John Updike :: essays research papers

Evolution of SireniaMany scientists believe that sirenians evolved around fifty-five million years ago from littler hoofed animals. The closests living relatives today are elephants and small asian mammals called hyraxes. Although there are vast differences between elephants, hyraxes, and sirenians, fossil evidence shows that all three evolved from a common ancestor. The manatees physical characteristics are visible remnants of their ancient heritage. The dugongs tusks are an some other link to elephants. Like elephants, sirenians are gigantic animals.There are five species of Sirenia, but scientists theorize that there were many more in the past. Early forms of manatees are thought to have originated near the virago basin in South America. Some remained there to become the Amazonian manatee, while others migrated up through the Caribbean, giving rise to the Antillean and Florida manatees. Another group managed to swim or where carried on currents across the Atlantic and became th e West African manatee.Dugongs thought to have evolved along with mantees once ranged from Europe to Africa, and along the East and West coasts of the Americas. At the bear time, they are only found in the Eastern Hemisphere in the Indian and Pacific Oceans. Stellers Sea Cow, another species of Sirenian became extinct by fisherman within 25 to 30 years of its discovery in the 1700s.Physical CharacteristicsAn adult manatee has a rounded body, usually light to minacious gray or black, that tapers to a horizontally flattened, rounded tail, which provides propulsion and also serves as a rudder. Manatees range in length from 8 to 14 ft. long, depending on the species and can weigh 440 to 1300lbs. Although males can grow larger than the females. The small head includes a straight snout and a cleft f number lip with bristly hairs. Whiskers can be found on the surface of the lips, each attached seperatly to nerve endings and its own blood supply in the follicle. A manatees only teeth are 24 to 32 molars located in the back of the mouth, in addition to molars, manatees have horny, ridged pads at the calculate of the upper and lower jaws. The manatees upper lip is cleft with two lobes this isn used for gathering food. There small eyes are located on the sides of the head. Its nostrils, decline on the upper surface of the snout are closed tightly by valves when the animal is under water. Their lungs and diagphram run lengthwise along the body, unlike other mammals, in which they run crosswise.