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

Abstract

A conglomerate bed from the Tournaisian Ballagan Formation of Scotland preserves a rich array of vertebrate and other non-marine fossils providing an insight into the wider ecosystem and palaeoenvironment that existed during this pivotal stage of Earth history. It challenges hypotheses of a long-lasting post-extinction trough following the end-Devonian extinction event. The fauna recovered includes a wide size range of tetrapods, rhizodonts and dipnoans, from tiny juveniles or small-bodied taxa up to large adults, and more than one taxon of each group is likely. Some fauna, such as actinopterygians and chondrichthyans, are rare as macrofauna but are better represented in the microfossil assemblage. The fauna provides evidence of the largest Carboniferous lungfish ever found.The specimens are preserved in a localised, poorly-sorted conglomerate which was deposited in the deepest part of a river channel, the youngest of a group of channels. In 24The fauna recovered includes a wide size range of tetrapods, rhizodonts and dipnoans, 25 from tiny juveniles or small-bodied taxa up to large adults, and more than one taxon of each 26 group is likely. Some fauna, such as actinopterygians and chondrichthyans, are rare as 27 macrofauna but are better represented in the microfossil assemblage. The fauna provides 28 evidence of the largest Carboniferous lungfish ever found. 29The specimens are preserved in a localised, poorly-sorted conglomerate which was 30 deposited in the deepest part of a river channel, the youngest of a group of channels. In addition 31 to the fossils (micro-and macro-), the conglomerate includes locally-derived clasts of palaeosols 32 and other distinctive elements of the surrounding floodplains. Charcoal fragments represent stem 33 and possible trunk tissue from arborescent pteridosperms. Preservation of the fossils indicates 34 some aerial exposure prior to transport, with abrasion from rolling. 35The findings presented here contrast with other published trends in vertebrate size that 36 are used to interpret a reduction in maximum sizes during the Tournaisian. The richness of the 37 fauna runs counter to the assumption of a depauperate non-marine fauna following the end-38 Devonian Hangenberg event, and charcoal content highlights the occurrence of fire, with the 39 requisite levels of atmospheric oxygen during that stage.40 41

The early diversification of ray-finned fishes (Actinopterygii): hypotheses, challenges and future prospects

“…This sharp rise is especially notable because the early Carboniferous (Tournaisian and early Visean) coincides with 'Romer's Gap', an apparent gap in the fossil record of tetrapods (and other animals) variably explained as either a period of poor sampling (Romer, 1956), low atmospheric oxygen (Ward et al, 2006) or recovery following the EDME (Sallan & Coates, 2010). Recent concerted efforts have begun to populate Romer's Gap, indicating that poor sampling accounted for most of the apparent paucity of the record (Clack et al, 2019;Otoo et al, 2019). The diversification of actinopterygians immediately following the EDME likely represents an adaptive radiation seeded by very few-or perhaps just one-actinopterygian lineages , although this hypothesis has not been explicitly tested.…”

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

“…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. In contrast, remains of very small individuals have been collected throughout the Ballagan Formation representing fish no more than 10 cm long.…”

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

“…data; Clack et al . ). There are also gyracanth and rhizodont fossils from outside the TIM that represent larger individuals (BKAO pers.…”

“…The only characteristic coal swamp taxon missing from the TIM are xenacanths, which first appear in the Visean with Diplodoselache at Wardie (this may be superseded by Tournaisian occurrences at Burnmouth; Clack et al . ) It would be interesting to know the extent to which the apparent transition of the more or less Devonian‐type floodplain fauna (as seen at Red Hill, East Greenland and, with minor changes, in the TIM at Burnmouth) to the coal swamp setting represents a physical move inland toward fresher water or if the swamps developed on top of the existing alluvial plains (the two scenarios are not mutually exclusive). With the addition of xenacanths, the key difference between the floodplain faunal association and the coal swamp association seems to be the increased abundance and diversity of tetrapods (and arthropods), at least in part driven by the increased resources and complexity of terrestrial habitats.…”