Sirenians first appeared in the Eocene and eventually spread world-wide through various time periods mostly preferring warm water environments. Interestingly enough, Florida, with its manatee population, is the only area in the world that can boast of continual habitation of sirenians since the Eocene! Fossil remains in Florida represent all known sea cow families that ever existed!
Manatees are members of the group of aquatic mammals called Sirenians. Only two genera live today and are nearly extinct. One is Trichechus with three living species in the world and one in Florida. The other genera is Dugong with only one living species. A third genus was living as recently as two hundred years ago. Hydrodamalis once lived near the coast of the Bering Sea but was hunted to extinction.
Due to the extreme buoyancy of the creature, sea cows have very dense bones that lack marrow cavities or spongy internal structure. This allows their skeletons to act as ballast thereby offsetting their buoyant body mass. They are exclusively herbivores and are highly docile creatures allowing human contact in the wild.