A cetacean limb from the Middle Eocene of Ukraine sheds light on mammalian adaptations to life in water.

This article focuses on the description and analysis of an isolated hindlimb from a fully aquatic cetacean dated to the Middle Eocene (43–42 million years ago) from Ukraine, representing the earliest cetacean reported from Europe. The limb, comprising flattened tibia, fibula, and phalanges, exhibits a unique morphology combining features of both forelimbs and hindlimbs, indicating functional, four-legged aquatic locomotion with advanced swimming adaptations. Microanatomical and morphological evidence suggests this cetacean used a transitional swimming style involving body undulation, tail propulsion, and possibly foot-based lift, differing from previously known cetaceans with reduced or vestigial hindlimbs. The specimen is taxonomically assigned to fully aquatic cetaceans (Pelagiceti incertae sedis) but with unclear precise affinities, highlighting evolutionary stages in cetacean limb reduction and aquatic adaptation during the Lutetian age.