The smallest known Devonian tetrapod shows unexpectedly derived features

Abstract

A new genus and species of Devonian tetrapod, Brittagnathus minutus gen. et sp. nov., is described from a single complete right lower jaw ramus recovered from the Acanthostega mass-death deposit in the upper part of the Britta Dal Formation (upper Famennian) of Stensiö Bjerg, Gauss Peninsula, East Greenland. Visualization by propagation phase contrast synchrotron microtomography allows a complete digital dissection of the specimen. With a total jaw ramus length of 44.8 mm, Brittagnathus is by far the smallest Devonian tetrapod described to date. It differs from all previously known Devonian tetrapods in having only a fang pair without a tooth row on the anterior coronoid and a large posterior process on the posterior coronoid. The presence of an incipient surangular crest and a concave prearticular margin to the adductor fossa together cause the fossa to face somewhat mesially, reminiscent of the condition in Carboniferous tetrapods. A phylogenetic analysis places Brittagnathus crownward to other Devonian tetrapods, adjacent to the Tournaisian genus Pederpes . Together with other recent discoveries, it suggests that diversification of ‘Carboniferous-grade’ tetrapods had already begun before the end of the Devonian and that the group was not greatly affected by the end-Devonian mass extinction.

High lineage survivorship across the end-Devonian Mass Extinction suggested by a remarkable new Late Devonian actinopterygian

“…This matches revised models of diversification for other vertebrate groups relative to mass extinction boundaries including the celebrated case of placental mammals, where a growing consensus between molecular and fossil datasets places many intra-ordinal divergences in the Late Cretaceous with the emergence of ecologically distinct radiations in the Cenozoic (50,51). The pattern of phylogenetic divergences that we recover for actinopterygians also appears to match evolutionary patterns in contemporary tetrapods and lungfishes, with numerous lineages diverging prior to and extending across the Devonian-Carboniferous boundary (30)(31)(32)(33)(34)(35).…”

“…This taxon prompts a reconsideration of trait evolution and tree topology that together provide evidence that numerous lineages of ray-finned fishes radiated before and later survived the end-Devonian mass extinction. This revised perspective on ray-finned fish evolution complements emerging pictures of cryptic diversification before, and potentially high survivorship during, ecological crises at the end of the Devonian (30)(31)(32)(33)(34)(35).…”

The Making of Calibration Sausage Exemplified by Recalibrating the Transcriptomic Timetree of Jawed Vertebrates

“…However, as stressed by Pardo et al (2020) , they represent a very small region of the Carboniferous globe, so I continue ( Marjanović and Laurin, 2019 ) to caution against this regardless of the phylogenetic issues. Rather, the richer and better studied Famennian (end-Devonian) record, which has not so far yielded close relatives of Tetrapoda but has yielded more rootward stegocephalians and other tetrapodomorphs ( Marjanović and Laurin, 2019 ; Ahlberg and Clack, 2020 ; and references therein), should be used to place a soft maximum age around very roughly 365 Ma.…”

Jennifer A. Clack. 3 November 1947—26 March 2020

“…Mindful of the limited available time and driven to complete as much of her research as possible, Jenny continued working energetically until her final days. Several papers were in press or nearing completion at the time of her death (26). As a final honour, in 2020, she was posthumously awarded the Romer-Simpson Medal -the lifetime achievement award of the Society of Vertebrate Paleontology.…”

“…Much later, during the first two decades of the 21st century, the development of powerful techniques for x-ray tomography – initially conventional CT (Computed Tomography) (13) and later synchrotron microtomography – would open up startling new possibilities for studying the Greenland tetrapod material. Tomographic investigation of limb bones from the Acanthostega locality would suggest that the animals are juveniles (22), while an isolated tiny lower jaw from the same site, which was noticed already in 1987 but could not be prepared, would prove to represent a new genus Brittagnathus (26). However, at the end of the 1980s all this lay in the future, as a tangle of semi-complete Acanthostega skeletons slowly began to emerge from the encasing rock (figure 2).…”