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Laotian rock rat

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Laotian Rock Rat
File:Laonastes aenigmamus.jpg
Laonastes drawing by R.J. Timmins
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Mammalia
Order:
Rodentia
Suborder:
Hystricomorpha
Family:
Diatomyidae
Genus:
Laonastes

P. D. Jenkins et al., 2005
Species:
L. aenigmamus
Binomial name
Laonastes aenigmamus
(Jenkins, Kilpatrick, Robinson, Timmins, 2005)

The Laotian rock rat or kha-nyou (Laonastes aenigmamus), sometimes called the "rat-squirrel") is a rodent species of the Khammouan region of Laos. The species was first described in a 2005 article by Paulina Jenkins, et al., who considered the animal to be so distinct from all living rodents that they placed it in a new family, Laonastidae. This has since been refuted by Mary Dawson, et al. (2006) who argued that it belongs to an ancient fossil family, Diatomyidae, which had been thought to have been extinct for 11 million years. It would thereby represent a Lazarus taxon. The animals resemble large dark rats with a haired thick tail. Their skulls are very distinctive and have features that separate them from all other living mammals.

A new family or a Lazarus taxon

Upon their initial discovery, Jenkins et al. (2004) considered the Laotian Rock Rat to represent a completely new family. The discovery of a new species of extant mammal happens periodically (see Leaf muntjac or Saola). The discovery of a completely new family by comparison is much more unusual. The most recent incident prior to the discovery of the family Laonastidae of the Laotian Rock Rat by western science was the discovery of the Bumblebee Bat (Craseonycteris thonglongyai; family Craseonycteridae) in 1974. The only other examples from the 1900s are represented by species that are only considered distinct families by a few authorities. These discoveries are: Chinese River Dolphin (Lipotes vexillifer; family Lipotidae) in 1918, Zagros Mouse-like Hamster (Calomyscus bailwardi; family Calomyscidae) in 1905, and Goeldi's Marmoset (Callimico goeldii; family Callimiconidae) in 1904. Kalinowski's Opossum (Hyladelphys kalinowskii), described only in 1992, is currently assigned to the family Didelphidae, but it may represent its own family. Representatives from all the remaining mammal families with living representatives (~30) were discovered prior to 1900.

Jenkins et al. (2004) did not compare the specimens to known rodent fossils. After such a comparison, Dawson et al. (2006) determined that the Laotian Rock Rat belongs to a previously described family, that had only been known from fossils, the Diatomyidae. The Diatomyidae are known from a series of fossils from the Early Oligocene (~32.5 Ma) until the Miocene (~11 Ma). The discovery of the Laotian Rock Rat means that an 11 million year gap exists in the fossil record where no diatomyids have been found. Dawson et al. (2006) described the Diatomyidae as a Lazarus taxon due to this gap. The only other comparable length of time for a mammal Lazarus taxon is the monito del monte, which is part of a family (Microbiotheriidae) also most recently known from Miocene deposits. Mary Dawson described Laonastes as the "coelacanth of rodents." (MSNBC), (Science)

Etymology

The genus name for this animal, Laonastes, means "inhabitant of stone". This is in reference to its presence around limestone rocks. The specific epithet aenigmamus means "enigmatic mouse" referring to its unknown position among the rodents.

Discovery

The first specimens were found for sale as meat at a market in Thakhek, Khammouan in 1996. Remains of three additional animals were obtained in 1998 from villagers and in an owl pellet. As of 2005, no Laotian Rock Rats have been observed alive by Western scientists. Interestingly, the researchers also obtained two additional rodent species and one insectivore on that expedition that were unknown to science. These animals, however, could be assigned to known genera (Leopoldamys, and the insectivore Hylomys) or subfamilies (as in the case of Saxatilomys in the rodent subfamily Murinae).

Description

The animals look like rats with furred thick tails. They are about 26 cm long with a 14 cm tail and weigh about 400 g. Jenkins et al. (2004) described the jaw as hystricognathous, but Dawson et al. (2006) argued it is sciurognathous. The infraorbital foramen is enlarged, consistent with a hystricomorphous zygomasseteric system. The pterygoid fossa do not connect to the orbit, setting them apart from the hystricognathous rodents.

Natural history

Laotian Rock Rats are found in regions of karst limestone. They appear to be found only among limestone boulders on hillsides. Villagers know the animal, calling it kha-nyou, and trap the rodents for food. The animals are presumed to be nocturnal.

These rock rats appear to be predominantly herbivores, eating leaves, grass and seeds. Insects may be eaten as well, but probably not in high abundance. Females may give birth to a single young.

References

  • Dawson, M. R., L. Marivaux, C.-k. Li, K. C. Beard, and G. Métais. 2006. Laonastes and the "Lazarus effect" in Recent mammals. Science, 311:1456-1458.
  • Jenkins, P. D., C. W. Kilpatrick, M. F. Robinson, and R. J. Timmins. 2004. Morphological and molecular investigations of a new family, genus and species of rodent (Mammalia: Rodentia: Hystricognatha) from Lao PDR. Systematics and Biodiversity, 2:419-454.
  • Wilson, D. E. and D. M. Reeder. 1993. Mammal Species of the World: A Taxonomic and Geographic Reference. Washington D.C., Smithsonian Institution Press.
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