Phylogenetic and ecological correlates of inner ear morphology and ontogeny in pinnipeds (Mammalia, Carnivora).
Pinnipeds are caniform carnivorans secondarily adapted to an aquatic environment, with fossils such as the late Oligocene Enaliarctos providing important evidence on the early evolution of this clade. Ecology and locomotory performance on land and in water differs markedly among the three extant families (Phocidae, Otariidae, and Odobenidae), and understanding the ecology and degree of land-dependency of fossil species is crucial to reconstructing this major transition. While most studies of pinnipeds have focused on cranial, dental, and postcranial traits, the inner ear provides key information on hearing and locomotory ecology. The cochlea and semicircular canals (SCC) of the inner ear are well studied in living mammal species, and as the petrosal is well preserved in fossils, study of these elements could help reconstructing the phylogeny and ecology of fossil specimens.
Skulls from fetal to adult specimens of 13 species of living pinnipeds were CT scanned, and 3D reconstructions were made of the SSC and cochlea. The holotype skull of Miocene-Pliocene monachine phocid Acrophoca was also scanned and the cochlea was reconstructed. Three sets of size-corrected measurements were analysed with Principal components analyses (PCA): all measurements, SSC measurements only, and cochlear measurements only. In all PCAs, the first component separated the three families with no overlap.
Adding younger ontogenetic stages to the PCAs revealed divergent routes of ontogenetic direction, with no consistent pattern within or across families. Addition of Acrophoca to the cochlea PCA showed that this specimen was consistently closest to Leptonychotes weddellii, a deep diving monachine.
Phylogenetic least-squares regressions of adult-only size-corrected log inner ear measurements and PC scores against 17 ecological and life history variables found significant correlations of PC scores with latitude (PC1-all), mean diving depth (PC3-all, and PC3-cochlea), and temperature (PC1-SSC). Measurements of the anterior SSC were significantly correlated with temperature and gestation, while latitude, growth rate, and mean diving depth were significantly correlated with cochlear measurements. Although no relationships were significant following a Bonferroni correction, the correlation between cochlear measurements and diving depth, and the similarity of Acrophoca to L. weddelli in most aspects of cochlear morphology suggests that this extinct phocid may have also achieved moderate to deep diving depths.