Does Inner and Middle Ear Morphology Link to an Aquatic Adaptation in Crocodylomorphs (Crocodylomorpha: Thalattosuchia)?

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

Praga

Congreso; 12th International Congress of Vertebrate Morphology; 2019

Bony labyrinth morphology and tympanic air spaces provide powerful proxies for ecological adaptations in vertebrates. The labyrinth is responsible for the sense of hearing, and balance and equilibrium. Semicircular canal morphology is influenced by locomotion and behav- ior, the cochlear duct however, by hearing sensitivity. Here, we investi- gate ancient crocodylomorphs that underwent a major evolutionary transition from semi-aquatic to marine environments. Thalattosuchia thrived from the Early Jurassic to the Early Cretaceous. They originated in shallow marine environments, but one subclade, Metriorhynchidae, evolved a streamlined body plan with paddle shaped limbs and a verti- cally orientated tail fluke. Besides obvious osteological changes, under- standing shifts in cephalic sensory organs will be key to understanding how these ancient animals adapted to a radically new ecosystem. Using μCT-scanning we digitally segmented the bony labyrinths of various extinct and extant crocodylomorphs. This allows us an insight into how their morphology changed and adapted during the land-water transi- tion. In protosuchians and extant crocodylians, the middle ears are acoustically coupled by two sinus pathways, dorsal and ventral to the brain endocast. Thalattosuchians, however, lack the dorsal pathway. The ventral pathway in basal thalattosuchians is formed by either: the basioccipital diverticulum (BaD) and median pharyngeal sinus (MPs), or by the recessus epitubaricum and the MPs (or both in some taxa). In Metriorhynchidae the BaD is lost, indicating lower sensitivity to low- frequency sounds. Within Metriorhynchidae the anterior and posterior semicircular canals are dorsoventrally shortened. Similar adaptations occur in other marine reptiles. Furthermore, the cochlear duct shortens in metriorhynchids, possibly related to a reduction in hearing sensitivity. Our results indicate that metriorhynchid ear morphology can be corre- lated with an adaptation to marine ecosystems.