69 - Early Insights into "Miacid" Inner Ears: Assessing Locomotor Agility Based on Semicircular Canal Size and Body Mass

Poster Board Number: 69
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Abstract Body : The “Miacidae” are a paraphyletic assemblage of stem taxa that variably approximate members of crown Carnivora. Together, the “Miacidae” plus crown carnivorans form the monophyletic Carnivoraformes. Relationships among carnivoraforms have previously been based on a combination of dental and basicranial features, with particular emphasis on anatomical transitions in the ear region. The inner ears of various mammal groups have also received considerable attention for their potential to reconstruct behavioral or locomotory patterns among both extant and extinct groups. Even so, the inner ear of a “miacid” has yet to be described. We here report the discovery of a partial “miacid” basicranium from the Willwood Formation (Early Eocene) in the Bighorn Basin, Wyoming. This specimen, identified herein as Uintacyon sp., is comprised of fragmentary cranial elements, including both left and right petrous fragments of the temporal bones, partial braincase, a complete and a partial occipital condyle, a partial zygomatic arch, and an incomplete maxilla preserving the upper fourth premolar. Given the specimen age, it currently represents the oldest well-preserved basicranium of a carnivoraform. Micro-CT scans of the specimen additionally reveal the preservation of both inner ear chambers, including the intact bony labyrinths. Using 3D Slicer, these scans were manually segmented to construct 3D models of the bony labyrinths, representing the first ever documentation of the inner ear of a “miacid”. Measurements were then taken of the radius of curvature of each semicircular canal, along with bicondylar width measurements used to estimate body mass. The average size of the canals was then plotted against the body mass estimate and compared to previously published datasets to infer the relative agility of Uintacyon. Results indicate that Uintacyon was likely a moderately agile taxon, with a canal size to body mass ratio that is fairly middling relative to other mammals. Its ratio is well below that of highly-agile animals such as primates, for instance, but above that of exceptionally slow-movers such as sloths. Such a pattern casts doubt on a highly-agile, arboreal interpretation of early “miacids”, but does not necessarily rule out the possibility of occasional tree-climbing propensities. This latter point is especially palatable considering the large size of the lateral semicircular canal in Uintacyon, which, among primates, seems to indicate a modest degree of agility. Taken together, these results suggest that Uintacyon was likely a versatile, although predominantly ground-dwelling predator.