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.

Keywords: Devonian; Greenland; jaw; tetrapod; vertebrate.

Figure 1.

(a) Map of Greenland showing locality where NHMD 116368 was collected (red asterisk). Modified from [15]. (b) Photograph of specimen in internal and external view. (c–e) Images from scan dataset. (c) Series of transverse sections of the jaw, arranged from anterior (left) to posterior (right). In each section, the lateral surface of the jaw is to the right. (d) Longitudinal section through dentary, showing teeth. (e) Longitudinal section through entire jaw, showing articular and Meckelian fenestrae. (d) and (e) are shown to the same scale. Scale bars in (b) and (e), 10 mm.

Figure 2.

Digital reconstruction of the jaw, subdivided into separate bones, segmented in Mimics Research 19.0 and rendered in Blender 2.79b. (a) Lateral view. (b) Dorsal view. (c) Medial view. (d) Ventral view. (e) Dorsal view without prearticular, Meckelian bone and infradentaries. (f) Medial view without prearticular and Meckelian bone. Scale bar, 10 mm.

Figure 3.

Phylogenetic analysis. (a) Strict consensus of 72 trees of 271 steps, branch-and-bound analysis of 26-taxon matrix. Consistency Index 0.491, retention Index 0.700, rescaled consistency index 0.344, Homoplasy Index 0.509. (b) Strict consensus of 130 trees of 283 steps, heuristic analysis (see Material and methods for analytical parameters), 29-taxon matrix. Consistency index 0.470, retention index 0.688, rescaled consistency index 0.323, homoplasy index 0.530. (c) Bootstrap analysis of 26-taxon matrix, 1000 replicates (see Material and methods for analytical parameters). Numbers at nodes indicate bootstrap support values. In all trees, Devonian taxa are shown in blue, Carboniferous taxa in orange.