A partial lower jaw of a tetrapod from “Romer's Gap”

Abstract

The first half of the Mississippian or Early Carboniferous (Tournaisian to mid- Viséan), an interval of about 20 million years, has become known as “Romer's Gap” because of its poor tetrapod record. Recent discoveries emphasise the differences between pre-“Gap” Devonian tetrapods, unambiguous stem-group members retaining numerous “fish” characters indicative of an at least partially aquatic lifestyle, and post-“Gap” Carboniferous tetrapods, which are far more diverse and include fully terrestrial representatives of the main crown-group lineages. It seems that “Romer's Gap” coincided with the cladogenetic events leading to the origin of the tetrapod crown group. Here, we describe a partial right lower jaw ramus of a tetrapod from the late Tournaisian or early Viséan of Scotland. The large and robust jaw displays a distinctive character combination, including a significant mesial lamina of the strongly sculptured angular, an open sulcus for the mandibular lateral line, a non-ossified narrow Meckelian exposure, a well-defined dorsal longitudinal denticle ridge on the prearticular, and a mesially open adductor fossa. A phylogenetic analysis places this specimen in a trichotomy with Crassigyrinus and baphetids + higher tetrapods in the upper part of the tetrapod stem group, above Whatcheeria, Pederpes, Ossinodus, Sigournea and Greererpeton. It represents a small but significant step in the gradual closure of “Romer's Gap”.

The smallest known Devonian tetrapod shows unexpectedly derived features

“…The overall effect of the last two features is that the opening of the adductor fossa faces somewhat mesially in Brittagnathus, but purely dorsally in Acanthostega. In this regard, Brittagnathus differs from all previously known Devonian tetrapods but resembles a range of early Carboniferous forms [12,19]. This transformation may relate to the beginning of the differentiation of the jaw adductor musculature towards the crown tetrapod condition [12].…”

“…The matrix contains 109 characters scored for 29 taxa (the new taxon plus the taxa used by [19]). Aytonerpeton, Diploradus and Perittodus, which were not present in the matrix of [19], were coded from [18]. The recently described Parmastega was coded from [20].…”

“…The one respect in which Brittagnathus differs strikingly from the majority of Carboniferous and later tetrapods, and instead aligns with Devonian taxa, is the absence of a mesial lamina of the angular. Such a lamina, which wraps around the ventral jaw margin and sutures with the prearticular on the mesial face of the jaw ramus, is present--where this region is known--in almost all post-Devonian tetrapods except Whatcheeria and possibly Diploradus [12,18,19,22,25].…”

What made Stan Wood a great collector?

“…The original Tantallon material had been collected in the 1970s, and included a partial tetrapod jaw (Chen et al 2018 [this volume]), but little was known about the site and there was no record of where the specimens had been collected. Armed with a sample of the very distinctive red conglomerate, he went in search.…”

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

“…Subsequently, from 2006, more regular collecting was undertaken with the late Stan Wood, and in 2012, a NERC-funded consortium project focusing on the Tournaisian Ballagan Formation of Scotland, known as the TW:eed Project (Tetrapod World: early evolution and diversification), began an intensive exploration of Burnmouth and other Tournaisian localities. Several papers relating to the Burnmouth sediments and associated Tournaisian and other Early Carboniferous sites have already resulted from this project (Bennett et al, 2016; Clack et al, 2016; Coates & Teitjen 2018; Kearsey et al, 2016; Smithson, Richards & Clack, 2015; Smithson et al, 2017; Chen et al, 2018; Clack, Porro & Bennett, 2018; Clark, Miller & Ross, 2018; Millward et al, 2018a, 2018b; Richards et al, 2018; Ross et al, 2018; Smithson & Clack, 2018; Marshall et al, in press).…”