EVOLUTIONARY TRANSITION - NEUROANATOMICAL ADAPTATIONS IN THALATTOSUCHIAN CROCODYLOMORPHS

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

Congreso; 78th ANNUAL MEETING OF THE SOCIETY OF VERTEBRATE PALEONTOLOGY; 2018

Thalattosuchia was a group of globally distributed crocodylomorphs that lived from the Early Jurassic to the Early Cretaceous. They are an excellent example of a major evolutionary transition, in this case secondary adaptation to a pelagic marine environment. Thalattosuchians evolved from being semi-aquatic and superficially gharial-like (e.g., teleosauroids and basal metriorhynchoids) to being pelagic and cetacean-like (i.e., metriorhynchids). Although changes in their osteology during this transition are well known, how their sensory systems changed during their adaptation to living in the marine realm is not well understood. However, insight into sensory evolution is essential for understanding how extinct animals could have interacted with their environment, and in this case, how they adapted as they moved into the open sea. We digitally segmented the endocranial structures (such as the brain cavity, bony labyrinth, and sinuses) from CT datasets of several thalattosuchians, enabling us to draw inferences about their behaviour and sensory capability. We hypothesized that thalattosuchians underwent a similar land-to-sea neuroanatomical transition as cetaceans, with regression of bone-enclosed sinuses and shifts in bony labyrinth shape. Our results thus far show that in basal thalattosuchians (e.g., Pelagosaurus typus, Steneosaurus cf. S. gracilirostris), the pharyngotympanic sinus system reduced and is poorly differentiated. The bony labyrinth and brain cavity are not dorsally enclosed by the pharyngotympanic sinus system (unlike in protosuchians and most other crocodyliforms, including extant crocodylians). In the pelagic metriorhynchids (e.g., Cricosaurus araucanensis), there is an even greater reduction in the pharyngotympanic sinus system compared to extant species, and the three semicircular canals and the overall bony labyrinth shape are considerably reduced. The extreme changes in the bony labyrinth are convergent with other secondarily aquatic vertebrates (such as cetaceans and various clades of pelagic marine reptiles), and is likely linked to reduced neck mobility?thereby potentially allowing the animal to better handle fast body rotations. Our results hint that thalattosuchians quickly became specialised to a marine lifestyle prior to the teleosauroid-metriorhynchid split. It would appear that thalattosuchians quickly adapted to living in a pelagic marine environment, and that their neuroanatomical adaptations might underpin their extraordinary success.