Temnospondyls are large extinct fossil tetrapods; superficially resembling crocodiles in their general size, appearance and lifestyle. Temnospondyls are a group of early tetrapods, the oldest fossils are more than 340 million years old, and they existed for more than 200 million years. This doctoral thesis examined the postcranial skeleton of temnospondyls and its evolutionary history and diversification. Standard taxonomic techniques were used to distinguish between the types of variation observed in the postcranial skeleton and for phylogenetic analysis. The thesis consists of a series of published articles, three describing the postcranial skeletons of various temnospondyls, and three summary articles, all with extensive illustrations. To provide data, the postcranial skeletons of three temnospondyl taxa were described. The articulated postcranial skeleton of a basal stereospondyl (rhinesuchid) is immature, and paedomorphism of the postcranial skeleton in stereospondyls is discussed. The robust appendicular skeleton of Eryops megacephalus is plesiomorphic, well-ossified, and terrestrially adapted. The paedomorphic postcranial skeleton of Trimerorhachis insignis is plesiomorphic, and secondarily aquatic, the description includes growth stages. This study found that extensive morphogenetic variation is present in the postcranial skeleton of temnospondyls. Many phylogenetically significant characteristics develop with morphogenesis, they may be absent in early growth stages, and may never develop even in the largest growth stages of taxa with paedomorphic postcranial skeletons. Consequently, assessment of the presence or absence of a phylogenetically significant characteristic in any taxon may be dependant on the morphogenetic stage of the specimen examined. This finding has major implications for the phylogenetic analysis of temnospondyls and other early tetrapods. An overview of phylogenetic variation in the postcranial skeleton is presented, including a large phylogenetic analysis of the Temnospondyli. The most primitive temnospondyls possess fully ossified postcranial skeletons, well adapted for terrestrial locomotion, but some of the derived clades of temnospondyls have paedomorphic postcranial skeletons and are exclusively aquatic. For the first time, the postcranial skeleton of temnospondyls is comprehensively compared with that of other early tetrapods in the largest phylogenetic analysis to date, resulting in the unexpected discovery that temnospondyls are most closely related to the ancestors of amniotes. The Temnospondyli plus Neospondyli (Seymouriamorpha plus Cotylosauria plus Lepospondyli) forms a large new clade, the Terrapoda, defined by the presence of many derived synapomorphies. Some of the cranial synapomorphies of the Terrapoda are most likely related to improvements in hearing. The postcranial synapomorphies indicate that the Terrapoda are the first vertebrates to have evolved limbs that are well adapted for terrestrial locomotion. The Terrapoda are the first truly terrestrial vertebrates, their postcranial adaptations facilitated their colonisation of the land and consequent phylogenetic radiation during the early Carboniferous. Both analyses incorporate characters from previous analyses and many new postcranial characters. The results of the phylogenetic analyses are statistically more parsimonious than previous analyses and have much lower levels of homoplasy. Comparative analyses indicate that the distinctive results are most likely due to the increased use of characters pertaining to temnospondyls, increased use of postcranial characters, and differentiation between sources of morphological variation to minimise morphogenetic and phenotypic variation and elucidate true phylogenetic signal.

The impact of the end-Devonian mass extinction (the Hangenberg extinction) on tetrapods is not well understood. One issue is that we have very little knowledge of the Devonian tetrapod fauna that immediately preceded, and experienced, the mass extinction. New specimens from an early tetrapod bone bed of the latest Famennian, Stensiö Bjerg Formation of Celsius Bjerg, East Greenland have the potential to shed light on this problem. In this study, five new well-preserved specimens are presented: a partial skull, two humeri, a left pelvis, and a strange vertebral element. The specimens were imaged using propagation phase-contrast synchrotron microtomography (PPC-SRμCT), and then virtually segmented and rendered.  The partial skull and left pelvis share many similarities with Ichthyostega and Acanthostega, but also have some distinctive features. Notably, the epipterygoid does not articulate with the skull roof, the fenestra vestibuli appears to have an anterior lobe, the postorbital has a posteroventral process, and there is a sharp contrast between the rugose dermal ornament of the skull roof and the unornamented cheek. In addition, the partial skull is box-like in shape and has a laterally facing orbit, which are features seen in some Carboniferous tetrapods. Among other characters, the left pelvis has a differently curving posterior iliac process, and differently shaped ischium and anterior pubic margin compared to Ichthyostega and Acanthostega. The acetabulum of the left pelvis is also lacking a posterior notch, a feature seen in many other early tetrapods. The humeri presented in this thesis are curiously similar to the isolated tetrapod humerus from the Catskill Formation, Pennsylvania, USA known as ANSP 21350. In fact, they are more similar to each other and to ANSP 21350 than to any other early tetrapod humeri. This is in large part due to a distinctly distal supinator ridge which was previously only known from ANSP 21350. The mysterious vertebral element does not resemble any known early tetrapod bone due to its large, ventral, concave saddle-shaped projections. It consists of five fused vertebrae and is interpreted here to be a unique sacrum of an early tetrapod. The specimens described and interpreted in this study represent at least two new species, none of which can be assigned to known Devonian early tetrapods from Greenland. It is clear that a new and important faunal assemblage is emerging.