1) Transitional fossil 
Pezosiren portelli is a basal sirenian from the early Eocene of Jamaica, 50 million years ago. The type specimen is represented by a Jamaican fossil skeleton, described in 2001 by Daryl Domning, a marine mammal paleontologist at Howard University in Washington, DC. It is believed to have a hippopotamus-like amphibious lifestyle, and is a perfect e...