Desmostylia is an extinct order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of marine mammal
Marine mammal
Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...

s which existed from the Arikareean
The Arikareean North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 30,600,000 to 20,800,000 years BP, a period of . It is usually considered to overlap the Oligocene and Miocene epochs...

 age of the late Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

Epoch (reference date)
In the fields of chronology and periodization, an epoch is an instance in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured...

 (30.8 million years ago) to the Tortonian
The Tortonian is in the geologic timescale an age or stage of the late Miocene that spans the time between 11.608 ± 0.005 Ma and 7.246 ± 0.005 Ma . It follows the Serravallian and is followed by the Messinian....

 age of the late Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 epoch (7.25 million years ago). Their dental and skeletal form suggests desmostylians were amphibious
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

s dependent on littoral
The littoral zone is that part of a sea, lake or river that is close to the shore. In coastal environments the littoral zone extends from the high water mark, which is rarely inundated, to shoreline areas that are permanently submerged. It always includes this intertidal zone and is often used to...

 habitats. Their name refers to their highly distinctive molars, in which each cusp was modified into hollow columns, so that a typical molar would have resembled a cluster of pipes, or in the case of worn molars, volcanoes. (This shows the close relationship between the Paenungulata
Paenungulata is a taxon that groups some remarkable mammals, including three orders that are extant: Proboscidea , Sirenia , and Hyracoidea . At least two more orders are known only as fossils, namely Embrithopoda and Desmostylia...

, to which this group belongs, and the Tubulidentata.)

Desmostylian fossils are known from the northern Pacific rim
Pacific Rim
The Pacific Rim refers to places around the edge of the Pacific Ocean. The term "Pacific Basin" includes the Pacific Rim and islands in the Pacific Ocean...

, from southern Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 through Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

, the Aleutian Islands and the Pacific coast of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 to the southern tip of Baja California
Baja California
Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...

. Though the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...

 Desmostylus hesperus was originally classified from a few teeth and vertebrae as a sirenian by Marsh in 1888, doubts arose a decade later when more complete fossils were discovered in Japan. In 1898, American geologist Henry Fairfield Osborn
Henry Fairfield Osborn
Henry Fairfield Osborn, Sr. ForMemRS was an American geologist, paleontologist, and eugenicist.-Early life and career:...

, working with Japanese paleontologists S. Yoshiwara and J. Iwasaki, suggested a proboscidean origin based on skull and tusk similarities to early mastodons. One of the most comprehensive collections of desmostylian teeth was amassed by paleontologist John C. Merriam
John C. Merriam
John Campbell Merriam was an American paleontologist. The first vertebrate paleontologist on the West Coast of the United States, he is best known for his taxonomy of vertebrate fossils at the La Brea Tar Pits in Los Angeles, particularly with the genus Smilodon, more commonly known as the...

, who concluded based on the molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....

 structure and repeated occurrence in marine beds that the animals had been aquatic, and were probably sirenian. Other scientists suggested origins with monotremes like the duck-billed platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...

. Because desmostylians were originally known only from skull fragments, teeth and bits of other bones, general agreement was that they had had flippers and a fin-like tail. 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'
The hippopotamus , or hippo, from the ancient Greek for "river horse" , is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae After the elephant and rhinoceros, the hippopotamus is the third largest land mammal and the heaviest...

, and justified the creation of a new order for the desmostylians, described by Roy H. Reinhart in 1953.

Despite their similarities to manatees and elephants, desmostylians were entirely unlike any living creatures. Douglas Emlong
Douglas Emlong
Douglas Ralph Emlong was an amateur fossil collector from the Oregon Coast in the northwestern United States. His collections contributed to the discovery and description of numerous extinct marine mammal species, many of which are ancestral to extant groups...

's 1971 discovery of the new genus Behemotops
Behemotops is an extinct genus of herbivorous marine mammal of the family Desmostylidae living from the Rupelian stage of the Early Oligocene subepoch through the Late Oligocene subepoch and in existence for approximately ....

from Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...

showed that early desmostylians had more proboscidean-like teeth and jaws than later ones. Despite this discovery, their relationships to manatees and proboscids remain unresolved.

Desmostylians grew to 1.8 metres (5.9 ft) in length and are thought to have weighed more than 200 kilograms (440.9 lb).

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