Desmostylia

Desmostylia (from Greek δεσμά desma, "bundle", and στῦλος stylos, "pillar")[1] is an extinct order of aquatic mammals that existed from the late Oligocene (Arikareean) to the late Miocene (Tortonian) (30.8 to 7.25 million years ago).

Desmostylians are the only known extinct order of marine mammals.[2]

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. The relationship between Desmostylia and the other orders within Tethytheria has been disputed; if the common ancestor of all tethytheres was semiaquatic, the Proboscidea became secondarily terrestrial; alternatively, Desmostylia and Sirenia could have evolved independently into aquatic mammals.[3] 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 Afrotheria while Desmostylia has only been found along the Pacific Rim. That assignment has been seriously 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.[4]

Description

Restoration of Paleoparadoxia

Desmostylians were large amphibious quadrupeds with massive limbs and a short tail.[2] 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).[2]

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 3.1.4.3, 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.[2]

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 (digit I is vestigial.)[2]

Behaviour

Restoration of Desmostylus on land

Their dental and skeletal form suggests that 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 may reflect the close relationship between the Paenungulata, to which this group has been assigned, and the Tubulidentata.)

Desmostylus does not chew or eat like any other known animal. It clenches its teeth, roots up plants with the help of tusks and powerful neck, and then sucks them in using strong throat muscles and the shape of the roof of the mouth.[5]

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:[1]

Based on a comparison of trunk and limb proportions, Gingerich 2005 concluded[6] 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,[7] with their limbs being incapable of supporting their own weight on land.

Distribution

Desmostylian fossils are known from the northern Pacific Rim,[8] from southern Japan through Russia, the Aleutian Islands and the Pacific coast of North America to the southern tip of Baja California.

Classification

Restoration of Desmostylus and Paleoparadoxia
Neoparadoxia cecilialina on display at the Natural History Museum of Los Angeles County
Paleoparadoxia keleton
Skull of Desmostylus

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.[9] 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.

In 1926, the Austrian palaeontologist Othenio Abel suggested origins with monotremes, like the duck-billed platypus, and in 1933 he even created the order "Desmostyloidea", which he placed within Multituberculata. Abel died shortly after WW2 and his classification won few supporters and has been ignored since.[10]

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.

A major find was announced in October 2015 after scientists examined an extensive group of giant, tusked, quadruped, marine mammal fossils. This northernmost to date species discovery had been unearthed during excavation for the construction of a school in Unalaska, in the Aleutian Islands.[11] A rendition of a group was drawn by Alaskan artist Ray Troll.[12]

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. The analysis of Cooper et al. (2014) indicates the similarities with manatees and elephants may be a result of convergence.[4]

There are six accepted genera of desmostylians according to Gingerich 2005:[1]

From the late Oligocene:

From the early to middle Miocene:

Barnes 2013 proposed a new classification of Paleoparadoxiidae:[13]

Notes

  1. 1 2 3 Gingerich 2005, Introduction
  2. 1 2 3 4 5 Gheerbrant, Domning & Tassy 2005, pp. 95–6
  3. ↑ Uhen 2007, p. 515
  4. 1 2 Cooper et al. 2014
  5. ↑ "New fossils intensify mystery of short-lived, toothy mammal found in ancient North Pacific, Oddball creature, Desmostylia". www.sciencedaily.com. Southern Methodist University. Oct 6, 2015. Retrieved October 11, 2015.
  6. ↑ Gingerich 2005, Discussion
  7. ↑ Hayashi et al. 2013
  8. ↑ Gingerich 2005, Abstract
  9. ↑ H. F. Osborn 1905 in the Paleobiology Database. Retrieved March 2013.
  10. ↑ Domning, Ray & McKenna 1986, p. 34
  11. ↑ Giant extinct mammal identified from Unalaska fossils, Alaska Dispatch News, Mike Dunham, October 7, 2015. Retrieved 8 October 2015.
  12. ↑ Photos: New species of extinct marine mammal identified from Unalaska fossils, Alaska Dispatch News, Mike Dunham, October 7, 2015. Retrieved 8 October 2015.
  13. ↑ Barnes 2013, pp. 103, 108–109

References

External links

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