Meeting Abstract

P2.150  Thursday, Jan. 5  Natural Orientation of the Lateral Semicircular Canal in Alligator mississippiensis GEORGI, Justin A; Arizona College of Osteopathic Medicine, Midwestern University

Head orientation is useful for the study of extant vertebrates and the reconstruction of life history in fossils. The organs of rotational balance, the semicircular ducts, are fixed in bony canals of the skull and are integral to many systems, including stable vision. Previous researchers have postulated that the semicircular ducts are tightly linked to head orientation and that the lateral duct should approximate earth horizontal. Studies in many mammalian models support this conclusion, but data for non-mammals are restricted to only pigeon and slider turtle. This study investigates the lateral semicircular canal in Alligator missisippiensis. Using 3D motion capture, the orientation of the head during terrestrial locomotion was recorded for five specimens ranging in size from 0.89 to 1.39 m. Using CT imaging it is shown there is no ontogenetic variation in the orientation of the lateral canal relative to an external skull plane. Thus, the orientation of the lateral canals can be tracked relative to the motion capture landmarks. Four of the five subjects maintained a mean lateral canal orientation of 5.3° - 8.4° above the horizontal anteriorly. This is consistent with the values reported for the turtle (3° - 4°) and the pigeon (10°). One subject’s mean orientation was higher: 15.6° evidence points to intraspecific display and this is not considered normal position. These results continue to support the hypothesis of general correlation between lateral semicircular canal orientation and earth horizontal during typical behaviors. The addition of alligator data to the non-mammalian groups not only reinforces the general nature of this trend, but also provides stronger support for the numerous studies that seek to use this method to reconstruct head orientation in fossil archosaurs.