Trichechus manatus latirostris (Harlan, 1824) - Florida manatee in Florida, USA. (July 2009)

Mammals are the dominant group of terrestrial vertebrates on Earth today. The group is defined based on a combination of features: endothermic (= warm-blooded), air-breathing, body hair, mother's milk, four-chambered heart, large brain-to-body mass ratio, two teeth generations, differentiated dentition, and a single lower jawbone. Almost all modern mammals have live birth - exceptions are the duck-billed platypus and the echidna, both of which lay eggs.

Mammals first appear in the Triassic fossil record - they evolved from the therapsids (mammal-like reptiles). Mammals were mostly small and a minor component of terrestrial ecosystems during the Mesozoic. After the Cretaceous-Tertiary mass extinction at 65 million years ago, the mammals underwent a significant adaptive radiation - most modern mammal groups first appeared during this radiation in the early Cenozoic (Paleocene and Eocene).

Three groups of mammals exist in the Holocene - placentals, marsupials, and monotremes. Other groups, now extinct, were present during the Mesozoic.

Shown above is the tail of a Florida manatee, Trichechus manatus latirostris, in Pine Island Sound, Florida. Manatees are members of Order Sirenia, a small group of entirely aquatic mammals. Four species are alive on Earth today. The sirenians are manatees, dugongs, and sea cows. They occur in the tropical Atlantic Ocean, the Gulf of Mexico, the Caribbean Sea, the tropical Indian Ocean, and the western Pacific Ocean. They have large bodies, a paddle-like tail, rounded front flippers, and few body hairs (mostly concentrated on the face). Sirenians are principally herbivores - they feed on shallow-water vegetation.

Classification: Animalia, Chordata, Vertebrata, Mammalia, Sirenia, Trichechidae

Locality: near the western margin of Pine Island Sound at McCarthy's Marina, just offshore from the eastern shoreline of Captiva Island, Gulf of Mexico coast of southern Florida, USA

More info. at: <a href="https://en.wikipedia.org/wiki/Sirenia" rel="nofollow">en.wikipedia.org/wiki/Sirenia</a> and