Desmostylia

Desmostylia (from Ancient Greek δεσμά (desmá), meaning "bundle", and στῦλος (stûlos), meaning "pillar") is an extinct order of aquatic mammals native to the North Pacific from the early Oligocene (Rupelian) to the late Miocene (Tortonian) (30.8 to 7.25 million years ago). 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 afrotherian clade Tethytheria, 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 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.