A fish and tetrapod fauna from Romer's Gap preserved in Scottish Tournaisian floodplain deposits

Abstract

The 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.

A lungfish survivor of the end-Devonian extinction and an Early Carboniferous dipnoan radiation

“…It is the largest of all the lungfish taxa collected from the horizons in the lower half of the Tournaisian. This includes specimens from Bute (Carpenter et al 2014), Willie's Hole and most of the other material from Burnmouth (Smithson et al 2012, 2016Clack et al 2016, Otoo et al 2018. Evidence of much larger taxa has been found in the highest tetrapod-bearing horizon at Burnmouth (Clack et al 2016, Clack et al 2019 with individual bones representing lungfish up to 3 m long.…”

“…Naiadites) and rare eurypterids, shrimps, Spirorbis sp. Serpula sp., orthocones and scolecodonts , Otoo et al 2018. No direct evidence has been found indicating the diet of different Palaeozoic lungfish though they are generally considered to have been durophagous (Clement et al 2016).…”

“…The innovations in the pectoral girdle outlined by represented a change to a more varied omnivorous feeding strategy. The prevalence of plant material in the Ballagan Formation coupled with the abundance of small actinopterygian fish (Otoo et al 2018) and the diversity and innovations in lungfish tooth plate morpholgies encountered in the Tournaisian (Smithson et al 2015) may indicate a more varied diet in some of the lungfish at this time compared to lungfish from the Devonian. The diversity of new ecological niches in the Tournaisian Ballagan Formation represents a step change from the relatively uniform environments of the marine realm or large lacustrine habitats (e.g.…”

A Tournaisian (earliest Carboniferous) conglomerate-preserved non-marine faunal assemblage and its environmental and sedimentological context

“…One particularly rich bed and its context is described by B.K.A. Otoo and members of the consortium team (Otoo, 2015; Otoo et al, 2018).…”

A lower Carboniferous (Visean) tetrapod trackway represents the earliest record of an edopoid amphibian from the UK

“…The Carboniferous Period was a key interval in the diversification of land vertebrates (tetrapods). Following the appearance of the first tetrapods in the Late Devonian (Clack 2012), the group underwent a substantial diversification in the early part of the Carboniferous and was well established across Euramerica by the Tournaisian-Visean on the basis of body fossil remains of several tetrapods from Nova Scotia (Clack & Carroll 2000;Anderson et al 2015) and Scotland (Smithson et al 2012(Smithson et al , 2017Clack et al 2016;Pardo et al 2017;Otoo et al 2018). Tetrapod trackways have been known for over a century from the Horton Bluff Formation (Tournaisian) of Nova Scotia.…”