VESTIBULAR MORPHOLOGY REVEALS ECOMORPHOLOGICAL TRENDS AND ONTOGENETIC CHANGES IN CROCODYLOMORPHS

SCHWAB J.; YOUNG M.; NEENAN J.; WALSH S.; HERRERA Y.; WITMER L.; BROCHU, C.A.; CHOINIERE J.; TURNER, A.H.; BRUSATTE S.

evento virtual

Congreso; The Society of Vertebrate Paleontology 80th Annual Meeting · Virtual 2020; 2020

The Society of Vertebrate Paleontology

One prime example of a major evolutionary transition is thalattosuchian crocodylomorphs, which evolved from terrestrial ancestors and moved into the open ocean. Their transition began in the Early Jurassic in semiaquatic environments, before they became the only group of archosaurs that fully adapted to a pelagic lifestyle, with paddle shaped limbs, a hypocercal tail and a streamlined body. Sensory systems, such as the inner ear, are key to understanding such transitions, and can reveal how ecology changed over long time scales. The bony labyrinth, the cavity housing the membranous system of the inner ear, has previously been linked to ecomorphological adaptations, such as locomotor behavior and habitat preferences, and can also be used to study developmental changes. Due to its involvement in orientation this system needs to change and adapt when animals transition into new environments. Using CT scans and geometric morphometrics we examined the bony labyrinths of a broad sample of taxa, including 14 extant and 18 fossil crocodylomorphs (adults and subadults) of different habitats, and an additional six hatchlings and seven juveniles of extant species to study ontogenetic differences in the labyrinth morphology. Our results show that there are both ecomorphological and ontogenetic differences. Pelagic metriorhynchids developed a dorsoventrally short labyrinth with thicker semicircular canals compared to their terrestrial ancestors, with the most extreme pelagic morphology present in the derived metriorhynchids Cricosaurus and Torvoneustes. Similar adaptations have previously been noted in other secondarily aquatic vertebrates such as pliosaurs, turtles and cetaceans. In thalattosuchians this pelagic labyrinth evolved after major osteological changes that permitted swimming (e.g., flippers, loss of osteoderms, tail fluke). Modern hatchling and juvenile crocodylians, compared to adults have a more compact labyrinth with smaller semicircular canals and a particularly reduced lateral canal. This is somewhat similar to the pelagic metriorhynchid labyrinth morphology, indicating possible heterochrony. Changes in developmental timing therefore, may have helped thalattosuchians adapt their sensory systems to the water