Abstract
AbstractThe end‐Devonian mass extinction has been framed as a turning point in vertebrate evolution, enabling the radiation of tetrapods, chondrichthyans and actinopterygians. 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 unexpected tetrapod and lungfish diversity. 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 also represented. Gyracanths, holocephalans, and actinopterygian fishes are represented by rarer fossils. This material compares well with vertebrate fossils from other Ballagan deposits. Faunal similarity analysis using an updated dataset of Devonian–Carboniferous (Givetian–Serpukhovian) sites corroborates a persistent Devonian/Carboniferous split. Separation of the data into marine and non‐marine partitions indicates more Devonian–Carboniferous faunal continuity in non‐marine 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.
Highlights
The end-Devonian mass extinction has been framed as a turning point in vertebrate evolution, enabling the radiation of tetrapods, chondrichthyans and actinopterygians
ROMER’s Gap (Romer 1956; Coates & Clack 1995) is a hiatus in the non-marine fossil record that currently spans the Tournaisian stage of the Carboniferous (~359– 345 Ma)
Named for the paucity of tetrapod fossils throughout this interval, the gap has long obscured cladogenic events and ecological transitions implied by differences between the few, mostly aquatic, limbed forms present at the end of the Devonian and the diverse and disparate tetrapods known from the Visean (Wellstead 1982; Clarkson et al 1994; Pardo et al 2017)
Summary
The end-Devonian mass extinction has been framed as a turning point in vertebrate evolution, enabling the radiation of tetrapods, chondrichthyans and actinopterygians. Few Tournaisian fish localities have been known until recently (Clarkson 1985; Long 1989; Sallan & Coates 2010; Mansky & Lucas 2013; Sallan & Galimberti 2015; Mickle 2017, Richards et al 2018), and these, too, appear impoverished relative to subsequent Visean faunas Tournaisian tetrapod discoveries include a mix of morphologies (Anderson et al 2015) that are distributed across multiple nodes in recent phylogenetic analyses (Clack et al 2016) and a recentlyrecognized Tournaisian radiation in lungfish (Smithson et al 2015) contradicts a historical narrative of postDevonian decline and stasis
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