The Hind Limbs of Sobrarbesiren cardieli (Eocene, Northeastern Spain) and New Insights into the Locomotion Capabilities of the Quadrupedal Sirenians

In the transition from a terrestrial to an aquatic environment, sirenian marine mammals reduced and lost their hind limbs and developed a horizontal caudal fin, the main propulsive organ in extant sirenians. Quadrupedal forms are only known from the Eocene and are represented by three different clades: the amphibious “prorastomids,” the aquatic quadrupedal protosirenids, and Sobrarbesiren cardieli, a four-legged sirenian from the middle Eocene...