PHYLOGENETIC AND FUNCTIONAL IMPLICATIONS OF INNER EAR MORPHOLOGY OF FOSSIL AND EXTANT PORPOISES (CETACEA: PHOCOENIDAE)

The inner ear anatomy of cetaceans, now readily accessible by means of nondestructive high-resolution X-ray CT scanning, provides a window into their acoustic and locomotor abilities and a source for additional morphological characters for phylogenetic analyses. Digital endocasts of the inner ear labyrinths of representatives of fossil and recent porpoises (Phocoenidae), a clade of some of the smallest odontocete cetaceans, which produce some of the highest-frequency clicks for biosonar and communication, show variation in shape of the vestibule and semicircular canals more than in the cochlear region. Fossil and later-diverging phocoenids have tightly coiled cochlea with fewer cochlear turns (1.5-2) than outgroup odontocetes (2-2.5). The vestibular region is particularly intriguing in the fossils, with the fossil Piscolithax boreios having more compact semicircular canals and vestibule, similar to the extant Phocoenoides dalli and Neophocoena phocoenoides. The fossil Piscolithax tedfordi, an undescribed fossil porpoise, and the earlier diverging extant species Phocoena spinipinnis have more broadly expanded semicircular canals and vestibules. The more compact shape of the vestibular region could be related to behavioral preferences of more pelagic (P. dalli) versus coastal (Neophocoena phocoenoides) taxa, or implies phylogenetic differences. The similar measurements and shape of the cochlear regions indicate that all specimens examined could hear high-frequency sounds, thus the group had already acquired high-frequency hearing capabilities by the Pliocene at the latest.