Sirenia

The Sirenia, 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 and
Trichechidae with a total of three species.

The Protosirenidae and Prorastomidae families are extinct. Sirenians are classified in the clade Paenungulata, alongside the elephants and the hyraxes, and evolved in the Eocene 50 million years ago. The Dugongidae diverged from the Trichechidae in the late Eocene or early Oligocene.
Sirenians grow to between in length and in weight. The recently extinct Steller's sea cow was the largest known sirenian to have lived, reaching lengths of and weights of.
Sirenians have a large, fusiform body which reduces drag through the water and heavy bones that act as ballast to counteract the buoyancy of their blubber. They have a thin layer of blubber and consequently are sensitive to temperature fluctuations, which cause large-scale migrations when water temperatures dip too low. Sirenians are slow-moving, typically coasting at, but they can reach in short bursts. They use their strong lips to pull out seagrasses, consuming 10–15% of their body weight per day.
While breathing, sirenians hold just their nostrils above the surface, sometimes standing on their tails to do so. They typically inhabit warm, shallow, coastal waters, or rivers. They are mainly herbivorous, but have been known to consume animals such as birds and jellyfish. Males typically mate with more than one female and may gather in leks to mate. Sirenians are K-selected, displaying parental care.
File:Dugong_skeleton_displayed_at_Philippine_National_Museum.jpg|thumb|Dugong skeleton displayed at Philippine National Museum
The meat, oil, bones, and skins of sirenians are commercially valuable. Mortality is often caused by direct hunting from humans or by other human-induced causes, such as habitat destruction, entanglement in fishing gear, and watercraft collisions. Steller's sea cow was finally driven to extinction due to overhunting in 1768.

Taxonomy

Etymology

Sirenia, commonly sirenians, are also referred to by the common name sirens, deriving from the sirens of Greek mythology.

Classification

Sirenians are classified within the cohort Afrotheria in the clade Paenungulata, alongside Proboscidea, Hyracoidea, Embrithopoda, Desmostylia, and Afroinsectiphilia. This clade was first established by George Gaylord Simpson in 1945 on the basis of anatomical evidence, such as testicondy and similar fetal development. The Paenungulata, along with the Afrotheria, are one of the most well-supported mammalian clades in molecular phylogeny. Sirenia, Proboscidae, and Desmotylia are grouped together in the clade Tethytheria. On the basis of morphological similarities, Tethytheria, Perissodactyla, and Hyracoidea were previously thought to be grouped together as the Altungulata, but this has been invalidated by molecular data.

Sirenia families

† = Extinct
Family Dugongidae:

Genus Dugong
*D. dugon
Genus †Anisosiren
*†A. pannonica
Genus †Indosiren
*†I. javanense
Genus †Bharatisiren
*†B. indica
Genus †Callistosiren
*†C. boriquensis
Genus †Crenatosiren
*†C. olseni
Genus †Corystosiren
*†C. varguezi
Genus †Dioplotherium
*†D. allisoni
*†D. manigualti
Genus †Domningia
*†D. sodhae
Genus †Kutchisiren
*†K. cylindrica
Genus †Nanosiren
*†N. garciae
*†N. sanchezi
Genus †Rytiodus
*†R. capgrandi
*†R. heali
Genus †Xenosiren
*†X. yucateca
Genus †Caribosiren
*†C. turneri
Genus †Halitherium
*†H. alleni
*†H. schinzii
Genus †Paralitherium
*†P. tarkanyense
Genus †Priscosiren
*†P. atlantica
Genus †Sirenavus
*†S. hungaricus
Genus †Metaxytherium
*†M. albifontanum
*†M. arctodites
*†M. crataegense
*†M. floridanum
*†M. krahuletzi
*†M. medium
*†M. serresii
*†M. subapenninum
Genus †Dusisiren
*†D. dewana
*†D. jordani
*†D. reinharti
*†D. takasatensis
Genus †Hydrodamalis
*†H. cuestae
*†H. gigas

Family Trichechidae:

Genus Trichechus
*T. manatus
*T. senegalensis
*T. inunguis
Genus †Anomotherium
*†A. langewieschei
Genus †Miosiren
*†M. canhami
*†M. kocki
Genus †Potamosiren
*†P. magdalenensis
Genus †Ribodon
*†R. limbatus

†Family Protosirenidae:

Genus †Ashokia
*†A. antiqua
Genus †Libysiren
*†L. sickenbergi
Genus †Protosiren
*†P. eothene
*†P. fraasi
*†P. minima
*†P. sattaensis
*†P. smithae

†Family Prorastomidae:

Genus †Pezosiren
*†P. portelli
Genus †Prorastomus
*†P. sirenoides

Common name	Genus	Scientific name	Status	Distribution	Picture
West Indian manatee	Trichechus	T. manatus Linnaeus, 1758
African manatee	Trichechus	T. senegalensis Link, 1795
Amazonian manatee	Trichechus	T. inunguis Natterer, 1883
Dugong	Dugong	D. dugon Müller, 1776

Distribution
The warm shallow waters of the equator have been the center of sirenian habitation. The northernmost living population, the Florida subspecies of the West Indian manatee, inhabits the coast and frequents freshwater springs, power plants, and canals in Florida to stay warm during the winter. Individuals may migrate north in the warm summer months, some up to 1,000 kilometers from their winter range. The Antillean subspecies occurs in the Caribbean, South America, and Central America and frequent drowned cays, mangroves, lagoons, and sea grass beds.
The Amazonian manatee has been documented in all parts of the Amazon River Basin in South America. River channels that connect allow easy travel to other waterways where food may be plentiful. The Amazonian manatee lives only in freshwater.
The West African manatee lives in murky isolated inland mangroves and coastal flats in West Africa. It is found in waters above, and its range spans Senegal to Angola.
The dugong, the closest living relative of Steller's sea cow, lives in the Indo-West Pacific Ocean in more than 40 different countries. They are coastal animals supported by wide protected sea grass meadows.
Steller's sea cow was discovered in 1741 around islands in the Bering Sea and was specialized for cold subarctic temperatures. It ranged from Alaska through the Amchitka and Aleutian Islands, and even to Japan. Steller's sea cow was reported to have congregated in shallow, sandy areas along coastline and mouths of rivers and creeks to feed on kelp.

Evolution

The evolution of sirenians is characterized by the appearance of several traits that are found in all sirenians. The nostrils are large and retracted, the upper-jaw bone contacts the frontal bone, the sagittal crest is missing, the mastoid fills the supratemporal fenestra, there is a drop-like ectotympanic, and the bones are pachyosteosclerotic.
Sirenians first appeared in the fossil record in the Early Eocene and diversified throughout the epoch. They inhabited rivers, estuaries, and nearshore marine waters. Sirenians, unlike other marine mammals such as cetaceans, lived in the New World. In Western Europe the first and oldest sirenian remains have been found in a new paleontological site, in Santa Brígida, Amer. One of the earliest aquatic sirenians discovered is Prorastomus, which dates back to 40 million years ago, and the first known sirenian, the quadruped Pezosiren, lived 50 million years ago. An ancient sirenian fossil of a petrosal bone was found in Tunisia, dating back to approximately the same time as Prorastomus. This is the oldest sirenian fossil to be found in Africa and supports molecular data suggesting that sirenians may have originated in Africa. Prorastomidae and Protosirenidae, the earliest sirenian families, consisted of pig-like amphibious creatures who died out at the end of the Eocene. With the appearance of the Dugongidae at this time, sirenians had evolved the characteristics of the modern order, including an aquatic, streamlined body with flipper-like fore limbs and no hind limbs, and a powerful tail with horizontal caudal fins which uses an up-and-down motion to move them through the water.
The last of the sirenian families to appear, Trichechidae, apparently arose from early dugongids in the late Eocene or early Oligocene. In 1994, the family was expanded to include not only the subfamily Trichechinae, but also Miosireninae. The African manatee and the West Indian manatee are more closely related to each other than to the Amazonian manatee.
Dugongidae comprises the subfamilies Dugonginae and Hydrodamalinae and the paraphyletic Halitheriinae. The tusks of modern-day dugongs may have originally been used for digging, but they are now used for social interaction. The genus Dugong probably originated in the Indo-Pacific.