4 research outputs found
Phylogenetic and environmental context of a Tournaisian tetrapod fauna
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 (, , , and ) 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.NERC consortium grants NE/J022713/1 (Cambridge), NE/J020729/1 (Leicester), NE/J021067/1 (BGS), NE/J020621/1 (NMS) and NE/J021091/1 (Southampton
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A fish and tetrapod fauna from Romer's Gap preserved in Scottish Tournaisian floodplain deposits
The end-Devonian mass extinction has been framed as a turning point
in vertebrate evolution, enabling the radiation of tetrapods, chondrichthyans, and
actinopterygians in the Carboniferous and beyond. Until very recently ‘Romer’s
Gap’ rendered the Early Carboniferous a black box standing between the
Devonian and the later Carboniferous, but now new Tournaisian localities are
filling this interval. Recent work has recovered tetrapod and lungfish diversity in
contrast with previous expectations. However, the composition of Tournaisian
faunas remains poorly understood.
Here we report on a Tournaisian vertebrate fauna from a well-characterized,
narrow stratigraphic interval from the Ballagan Formation exposed at Burnmouth,
Scotland. Microfossils suggest brackish conditions and the sedimentology
indicates a low-energy debris flow on a vegetated floodplain. A range of
vertebrate bone sizes are preserved. Rhizodonts are represented by the most
material, which can be assigned to two taxa. Lungfish are represented by several
species, almost all of which are currently endemic to the Ballagan Formation.
There are two named tetrapods, Aytonerpeton and Diploradus, with at least two
others represented by additional specimens. Gyracanths, holocephalans, and
actinopterygian fishes are represented by rarer fossils. This material compares
well with vertebrate fossils from other Ballagan deposits elsewhere.
The faunal provides an opportunity to revisit previous work on the end-Devonian
extinction. Faunal similarity analysis using an updated dataset of DevonianCarboniferous
(Givetian-Serpukhovian) sites corroborates a persistent
Devonian/Carboniferous split. Separation of the data into marine and nonmarine
partitions indicates more Devonian-Carboniferous faunal continuity in nonmarine
settings compared to marine settings. These results agree with the latest fossil
discoveries, and suggest that the Devonian-Carboniferous transition proceeded
differently in different environments and among different taxonomic groups