Your Daily Fossil

Pezosiren

Photo from temporary display at the  National Museum of Natural History in Paris, France

Sadly, I could find no suitable reconstruction of Pezosiren to show, so just imagine a manatee with legs on. 

When: Early Eocene (~50 million years ago)

Where: Jamaica

What: Pezosiren is a legged ancestor of the modern manatees and dugong. It is an amazing example of a transitional fossil. The head and axial skeleton of Pezosiren are already unmistakably sirenian, with its dense pachyostotic ribs and skull with a relatively posterior nasal opening that would not look out of place on a living manatee. These ribs would have decreased its buoyancy and allowed it to easily stay submerged for long periods of time as it munched on water plants. The limbs of Pezosiren show this animal was also well adapted for life out of the water. It shows no evidence of reduction of the forelimb to a flipper as is seen in all living sirenians and its pelvic girdle and was strong, clearly meant to support the weight of the 7 feet (~2 meter) long animal on its hind limbs outside of the water. It also lacked the expanded processes on the vertebrae that we seen in modern sirenans, which are the site of attachment for muscles that move the large flattened tail up and down as these animals swim. 

All of these features together built a great picture of how Pezosiren lived in the early Eocene. This animal spent a great deal of time in the water, but did emerge frequently on land. It probably was very much like the living Hippopotamus in both its amphibious nature. Pezosiren shows had no evidence of the tail powered locomotion we see in the modern manatees and dugongs; instead it appears to have paddled with its hind feet to swim, as it flexed its spin up and down, much like the living otters swim. Why did the Pezosiren lineage become more and more aquatic when the modern hippo has been amphibious for millions of years? One hypothesis is that it is mostly due to diet. Hippos, for all the time they spend in the water, emerge to eat a huge amount of terrestrial vegetation. Manatees are the only secondarily aquatic herbivorous mammals, eating soft water plants, and the skull of Pezosiren  already shows simular adaptations to water feeding as are in these living sirenians. Therefore, as the sirenian linuage became more and more aquatic, more and more of these plants became within reach of feeding, as there was less and less need to emerge onto land. By the close of the Eocene, 15 million years later, the first fully aquatic sirenians had appeared.