Skull osteology of the holotype of the rhynchosaur Hyperodapedon sanjuanensis (Sill, 1970) from the Upper Triassic Ischigualasto Formation of Argentina.

Rhynchosaurs are bulky quadrupedal herbivores that achieved a cosmopolitan distribution during the Middle and Late Triassic. Rhynchosaurids are characterized by a pair of premaxillae modified into an edentulous beak that had a bone-to-bone occlusion with the tips of the dentaries, and a specialized masticatory apparatus composed of groove(s) on the maxilla and ridges(s) on the dentary. The Argentinian fossil record of rhynchosaurs is abundant, but only two nominal species have been named so far. One of them, the hyperodapedontine Hyperodapedon sanjuanensis, is the most abundant tetrapod of the Ischigualasto Formation of northwestern Argentina. However, the anatomy of H. sanjuanensis remains poorly studied. The repreparation and computed tomographic (CT) scanning of its holotypic skull (MACN-Pv 18185) allowed a detailed description of its anatomy. Among the novel anatomical data presented here, we provide the first tridimensional complete reconstruction of a rhynchosaurid palate. This structure is extremely complex in comparison to the plesiomorphic archosauromorph condition, and more data on other rhynchosaurid palates are needed to shed light on the sequence of character states acquisition that resulted in this peculiar anatomy. The presence of a pair of septomaxillae in H. sanjuanensis is the first report of these bones in a rhynchosaurid, and this new information allowed to identify septomaxillae also in the hyperodapedontines Teyumbaita sulcognathus and Hyperodapedon mariensis. The better understanding of the skull anatomy of the holotype of H. sanjuanensis is a first step towards an improved knowledge of the morphology and taxonomy of the South American rhynchosaurs.

Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria.

Pterosaurs were the first vertebrates to evolve powered flight1 and comprised one of the main evolutionary radiations in terrestrial ecosystems of the Mesozoic era (approximately 252-66 million years ago), but their origin has remained an unresolved enigma in palaeontology since the nineteenth century2-4. These flying reptiles have been hypothesized to be the close relatives of a wide variety of reptilian clades, including dinosaur relatives2-8, and there is still a major morphological gap between those forms and the oldest, unambiguous pterosaurs from the Upper Triassic series. Here, using recent discoveries of well-preserved cranial remains, microcomputed tomography scans of fragile skull bones (jaws, skull roofs and braincases) and reliably associated postcrania, we demonstrate that lagerpetids-a group of cursorial, non-volant dinosaur precursors-are the sister group of pterosaurs, sharing numerous synapomorphies across the entire skeleton. This finding substantially shortens the temporal and morphological gap between the oldest pterosaurs and their closest relatives and simultaneously strengthens the evidence that pterosaurs belong to the avian line of archosaurs. Neuroanatomical features related to the enhanced sensory abilities of pterosaurs9 are already present in lagerpetids, which indicates that these features evolved before flight. Our evidence illuminates the first steps of the assembly of the pterosaur body plan, whose conquest of aerial space represents a remarkable morphofunctional innovation in vertebrate evolution.