Thalattosuchian crocodylomorphs - A major evolutionary transition from land to water.

SCHWAB JULIA; YOUNG MARK; WALSH STIG; WITMER LAWRENCE; HERRERA YANINA; BRUSATTE STEPHEN

Caparica

Congreso; XVI Annual Meeting of the European Association of Vertebrate Palaeontology; 2018

Thalattosuchians are extinct clade of crocodylomorphs that underwent a major evolutionary transition from terrestrial to aquatic environments during the Jurassic. Not only did they achieve a wide geographical distribution, they evolved from gharial-like, semi-aquatic forms into pelagic cetacean-like species (the Metriorhynchidae). While the osteological underpinning of the metriorhynchids? success is well understood (evolving a hypocercal tail, hydrofoil-like forelimbs, loss of osteoderm cover), their neuroanatomical and physiological adaptations to an aquatic environment are still poorly understood and about to be investigated (with only the evolution of enlarged cranial exocrine glands known). Non-invasive μCT scans of three basal thalattosuchians (the gharial-like species Pelagosaurus typus, Steneosaurus cf. gracilirostris and Steneosaurus bollensis) were digitally segmented to reveal the internal anatomy. These were then compared with the extant gharial, a habitual aquatic living species. Early findings reveal basal thalattosuchians are unique in having a reduced and poorly differentiated pharyngotympanic sinus system, hypertrophy of the transverse dural venous sinuses, and hypertrophied orbital and internal carotid arteries. Moreover, basal Thalattosuchia have bony labyrinths in which the anterior and posterior semicircular canals form an ?M? shape in lateral view and generally show a reduced curvature. The extant gharials possess a curved canal shape. This suggest an early specialisation for an aquatic lifestyle in thalattosuchian, occurring prior to the teleosauroid-metriorhynchoid split.