Desmostylia

Desmostylia (from Ancient Greek δεσμά (desmá), meaning "bundle", and στῦλος (stûlos), meaning "pillar") is an extinctorder of aquatic mammals native to the North Pacific from the Early Oligocene (Rupelian) to the Late Miocene (Tortonian) from 33.9 to 7.2 or 5.3 Ma. However, phylogenetic models suggests the order first appeared during Middle Eocene between 47 and 39 Ma. Desmostylians are the only known extinct order of marine mammals. Their taxonomic placement within Placentalia is subject to considerable debate.

The Desmostylia, was traditionally assigned to the afrotheriancladeTethytheria, together with Sirenia (manatees and dugongs) and Proboscidea (elephants and their extinct relatives) and the extinct Embrithopoda. 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 been found only along the Pacific Rim. That assignment has been potentially undermined by a 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 reached their highest diversity during the Late Oligocene with roughly 9 contemporary species, with another peak in diversification during the Middle Miocene Climatic Optimum. Following the Middle Miocene Climatic Optimum, the order saw a terminal decline before going extinct. The extinction of the order is widely considered to have been the result of changing ecosystems and competition with sirenians.