The radiation of baleen whales (Cetacea: Mysticeti): insights from the Miocene record of Patagonia (Argentina)

MARTA S.FERNÁNDEZ; MÓNICA R. BUONO; JOSE CUITIÑO

París

Congreso; 5TH INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2018

Mysticete (baleen whales) evolution is marked by major turnover events. The first one occurredduring the Late Oligocene-early Miocene, with the extinction of toothed forms and the diversification of the baleen-bearing groups (= Chaeomysticeti). This is a critical time in the evolutionary history of the group, because it represents the time of diversification of crown lineages. However, there is an important early Miocene gap where fossil representatives of the living families Balaenopteridae (rorquals), Eschrichtiidae (grey whale), Neobalaeninae (pygmy right whale) and Balaenidae (right whales) are virtually unknown. Fossil baleen whales from the early Miocene of Patagonia (Gaiman Formation, Argentina) provide one of the best known records of mysticetes for this age interval. Some of the key species recorded include one of the most archaic rorquallike whales (Aglaocetus moreni) and the oldest right whale (Morenocetus parvus).These are pivotal fossils for the calibration and estimation of living mysticetes molecular clock divergences. The second important diversification event in baleen whale evolution is during the late Miocene-Pliocene, with the establishment of the modern mysticetes lineages. Fossil mysticetes from the late Miocene of Patagonia (Puerto Madryn Formation) also shed light on this critical time in mysticete evolution. Among the most relevant records are one the oldest Eubalaena -like right whales and a pygmy right-like whale (Neobalaeninae indet), as well as a new species of Balaenidae. In particular, the anatomical study of balaenids show that some of the specialized morphological traits of modern balaenids were acquired by the early Miocene and have remained essentially unchanged up to the present. Moreover, the study of an ontogenetic sequence of fossil balaenids indicates that an overall juvenile morphology is retained in the adult stage, also sharing some similarities with the juvenile form of Eubalaena australis. These observations suggest that a heterochornic process (i.e. paedomorphy) operated during the evolution of Miocenebalaenids. The results of our studies provide novel insights into the evolution of living mysticete lineages and ultimately will contribute to elucidate the evolutionary driving forces of the Miocene baleen whale radiation.