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paleobiogeographic

Diatomyidae

Diatomyidae is a family of hystricomorphous, sciurognathous rodents found in Asia. It is currently represented by a single known living species, Laonastes aenigmamus.

"Lazarus effect"

Prior to the discovery of Laonastes, the family Diatomyidae was known only from fossils. The family has a nearly continuous fossil range from Early Oligocene fossils of Fallomus from Lower Chitarwata Formation (32.5 Ma; Bugti Member, Bugti Hills; Flynn et al., 1986) in Balochistan, Pakistan to Middle/Late Miocene fossils (11 Ma) of Diatomys.

Jenkins et al. (2004) reported the discovery of a wholly unique new species of rodent, Laonastes aenigmamus, for which they created a new family, Laonastidae. They suggested that it was a hystricognath rodent, but basal to all other hystricognaths. Dawson et al. (2006) re-evaluated the phylogenetic position of Laonastes based on morphology and included fossil taxa in their analysis. They determined that Laonastes is actually sciurognathous and that it belongs to the Diatomyidae.

Dawson et al. (2006) described the Diatomyidae as a Lazarus taxon due to the 11 million year gap between the most recent diatomyid in the fossil record and the existence of Laonastes today. 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"

Characteristics

The Diatomyidae are similar to both the Ctenodactylidae and the Anomaluromorpha in being simultaneously hystricomorphous and sciurognathous. The masseteric fossa in diatomyids is enlarged and extends to below the first cheek tooth (p4). The enamel on incisors is multiserial (similar to the springhare, Ctenodactylidae (gundis), and Hystricognathi). The single premolar on both the upper and lower toothrows is enlarged (unlike the reduced state in Ctenodactylidae). 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.

Living diatomyids are only represented by the Laotian Rock Rat from the Khammouan region of Laos. Fossil diatomyids have been recovered in Pakistan, India, Thailand, China, and Japan.

Relationship to other rodents

The uniqueness of the Laotian Rock Rat was clear upon its initial discovery. The results of the phylogenetic analyses performed by Jenkins et al. (2004) were somewhat inconclusive and contradictory. Both 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. Fossil taxa were not included in the morphological analysis.

Analysis of mtDNA 12S rRNA and cytochrome b sequence, however, suggested that Laonastes is related to such living African hystricognaths such as the Dassie Rat and the Naked Mole Rat. Another type of analysis on the cytochrome b sequence data produces the same result as morphology; Laonastes represents the earliest split among hystricognaths. Neither analysis, however, shows entirely robust statistical support for the position of Laonastes within the hystricognaths; altogether, it appeared to belong among the basal African radiation.

Dawson et al. (2006) refuted the notion that Laonastes is a hystricognath and instead argued that the mandible is sciurognathous. They evaluated Laonastes in comparison 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 that 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 (1986) and McKenna and Bell (1997) placed Diatomys in the Pedetidae (the springhare family). Flynn et al. (1986) considered Fallomus to belong to the Chapattimyidae (a completely fossil group). Mein and Ginsburg (1997) erected the family Diatomyidae and considered it to be a member of the superfamily Ctenodactyloidea. Marivaux et al. (2004) united the two into a single family (Diatomyidae), but also suggested that this family might be related to the Pedetidae.

The fossil study of Dawson et al. was corroborated by the more comprehensive DNA sequence analyses of Huchon et al. (2007), which suggested a roughly Lutetian (c.44 mya, Early/Middle Eocene) divergence date between the ancestors of the Laotian Rock Rat and the African gundis (Ctenodactylidae), which are each other's closest living relatives. Considering the present-day day distribution,m the fossil record, and Eocene paleogeography, this divergence probably took place in one of three regions. Either the lineages split in Eurasia, somewhere in today's Zagros Mountains or adjacent ranges of the Alpide belt. These at that time formed a rugged and broken coastline with many offshore islands, as they emerged from the shrinking Tethys Sea. Alternatively, the entire Ctenodactyloidea might be of African origin, or the lineage split took place on India as is joined the Asian mainland, the gundis reaching Africa via the Mascarene Plateau's archipelagos and island continents. Each hypothesis would unite the paleontological, anatomical and molecular findings into a robust model. Which one is preferred depends on whether the Hystricomorpha are considered Laurasian or Gondwanan in origin.

Species

References

  • 2006. Laonastes and the "Lazarus effect" in Recent mammals. Science, 311:1456-1458.
  • (2007): Origin and evolution of the Diatomyidae, with clues to paleoecology from the fossil record. Bulletin of Carnegie Museum of Natural History 39(1): 173-181. (HTML abstract)
  • 1986. Baluchimyinae, a new ctenodactyloid subfamily from the Miocene of Baluchistan. American Museum Novitates, 2841:1-58.
  • 2005. An Unusual Diatomyid Rodent from an Infrequently Sampled Late Miocene Interval in the Siwaliks of Pakistan, Palaeontologia Electronica Vol. 8, Issue 1; 17A:10p,
  • (2007): Multiple molecular evidences for a living mammalian fossil. PNAS 104(18): 7495-7499. (HTML abstract)
  • (2004): Morphological and molecular investigations of a new family, genus and species of rodent (Mammalia: Rodentia: Hystricognatha) from Lao PDR. Systematics and Biodiversity 2(4): 419-454. (HTML abstract). Erratum: Systematics and Biodiversity 3(3):343.
  • 2004. High-level phylogeny of early Tertiary rodents: dental evidence. Zoological Journal of the Linnean Society, 142:105-134.
  • 2003. New diatomyid and baluchimyine rodents from the Oligocene of Pakistan (Bugti Hills, Balochistan): Systematic and paleobiogeographic implications. Journal of Vertebrate Paleontology 23:420-434.
  • 2002. The role of Asia in the origin and diversification of hystricognathous rodents. Zoologica Scripta, 31:225-239.
  • 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8
  • 1985. Les rongeurs miocènes de Li (Thailande). Compte Rendus de l’Académie des Sciences, Paris, Série II, 301:1369-1374.
  • 1997. - Les mammifères du gisement miocène inférieur de Li Mae Long, Thaïlande : systématique, biostratigraphie et paléoenvironnement. Geodiversitas 19 (4) : 783-844.
  • 1998. Ctenodactyloid rodent assemblage from Kargil Formation, Ladakh molasses group: Age and paleobiogeographic implications for the Indian subcontinent in the Oligo-Miocene. Geobios 31:533-544.

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