The inner ear of Diacodexis, the oldest artiodactyl mammal

Abstract

We provide the first detailed description of the inner ear of the oldest artiodactyl, Diacodexis, based on a three-dimensional reconstruction extracted from computed tomography imagery of a skull of Diacodexis ilicis of earliest Wasatchian age (ca. 55 Ma). This description provides new anatomical data for the earliest artiodactyls, and reveals that the bony labyrinth of Diacodexis differs greatly from that of modern artiodactyls described so far. The bony labyrinth of Diacodexis 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. Diacodexis strongly resembles small living tragulid ruminants in its overall body shape and hindlimb proportions. Comparison of the bony labyrinth of Diacodexis to that of the tragulid Moschiola meminna (Indian mouse deer) reveals great morphological difference in cochlear shape and semicircular canal disposition. The shape of the cochlea suggests that Diacodexis 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 Moschiola meminna is much lower (186.0 Hz at 60 dB). We also assess the locomotor agility of Diacodexis based on measurements of the semicircular canals. Locomotor agility estimates for Diacodexis 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.

Comparative Anatomy of the Bony Labyrinth (Inner Ear) of Placental Mammals

“…A clear positive correlation between the arc radius of curvature and sensitivity is evident [29], with absolutely larger canals being more sensitive to rotational movement than smaller canals. Additionally, the size of the canals has been related to locomotor behavior in extant and extinct mammals [10], [23], [26], [250]–[251].…”

Morphological variation among the inner ears of extinct and extant baleen whales (Cetacea: Mysticeti)

“…A correlation has been proposed between low frequency thresholds and the graded curvature of the cochlea, or rather the ratio between the radii of the apical and basal turns (Manoussaki et al, ). The calculations made using extant placental mammals have been extended to the fossil record to reconstruct the lower frequency limits of hearing for an array of extinct mammals, including some cetaceans (e.g., Orliac et al, ; Macrini et al, ; Ekdale and Racicot, ). Low frequency thresholds calculated for several odontocetes by Park et al () based on Manoussaki et al's () equation are lower than the thresholds measured via audiograms for most extant odontocetes (e.g., Ketten, ), and two species were calculated for sensitivity to infrasonic sounds, below 20 Hz (beaked whales Mesoplodon cf.…”

Virtual endocranial and inner ear endocasts of the Paleocene ‘condylarth’ Chriacus: new insight into the neurosensory system and evolution of early placental mammals

“…pelvidens and Ch. baldwini for the dataset, and also added other taxa: D. ilicis (AMNH VP 16141; Orliac et al., 2012b), Protungulatum sp. (AMNH VP 118359; Orliac & O'Leary, ), A. inopinata (MHNC 8372; Muizon et al., ), Ca.…”