Baurusuchid crocodyliforms as theropod mimics: clues from the skull and appendicular morphology ofStratiotosuchus maxhechti(Upper Cretaceous of Brazil)

Abstract

The Baurusuchidae crocodyliforms are usually interpreted as active terrestrial predators, but only some positive evidence of such habits has been described to date, mainly the relative position of external nares and orbits. Here we describe features that support this view in a complete specimen of the Baurusuchidae Stratiotosuchus maxhechti, and have executed a parsimony analysis to confirm their phylogenetic position. S. maxhechti exhibits theropodomorph features that have been previously recognized in skulls of the Baurusuchidae, as well as postcranial characteristics related to a parasagittal gait, showing that the similarities between the Baurusuchidae and theropods extend beyond the cranial morphology. These include a well-developed supracetabular crest, a relatively medially offset femoral head and a caudally orientated calcaneal tuber. The orientations of the surfaces for muscular attachments imply that the appendicular movements of S. maxhechti were mainly anteroposterior, with abduction significantly constrained. S. maxhechti presents features that mimic some present in theropods, including a ‘fossa brevis’ on the ilium and tubercles on the ischium and femur similar to the obturator process and accessory trochanter. The relative proportions of the femur, tibia, and longer metatarsal are more similar to those of Postosuchus than to other Crocodylomorpha. In the skull, besides the theropodomorph (ziphodont) dentition concentrated in the anterior half of the rostrum, the baurusuchids are remarkable by the fusion of the nasals, which can be related to a large resistance against feeding forces acting on a high-profile skull. The appendicular morphology of S. maxhechti strengthens the interpretation that the Baurusuchidae were active land-dwelling predators in the Upper Cretaceous of south-eastern Brazil, occuping ecological niches typical of small to medium-sized theropod dinosaurs.