The end-Devonian to mid-Mississippian time interval has long been known for its depauperate palaeontological record, especially for tetrapods. This interval encapsulates the time of increasing terrestriality among tetrapods, but only two Tournaisian localities previously produced tetrapod fossils. Here we describe five new Tournaisian tetrapods (Perittodus apsconditus, Koilops herma, Ossirarus kierani, Diploradus austiumensis and Aytonerpeton microps) from two localities in their environmental context. A phylogenetic analysis retrieved three taxa as stem tetrapods, interspersed among Devonian and Carboniferous forms, and two as stem amphibians, suggesting a deep split among crown tetrapods. We also illustrate new tetrapod specimens from these and additional localities in the Scottish Borders region. The new taxa and specimens suggest that tetrapod diversification was well established by the Tournaisian. Sedimentary evidence indicates that the tetrapod fossils are usually associated with sandy siltstones overlying wetland palaeosols. Tetrapods were probably living on vegetated surfaces that were subsequently flooded. We show that atmospheric oxygen levels were stable across the Devonian/Carboniferous boundary, and did not inhibit the evolution of terrestriality. This wealth of tetrapods from Tournaisian localities highlights the potential for discoveries elsewhere. The term ‘Romer’s Gap’ was coined1,2 for a hiatus of approximately 25 million years (Myr) in the fossil record of tetrapods3, from the end-Devonian to the mid-Mississippian (Viséan). Following the end-Devonian, the earliest terrestrial tetrapod fauna was known from the early Brigantian (late Viséan) locality of East Kirkton near Bathgate, Scotland4,5. By that time, tetrapods were ecologically diverse, and were terrestrially capable. With five or fewer digits, some had gracile limbs6,7—unlike the polydactylous, predominantly aquatic, fish-like tetrapods of the Late Devonian8. Fossils representing transitional morphologies between these disparate forms were almost entirely lacking, limiting both understanding of the acquisition of terrestrial characteristics and the relationships between the diverse mid-Carboniferous taxa. Alternative hypotheses to explain the hiatus have included a low oxygen regime9 or lack of successful collecting in Tournaisian strata2. Although isolated tetrapod limb bones, girdle elements and trackways are known from the Tournaisian of the Horton Bluff Formation at Blue Beach, Nova Scotia10,11, only a small fraction has been fully described12. The only other Tournaisian tetrapod material was the articulated skeleton of Pederpes finneyae, from the Tournaisian Ballagan Formation near Dumbarton, western Scotland13,14. More recently, new taxa from this formation in the Scottish Borders region were reported2. Further collecting from five localities (Supplementary Fig. 1) has since produced more data about the fauna, its environment and climatic conditions. Our analysis shows that the Tournaisian included a rich and diverse assemblage of taxa, which included close relatives of some Devonian forms on the tetrapod stem and basal members of the amphibian stem. We diagnose, name and analyse five taxa (Figs 1,2,3,4,5), and summarize at least seven others that are distinct but undiagnosable at present (Fig. 6 and Supplementary Figs 2–6). Tetrapods occupied a mosaic of juxtaposed microhabitats including ponds, swamps, streams and floodplains, the last of these with highly variable salinity and water levels in a sharply contrasting seasonal climate. Their fossils are most closely associated with palaeosols and the overlying sandy siltstones. These indicate exposed and vegetated land surfaces that were then flooded15,16 (Supplementary Fig. 7). This varied environment persisted over the 12 Myr of the Tournaisian3. In contrast to a previous study9, we show that atmospheric oxygen levels were stable across the Devonian/Carboniferous boundary and did not therefore compromise terrestrial faunal life. Differential diagnoses below give the characters in which each taxa differs from all other tetrapods in its combination of autapomorphic and derived (relative to Devonian taxa) characters.
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National Museums Scotland
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Nature Ecology & Evolution
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