Pezosiren skeleton. Pezosiren portelli is an early sirenian (sea-cow) type mammal that is believed to have lived about 50 million years ago in what is now the Caribbean. It is thought that it may have had a hippopotamus-like amphibious lifestyle, and is a perfect example of a transitional form between a land mammal and a sea mammal. Photographed at the In the Shadow of the Dinosaurs exhibition at the National Museum of Natural History, Paris, France, in 2011.