a new group of late oligocene mysticetes from méxico

The oligocene cetacean fossil record from mexico represents an important element to understand the cetacean evolutionary history in the pacific basin. however, our knowledge of these fossils is poor, as the specimens have not yet been properly described. nonetheless, recent observations on oligocene fossils from the state of baja california sur offer new ideas with regard to the cetacean taxonomic composition and suggest a high diversity of mysticete fossils with several forms of toothless mysticetes. consequently, a new group of extinct mysticetes from the late oligocene (chattian) of mexico is described based on two different specimens (partial skulls), which share a phenetically similar periotic bone. these fossils were collected from the marine units of the san juan member (30 to 23 ma) of the el cien formation in baja california sur. furthermore, this new group of archaic mysticetes is distinct to eomysticetid-like animals, mauicetus, horopeta, and whakakai mainly in the periotic morphology specifically in: a longer and anteroposteriorly flattened compound posterior process of the periotic, and a periotic (pars cochlearis, anterior process and body of the periotic) with an ovoid shape in lateral and medial views. phylogenetic analysis suggests a closer relation to sitsqwayk and eomysticetes than to horopeta+whakakai and crown mysticeti. oligocene cetaceans from mexico are still poorly known in terms of their paleoecology and phyletic relationship. this work represents the first description of oligocene mysticetes from baja california sur and shows the potential to further understanding of the biogeographic history of mysticetes in the eastern pacific.