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Trichechiformes Hay, 1923. The Sirenia (/ saɪˈriːni.ə /), commonly referred to as sea cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The extant Sirenia comprise two distinct families: Dugongidae (the dugong and the now extinct Steller ...
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sirenian, (order Sirenia), any of four large aquatic mammalian species now living primarily in tropical waters where food plants grow. The three species of manatee (genus Trichechus) occupy warm latitudes of the coastal Atlantic and associated rivers, and the dugong (Dugong dugon) inhabits the coastlines of the Indian and Pacific oceans. The extinc...
The order Sirenia was named after the Sirens of Greek mythology, and sirenians are believed to be the basis for the mermaid myth. Modern sirenians have two front limbs in the form of flippers but no hind limbs; even the pelvis is vestigial, and there are no skeletal remnants of leg or foot bones at all. Sirenian bodies are basically round in cross-section and taper toward the tail, which is flattened horizontally and provides propulsion. Dugong tails are deeply notched, similar to those of whales, whereas in manatees the tail is rounded outward like a paddle. Sirenians in general are slow-moving but capable of short bursts of speed. There are no marked differences between the sexes; females have two inconspicuous mammary glands with one teat near the base of each flipper. Both dugongs and manatees have thick tough skin (that of Steller’s sea cow was even tougher). All sirenians are nearly hairless except at the muzzle, where there are thick sensory hairs called vibrissae. The upper lip is enlarged and muscular and serves to grasp and manipulate food plants. The eyes are small, and the external ears consist only of tiny pits. Nostrils on the upper surface of the snout function as valves to exclude water. Sirenian bones are exceptionally dense, most of them lacking marrow. This adaptation is thought to help maintain neutral buoyancy.
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Manatees and dugongs differ in a number of traits besides the shape of the tail. Manatees lack incisor teeth, but incisors do occur in dugongs, erupting as tusks in the males. Manatees grow a never-ending series of molars that continuously move forward in the jaw to replace those that wear and fall out at the front. Dugongs, on the other hand, have a finite number of six molars and premolars in each jaw, and they are not replaced; in older individuals only two remain. Manatees have six neck vertebrae instead of the seven typical of mammals. Dugongs are exclusively marine, but manatees inhabit both marine and freshwater systems.
Sirenians arose from terrestrial hoofed mammals (tethytheres) during the Paleocene Epoch (65 million to 54.8 million years ago) in what is now the Old World. Tethytheres also gave rise to elephants (order Proboscidea), and early sirenians spread to shallow waters throughout the tropics. The family Dugongidae, which eventually included Steller’s sea cow, was an early offshoot, giving rise to the Trichechidae about 40 million years ago.
•Order Sirenia (sea cows) Four living species in two families
•Family Trichechidae (manatees)
•Genus Trichechus Three species
•Family Dugongidae Two genera, one now extinct
•Genus Dugong (dugong) One species
Jul 28, 2019 · Sirenians (Sirenia), also known as sea cows, are a group of mammals that includes dugongs and manatees. There are four species of sirenians alive today, three species of manatees and one species of dugong. A fifth species of sirenian, the Stellar's sea cow, became extinct in the 18 th century due to over-hunting by humans.
Sirenians are classified in the clade Paenungulata, alongside the elephants and the hyraxes, and evolved in the Eocene 50 million years ago (mya). The Dugongidae diverged from the Trichechidae in the late Eocene or early Oligocene (30–35 mya). Sirenians grow to between 2.5 and 4 metres (8.2 and 13.1 feet) in length and 1,500 kilograms (3,300 ...
There are many extinct sirenian species. Four subfamilies of Dugongidae are now extinct, including species such as Steller's sea cows (Hydrodamalis gigas), which likely went extinct around 1768 due to over-hunting. There are many laws to protect these creatures but they are often incompletely enforced.
Publisher Summary. Sirenia are the order of placental mammals comprising modern seacows and their extinct relatives. They are the only herbivorous marine mammals now living, and the only herbivorous mammals ever to have become totally aquatic. Sirenians have a known fossil record extending over some 50 million years.
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There are only four species left of the order Sirenia – one dugong and three manatees. The manatee weighs about a ton and a half when fully grown, but the last of the giant 6-ton sea cows, formerly seen in the North Pacific, was slaughtered by man and has been extinct since the 18th century.
