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).