An Early Miocene basal chaeomysticete from the East Pisco Basin of Peru

The evolution of baleen whales (Mysticeti) comprises a Paleogene phase, which saw the diversification of stem lineages like the aetiocetids, mammalodontids and eomysticetids; and a Neogene phase dominated by modern - looking, toothless, baleen - bearing forms. Separating these two phases is an apparent global turnover event coinciding with a global gap (or ‘dark age’) in the mysticete fossil record. The latter begins at 23 Ma and, regionally, lasts until about 18 Ma. Here, we report on a new baleen whale fossil from the Lower Miocene (Aquitanian to lower Burdigalian) sediments of the Chilcatay Formation exposed at Cerro Tiza (East Pisco Basin, Peru) that may help to elucidate the nature and extent of the Oligocene/Miocene turnover event. The same formation had a lready yielded two poorly preserved mysticete specimens. Our specimen preserves parts of the supraoccipital, the frontals (including the isolated left preorbital process), the basioccipital, the parietals, the basisphenoid, and natural endocasts of the brain cavity, the mesorostral groove and the nasal passages . Although the cranium is incomplete and heavily eroded, the combination of the following traits suggests phenetic similarities with basal Chaeomysticeti: poorly telescoped, roughly triangular occipital shield; estimated minimum length of the nasals longer than the combined exposure of the frontals and 4th_circular.pdf 89 parietals on the vertex; and parietals seemingly more elevated than the frontals. A large space between the parietals, possibly originally occupied by the anterior medial interparietal, suggests a young ontogenetic stage. Although the taxonomic affinities of the Cerro Tiza specimen are debatable, our find represents a globally precious record from the mysticete ‘dark age’.