A new chaeomysticete from the Late Oligocene of Baja California Sur, Mexico: expanding the diversity of early baleen whales

Chaeomysticeti includes the filter-feeding mysticetes, encompassing all extant baleen whales as well as early diverging mysticete fossils. Its origins date back to the early Oligocene Epoch, a period that marked a significant upswing for mysticetes. During this epoch, both toothed and toothless mysticetes were represented, alongside forms closely related to the crown group of mysticetes. In recent years, several taxa from the Oligocene of the El Cien Formation (Baja California Sur, Mexico) have been described, revealing a diversity of forms and adaptations that can be compared to those from other Oligocene localities in the Pacific and Atlantic. We describe the partially preserved skeleton of a new taxon of Chaeomysticeti, Calafiacetus longipinna, from the Late Oligocene of the El Cien Formation. This specimen includes the skull, tympanic bullae, vertebrae, and fragments of ribs and scapula, with both flippers notably preserving the carpal region. This taxon represents the earliest branching lineage of toothless whales, along with Maiabalaena nesbittae and Sitsqwayk cornishorum. Calafiacetus longipinna retained ancestral traits in forelimb morphology, including a prominent deltopectoral crest and proportionally similar radius and ulna, marking a transitional stage towards modern mysticete flipper adaptations. Its postcranial morphology indicates a wave-like locomotion pattern, consistent with other chaeomysticetes, facilitated by the elongation of the vertebral centra. This specimen provides key insights into the evolution of anatomical features in early diverging chaeomysticetes during the Oligocene in the North Pacific.https://zoobank.org/urn:lsid:zoobank.org:pub:89127B8E-E432-43C6-84A2-8505C22082C8