, the small triangular supinator process becomes prominent

FIGURE 30. Left radius and ulna of Trimerorhachis insignis. 1, radius MCZ 8244, in 1.1, extensor; 1.2, anterior; 1.3, flexor; 1.4, posterior; 1.5, proximal; and 1.6, distal views. 2, ulna MCZ 8413 (specimen 1), in 2.1, extensor; 2.2, anterior; 2.3, flexor; 2.4, posterior; 2.5, proximal; and 2.6, distal views. 3, ulna MCZ 8413 (specimen 2), in 3.1, posterior; and 3.2, proximal views. In 1.5, 1.6, 2.5, 2.6 and 3.2, the extensor surface is uppermost. Abbreviations: ol, olecranon process; puc, posterior ulnar crest; rfr, radial flexor ridge; tri, attachment area for triceps muscle; uek, ulnar extensor keel; vmrr, ventromesial radial ridge; vrc, ventral radial crest. CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 101 and separated from the distal articulation surface by periosteal bone (Figure 28.1.7); the distal edge remains unossified in all specimens examined. The ventral humeral ridge passes from the pectoral crest and across the shaft to the proximal part of the entepicondyle (Figure 29.4), it is present as a low ridge only in the largest specimen (Figure 29.3). In one of the most morphogenetically mature specimens, a partial humerus (Figure 28.2.7), the proximal edge of the radial condyle is partly defined, but the degree of ossification is insufficient for the orientation of the radial condyle to be determinable, and there is no evidence for the location of the ulnar condyle.

Radius

In transverse section, the shaft of the columnar radius (Figure 30.1) is convex on the extensor surface (Figure 30.1.1) and concave on the flexor surface (Figure 30.1.3). As only one radius was found, no morphogenetic series is available. Both the proximal and distal ends of the radius are finely striated. The convex extensor surface (Figure 30.1.1) is subtriangular in transverse section; the angle of the anterior edge is more acute than that of the posterolateral edge. The dorsomedial radial ridge and proximoventral radial ridge common to basal tetrapods (sensu Warren and Ptasznik, 2002) are absent. On the anterior side (Figure 30.1.2), the ventral radial crest originates distal to the expanded humeral articulation surface, and passes medial to the posterior edge, into the distal border. On the flexor surface (Figure 30.1.3), the proximal and distal portions are convex, whereas the shaft is concave. A slightly raised radial flexor ridge is located near the posterior distal border. The sharp-edged ventromesial radial ridge (sensu Warren and Ptasznik, 2002) forms the posterior edge of the flexor surface (Figure 30.1.3). It should be noted that the radius of Ossinodus pueri Warren and Turner, 2004, described by Warren and Ptasznik (2002), is here reinterpreted as a left radius, following comparison with Acanthostega gunnari Jarvik, 1952 (Coates, 1996). The proximal articulation surface of the radius (Figure 30.1.5) is ovoid, with a pointed corner on the extensor edge. The distal surface (Figure 30.1.6) is flattened between the extensor and flexor surfaces and broader than the proximal surface.

Ulna

The ulna (Figure 30.2) is a slender, anteroposteriorly flattened element; it exceeds the length of the radius by the length of a moderately developed olecranon process. The distal articulation surface is in line with the proximal articulation surface (Figure 30.2.5). In extensor view (Figure 30.2.1), the most prominent feature is the proximal portion of the ulnar extensor keel, which is highly rugose for the attachment to the triceps muscle. Distally it narrows into a steep keel that terminates at the distal articulation surface. In anterior view (Figure 30.2.2), the humeral articulation surface forms an angle of approximately 45º to the shaft, and the distal surface is broadly flared and as wide as the proximal surface. The olecranon process is affected by morphogenetic stage, it is better developed in the larger specimens (Figure 30.2.3). A low posterior ulnar crest originates below the posterior humeral articulation surface; it curves distally down the midline of the posterior surface, terminating on the extensor side of the distal edge. The posterior ulnar crest (Figure 30.2.4) develops later inmorphogenesis, and is only present in larger specimens. The humeral articulation surface (Figure 30.2.5) is roughly quadrangular in outline, with the medial edge more expanded than the lateral edge. In the larger specimen (Figure 30.3.2), the flexor edge is more deeply indented. The distal articulation 102 K. PAWLEY PHD THESIS

FIGURE 31. Morphogenetic series of left ilium and left ischium of Trimerorhachis insignis. 1, ilium AMNH 4270 (reversed); 2, ilium TMM 40031-61 in 2.1, lateral; 2.2, anterior; 2.3, medial; 2.4, posterior views. 3, ilium AMNH 4578 in 3.1, lateral; 3.2, anterior; 3.3, medial; and 3.4, posterior views. 4, ischium UMCP 174894 in 4.1, dorsal; 4.2, ventral; 4.3, lateral; 4.4, anterior; 4.5, medial; and 4.6, posterior views. Abbreviations: asan, anterior supracetabular notch; dip, dorsal iliac process; mir, mesial iliac ridge; psan, posterior supracetabular notch; sab, supracetabular buttress; tpr, transverse pelvic ridge. CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 103 surface (Figure 30.2.6) is anteroposteriorly flattened like the shaft, with the medial edge narrower than the lateral edge. In lateral view, the distal portion of the shaft bows gently posteriorly.

Pelvic girdle

The pelvic girdle (Figure 31) is composed of two ossified elements, the ilium, and the ischium. In all observed morphogenetic stages these remain separate elements, no ossified pubis was present in any specimen. Due to the highly paedomorphic nature of the pelvic girdle, only minor morphogenetic changes are observable.

Ilium

The dorsal iliac process (Figure 31) is anteroposteriorly expanded and flattened laterally; the dorsal surface, capped with cancellous bone, is irregular in outline. There is considerable phenotypic variation in the shape of the dorsal iliac process. In one of the smaller specimens (Figure 31.2), the posterior edge is truncated, whereas in the larger specimen (Figure 31.3), the dorsal border is notched anteriorly. Deep dorsoventrally directed ridges cover the lateral surface of the dorsal iliac process (Figure 31.3.1). These ridges are phenotypically variable and develop with morphogenesis, in larger specimens they are more pronounced. A short transverse pelvic ridge is apparent in some specimens (i.e. AMNH 4763, MCZ 8515) (Figure 31.3.1), as a crest located directly anterior to, but not extending posterior to, the prominent supracetabular buttress.

Ischium

The ischium (Figure 31.4) is relatively featureless; it is semicircular and thickest at the anterior margin. The anterior (Figure 31.4.4), medial (Figure 31.4.5), and posterior (Figure 31.4.6) margins are finished in cancellous bone, whereas periosteal bone forms the sharp edged lateral margin (Figure 31.4.3). Fine ridges striate both the dorsal (Figure 31.4.1) and ventral (Figure 31.4.2) surfaces of the ischium, radiating out from the center of ossification, which is located in the middle of the lateral border. In dorsal view (Figure 31.4.1), the ischium is convex in the anterolateral and anteromedial corners. In ventral view (Figure 31.4.2), the ischium is convex in the anterolateral corner, whereas the middle of the ischium is concave. All observed ischia were similar in shape, regardless of size.

Femur

The shape of the femur changes during morphogenesis (Figure 32, Figure 33), with the result that the increasing degree of ossification of the proximal and distal ends of the femur leads to the emergence of characteristics not present in earlier morphogenetic stages. The femur is longer than the humerus and comparatively more slender, with a fibular condyles. A deep intercondylar fossa on the distal extensor surface is steep sided on the posterior side and shallowest on the anterior side above the tibial condyle. The long narrow shaft and expanded ends. In extensor view (Figure 32.2.1), the shaft narrows directly above the adductor blade and flares distally towards the tibial and intercondylar fossa develops with morphogenesis; it is a small, shallow, vaguely defined depression in smaller specimens, but deeper and more sharply defined in larger specimens (Figure 33). 104 K. PAWLEY PHD THESIS

FIGURE 32. Morphogenetic series of left femur of Trimerorhachis insignis. 1, TMM 40031-64, in 1.1, extensor; 1.2, anterior; 1.3, flexor; 1.4, posterior; 1.5, proximal; and 1.6, distal views. 2, TMM 40031-61, in 2.1, extensor; 2.2, anterior; 2.3, flexor; 2.4, posterior; 2.5, proximal; and 2.6, distal views. In 1.5, 1.6, 2.5, and 2.6 the extensor surface is uppermost. Abbreviations: adb, adductor blade; adc, adductor crest; ffo, fibula fossa; fibc, fibular condyle; icf, intercondylar fossa; intr, internal trochanter; itf, intertrochanteric fossa; pir, posterior intertrochanteric ridge; ppa, popliteal area; tibc, tibial condyle; tr4, fourth trochanter. CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 105

In flexor view (Figure 32.2.3), the intertrochanteric fossa is gently concave on the posterior side. The anterior and posterior caput femoral fossae on the proximal femoral articulation surface deepen as the proximal head of the femur ossifies, and aremost prominent in the largest specimens (Figure 33). Increasing in prominence withmorphogenesis, the low, rounded, posterior intertrochanteric ridge joins the adductor blade at the junction of the adductor blade and adductor crest. The short adductor blade deepens with morphogenesis, and bears the typical internal and fourth trochanters on its ventral surface. The large internal trochanter is cylindrical in transverse section; the boss is shallowly concave with the most proximal tip surfaced in cancellous bone. In smaller specimens, the internal trochanter lacks a distinct process and is not distinct from the proximal articulation surface (Figure 32). In the morphogenetically more mature specimen (Figure 33), a deep trough of periosteal bone separates the internal trochanter from the proximal articulation surface. Distal to the internal trochanter, on the adductor blade, the fourth trochanter is well developed and highly rugose with deep muscle scars (Figure 33.1.3), and projects ventral to the internal trochanter. The degree of rugosity of the fourth trochanter is phenotypically variable, being highly rugose only in some of the larger specimens. The narrow ridge of the adductor crest passes from the distal surface of the adductor blade across the flexor surface of the femur towards the fibular condyle (Figure 33.2.3); it fades out on the lateral side of the poorly defined shallow popliteal area. The fibula fossa is located on the ventrolateral surface of the femur above the fibular condyle (Figure 33.2.3), so that it is visible ventrally. The popliteal area increases in size, and the fibula fossa is deeper in more mature specimens, consistent with increasing ossification of the distal end of the femur. A short rugose ridge, the femoral fibular ridge (new term, abbreviation = ffr), forms the dorsal border of the narrow fibula fossa (Figure 33.2.4) in larger specimens. In distal view (Figure 33.2.5), the deeply incised intercondylar fossa is oriented ventrally towards the midline of the popliteal area.

Tibia

The tibia (Figure 20, Figure 34) has a greatly expanded femoral head, which narrows rapidly to a thin shaft. The proximal and distal ends are set at approximately 45º to each other (Figure 34.1.6). The shaft is somewhat compressed between the extensor and flexor surfaces. In extensor view (Figure 34.1.1), the low cnemial crest dominates the anterior side with a broad, shallow cnemial trough lying posterior to it along the midline of the tibia. While the posterior border of the shaft curves deeply around the interepipodial space, the vertical axis through the center of the tibia is straight, in contrast to the bowed midline of the fibula. The anterior surface of the shaft (Figure 34.1.2) bears a sharp-edged ridge, the anterior tibial ridge (new term, abbreviation = atr). On the proximal side of the flexor surface (Figure 34.1.3), the surface between the poorly defined anterior tibial flexor crest and posterior tibial flexor crest is heavily marked with scars for muscle attachment. In contrast, the broad, flattened distal tibial flexor crest has a smooth, unmarked surface. From its junction in the midline of the flexor surface with the anterior and posterior tibial flexor crests, the distal tibial flexor crest continues down the midline to the distal articulation surface. The posterior tibial ridge (new term, abbreviation = ptr) passes directly down the midline of the posterior surface of the shaft (Figure 34.1.4). Together with the anterior tibial ridge, the posterior tibial ridge defines the edges of the extensor and flexor muscle attachments.

106 K. PAWLEY PHD THESIS

FIGURE 33. Morphogenetic series of left femur of Trimerorhachis insignis, cont.’d. 1, Proximal femur AMNH 23321 (reversed), in 1.1, extensor; 1.2, anterior; 1.3, flexor; 1.4, posterior; and 1.5, proximal views. 2, distal left femur AMNH 23302 in 2.1, extensor; 2.2, anterior; 2.3, flexor; 2.4, posterior; and 2.5, distal views. In 1.5 and 2.5 the extensor surface is uppermost. Abbreviations: acff, anterior caput femoral fossa; adb, adductor blade; adc, adductor crest; ffo, fibula fossa; ffr, femoral fibular ridge; fibc, fibular condyle; icf, intercondylar fossa; intr, internal trochanter; itf, intertrochanteric fossa; pcff, posterior caput femoral fossa; pir, posterior intertrochanteric ridge; ppa, popliteal area; tibc, tibial condyle; tr4, fourth trochanter. CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 107

The femoral articulation surface (Figure 34.2.3) is dumbbell-shaped, its anterior surface is gently convex in proximal view, and its posterior surface is concave. The cnemial trough forms a deep constriction on the extensor side of the femoral articulation surface, marking the boundary between the anterior and posterior articulation surfaces, but the flexor side of the tibial head (Figure 34.2.3) is gently constricted. The distal surface (Figure 34.1.6) is oval, with a low ridge in the anteromedial corner formed by the anterior tibial ridge.

FIGURE 34. Left tibia and metacarpal / metatarsal / phalangeal elements of Trimerorhachis insignis. 1, Tibia MCZ 8413 in 1.1, extensor; 1.2, anterior; 1.3, flexor; 1.4, posterior; 1.5, proximal; and 1.6, distal views. 2, Tibia TMM 40998-39 in 2.1, extensor; 2.2, flexor; and 2.3, proximal views. 3, Metacarpals / metatarsals / phalanges AMNH 4763 in 3.1, extensor, and 3.2, flexor views. In 1.5, 1.6, and 2.3 the extensor surface is uppermost, in 3, the proximal surface is uppermost. Abbreviations: atfc, anterior tibial flexor crest; atr, anterior tibial ridge; cn, cnemial crest; cnt, cnemial trough; dtfc, distal tibial flexor crest; ptfc, posterior tibial flexor crest; ptg, phalangeal tendon groove; ptr, posterior tibial ridge. 108 K. PAWLEY PHD THESIS

Fibula

The fibula (Figure 35) is laterally flattened, and approximately the same length as the tibia. The extensor surface (Figure 35.1.1) is smooth and relatively featureless, with the distal end wider than the proximal end. The anterior and posterior edges are both concave, but the anterior edge is much more deeply curved than the posterior. The flexor surface of the shaft (Figure 35.2.3) is convex along most of its length; a deep fibular sulcus bisects the anterodistal corner. The posterior fibular ridge extends along the posteromedial edge of the bone from just below the proximal third of the shaft to the distal facet for the fibulare. In posterior view (Figure 35.1.4), the bone narrows below the proximal head, whereas distally it is constant in width. In proximal view (Figure 35.1.5), the proximal surface twists at approximately 45º to the distal surface. The proximal articulation surface (Figure 35.2.4) is convex on the extensor side and concave on the flexor side. The distal end of the fibula is curved in smaller specimens (Figure 35.1.3), and with increasing ossification becomes sigmoid in ventral view (Figure 35.2.3). The intermedial facet becomes more deeply recessed, and the ventral outline of the fibulare facet flattens during morphogenesis (Figure 35.2.3). The distal articulation surface (Figure 35.1.6) is flat, the anterior corner bulges anteromedial to the fibular sulcus.

FIGURE 35. Left fibula of Trimerorhachis insignis. 1, MCZ 8164 (reversed) in 1.1, extensor; 1.2, anterior; 1.3, flexor; 1.4, posterior; 1.5, proximal; and 1.6, distal views. 2, fibula TMM 40998-39 in 2.1, extensor; 2.2, flexor; 2.3 flexor, angled to rear right; and 2.4, proximal views. In 1.5, 1.6, and 2.4 the extensor surface is uppermost. Abbreviations: ff, fibulare facet; fs, fibular sulcus; intf, intermedial facet; pfr, posterior fibular ridge. CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 109

Metacarpals/Metacarpals/Phalanges

Ossified carpal or tarsal elements were not present in any specimen observed. It is not possible to distinguish between disarticulated metacarpals, metatarsals and phalanges (Figure 34.3). The elements of the digits are swollen at the proximal and distal ends, prominently waisted, and flattened between the extensor and flexor surfaces. Deep notches are present on the proximal and distal ends of the flexor surfaces for the flexor tendons. The metatarsals (Figure 23) are flat proximally, most likely due to the low degree of ossification. In well-ossified, morphogenetically mature temnospondyls such as Cacops aspidephorus (Williston, 1910a), the metacarpals and metatarsals are angled, this characteristic was associated with turning the foot anteriorly during terrestrial locomotion by Clack and Finney (Clack and Finney, 2005). The reconstruction of the manus by Case (1935: figure 26) can be revised to a phalangeal formula of 2-2-3-3, and that of the pes (Case, 1935: figure 27) can be revised to 2-2-3-4-5, as is typical for temnospondyls (e.g. Carroll, 1964a; Boy, 1972; Hook and Baird, 1984; Daly, 1994; Milner and Sequeira, 1994).

Dermal ossifications

Overlapping cycloid dorsal scales were described by Colbert (1955). These scales cover the entire dorsal surface including the tail and limbs (Olson, 1979). Spindle shaped ventral scutes were also observed in several specimens, presumably these covered the ventral surface between the interclavicle and posterior limbs, as is typical for temnospondyls.

DISCUSSION

Comparisons with previous work

This description of the postcranial skeleton of Trimerorhachis insignis departs from previous descriptions, particularly those of Case (1911a; 1935) and Williston (1915a), in that this study incorporates later morphogenetic stages, which display an increased degree of ossification and thus significant differences in morphology from those previously described as typical for T. insignis.

Paedomorphosis of the postcranial skeleton in Trimerorhachis insignis

Heterochronic processes are usually determined within a group of organisms relative to the ancestral condition (McNamara, 1986). As current conflicting large scale phylogenies of temnospondyl taxa (Yates and Warren, 2000; Ruta et al., 2003) make determination of the ancestor-descendant relationships within temnospondyl taxa uncertain, heterochronic processes such as paedomorphosis in temnospondyls are here defined in terms of the comparative extent of morphogenetic maturity of the postcranial skeleton, among the morphogenetic stages observable within the Temnospondyli, as discussed by Pawley and Warren (2006). The endochondral postcranial elements, preformed in cartilage, undergo ossification during morphogenesis; the degree of ossification achieved determines the extent of morphogenetic maturity of the postcranial skeleton. Morphogenetic changes are not determinable in the dermal postcranial elements (clavicle, interclavicle and cleithrum). Indications of morphogenetic maturity of the endochondral postcranial 110 K. PAWLEY PHD THESIS skeleton in temnospondyls are a well-ossified coracoid and pubis, and fully ossified carpus and tarsus. Despite the large number of specimens, no evidence is available for the existence of any of these features in T. insignis. The endochondral postcranial skeleton remains osteologically immature throughout all observed morphogenetic stages with poorly ossified ends to the limb bones even in the largest specimens, indicating paedomorphosis (sensu McNamara, 1986). In T. insignis, increasing ossification of the endochondral postcranial elements correlates with increasing size. The variation observed in the endochondral postcranial skeleton of this taxon is thus mostly due to ontogenetic variation (morphogenetic changes) rather than phenotypic variation in the adult condition. The extent of ossification does not plateau in the largest specimens. Therefore the process of morphogenetic retardation though neoteny, rather than truncation of morphological development (progenesis) (sensu McNamara, 1986) has produced the paedomorphic morphology seen in T. insignis. No specimen has a fully ossified postcranial skeleton because the delayed development of the endochondral postcranial skeleton results in an increase in the developmental time needed to produce full ossification, which exceeds the lifespan of any known individual of T. insignis. The morphogenetic changes observed in the endochondral postcranial skeleton of T. insignis are consistent with those of other temnospondyls (discussed by Pawley and Warren, 2004). Derived stereospondyls, including the mastodonsaurid Benthosuchus sushkini Efremov, 1937 (Bystrow and Efremov, 1940) and the metoposaurs Buettneria perfecta Case, 1922, and Dutuitosaurus (Metoposaurus) ouazzoui Dutuit, 1976 have paedomorphic endochondral postcranial skeletons, with similar growth trajectories, displaying a linear increase in size and degree of ossification throughout growth stages (Olsen, 1951; Steyer et al., 2004). Cartilage has less than half the specific gravity of bone; thus retention of a high percentage of cartilage in the postcranial skeleton contributes to neutral buoyancy. In a fully aquatic animal, neutral buoyancy will be important throughout its lifespan, which may a reason for the persistent occurrence of paedomorphic postcranial skeletons in aquatic temnospondyls. Bone is metabolically more expensive to form and maintain than cartilage, so that the delayed ossification of endochondral elements would thus achieve an energy saving if a high degree of ossification was not ecologically important for Trimerorhachis insignis. A high degree of ossification of the endochondral postcranial skeleton would only be necessary if the limbs needed to support the weight of the animal on land rather than in water. The low degree of ossification of the endochondral postcranial skeleton of Trimerorhachis insignis, even in large specimens, is an adaptation for an aquatic lifestyle.

Implications for cladistic analysis

The range of morphogenetic variation observed in the endochondral skeleton of T. insignis could create problems for interpretation of postcranial characteristics used in phylogenetic analysis. Determination of the absence or presence of particular characteristics may depend on the morphogenetic stage of specimens examined (also discussed by Pawley and Warren, 2004, 2006). The basis for hypotheses of phylogenetic relationships is phylogenetic variation, not phenotypic or ontogenetic variation. An example of potential confusion of ontogenetic with phylogenetic variation would be the coding of the cladistic character FEM 2: Absence (0) or presence (1) of condition: internal trochanter separated from femur by a distinct trough-like space (‘FEM 2’ is the unique character identifier for the character state) (Ruta et al., 2003). Both states of this character CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 111 are observable in the specimens of T. insignis. This is not a polymorphism in T. insignis, because the character states describe morphogenetic (ontogenetic), not phylogenetic, variation in this taxon. The earlier morphogenetic stages display state (0) the later morphogenetic stages, state (1). Specimens displaying state (0) are of a morphogenetically less mature stage, rather than plesiomorphic, specimens displaying state (1) are of a morphogenetically more mature stage, rather than derived. It is probable that all individuals of T. insignis would have had the genetic potential to achieve the derived state, should they have lived long enough. In this instance, the only valid state for T. insignis is state (1), the derived state. Other examples of confusion of morphogenetic and phylogenetic variation exist in the literature, but the range of morphogenetic variation within the postcranial skeleton of the Temnospondyli, and the implications for character states used in phylogenetic analysis, will be described elsewhere.

Morphological comparisons

Despite the paedomorphic state of the postcranial skeleton of T. insignis, it displays many characteristics that are derived, relative to basal tetrapods (sensu Ruta et al., 2003), and common to all temnospondyls. Some of the characteristics common to all temnospondyls as listed by Pawley and Warren (2006) are observable only in well ossified temnospondyls, and are absent in T. insignis due to paedomorphosis of the postcranial skeleton. Those postcranial characteristics of temnospondyls affected by morphogenetic stage will be described in more detail elsewhere. Few differences exist between the postcranial skeleton of T. insignis and those of other taxa within the Dvinosauria but, where present, they are distinctive. The only non- morphogenetic difference between T. insignis and T. sandovalensis (Berman and Reisz, 1980) is the ‘V’ shaped dorsolateral cleft of the scapula in the latter. The ‘median precoracoid’, noted by Berman and Reisz (1980), is not observable in any other temnospondyl, and is most likely the crushed unornamented anterior border of the interclavicle, present in T. insignis. The anterior border of the interclavicle often extends beyond the anterior borders of the clavicles as a pectinate fringe in other temnospondyls, notably Acroplous vorax (Hotton, 1959); Isodectes (Eobrachyops, Saurerpeton) obtusus Sequeira, 1998 (AMNH 6919); Eryops megacephalus Cope, 1877 (Pawley and Warren, 2006); Archegosaurus decheni Goldfuss, 1847 (Meyer, 1857); Cheliderpeton latirostre Fritsch, 1877 (Boy, 1993); Sclerocephalus spp. Goldfuss, 1847 (Broili, 1926; Boy, 1988; Werneburg, 1992) and an unnamed rhinesuchid (Pawley and Warren, 2004). The right atlantal spine of T. sandovalensis is described as such in the text, but mislabelled in the figure as an ossified ‘proatlas’(Berman and Reisz, 1980: figure 7). As prezygapophyses are present on the atlas, the proatlas of T. sandovalensis was presumably present but unossified, as in T. insignis (Cope, 1884). The postcranial skeletons of Neldasaurus wrightae (Chase, 1965); Acroplous vorax (Hotton, 1959; Coldiron, 1978), and Isodectes obtusus (Watson, 1956; Sequeira, 1998), are similar to that of T. insignis. Dvinosaurus primus Amalitzky, 1921 (Bystrow, 1938) differs from T. insignis in that it possesses narrow, medially oriented clavicles and a broad interclavicle with a long, narrow, parallel edged parasternal process. All species of Dvinosaurus possess a scapular tubercle (Nikitin, 1997; Gubin, 2004) similar to that of T. insignis. Some of the endochondral postcranial elements of Dvinosaurus spp. become more highly ossified than is observed in T. insignis. The humerus of Dvinosaurus spp. (Nikitin, 1995) develops an ossified radial condyle and the scapula (Nikitin, 1997) develops an expanded dorsal blade, making Dvinosaurus spp. less paedomorphic than T. insignis. The tupilakosaur Thabanchuia oomie (Warren, 1998b), also has an interclavicle 112 K. PAWLEY PHD THESIS with long parallel-sided parasternal process, as does Tupilakosaurus spp. (Nielsen, 1954; Shishkin, 1961). Both these tupilakosaurs possess embolomerous vertebrae (pleurocentra and intercentra form complete amphicoelous discs), which are not present in T. insignis. Due to the lack of detailed morphological descriptions of other taxa within the Dvinosauria, it is difficult to determine whether any of the features of T. insignis not previously described in temnospondyls, are apomorphies of this taxon. Our knowledge of the postcranial skeleton of the Brachyopomorpha was summarized by Warren and Marsicano (2000). Relevant taxa within the Brachyopomorpha are the brachyopoid Batrachosuchus concordi (Chernin, 1977) and the chigutisaurs Siderops kehli (Warren and Hutchinson, 1983), Pelorocephalus sp. Cabrera, 1944 (Marsicano, 1993), Compsocerops cosgriffi (Sengupta, 1995) and Koolasuchus cleelandi (Warren et al., 1997). Several postcranial synapomorphies unite derived stereospondyls, including the Brachyopomorpha. The distribution of some of these is listed in Pawley and Warren (2005). The presence of these postcranial synapomorphies in stereospondyls precludes a sister taxon relationship between the Dvinosauria (including T. insignis) and the Brachyopomorpha. Postcranial features of derived stereospondyls that are not found in the Dvinosauria include: fusion of the atlantal neural arch to the intercentrum, dorsal fusion of the atlantal neural arches; the center of ossification of the interclavicle is located posterior to the posterior border of the clavicular facets, the latissimus dorsi process of the humerus is highly reduced or absent, and the presence of a ventrally open supraglenoid foramen. The anterior clavicular flange of Trimerorhachis insignis is convergent with that of mastodonsaurids, since T. insignis is not closely related to these taxa. Despite the independent development of the anterior clavicular flange in these taxa, the structure of this feature is sufficiently similar that a separate name for the structure in each taxon is not justifiable. In some mastodonsaurids, such as Eryosuchus (Parotosuchus) pronus Damiani, 2001 (Howie, 1970) and Mastodonsaurus giganteus Jaeger, 1828 (Schoch, 1999a), the clavicular flange is located across the junction of the ventral blade and dorsal clavicular process, although in others, such as Benthosuchus sushkini Efremov, 1937 (Bystrow and Efremov, 1940), it is located nearer the tip of the dorsal process, as in T. insignis. Interestingly, mastodonsaurids and T. insignis have flattened profiles, and both are obligatorily aquatic. The flanges may function to strengthen and stiffen the neck as an adaptation to aquatic locomotion. Within the Temnospondyli, the postcranial skeleton of Trimerorhachis insignis is derived because it lacks an entepicondylar foramen of the humerus and the post-iliac process of the ilium, which are plesiomorphically retained in the basal taxa Balanerpeton woodi (Milner and Sequeira, 1994) and Dendrerpeton acadianum Owen, 1853 (Carroll, 1967; Holmes et al., 1998). T. insignis is less derived than the Euskelia Yates and Warren, 2000 (Eryops Cope 1877 plus Zatrachydidae Williston, 1910a, and Dissorophus Cope, 1895 plus Trematopidae Williston, 1910b), and the Stereospondylomorpha Yates and Warren, 2000 (archegosaurs plus stereospondyls). Within the Euskelia and the Stereospondylomorpha, the pelvis is shortened anteriorly, so that the mesial iliac ridge forms the anterior edge of the ilium and pubis, and the adductor crest of the femur passes down the midline of the flexor surface, rather than diagonally towards the fibular condyle. The morphological characteristics of Trimerorhachis insignis observed in this study support the cladistic analysis of Yates and Warren (2000) in that the postcranial skeleton is most like that of other members of the Dvinosauria. However, the morphological characteristics of the postcranial skeleton of T. insignis observed in this study do not support the phylogenetic position of the Dvinosauria within the Temnospondyli as found in the results of the large scale cladistic analyses of Yates and Warren (2000) or Ruta et al. CHAPTER 3: POSTCRANIAL SKELETON OF TRIMERORHACHIS 113

(2003). Yates and Warren considered the Dvinosauria more derived than the Euskelia, but less derived than the Stereospondylomorpha. Ruta et al. (2003) considered T. insignis to be one of the most basal taxa within the Temnospondyli, basal to Balanerpeton woodi and Dendrerpeton acadianum. The morphology of the postcranial skeleton of the Dvinosauria, including T. insignis, is consistent with a phylogenetic position more derived than Balanerpeton woodi and Dendrerpeton acadianum, but less derived than the Euskelia plus Stereospondylomorpha.

Humeral torsion

Most temnospondyls have the proximal and distal ends of the humerus set at approximately right angles to each other. Possibly the most contentious finding of this description of the postcranial skeleton of T. insignis is that the angle between the proximal and distal ends of the humerus varies between specimens; some display a reduced degree of torsion (Figure 28, Figure 29). The degree of torsion is difficult to assess due to the changes in form of the proximal and distal ends during morphogenesis, but is not related to the morphogenetic stage of the specimen in the specimens observed. The two smallest specimens (Figure 28.3.1, Figure 28.3.2) have the proximal and distal ends set at approximately right angles, as does one of the medium sized specimens (Figure 28.3.4), and the largest specimen (Figure 29). In contrast, one of the smaller specimens (Figure 28.3.3), and two of the larger specimens (Figure 28.3.5, Figure 28.3.6) display low degrees of torsion. Further analysis of the connection between morphogenetic stage and degree of humeral torsion would be of value, but as most humerus specimens are broken at the narrowest point of the shaft (Appendix 4) the sample size of complete humeri is consequently too small for statistical analysis. Variation in the degree of humeral torsion is known to occasionally occur in other obligatory aquatic temnospondyls such as Dvinosaurus primus Amalitzky, 1921 (Nikitin, 1995), Buettneria perfecta Case, 1922 (Olsen, 1951), and Mastodonsaurus giganteus Jaeger, 1828 (Schoch, 1999a). The reason for the variation in degree of humeral torsion may be related to the aquatic lifestyle of these taxa, but it is not known what advantages this would provide, or why the degree of torsion is so variable. Alternatively, the variation in the degree of humeral torsion may be the result of taphonomic processes that have flattened some humeri and not others. Distortion during preservation may result in marked differences in the morphology of otherwise identical elements, such as the left and right mandibles of the brachyopoid Koolasuchus cleelandi (Warren et al., 1997).

Characteristics of primarily vs. secondarily aquatic early tetrapods

The most plesiomorphic tetrapod known to date, Acanthostega gunnari Jarvik, 1952, is primarily rather than secondarily aquatic, and it retains many plesiomorphic tetrapod characteristics otherwise known only in osteolepiform fish (Coates and Clack, 1990, 1991; Clack and Coates, 1995; Coates, 1996). These plesiomorphic features are not found in aquatic temnospondyls. The most obvious postcranial adaptations for an aquatic existence observed in Trimerorhachis insignis are the extensive ventral area of the interclavicle and clavicles, and the poorly ossified, paedomorphic condition of the endochondral bones. Otherwise, the postcranial skeletons of T. insignis, and other aquatic temnospondyls, are generally most similar to those of terrestrial temnospondyls (as defined by Pawley and Warren, 2006). This evidence supports the hypothesis that aquatic 114 K. PAWLEY PHD THESIS temnospondyls probably had terrestrial ancestors, and are thus secondarily rather than primarily aquatic.

ACKNOWLEDGEMENTS

This manuscript forms part of the PhD dissertation of K. Pawley, who sincerely thanks her supervisor, A. Warren, for her support and encouragement. An American Museum of Natural History collections visitation grant and an Australian Postgraduate Award provided funding. K. Parker provided invaluable assistance and discussion on taphonomic processes. Thanks are extended to G. Gaffney of the American Museum of Natural History, New York. C. Schaff of the Museum of Comparative Zoology, Harvard University, Massachusetts, and H. Fourie of the Transvaal Museum, Pretoria, South Africa, and J. Welmann of the National Museum at Bloemfontein, Bloemfontein, South Africa for facilitating collections visitation. For assistance with the loan of specimens I would like to thank L. K. Murray and T. Rowe of the Texas Memorial Museum, Austin; P. Holroyd of the Museum of Paleontology, University of California, Los Angeles; and G. Gunnell of the University of Michigan Museum of Paleontology, Ann Arbor. This manuscript benefited from the critiques of two anonymous reviewers. All illustrations are by K. Pawley.

This manuscript is reproduced with written permission from the copyright owner, the Paleontological Society, which publishes the Journal of Paleontology.