The evolution of thalattosuchian crocodylomorphs, neuroanatomical insights into the land-to-sea transition

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

Congreso; 5th International Palaeontological Congres; 2018

Thalattosuchians were a widely distributed group of marine crocodyliforms that thrived during the Early Jurassic?Early Cretaceous. This radiation evolved a wide diversity of body plans and ecomorphotypes ranging from gharial-like species to those superficially similar to living killer whales. While their osteological adaptations to a marine lifestyle are well documented, the full extent of their soft-tissue and neuroanatomical adaptations are still poorly understood. It is also unclear whether these soft-tissue adaptations evolved first, and therefore underpinned the thalattosuchian radiation, or evolved in concert with the osteology. Based on digitally segmented CT scans of thalattosuchian skulls, we begin to rectify this knowledge deficit. Due to the regression and loss of bone-enclosed craniomandibular sinuses in cetaceans and pursuit-diving birds, we hypothesise that thalattosuchians underwent a similar trajectory. Our early findings show exactly that, with the pharyngotympanic sinus system already reduced and simplified in the basalmost thalattosuchians on either side of the Metriorhynchoidea Teleosauroidea split. The intertympanic diverticula were absent, with the dural venous system hypertrophied dorsal to the brain cavity. The long-noted absence of aërum foramina in Thalattosuchia is here confirmed to be due to the quadrate diverticula regression, and the resultant lack of siphonia + articular diverticula. The infundibular diverticula still fill the dorsal quadrate, but also as long noted in Thalattosuchia, there are no subtympanic foramina. We can confirm the presence of basioccipital diverticula, and the recessus epitubaricum + pterygoid diverticula in thalattosuchians. Both the quadrate and pterygoid diverticula are vestigial. Overall, the various pharyngotympanic diverticula are not well differentiated from one another, unlike in extant crocodylians. Moreover, the pharyngotympanic system in thalattosuchians does not dorsally enclose the bony labyrinths and brain cavity (as they do in extant crocodylians), with the semi-circular canals largely visible in lateral view when the neuroanatomy is digitally segmented out.Thus at the base of Thalattosuchia, there was already a generalised simplification of the pharyngotympanic sinus system, a loss of diverticula and regression of others. This fits with osteological data, that show thalattosuchians had already become specialised for an aquatic lifestyle prior to the teleosauroid-metriorhynchoid split.