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The inner ear of D iacodexis , the oldest artiodactyl mammal
Author(s) -
Orliac M. J.,
Benoit J.,
O'Leary M. A.
Publication year - 2012
Publication title -
journal of anatomy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.932
H-Index - 118
eISSN - 1469-7580
pISSN - 0021-8782
DOI - 10.1111/j.1469-7580.2012.01562.x
Subject(s) - cochlea , semicircular canal , inner ear , anatomy , skull , basal (medicine) , posterior semicircular canal , dorsum , biology , mammal , ear canal , middle ear , vestibular system , paleontology , acoustics , physics , neuroscience , insulin , endocrinology
We provide the first detailed description of the inner ear of the oldest artiodactyl, D iacodexis , based on a three‐dimensional reconstruction extracted from computed tomography imagery of a skull of D iacodexis ilicis of earliest W asatchian age (ca. 55 Ma). This description provides new anatomical data for the earliest artiodactyls, and reveals that the bony labyrinth of D iacodexis differs greatly from that of modern artiodactyls described so far. The bony labyrinth of D iacodexis presents a weakly coiled cochlea (720 °), a secondary common crus, a dorsal extension of the anterior semicircular canal more pronounced than that of the posterior one, and a small angle between the basal turn of the bony cochlear canal and the lateral semicircular canal. This suite of characters also occurs in basal eutherian mammals. D iacodexis strongly resembles small living tragulid ruminants in its overall body shape and hindlimb proportions. Comparison of the bony labyrinth of D iacodexis to that of the tragulid M oschiola meminna (Indian mouse deer) reveals great morphological difference in cochlear shape and semicircular canal disposition. The shape of the cochlea suggests that D iacodexis was a high‐frequency hearing specialist, with a high low‐frequency hearing limit (543 Hz at 60 dB). By comparison, the estimated low‐frequency limit of M oschiola meminna is much lower (186.0 Hz at 60 dB). We also assess the locomotor agility of D iacodexis based on measurements of the semicircular canals. Locomotor agility estimates for D iacodexis range between 3.62 and 3.93, and suggest a degree of agility compatible with a nimble, fast running to jumping animal. These results are congruent with the postcranial functional analysis for this extinct taxon.