Sirenians are members of the group known as subungulates, thought to be distantly related to hyraxes , elephants , and perhaps, artiodactyls and perissodactyls . Their fossil record goes back to the Eocene, but at that time both families were distinct and specialized for aquatic life, so their origin is likely to have been considerably earlier. A fifth species of sirenian is extinct. Stellar sea cows were exterminated by sailors in the mid 1700's, shortly after their discovery. The remaining sirenians, manatees and dugongs, are seriously threatened by hunting, habitat degradation, and in the case of manatees, collisions with boats in the shallow coastal areas they prefer.
Sirenians are vegetarians, feeding on a variety of marine algae and higher plants.
Members of both families are social, occuring in large aggregations and interacting frequently with one another. L. C. Drickamer, S. H. Vessey, and J. F. Merritt. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. Rathbun, G. B. 1984.