Simplicidentata is a group of

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s that includes the

rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...

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

lagomorph The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families: the Leporidae (hares and rabbits) and the Ochotonidae (pikas). The name of the order is derived from the Ancient Greek ''lagos'' (λαγ� ...

s (which have two). However, Simplicidentata is now defined as including all members of

Glires Glires (, Latin ''glīrēs'' 'dormice') is a clade (sometimes ranked as a grandorder) consisting of rodents and lagomorphs (rabbits, hares, and pikas). The hypothesis that these form a monophyletic group has been long debated based on morphologic ...

(the

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

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 In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

that is more inclusive than Rodentia, a

crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

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 The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth ...

(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 Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

including Rodentia and the extinct Mixodontia, contrasted with the superorder

Duplicidentata The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families: the Leporidae (hares and rabbits) and the Ochotonidae (pikas). The name of the order is derived from the Ancient Greek ''lagos'' (λαγ� ...

(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 Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

within the

grandorder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

Anagalida Anagaloidea is an extinct order of placental mammals that first appeared during the Paleocene epoch. Taxonomy According to the traditional (morphological) view, Anagaloidea is part of the superorder Anagalida, along with the elephant shrews, rod ...

(also including lagomorphs, macroscelideans, and some additional extinct groups). Within Simplicidentata, they recognized the orders Mixodontia (including only the extinct family

Eurymylidae Eurymylidae is a family (biology), family of extinct Simplicidentata, simplicidentates. Most authorities consider them to be Basal (phylogenetics), basal to all modern rodents and may have been the ancestral stock whence the most recent common a ...

from the

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...

and

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

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.Rose, 2006, pp. 9, 315

References

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. *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. *Szalay, F.S. 1999
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