Laotian rock rat

TheLaotian rock ratorkha-nyou(Laonastes aenigmamus,Lao: ຂະຍຸ), sometimes called the "rat-squirrel", is a species ofrodentfound in theKhammouanregion ofLaos. The species was first described in a 2005 article byPaulina Jenkinsand coauthors, who considered the animal to be so distinct from all living rodents, they placed it in a newfamily, Laonastidae. It is in themonotypicgenusLaonastes.
In 2006, the classification of the Laotian rock rat was disputed byMary Dawsonand coauthors. They suggested the rat belongs to the ancient fossil familyDiatomyidae, which was thought to have been extinct for 11 million years,since the lateMiocene. It would thereby represent aLazarus species. The animals resemble large, darkratswith hairy, thicktailslike those of asquirrel. Theirskullsare very distinctive and have features that separate them from all other livingmammals.
Upon their initial discovery, Jenkins and coauthors (2005) considered the Laotian rock rat to represent a completely new family. The discovery of a new species of an extant mammalgenus, or a new genus, happens periodically, such as with theleaf muntjacor thesaola. The discovery of a completely new family is, by comparison, much more unusual. The most recent comparable incident before this discovery by Western science was the discovery of thebumblebee bat(Craseonycteris thonglongyai; familyCraseonycteridae) in 1974. The only other examples from the 20th century are species that are only considered distinct families by a few authorities. These discoveries are: theChinese river dolphin(Lipotes vexillifer; family Lipotidae) in 1918, theZagros mouse-like hamster(Calomyscus bailwardi; family Calomyscidae) in 1905, andGoeldi's marmoset(Callimico goeldii; family Callimiconidae) in 1904. Representatives from all the remaining rodent families with living representatives (approximately 30) were discovered before 1900.
Jenkinset al.did not compare the specimens to known rodent fossils.After such a comparison, Dawsonet al.believed the Laotian rock rat belongs to a previously described family, which had only been known from fossils, theDiatomyidae, so not a new family at all, but simply one that had been believed extinct.The Diatomyidae are known from a series of fossils from the earlyOligocene(~32.5mya) until theMiocene(~11 mya). The discovery of the Laotian rock rat means an 11 million-year gap exists in the fossil record where no diatomyids have been found. Dawsonet al.described the Diatomyidae as aLazarus taxondue to this gap.The only other mammal Lazarus taxon with a comparable time gap between it and its most recently known fossil relative is themonito del monte, which is part of a marsupial family (Microbiotheriidae) also most recently known from Miocene deposits. Mary Dawson describedLaonastesas the "coelacanthof rodents".
The analysis ofmtDNA12S rRNAandcytochromebsequenceby Jenkinset al.alliedLaonasteswith Africanhystricognathrodents, namely theblesmolsand thedassie rat.Support for such a placement was fair, but the exact position could not be resolved. Huchonet al.conducted a large-scale molecular phylogeny of rodents, including representatives of all major rodent taxonomic groups, based on 5.5 kb of sequence data from four nuclear and two mitochondrial genes, and a short, interspersed element, insertion analysis including 11 informative loci. Their molecular data placeLaonastesrobustly as a sistercladeofCtenodactylidae, and support an ancient divergence during theLutetian(Early/Middle Eocene, ~44 mya). The earlier molecular study was in error due tolong branch attractionand inadequate sampling.
The genus name for this animal,Laonastes, means "inhabitant of stone" (fromGreekλαας =laas= stone, gen: λαος =laos= of stone andGreekναστης =nastes= inhabitant). This is in reference to its presence around limestone rocks and also to the country where it was recently discovered. The specific epithetaenigmamusmeans "enigma mouse" (from Greek αινιγμα =ænigmaand μυς =mus, "mouse") referring to its unknown position among the rodents (Jenkinset al., 2004).
The first specimens were found for sale as meat at a market inThakhek, Khammouan, in 1996.Remains of three additional animals were obtained in 1998 from villagers and in anowl pellet.Return trips to Laos by theWildlife Conservation Societyresearchers uncovered several other specimens.These new discoveries have prompted the suggestion that the animals may not be as rare as once thought. On June 13, 2006, David Redfield, aprofessor emeritusofFlorida State University, and Thai wildlife biologist Uthai Treesucon announced they had captured, photographed, and videotaped a live specimen of the species in the village ofDoyin Laos.
The animals look generally like rats, with thick, furred tails similar to a squirrel's, but limp. The head is large, with round ears and a somewhat bulbous bridge of the nose, and very longwhiskers. Their fur is darkslate grey, with a blackish tail. The belly is lighter, with a small, whitish area in the center. Their eyes are beady and black. They are about 26 cm (10 in) long with a 14 cm (5.5 in) tail and weigh about 400 g (14 oz). Jenkinset al.(2004) described the jaw ashystricognathous, but Dawsonet al.argued it issciurognathous. Theinfraorbital foramenis enlarged, consistent with ahystricomorphouszygomasseteric system. Thepterygoid fossaedo not connect to theorbit, setting them apart from the hystricognathous rodents.
The Laotian Rock rat is found inlimestonekarstsofKhammouan Provinceand southernBolikhamxai Province,Laos, and also in a small area ofMinh Hóa District, westernQuảng Bình Province,Vietnam.In Vietnam, it is found in the 5 communes (xã) ofThượng Hoá,Hóa Sơn,Trung Hoá,Hóa Hợp, andDân Hoá, and inPhong Nha – Kẻ Bàng National Park, near the villages of theVietic-speakingRuc,Sach, andChutethnic groups.It is also found directly across the border inHin NamnoNational Biodiversity Conservation Area, Laos. In Laos, it is most common in thePhou Hin Poun National Biodiversity Conservation Area. Nguyen et al.suggested that the Quảng Bình and Phou Hin Poun populations may be distinct and genetically isolated from each other.
Nicolas et al. found 8 major geographicalcladesand 8 to 16evolutionary significant unitsamong Laotian rock rat populations in Laos, and suggested thatLaonastesmay in fact consist of various undescribedcryptic species.
Laotian rock rats are found in regions ofkarstlimestone. They appear to be found only among limestone boulders on hillsides. Villagers in the area are familiar with the animal, calling itkha-nyou, and trapping it for food.The animals are presumed to benocturnal.
These rock rats appear to be predominantlyherbivores, eatingleaves,grass, andseeds. They may eatinsects, as well, but probably not in high abundance. Females may give birth to a single young.
Laotian rock rats appear to be quite docile and slow-moving over open ground. They walk with feet splayed outward in a gait described asduck-like. Although not ideal for mobility on open surfaces, this appears to be efficient when scrambling up and across large rocks. The sideways angle allows for greater surface area for their feet to find purchase on tilted or parallel surfaces.