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 of Spain, considered the sister taxon of the fully aquatic Dugongidae. This ecological shift from terrestrial to an aquatic environment was naturally associated with adaptations, among others, of the skeleton. However, sirenian hind limb bones have been poorly studied because of the scarce material available in the fossil record. Here, we describe in detail the hind limb bones of Sobrarbesiren, analyzing their functional morphology and comparing them with other basal sirenians and cetaceans, and with related terrestrial mammals such as proboscideans and hyracoids. The hind limbs of Sobrarbesiren were capable of a great variety of movements. Based on the presence of a strong sacroiliac articulation, we propose that it swam by dorsoventral pelvic undulation combined with pelvic paddling analogous to extant otters and the “prorastomid” Pezosiren. We also conduct the first microanatomical analysis of hind limb bones of an Eocene sirenian. Data reveal extreme inner compactness in the Sobrarbesiren innominate and femur, with the first description of osteosclerosis in an amniote innominate combined with the highest degree of osteosclerosis observed in amniote femora. The results confirm that the microanatomical changes precede the external morphological changes in such ecological transitions. The process of adaptation of sirenians to an aquatic life was thus a more complex process than previously thought.