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 four 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 (mya). The Dugongidae diverged from the Trichechidae in the late Eocene or early Oligocene.
Sirenia
Dugong skeleton displayed at Philippine National Museum
Dugongs sift through the seafloor in search of seagrasses.
West Indian manatees in a conservation project in Brazil
A mammal is a vertebrate animal of the class Mammalia. Mammals are characterized by the presence of milk-producing mammary glands for feeding their young, a neocortex region of the brain, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 29 orders.
Restoration of Juramaia sinensis, the oldest-known Eutherian (160 mya)
Fossil of Thrinaxodon at the National Museum of Natural History
Hyaenodon horridus, a North American species of hypercarnivore within the now-extinct order Hyaenodonta, at the Royal Ontario Museum. The genus Hyaenodon was amongst the most successful mammals of the late Eocene-early Miocene epochs spanning for most of the Paleogene and some of the Neogene periods, undergoing many endemic radiations in North America, Europe, and Asia.
Sexual dimorphism in aurochs, the extinct wild ancestor of cattle
