Turning Semicircular Canal Function on Its Head: Dinosaurs and a Novel Vestibular Analysis

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.

New tyrannosaur from the mid-Cretaceous of Uzbekistan clarifies evolution of giant body sizes and advanced senses in tyrant dinosaurs

“…The semicircular canals and cochlear duct are extremely robust compared with all other theropods with comparative CT data, including both juvenile and adult tyrannosaurids (20,26,29). This is an autapomorphy of Timurlengia whose functional implications are unclear, but it could potentially be related to increased agility (36). The cochlear duct is elongated, such that it extends far ventral to the brain endocast and is approximately as long dorsoventrally as the depth of the semicircular canals.…”

Form and function of the mammalian inner ear

“…; Clarke, ; Georgi & Sipla, ; Georgi et al. ). However, phylogeny has a strong effect on the evolutionary development of semicircular canal morphology, so ecological comparisons among distantly related taxa may not be appropriate (Georgi & Sipla, ).…”

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“…Sauropod dinosaurs had the relatively smallest heads in length and mass of any non-avian dinosaur [113], [116] and likely of any terrestrial tetrapod, although comparative data across extinct and extant Tetrapoda have not been compiled. The small head of sauropods had to serve three major functions, the space required for all of which apparently shows a negative allometry with body mass.…”

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