The Desmostylia (from Greek δεσμά desma, "bundle", and στῦλος stylos, "pillar")[1] are 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).

[2] 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.

[4] 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.

[2] The smallest is Ashoroa laticosta, a relatively large animal at a body length of 168 centimetres or 5 feet 6 inches, while the largest species reached sizes comparable to Steller's sea cow.

[10] 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.

[13] Both desmostylians and North Pacific dugongids were apparently kelp specialists, as opposed to marine herbivorous mammals from other regions, with diets primarily composed of seagrass.

[6] The type species Desmostylus hesperus was originally classified from a few teeth and vertebrae as a sirenian by Marsh 1888, but doubts arose a decade later when more complete fossils were discovered in Japan.

[14] One of the most comprehensive collections of desmostylian teeth was amassed by paleontologist John C. Merriam, who concluded on the basis of the molar structure and repeated occurrence in marine beds that the animals had been aquatic, and were probably sirenian.

The discovery of a complete skeleton from Sakhalin Island in 1941, however, showed that they possessed four legs, with bones as stout as a hippopotamus', and justified the creation of a new order for the desmostylians, described by Reinhart 1959.