BACK TO THE SEA - ADAPTATIONS OF THE TYMPANIC SYSTEM IN THALATTOSUCHIAN CROCODYLOMORPHS

SCHWAB J.; YOUNG M.; NEENAN J.; WALSH S.; WITMER L.; HERRERA Y.; DOLLMAN K.; BRUSATTE S.

Brisbane

Congreso; SVP 2019 ANNUAL MEETING; 2019

Cranial sensory organs are a powerful ecological proxy, and give unique insights into ancient animalbehaviour and lifestyles. One of those sensory organs is the otic region, comprising the innerand middle ear, as well as the tympanic air space. The inner ear includes the membranous and thebony labyrinth, and besides its involvement in hearing, it is also involving in for equilibrium andhead stabilisation. Such sensory organs also played a key role in major evolutionary and environmentaltransitions within vertebrates. We here study one group that underwent a major transition:thalattosuchian crocodylomorphs, that underwent a major transition, evolving from terrestrial ancestors into pelagic marine species during the Jurassic. Their osteological changes are well known, such as their development of paddle shaped limbs, loss of osteoderms, and a vertically orientated tail fluke. However, little isknown about how their neurosensory systems changed during that transition.Using µCT scans we digitally segmented the endosseous labyrinth of a broad sample of taxa including eleven extant species, six thalattosuchians, Eopneumatosuchus colberti, and two non-crocodyliform crocodylomorphs.Basal thalattosuchians, such as the basal metriorhynchoid Pelagosaurus typus and teleosaurids, have?M?-shaped semi-circular canals and reduced canal curvature. The ?M?-shaped canals hint that thalattosuchians retained the basal crocodylomorph condition, as this shape is present in E. colberti and non-crocodyliform crocodylomorphs, but absent in extant crocodylians or derived metriorhynchids. The metriorhynchids ?Metriorhynchus? brachyrhynchus, Cricosaurus schroederi, and Cricosaurus araucanensis however, have dorsoventrally shortened anterior and posterior semicircular canals as well as increased canal diameters. Furthermore, they have a shortened cochlear duct possibly related to reduced hearing sensitivity or airborne sound. This difference may be explained by the different lifestyles of these animals: semi-aquaticin basal thalattosuchians, and pelagic marine in derived metriorhynchids. This parallels the trends seen in marine turtles and sauropterygians, suggesting a common pathway for pelagic reptiles to adapt to marine life and a strong correlation between aquatic lifestyle and semicircular canal morphology.