A taxonomic revision and cranial description of Terrestrisuchus gracilis (Archosauria, Crocodylomorpha) from the Late Triassic of Pant-y-ffynnon Quarry (southern Wales)

Non-crocodyliform crocodylomorphs, formerly referred to the informal group “Sphenosuchia”, are the earliest known crocodylomorph precursors of extant crocodylians. Therefore, they are crucial for our understanding of early crocodylian evolution and the origin of typical crocodylian characteristics, such as the formation of a secondary palate, complex cranial pneumaticity, and a reinforced braincase. Terrestrisuchus gracilis, known from the Late Triassic fissure fill deposits of Pant-y-ffynnon in southern Wales, is represented by almost 200 specimens, comprising articulated, partially associated, and isolated remains. In this contribution, we provide a taxonomic revision of Terrestrisuchus gracilis and redescribe its cranial anatomy in detail, based in part on novel μCT data. The posterior skull region is extensively pneumatised as indicated, among other features, by large pre- and postcarotid recesses on the parabasisphenoid and a large pneumatic cavity within the articular of the mandible. In contrast, the quadrate only forms a small, unfused contact with the prootic, suggesting that complex pneumatization of the postorbital region predated the coossification of the quadrate and braincase in Crocodylomorpha. Terrestrisuchus gracilis preserves an ossified basihyal, which represents the first occurrence of this bone among non-avemetarsalian archosaurs. Finally, we show that Terrestrisuchus gracilis was likely cathemeral (i.e. active in a range of light levels), based on a phylogenetic flexible discriminant analysis (pFDA) of the relative dimensions of the sclerotic ring and orbit.