Abstract
Previous investigations have correlated vestibular function to locomotion in vertebrates by scaling semicircular duct radius of curvature to body mass. However, this method fails to discriminate bipedal from quadrupedal non-avian dinosaurs. Because they exhibit a broad range of relative head sizes, we use dinosaurs to test the hypothesis that semicircular ducts scale more closely with head size. Comparing the area enclosed by each semicircular canal to estimated body mass and to two different measures of head size, skull length and estimated head mass, reveals significant patterns that corroborate a connection between physical parameters of the head and semicircular canal morphology. Head mass more strongly correlates with anterior semicircular canal size than does body mass and statistically separates bipedal from quadrupedal taxa, with bipeds exhibiting relatively larger canals. This morphologic dichotomy likely reflects adaptations of the vestibular system to stability demands associated with terrestrial locomotion on two, versus four, feet. This new method has implications for reinterpreting previous studies and informing future studies on the connection between locomotion type and vestibular function.
Highlights
Three semicircular canals are present in the neurocranium of all vertebrates
This study suggests that use of body mass as a comparison variable for semicircular canal size does not adequately reflect the functional adaptations of the semicircular canal system
Parameters of head size, are more theoretically justified choices for comparison variables and are shown to correlate more highly with semicircular canal area than body mass, and in several instances are shown to recover strong and statistically significant correlations where none exist with body mass
Summary
Three semicircular canals are present in the neurocranium of all vertebrates. The fluid-filled ducts within these canals sense angular accelerations of the head and operate in conjunction with reflex arcs to cervical and extraoccular muscles to stabilize the head and maintain visual fixation. Hypotheses linking the morphology of the semicircular canals to locomotor type go back to the first surveys of the vestibular system [1,2,3], it was not until 1963 that Jones and Spells [4] made the first comprehensive attempt to quantify this relationship This pioneering study assumed two possible scenarios of similarity in duct function relative to body proportions: geometrical similarity (all vertebrate heads are roughly the same mass relative to body mass), and dynamic similarity (all vertebrate heads produce the same stress on the neck and body relative to body mass) [4]. By reducing these assumptions to allometric equations, they predicted that the radius of curvature of the semicircular ducts should have an allometric scaling exponent
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