Terrestrialisation in vertebrates using evidence from synchrotron tomography

etrapods are a widespread group of animals, encompassing all terrestrial vertebrates. They comprise two major groups, the amphibians (frogs, salamanders and caecilians) and the amniotes (mammals and reptiles, including birds). Fossils provide unique data on the deep evolutionary divergence between these groups, but the affinities of many early fossil species, and their implications for the origins of amphibians and amniotes have been debated for over a century. Unquestioned crown amniotes, such as Hylonomus, are known from the late Carboniferous (312Ma; 1), demonstrating that the two clades had diverged by this time.Fossils groups such as temnospondyls, seymouriamorphs, diadectamorphs and lepospondyls possess anatomical traits and trait-combinations with important implications for early tetrapod evolution (e.g. 2; 3; 4). However, consensus on the affinities of these groups as stem-group tetrapods, stem-group amniotes or stem-group amphibians, is varied. Since the introduction of computerised phylogenetic analyses to the field in the late 1990s, many more phylogenies (evolutionary trees) have been published (see following for references), enhancing our understanding of how these extinct taxa relate to each other and to the early evolution of extant groups. However, there is still considerable uncertainty. (5).Two particularly large and diverse groups are typically recognised among early tetrapods: (1) Temnospondyls, which are often regarded as stem-group amphibians (6; 7); and (2) Lepospondyls, a wide range of generally small-bodied taxa united by anatomical similarities in their vertebrae (8). Many authors have suggested that lepospondyls are closely related to amniotes (e.g. 9). Two broad categories of hypothesis have emerged concerning amphibians in relation to these clades: the temnospondyl hypothesis, in which temnospondyls are stem-amphibians (e.g. 6); and the lepospondyl hypothesis, in which temnospondyls are stem-tetrapods and amphibians are closely related to a particular group of lepospondyls, the lysorophians (10). These hypotheses are substantially different large-scale hypotheses of tetrapod relationships, entailing major uncertainties about the deep evolutionary history of tetrapods.Many other taxa are also recognised among early tetrapods, outside of temnospondyls and lepospondyls. These provide phylogenetically relevant anatomical data, and other phylogenetic questions remain. In particular, there is debate whether Diadectamorpha and recumbirostan microsaurs (a clade of lepospondyls) fall outside or within the amniote-crown (11, 12), and themonophyly of long recognised clades, such as the aïstopods, or indeed, of lepospondyls (13).
My project will focus on resolving these phylogenetic issues, focusing on the anatomy key Carboniferous taxa of relevance to the amniote stem-lineage: Westlothiana (14), Casineria (15) and Solenodonsaurus (16). These have been placed in multiple locations within phylogenies in previous studies, and have hypothesised importance for resolving phylogenetic issues of deep-tetrapod relationships. Westlothiana has been considered close to amniotes (9) or a sister taxon to lepospondyls (6). Solenodonsaurus has been placed in various positions along the amniote stem-lineage (e.g. 17; 7). Casineria is poorly known and has been used in relatively few phylogenies WestlothianaAmniotesAmphibiansMicrosaursDiadectamorphsAïstopodsNectrideansLyrosophiansTemnospondylsLepospondyls1A1BLepospondylsWestlothianaAmniotesTemnospondyls, including amphibiansMicrosaursAïstopodsNectrideansLyrosophiansDiadectamorphsFig 1; Simplified phylogenies representing the temnospondyl and lepospondyl hypothesis, and differing positions of Westlothiana.