Temporal range: Oligocene-Miocene, 30.8–7.2Ma
|Families and genera|
Desmostylians are the only extinct order of marine mammals.
Desmostylia, together with Sirenia and Proboscidea (and possibly Anthracobunia and Embrithopoda), form the clade Tethytheria, a group named after the paleoocean Tethys around which they originally evolved. The relationship between Desmostylia and the other orders within Tethytheria is disputed; either the common ancestor of all tethytheres was semiaquatic and the Proboscidae became secondarily terrestrial, or Desmostylia and Sirenia evolved independently into aquatic mammals.
Desmostylians were large amphibious quadrupeds with massive limbs and a short tail. They grew to 1.8 metres (6 ft) in length and are thought to have weighed more than 200 kilograms (440 lb).
A desmostylian skull has an elongated and broadened rostrum with the nasal opening located slightly dorsally. The zygomatic arches are prominent (behind the eyes); the paroccipital processes elongated (downward-pointing processes behind the jaw-joints); and the epitympanic sinuses open into the temporal fossae (cavities above the ear holes).
The mandible and maxilla typically have forward-pointing incisors and canine tusks, followed by a long postcanine diastema, partly because of the reduced number of premolars. The cusps of the premolars and molars are composed of densely packed cylinders of thick enamel, giving the order its name ("bundle of columns"). The primitive dental formula is 184.108.40.206, with a trilobate fourth deciduous premolar. The cheek teeth are brachydont and bunodont in primitive genera, but hypsodont in later genera such as Desmostylus, which has many supernumerary cusps.
In the postcrania, the clavicle is absent and the sternum consists of a series of heavy, paired, plate-like sternebrae. In adults, the joints between the radius and ulna prevent any movements. The metacarpals are longer than metatarsals, and each foot has four digits, of which the first is vestigial.
Their dental and skeletal form suggests desmostylians were aquatic herbivores dependent on littoral 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, to which this group belongs, and the Tubulidentata.)
Desmostylians are believed to aquatic because of a combination of characters. Their legs seemed to be adapted for terrestrial locomotion, while a number of other parameters confirms their aquatic nature:
- fossils have been found in marine strata
- the nares are retracted and the orbits are raised like in other semi-aquatic mammals
- Levels of stable isotopes in their tooth enamel suggest an aquatic diet and environment (carbon and oxygen) and fresh or brackish water (strontium)
- Spongy bone structure similar to that of cetaceans
Based on a comparison of trunk and limb proportions, Gingerich 2005 concluded that desmostylians were more terrestrial than aquatic and clearly forelimb dominated swimmers, hence they were more "sea bears" than "sea sloths" (as proposed by other researchers.) However, a more recent and detailed analysis of desmostylian bone structure has revealed them to be fully aquatic, like sirenians and cetaceans, with their limbs being incapable of supporting their own weight on land.
Desmostylian fossils are known from the northern Pacific Rim, from southern Japan through Russia, the Aleutian Islands and the Pacific coast of North America to the southern tip of Baja California.
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. Osborn 1905 also proposed that they belonged to Sirenia. 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. Other scientists[who?] suggested origins with monotremes, like the duck-billed platypus.
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', and justified the creation of a new order for the desmostylians, described by Reinhart 1959.
Despite their similarities to manatees and elephants, desmostylians were entirely unlike any living creatures. Douglas Emlong's 1971 discovery of the new genus Behemotops from Oregon 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.
There are six genera of desmostylians:
From the late Oligocene:
From the early to middle Miocene:
- Domning, Daryl P.; Ray, Clayton Edward; McKenna, Malcolm C. (1986). "Two new Oligocene desmostylians and a discussion of Tethytherian systematics". Smithsonian Contributions to Paleobiology 59: 1–56. OCLC 12974759.
- Gingerich, Philip D. (2005). "Aquatic Adaptation and Swimming Mode Inferred from Skeletal Proportions in the Miocene Desmostylian Desmostylus". Journal of Mammalian Evolution 12 (1/2). doi:10.1007/s10914-005-5719-1. Retrieved March 2013.
- Hayashi, Shoji; Houssaye, Alexandra; Nakajima, Yasuhisa; Kentaro, Chiba; Ando, Tatsuro; Sawamura, Hiroshi; Inuzuka, Norihisa; Kaneko, Naotomo; Osaki, Tomohiro (2013). "Bone Inner Structure Suggests Increasing Aquatic Adaptations in Desmostylia (Mammalia, Afrotheria)". PLoS ONE 8 (4): e59146. doi:10.1371/journal.pone.0059146. OCLC 837402105.
- Marsh, O. C. (1888). "Notice of a new fossil sirenian, from California". American Journal of Science 25 (8): 94–96. OCLC 79838746.
- Osborn, H. F. (1905). "Ten years progress in the mammalian Palaeontology of North America". Extrait des Comptes rendus du 6e Congres international de Zoologie. Session de Berne 1904: 86–113. OCLC 18502464.
- Reinhart, Roy Herbert (1959). "A review of the Sirenia and Desmostylia". University of California Publications in Geological Sciences 36 (1): 1–146. OCLC 3474601.
- Gheerbrant, Emmanuel; Domning, Daryl P.; Tassy, Pascal (2005). "Paenungulata". In Rose, Kenneth D.; Archibald, J. David. The Rise of Placental Mammals: Origins and Relationships of the Major Extant Clades. JHU Press. ISBN 9780801880223. OCLC 55801049. Retrieved June 2013.
- Uhen, Mark D. (2007). "Evolution of Marine Mammals: Back to the Sea After 300 Million Years" (PDF). Anatomical record 290 (6): 514–22. OCLC 137264359. Retrieved June 2013.
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|Look up desmostylia in Wiktionary, the free dictionary.|
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- "Introduction to Desmostylia". University of California Museum of Paleontology. January 2009. Retrieved June 2013.