Laotian Rock Rat
Classification Mammal
Location Khammouan region of Laos
Conservation Status Endangered
Scientific Name Laonastes aenigmamus

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 and coauthors, who considered the animal to be so distinct from all living rodents, they placed it in a new family, Laonastidae. It is in the monotypic genus Laonastes.

In 2006, the classification of the Laotian rock rat was disputed by Mary Dawson and coauthors. Dawson and her colleagues suggested instead it belongs to the ancient fossil family Diatomyidae, that was thought to have been extinct for 11 million years, since the late Miocene. It would thereby represent a Lazarus taxon. The animals resemble large, dark rats with hairy, thick tails like those of a squirrel. Their skulls are very distinctive and have features that separate them from all other living mammals.

A new family or a Lazarus taxon?Edit

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 mammal genus happens periodically, such as with the leaf muntjac or saola. The discovery of a completely new family is, by comparison, 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 20th century are species that are only considered distinct families by a few authorities. These discoveries are: the Chinese river dolphin (Lipotes vexillifer; family Lipotidae) in 1918, the Zagros mouse-like hamster (Calomyscus bailwardi; family Calomyscidae) in 1905, and Goeldi's marmoset (Callimico goeldii; family Callimiconidae) in 1904. Representatives from all the remaining rodent families with living representatives (approximately 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) were of the opinion that the Laotian rock rat belongs to a previously described family which had only been known from fossils, the Diatomyidae. The Diatomyidae are known from a series of fossils from the early Oligocene (~32.5 mya) until the Miocene (~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. 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".

The analysis of mtDNA 12S rRNA and cytochrome b sequence by Jenkins et al. (2004) allied Laonastes with African hystricognath rodents, namely the blesmols and the dassie rat. Support for such a placement was fair, but the exact position could not be resolved. Huchon et al. (2007) 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 place Laonastes robustly as a sister clade of Ctenodactylidae, and support an ancient divergence during the Lutetian (Early/Middle Eocene, ~44 mya). The earlier molecular study was in error due to long branch attraction and inadequate sampling.