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Laotian rats (Family Diatomyidae)

The family Diatomyidae contains hystricomorphous, sciurognathous rodents found in Asia. There is one living species, Laonastes aenigmamus. Before this animal was discovered, the family was known only from fossils ranging from Early Oligocene fossils of Fallomus in Pakistan, 32.5 million years ago in the Early Oligocene (32.5 Ma; [2] ) to Middle/Late Miocene fossils of Diatomys dating back to 11 milion years ago. (11 Ma) of Diatomys.

Jenkins et al. (3) reported the discovery of a new species of rodent, Laonastes aenigmamus, for which they created a new family, Laonastidae. They suggested it was a hystricognath rodent, basal to all other hystricognaths. Dawson et al. (4) used morphology and fossil taxa to determine that Laonastes is sciurognathous and belongs to the Diatomyidae. They described the Diatomyidae as a Lazarus taxon as there was an the 11 million year gap between the most recent fossil diatomyid and the existence of Laonastes today. Mary Dawson called Laonastes the "coelacanth of rodents" [1].

The Diatomyidae are similar to the Ctenodactylidae and the Anomaluromorpha in being hystricomorphous and sciurognathous. The enlarged masseteric fossa in diatomyids extends to below the first cheek tooth (p4). The enamel on incisors is multiserial (similar to the springhare, gundis and Hystricognathi). The single premolar on both toothrows is enlarged (unlike the reduced state in gundis). Most diatomyids have cheek teeth with four roots except for p4. In Laonastes, the lower molars have four roots, but upper cheek teeth have three roots including a U-shaped anterior root that may be derived from the merging of two roots.

The Laotian rock rat occurs in the Khammouan region of Laos. Fossil diatomyids lived in Pakistan, India, Thailand, China, Japan, and Saudi-Arabia.

Phylogenetic analyses by Jenkins et al. (3) were inconclusive and contradictory. Morphological and molecular studies suggested Laonastes is a member of the rodent suborder Hystricognathi. The morphological analysis suggested that it is the most basal hystricognath.

Analysis of mtDNA 12S rRNA and cytochrome b sequence suggested that Laonastes may be related to living African hystricognaths, such as the dassie rat and naked mole rat. Analysis on the cytochrome b sequence data produces the same result as morphology. Neither analysis showed entirely robust statistical support for the position of Laonastes within the hystricognaths; it seemed to belong among the basal African radiation.

Dawson et al. (4) refuted the idea that Laonastes is a hystricognath and argued that the mandible is sciurognathous. They evaluated Laonastes compared to several fossil rodents and determined that it is closely related to the diatomyids, particularly Diatomys. Their results suggested that the Diatomyidae are a sister group to the Ctenodactylidae and this diatomyid/ctenodactylid clade (along with the Yuomyidae) is sister to the Hystricognathi.

Besides Laonastes, other diatomyids have also been placed in different families. Mein and Ginsburg (5) and McKenna and Bell (6) placed Diatomys in the Pedetidae). Flynn et al. (2) placed Fallomus in the extinct family Chapattimyidae. Mein and Ginsburg (7) erected the family Diatomyidae asa member of the superfamily Ctenodactyloidea. Marivaux et al. (8) united the two into one family (Diatomyidae), but also suggested that this family might be related to the Pedetidae.

The fossil study of Dawson et al. (4) was corroborated by the more comprehensive DNA sequence analyses of Huchon et al. (9), which suggested a roughly Lutetian, about 44 million years ago in the Early/Middle Eocene divergence date between the ancestors of the Laotian rock rat and the African gundis, which are each other's closest living relatives. The present-day distribution, fossil record and Eocene paleogeography indicate that this divergence probably occurred in one of three regions. The lineages may have split in Eurasia, somewhere in the Zagros Mountains or adjacent ranges of the Alpide belt, which formed a rugged and broken coastline with many offshore islands, as they emerged from the shrinking Tethys Sea. Alternatively, the entire Ctenodactyloidea may be of African origin or the lineage split took occurred on India as it joined the Asian mainland; the gundis reached Africa via the Mascarene Plateau's archipelagos and island continents. Each hypothesis would unite the paleontological, anatomical and molecular findings into a robust model and depends on whether the Hystricomorpha are considered Laurasian or Gondwanan in origin.

The family includes the following species: Fallomus razae, ginsburgi and quraishyi; Diatomys shantungensis and liensis; Marymus dalanae; Pierremus explorator and ladakhensis; Willmus maximus; Laotian rock rat (Laonastes aenigmamus)

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