English: Skeletal drawing of the Eocene sirenian Protosiren. Skeletal elements are based on two similarly sized fossil specimens of the species P. smithae (the holotype CGM 42292 (yellow) and UM 101224 (blue)).^[1][2] Note that some manual elements in this restoration are restored from known corresponding pieces from the left manus. The restored fluke is speculative.

While its overall body shape resembled sirenians of today, Protosiren possessed hindlimbs which were well developed ("functional"), but did not fuse to the sacrum. Therefore, it was most likely unable to walk on land. It is possible that it swam with a different technique and occupied a different ecological niche than the contemporaneous Eotheroides, which bore a closer resemblance to modern, fully marine sirenians.^[3][2]

1. ↑ Domning D.P. & Gingerich P.D. (1994), “Protosiren smithae, New Species (Mammalia, Sirenia), from the Late Middle Eocene of Wadi Hitan, Egypt”, Contributions from the Museum of Paleontology 29(3): p. 69-87
2. ↑ ^{a b} Zalmout I.S. & Gingerich P.D. (2012), “Late Eocene sea cows (Mammalia, Sirenia) from Wadi al Hitan in the western desert of Fayum, Egypt”, University of Michigan Papers on Paleontology No. 37
3. ↑ Domning D.P. (2000). "The Readaptation of Eocene Sirenians to Life in Water". Historical Biology 14(1-2): p. 115-119