Baleen whales from the late Oligocene of Baja California Sur, Mexico: clues to the evolution of forelimb anatomy in basal Chaeomysticeti

SOLIS-AÑORVE, AZUCENA; BUONO, MÓNICA R.; GONZÁLEZ-BARBA, GERARDO

Otro; Secondary Adaptations of tetrapods to life in water (SECAD meeting); 2024

Baleen whales (Mysticeti) notably diversified during the Oligocene, which saw theemergence of various toothed lineages and baleen-bearing chaeomysticetes. Several mysticetes- both toothed and baleen-bearing - have been reported from the upper Oligocene El CienFormation of Baja California Sur, Mexico, which provides a window into baleen whaleevolution in the ancient Northwest Pacific. Here, we report a new chaeomysticete fossil (MHNUABCS-EC/SJ29/42b) from the same formation, consisting of a partial skeleton preservingboth flippers (humerus, radius, ulna, phalanges). The forelimb appears elongate and slender,with a rounded humeral head, a well-defined lesser tubercle of the humerus featuring a lateralprojection, a posteriorly projected olecranon process, and no radial tuberosity on the radius.Overall, the flipper appears proportionally more elongate than in archaeocetes and resemblesthat of acrobatic whales like Megaptera novaeangliae, suggesting a high degree of agility.Cetacean postcrania have received comparatively little attention yet are crucial to inferring theevolution of cetacean swimming abilities. Knowledge of mysticete forelimb anatomy duringthe Oligocene remains especially poor, underlining the importance of finds like ours.