On Ossirarus kierani, a stem tetrapod from the Tournaisian of Burnmouth, Berwickshire, Scotland, and the phylogeny of early tetrapods

Abstract

Recent discoveries in the Scottish Borders have greatly expanded our knowledge of post-Devonian tetrapods. Six new taxa have been named and briefly described so far. One of these, Ossirarus kierani, is represented by a single specimen from the coastal section of the Tournaisian Ballagan Formation at Burnmouth. It comprises the disarticulated bones of the posterior half of the skull, the anterior portion of the axial skeleton, and parts of the pectoral girdle and forelimbs. It is relatively small, with an estimated skull length of 54 mm. Like some Devonian tetrapods it has a preopercular and a lateral line system represented by pores. It shares with embolomeres, a tabular-parietal suture, an intertemporal and a long tabular horn. The gastrocentrous vertebrae resemble those of Caerorhachis and the brachial foramen pierces the humerus through the posterior edge, as in Mesanerpeton. Phylogenetic analyses place Ossirarus on the tetrapod stem, crownward of some – but not all – Devonian taxa. The topology of the tetrapod stem suggests that numerous lineages of Carboniferous tetrapods extended back into the Devonian.