Semicircular canal morphology as evidence of locomotor environment in amniotes

The vestibular system is a critical component of the neural control of locomotion in vertebrates. In the vestibule, macular endorgans transduce linear movements of the head and the semicircular ducts transduce rotational movements. Integrated in the cerebellum with visual and proprioceptive inputs, the vestibular signals provide vital information about movement relative to the environment, and drive stabilization reflexes.;The semicircular ducts leave distinct canals through the bones of the posterior braincase. These bony semicircular canals preserve some of the morphologies that determine the functional parameters of the semicircular ducts: e.g., response time, signal gain and frequency range. Thus, the semicircular canals represent the function of a neurological system via discrete bony correlates. Therefore, because the semicircular ducts have morphologies that determine the functional response of the system, and because some of these morphologies can be determined by examination of the semicircular canals, it has been previously hypothesized that there are correlations between semicircular canal morphology and locomotion. Semicircular canals represent a possible way, independent of post-cranial morphology, to verify hypotheses about locomotion in extinct vertebrates.;To test some of the underlying assumptions of this hypothesis of semicircular canal adaptation, the semicircular canals of a broad array of amniotes were examined using Computed Tomography (CT). Shape analysis of the semicircular canals in carnivoran mammals, turtles, varanids and crocodilians shows that despite phylogenetic shape differences, there is a consistent pattern of shape change that correlates with the terrestrial-to-aquatic locomotor transition. This pattern is most observable in the anterior semicircular canal where the height of the common crus and the height of the peak of the canal adjacent to the common crus are reduced in aquatic taxa. It is further demonstrated that this change in anterior semicircular canal shape is strongly tied to factors of limb morphology that correlate with locomotion and not with other factors of skull morphology, thus supporting the hypothesis that this is adaptive change in the system and not coincidental change. On the basis of this highly correlated, adaptive change, the results of this study then are shown to be applicable to the prediction of locomotor environment in extinct organisms via examination of the semicircular canals preserved in fossil crocodilians.