Skull and postcranium of the bystrowianid Bystrowiella schumanni from the Middle Triassic of Germany, and the position of chroniosuchians within Tetrapoda

Abstract

Chroniosuchians form a mainly terrestrial or semi-terrestrial clade of Permian and Triassic crocodile- or varanid-like tetrapods, usually considered stem amniotes, but with disputed affinities within that grade. Two groups can be distinguished, the chroniosuchids and bystrowianids. Whereas the chroniosuchid skull and postcranium are well known, our knowledge of bystrowianids is restricted mainly to isolated vertebrae and osteoderms. The Middle Triassic bystrowianid Bystrowiella schumanni from south-western Germany was identified as the first taxon of this clade outside Russia and China, based on the morphology of its vertebrae and osteoderms. Here we expand on a full description of cranial and postcranial remains from this taxon, based on a partially articulated specimen and further isolated material. The material comprises large parts of the dermal skull roof, the pectoral girdle, ribs and limbs, and sheds light on many anatomical regions formerly unknown in bystrowianids. Among the autapomorphic features of Bystrowiella are the premaxilla with an edentulous crest lateral to the choana, the premaxillary teeth with conspicuous size differences, the jugal with an extremely long, narrow anterior process, and the enlarged postparietals and tabulars forming facets for articulation with the anteriormost osteoderm. As in chroniosuchids, the pre- and postfrontal are not in contact, but unlike in chroniosuchids, the internarial fontanelle and the antorbital fenestra are absent. In the amniote-like postcranium, the interclavicle is slender and has a conspicuous parasternal process, the humerus is waisted and bears a short supinator process, and the long, curved trunk ribs have widely separated rib heads and a slender shaft without blades or processes. Phylogenetic analysis, particularly based on numerous new postcranial features, supports the monophyly of chroniosuchians, even when the constituent synapomorphies (osteoderm and vertebral characters) are excluded from the matrix. Chroniosuchians nest at the base of the amniote stem, forming an unresolved polytomy with Silvanerpeton, embolomeres and more advanced stem amniotes.