New Paratethyan dwarf baleen whales mark the origin of cetotheres.

BACKGROUND

Family Cetotheriidae and several closely related taxa comprise the Cetotherioidea and represent a lineage of Neogene baleen whales that includes the smallest edentulous baleen whales in Earth history. Most of known cetotheriids came from the Late Miocene to Quaternary, and the earliest records from the latest Middle Miocene. The Paratethys region shows a great diversity of Middle to Late Miocene cetotheriids. That includes nominative taxon of the family, , and this suggests that the earliest cetotheriids may have lived in that region.

MATERIALS AND METHODS

Here, , a new genus and species from the Middle Miocene of southeastern Europe, is described as the chronologically earliest and earliest diverging member of Cetotheriidae. Also, a new specimen of , a basal Cetotherioidea sensu Gol'din & Steeman, 2015 is identified from the Late Miocene deposits of Caucasus and compared with from the Middle Miocene of the same region.

RESULTS AND DISCUSSION

is a dwarf whale displaying primitive traits: posterior ends of facial bones forming a single transverse line, a narrow occipital shield, and a relatively long interparietal region. Meanwhile, it shares some cetotheriid apomorphies: posteriorly telescoped wedge-shaped facial bones and an ovoid tympanic bulla with shallow lateral and medial furrows, a short anterior lobe and a short sigmoid process. Phylogenetic analysis suggests that and are branches diverging before the clade Cetotheriidae + Neobalaenidae. This is confirmed by the stepwise evolution of the anatomy of the squamosal, mandible, and ear bones across these groups. The re-described juvenile specimen of differs from in the more primitive anatomy of the mandible and the autapomorphic anatomy of the humerus. Records of the earliest cetotheriids and related taxa in the Paratethys support the idea that this could be the region where Cetotheriidae evolved before their worldwide dispersal and radiation.