A new archaic homodont toothed cetacean (Mammalia, Cetacea, Odontoceti) from the early Miocene of Peru

摘要

Apart from a few exceptions, extant odontocetes (toothed cetaceans) exhibit a roughly homodont dentition. The transition from basilosaurid-like double-rooted cheek teeth with accessory denticles to single-rooted conical teeth occurred during the late Oligocene-early Miocene. At that time, several clades of now extinct, homodont and predominantly long-snouted odontocetes appeared in the fossil record. Among them, members of the genera Argyrocetus Lydekker, 1893 and Macrodelphinus Wilson, 1935, from the early Miocene of the Northeast Pacific and Argentina, were tentatively attributed to the family Eurhinodelphinidae. However, due to the fragmentary state of the specimens, unambiguous apomorphies of the family could not be detected. Based on two well-preserved skulls with associated mandibular elements, discovered in early Miocene layers of the Chilcatay Formation (Pisco Basin, Peru), we report on a new genus and species of long-snouted homodont odontocete, Chilcacetus cavirhinus n. gen., n. sp. Characterized by, among others, the presence of alveoli on the anterior premaxillary portion of the rostrum, the lack of a lateral groove on the rostrum, anterodorsally elevated nasals, a possibly autapomorphic cavity between nasals and mesethmoid in the posterior wall of the bony nares, a high temporal fossa, and the absence of ankylosis along the mandibular symphysis, C. cavirhinus n. gen., n. sp. does not fit in any of the known odontocete families, but shares several morphological features with Argyrocetus spp. and Macrodelphinus. Our phylogenetic analysis, based on 77 characters for 35 odontocete taxa, suggests the existence of an early Miocene Eastern Pacific long-snouted homodont odontocete clade (with an hypothetical South Atlantic member, the poorly known Argyrocetus patagonicus Lydekker, 1893), distinct from the only superficially similar eoplatanistids and eurhinodelphinids. Furthermore, our consensus tree indicates an early branching of this new clade compared to other homodont odontocete lineages. Unfortunately, the results of the cladistic analysis presented here are not well supported; a reappraisal of Argyrocetus and Macrodelphinus is needed to more clearly define the new clade and bolster its phylogentic position.