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The Postcranial Skeleton of Temnospondyls (Tetrapoda: Temnospondyli) Phd Thesis

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The Postcranial Skeleton of Temnospondyls (Tetrapoda: Temnospondyli) Phd Thesis Pawley, K. 2006. 149 The Postcranial Skeleton of Temnospondyls (Tetrapoda: Temnospondyli) PhD Thesis. La Trobe University, Melbourne CHAPTER 5. PHYLOGENETIC VARIATION IN THE POSTCRANIAL SKELETON OF TEMNOSPONDYLS (TETRAPODA: TEMNOSPONDYLI) Abstract. Phylogenetically significant features of the postcranial skeleton within the Temnospondyli are described and illustrated. Considerable morphological variation is observable in vertebral structure and the configuration of the dermal pectoral girdle; otherwise, the postcranial skeleton is generally conservative. Although most temnospondyls are best described as amphibious, the most basal temnospondyls are well adapted for terrestrial locomotion, with one major derived clade, the Euskelia, displaying postcranial adaptations to the axial skeleton that may have improved the efficiency of their terrestrial locomotion. Two derived clades, the Dvinosauria and Superstereospondyli, were obligatorily aquatic. In general appearance and lifestyle, most temnospondyls superficially resemble extant crocodilians. The information provided by this overview of the postcranial skeleton is used to revise the postcranial characteristics used for classifying temnospondyls, and to create new characters for a comprehensive phylogenetic analysis of the Temnospondyli, using 57 taxa and 154 phylogenetic characters. This analysis is innovative for phylogenetic analysis of temnospondyls in that morphogenetic and phylogenetic variation in the postcranial skeleton is carefully distinguished and accounted for in the coding of the data matrix. The most basal taxa within the Temnospondyli are Caerorhachis bairdi and Edops craigi, with the Dendrerpetontidae, Cochleosauridae, Dvinosauria, and Eryopoidomorpha (Euskelia plus Stereospondylomorpha) as successively more derived clades. Comparison with the only previous computer-based analysis of the Temnospondyli indicates that the different tree topology presented here is significantly more parsimonious. Test analyses indicate that phylogenetic analyses based entirely on cranial characters produce quite different tree topologies to those including postcranial characters. INTRODUCTION The Temnospondyli (Zittel, 1888), named for a characteristic of their postcranial skeleton, the multipartite vertebral centra, are a large and diverse group of early tetrapods. Their known stratigraphic range spans more than 200 million years, starting in the Early Carboniferous with the basal temnospondyls Caerorhachis bairdi (Holmes and Carroll, 1977), and Balanerpeton woodi (Milner and Sequeira, 1994) from the Viséan (345-326 mya) of Scotland, and Dendrerpeton acadianum from the Namurian (313-314 mya) of Nova Scotia (Falcon-Lang et al., 2006). The latest known temnospondyl is the derived stereospondyl Koolasuchus cleelandi from the Early Cretaceous (110 mya) of Australia (Warren et al., 1997). Discovery of temnospondyl fossils on every continent, including Antarctica (e.g. Cosgriff and Hammer, 1984), emphasises their success in adapting to diverse environments. Given the extensive time range and diversification of temnospondyls, a corresponding range of phylogenetic variation in the postcranial skeleton is to be expected, thus providing a useful source of information for taxonomic purposes. 150 K. PAWLEY PHD THESIS Previous studies of the temnospondyl postcranial skeleton Although no previous work has attempted to assess the range of phylogenetic variation within the whole of the Temnospondyli, several partial overviews are available. Romer (1947) provided a basic outline of morphological variation of the postcranial skeleton within temnospondyls, Nilsson (1939) characterised the cleithrum and humerus, Warren and Snell (1991) provided a comprehensive overview of the range of variation in Mesozoic temnospondyls, and Shishkin (2000) described variation in cervical vertebrae. A thorough assessment of the postcranial skeleton of temnospondyls is necessary to enable comparative research. As part of long-term research on the postcranial skeleton of temnospondyls, the aim of this study is to characterise the temnospondyl postcranial skeleton, and define the extent of phylogenetic variation of the postcranial skeleton within the Temnospondyli. Previous phylogenetic analyses The only previous large scale, computer based analysis of temnospondyl relationships is that of Yates and Warren (2000), which also summarised previous hand generated phylogenetic analyses (Coldiron, 1978; Warren and Black, 1985; Boy, 1990; Foreman, 1990; Milner, 1990, 1991; Boy, 1993; Daly, 1994), and smaller computer based analyses (Trueb and Cloutier, 1991; Damiani and Warren, 1996; Holmes et al., 1998; Laurin and Soler-Gijón, 2006). The analysis of Yates and Warren (2000) included the ‘higher’ temnospondyls but excluded plesiomorphic temnospondyls. An analysis of early tetrapod relationships which did include several plesiomorphic temnospondyls (Ruta et al., 2003) produced a completely different tree topology to that of Yates and Warren (2000), but this analysis was still constrained because it included only a small number of plesiomorphic temnospondyls. Several recent studies of postcranial material, including the phylogenetic analysis of Pawley and Warren (2005), and the description of the postcranial skeletons of more plesiomorphic temnospondyls such as Eryops megacephalus (Pawley and Warren, 2006) and Trimerorhachis insignis (Pawley, in press), all provide evidence that their phylogenetic affinities are not congruent with those of Yates and Warren (2000). Sources of morphological variation Sources of morphological variation in the postcranial skeleton are both interspecific (phylogenetic) variation, and intraspecific (phenotypic and morphogenetic). None of the previous studies of the postcranial skeleton of temnospondyls has distinguished between these types of variation. Chapter 4 described the extensive morphogenetic variation of the postcranial skeleton in temnospondyls, particularly in the endochondral postcranial skeleton. Of particular concern was the finding that certain characteristics, usually considered derived, such as the height of the neural spines, supinator process and radial condyle of the humerus (to name a few), develop with morphogenesis. In practical terms, phylogenetic characters defining the presence or absence of these morphological features are sometimes simply describing the difference between morphogenetically immature and mature specimens, i.e. describing morphogenetic rather than phylogenetic variation. The problem of immaturity vs. maturity is particularly apparent when comparing taxa with paedomorphic postcranial skeletons with non paedomorphic taxa. In paedomorphic morphogenesis is delayed, so that characteristics that develop late in morphogenesis and are present in early growth stages of non paedomorphic taxa are often only present in the largest specimens of paedomorphic taxa, CHAPTER 5: POSTCRANIAL SKELETON OF TEMNOSPONDYLS 151 if at all (Pawley and Warren, 2004; Chapter 4; Pawley, in press). Unfortunately, for many temnospondyl genera, only single specimens are available, so it is usually impossible to determine whether morphogenetically immature specimens are truly immature or actually paedomorphic adults, which also means that the phylogenetic significance of heterochronic processes such as paedomorphism are difficult to objectively assess. Steyer (2000), in a study of cranial characters, found that that using characters that pertain to morphogenetic instead of phylogenetic variation had dramatic effects on the resulting cladogram topology, and that the use of these characters resulted in extremely inconsistent tree topologies that were dependant on the morphogenetic age of the specimens used. A diagnosis of the postcranial characters used by Yates and Warren (2000), which is the only other large scale computer based analysis of temnospondyls, indicates that several characters describe morphogenetic rather than phylogenetic variation. The presence or absence states of other postcranial characters are affected by the morphogenetic stage of specimens, additionally some did not separate into discrete character states within a larger sample of temnospondyls. The same concerns are applicable to the phylogenetic analysis of early tetrapods conducted by Ruta et al. (2003). These findings prompted a revision of the postcranial characters used in phylogenetic analysis of the Temnospondyli, presented here (Appendix 8). Potentially the use of morphogenetic variation rather than phylogenetic variation for phylogenetic analysis could increase the amount of homoplasy and mask true phylogenetic signal. In order to elucidate true phylogenetic signal, consideration and assessment of all sources of morphological variation is necessary, so that the effects of non-phylogenetic variation are minimised and phylogenetic analysis is based solely on phylogenetic variation. The main aim of this study is to summarise the extent of phylogenetic variation in the postcranial skeleton of the Temnospondyli. The additional postcranial data is used to revise existing postcranial characters used in phylogenetic analysis of temnospondyls, and to create new characters, in order to reassess the phylogenetic relationships of taxa within the Temnospondyli. MATERIALS AND METHODS Data sources As noted in the introduction, this study is only concerned with characteristics pertaining to phylogenetic variation within temnospondyls. Many specimens were examined personally, but this study was mainly literature based. Abbreviations
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