Gross and microscopic anatomy of a tibia tentatively attributed to a cetacean from the Middle Eocene of Europe, with a note on the artiodactyl Anoplotherium and on the perissodactyl Lophiodon

Transition of cetaceans from a semi-aquatic to a fully aquatic lifestyle during the Eocene involved numerous transformations of their anatomy, including reduction of the hind limbs. The earliest fully aquatic cetaceans retained externally protruding, possibly functional hind limbs. Being extremely rare, every find of archaeocete hind limb elements can improve our knowledge on the evolution of locomotor strategies in cetaceans. Here, we describe the anatomy and inner microstructure of an isolated tibia fragment, that has been found in the Middle Eocene of Helmstedt, present-day Germany, with comments on its affinities and function. Assignment to an archaeocete is most probable, as other, presently known taxa could be excluded, because of the shape of the distal epiphyseal side and the internal architecture. Nevertheless, it cannot be ruled out with complete certainty that this isolated tibia belongs to a taxon that is new, or is at this moment unknown from coeval sediments of Europe. Well-developed, distally extended tibial crests could indicate the capability of movements with the foot and thus feet-induced aquatic locomotion. The tibia fragment may belong to a large member of semi-aquatic cetaceans (Protocetidae) or to a member of Basilosauridae, early fully aquatic whales. It shares gross and inner morphological features with tibiae of Protocetidae. In this case, it could be from a large protocetid, certainly more than 4 m long, and would be the first protocetid find in Europe as well as the northernmost record of this family in the world. Alternatively, the tibia may belong to a representative of Basilosauridae, which have been found in different parts of Europe, including the region of Helmstedt. If so, it would imply that early basilosaurids had large, protocetid-like hind limbs. This record shows that large quadrupedal cetaceans inhabited the seas of Northwestern Europe by the late Middle Eocene. Not-related finds of our studies are that the terrestrial lifestyle of the artiodactyl Anoplotherium is confirmed by the inner structure of its long bones, and that some species of the perissodactyl Lophiodon were most probably semi-aquatic, as shown by the inner microstructure of the tibia.