Desmostylus was a large, hippopotamus-like creature of about 1.8 metres (6 ft) long which weighed about 200 kilograms (440 lb). It had a short tail and powerful legs with four hooves. Both the creature's jaws were elongated and sported forward-facing tusks, which were elongated canines and incisors.
Most likely amphibious, Desmostylus is thought to have lived in shallow water in coastal regions. Some paleontologists[who?] have argued it may have fed on seaweed during low tide. However, recent isotope work indicates that Desmostylus more likely lived (or spent a large amount of time) in freshwater or estuary ecosystems foraging for aquatic freshwater plants.
D. hesperus (synonyms and invalid names: D. watasei, D. cymatias, D. californicus, D. mirabilis, D. minor, Desmostylella typica), D. coalingensis (syn. Vanderhoofius coalingensis), and D. japonicus.
Marsh 1888 named the type specimen D. hesperus based on a set of isolated teeth that he had found near Mission San Jose, California (type locality:
37.6°N 121.9°W, paleocoordinates
36.8°N 117.6°W). Marsh described his specimen as a sirenian and proposed affinities with Metaxytherium (a genus of extinct dugongs) and Halicore (an obsolete name for dugong).
Several other species were later described based on minor differences in tooth morphology. Most or all of these species have been synonymized with D. hesperus since variation in tooth morphology between individuals assigned to one of these species has proven to be to greater than the differences between species.
D. japonicus was described by Yoshiwara & Iwasakai 1902 based on a well preserved partial skull and named by Tokunaga & Iwasaki 1914. It has been reproposed as distinct species based on cranial morphology.