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

There are a few mammalian lineages secondarily adapted to life in water throughout their evolutionary history. Of them, only sirenians and cetaceans evolved as fully aquatic organisms. This transition was accompanied by changes in swimming mode, from foot paddling to tail-powered propulsion, with the forelimbs acting as efficient hydrofoils and the gradual loss of external hindlimbs. Here, we describe an isolated limb from the Middle Eocene of Ukraine, 43–42 Mya, identified as a hindlimb of a fully aquatic cetacean and being the earliest cetacean reported from Europe. It is represented by flattened, jointly articulated bones, identified as the tibia and fibula with a partly reduced knee joint and loose connection to the pes, and by flattened phalanges. This anatomy reveals a hitherto only presumed cetacean morphotype, showing that some of the early fully aquatic cetaceans were four-legged animals with functional hindlimbs that could be involved in advanced styles of swimming. They used either body undulation or lift-based propulsion powered by the tail or feet and could also use a transitional swimming style combining these modes.