Simplicidentata is a group of mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s that includes the rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s (order Rodentia) and their closest extinct relatives. The term has historically been used as an alternative to Rodentia, contrasting the rodents (which have one pair of upper incisors) with their close relatives the lagomorphs (which have two). However, Simplicidentata is now defined as including all members of Glires

Glires

Glires is a clade consisting of rodents and lagomorphs . This hypothesis that these form a monophyletic group has been long debated based on morphological evidence, although recent morphological studies strongly support monophyly of Glires...

 (the clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 formed by lagomorphs and rodents) that share a more recent common ancestor with living rodents than with living lagomorphs. Thus, Simplicidentata is a total group that is more inclusive than Rodentia, a crown group

Crown group

A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

 that includes all living rodents, their last common ancestor, and all its descendants. Under this definition, the loss of the second pair of upper incisors is a synapomorphic (shared derived) feature of Simplicidentata. The loss of the second upper premolar

Premolar

The premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...

 (P2) has also been considered as synapomorphic for Simplicidentata, but the primitive simplicidentate Sinomylus does have a P2.

This sense of Simplicidentata was introduced by Chuankui Li and colleagues in 1987, who ranked Simplicidentata as a superorder including Rodentia and the extinct Mixodontia, contrasted with the superorder Duplicidentata (including Lagomorpha and the extinct Mimotonida). In their 1997 book Classification of Mammals, Malcolm C. McKenna and Susan K. Bell ranked Simplicidentata as a mirorder within the grandorder Anagalida (also including lagomorphs, macroscelideans, and some additional extinct groups). Within Simplicidentata, they recognized the orders Mixodontia (including only the extinct family Eurymylidae

Eurymylidae

Eurymylidae is a family of extinct simplicidentates. Most authorities consider them to be basal to all modern rodents and may have been the ancestral stock whence the most recent common ancestor of all modern rodents arose...

 from the Paleocene

Paleocene

The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

 and Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

of Asia) and Rodentia. McKenna and Bell's decision to use Simplicidentata was criticized by reviewer Frederick S. Szalay, who preferred to simply place the Mixodontia within Rodentia, which would leave Simplicidentata unnecessary. In The Beginning of the Age of Mammals (2006), Kenneth Rose recognized a mirorder Simplicidentata, including Mixodontia, Rodentia, and the genus Sinomylus (not placed in either order), within the superorder Anagalida.

Literature cited

• Landry, S.O., Jr. 1999. A proposal for a new classification and nomenclature for the Glires (Lagomorpha and Rodentia). Mitteilungen aus dem Museum für Naturkunde in Berlin, Zoologische Reihe 75(2):283–316.
• McKenna, M.C. and Bell, S.K. 1997. Classification of Mammals: Above the species level. New York: Columbia University Press, 631 pp. ISBN 978-0-231-11013-6
• McKenna, M.C. and Meng, J. 2001. A primitive relative of rodents from the Chinese Paleocene. Journal of Vertebrate Paleontology 21(3):565–572.
• Meng, J. and Wyss, A.R. 1994. Enamel microstructure of Tribosphenomys (Mammalia, Glires): Character analysis and systematic implications. Journal of Mammalian Evolution 2(3):185–203.
• Meng, J. and Wyss, A.R. 2001. The morphology of Tribosphenomys (Rodentiaformes, Mammalia): phylogenetic implications for basal Glires. Journal of Mammalian Evolution 8(1):1–71.
• Meng, J., Hu, Y. and Li, C. 2003. The osteology of Rhombomylus (Mammalia, Glires): implications for phylogeny and evolution of Glires. Bulletin of the American Museum of Natural History, 275:1–247.
• Meng, J., Kraatz, B.P., Wang, Y., Ni, X., Gebo, D.L. and Beard, K.C. 2009. A new species of Gomphos (Glires, Mammalia) from the Eocene of the Erlian Basin, Nei Mongol, China. American Museum Novitates 3670:1–11.
• Rose, K.D. 2006. The Beginning of the Age of Mammals. Baltimore: The Johns Hopkins University Press, 428 pp. ISBN 9780801884726
• Szalay, F.S. 1999. [Review of Classification of Mammals: Above the Species Level] (subscription required). Journal of Vertebrate Paleontology 19(1):191–195.