Sirenians, or seacows, are a group of marine mammals that include manatees and dugongs; Today, only one species of seacow is found in each world region. Smithsonian scientists have discovered that this was not always the case. According to the fossil record of these marine mammals, it was more common to find three or more different species of seacows living together at one time. This suggests that the environment and food sources for ancient seacows were also different than today.. Image credit: Carl Buell.