Comparative anatomy and function of the leptaucheniine oreodont middle ear

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Abstract
Leptaucheniine oreodonts (Merycoidodontidae, Leptaucheniinae) have extremely well-inflated auditory bullae—apparently the largest, relative to body size, among artiodactyls. Leptaucheniines have middle ear volumes (ranging from about 4.0 to 8.5 cc) several times those of extant bovids and cervids of comparable body size. A well-developed, smooth-walled, and laterally-elongate epitympanic sinus, underlies the leptaucheniine mastoid region and comprises 14–21% of total middle ear volume; it is a uniquely derived condition that parallels the enlarged epitympanic sinus seen in some members of other mammalian orders (e.g., Carnivora). Several other features characterize the cranium of leptaucheniines, including: low, flattened skull, rod-like basioccipital and basisphenoid, and prominent petrosal-squamosal contact. In a comparison of several extant and fossil artiodactyls, clear patterns emerge in the relationship of middle ear volume to basicranial axis length (used here as an estimator of overall skull size): the regression line expressing this relationship for leptaucheniines has a slope of 7.86, compared to slopes of 1.57 for antilocaprids and 0.77 for cervids. A sample of bovids is best fit by the quadratic regression equation y = 0.86x2 − 7.02x + 14.31. It is likely that the leptaucheniine ear was an adaptation to the development of open, dry environments in North America during the late Oligocene.