Жоғалған теңіз сүтқоректілерінің Desmostylia тобы

Extinct order of mammals

The Desmostylia (from Greek δεσμά desma, "bundle", and στῦλος stylos, "pillar") are an extinct order of aquatic mammals native to the North Pacific from the early Oligocene (Rupelian) to the late Miocene (Tortonian) Desmostylians are the only known extinct order of marine mammals. The Desmostylia, together with Sirenia and Proboscidea (and possibly Embrithopoda), have traditionally been assigned to the afrotherian clade Tethytheria, a group named after the paleoocean Tethys around which they originally evolved. The relationship between the Desmostylia and the other orders within the Tethytheria has been disputed; if the common ancestor of all tethytheres was semiaquatic, the Proboscidea became secondarily terrestrial; alternatively, the Desmostylia and Sirenia could have evolved independently into aquatic mammals. The assignment of Desmostylia to Afrotheria has always been problematic from a biogeographic standpoint, given that Africa was the locus of the early evolution of the Afrotheria while the Desmostylia have only been found along the Pacific Rim. That assignment has been seriously undermined by a 2014 cladistic analysis that places anthracobunids and desmostylians, two major groups of putative non African afrotheres, close to each other within the laurasiatherian order Perissodactyla. However, a subsequent study shows that, while anthracobunids are definite perissodactyls, desmostylians share the same number of characteristics necessary for either Paenungulata or Perissodactyla, making their former assessment as afrotheres a possibility.

Desmostylians were large, fully aquatic quadrupeds with massive limbs and short tails. A desmostylian skull has an elongated and broadened rostrum, with the nasal opening located slightly dorsally. The zygomatic arches are prominent (behind the eyes), the paroccipital processes elongated (downward pointing processes behind the jaw joints), and the epitympanic sinuses open into the temporal fossae (cavities above the ear holes). Desmostylians are believed to be aquatic because of a combination of characteristics. Their legs seemed to be adapted for terrestrial locomotion, while a number of other parameters confirms their aquatic nature: that desmostylians were more terrestrial than aquatic and clearly fore limb dominated swimmers, hence they were more similar to "sea bears" than "sea sloths" (as proposed by other researchers.) However, a more recent and detailed analysis of desmostylian bone structure has revealed them to be fully aquatic, like sirenians and cetaceans, with their limbs being incapable of supporting their own weight on land. More recent studies vindicate this assessment, as desmostylians had a thoracic morphology more similar to sirenians and modern cetaceans than to that of semiaquatic mammals. Its less dense bone structure suggests that Desmostylus had a lifestyle of active swimming and possibly feeding at the surface, while other desmostylians were primarily slow swimmers and/or bottom walkers and sea grass feeders.

Desmostylian fossils are known from the northern Pacific Rim, from southern Japan through Russia, the Aleutian Islands, and the Pacific coast of North America to the southern tip of Baja California. They range from the Early Oligocene to the late Miocene.