Muroid molar
Encyclopedia
Rodent
s of the superfamily Muroidea
, which includes mice, rats, voles, hamsters, bamboo rats, and many other species, generally have three molars
in each quadrant of the jaws. A few of the oldest species retain the fourth upper premolar
, and some living species have lost the third and even the second molars. Features of the molar crown are often used in muroid taxonomy
, and many different systems have been proposed to name these features.
, a smaller group of rodents that includes the jerboa
s, birch mice
, and jumping mice. Jerboas have a dental formula of , including incisor
s in the upper and lower jaws, three molars in the upper and lower jaw, and in most species a small premolar
(the fourth upper premolar, P4) in the upper jaw only. In contrast, all muroids lack the P4, but some species of Pappocricetodon from the Eocene
of Asia, one of the earliest known muroids, do have a P4. Some have suggested that the first molar in muroids is in fact a retained deciduous premolar, but this hypothesis has been discredited. Several species have lost the M3 and/or m3, and four species of the New Guinea murine
genus Pseudohydromys
have also lost the M2 and m2, so that they only have four minute molars.
and for other, smaller groups.
Ontogeny
Muroids are often considered adult when the third molar has erupted. After the molars erupt, wear progressively obliterates the distinct features of the molar crown.
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 of the superfamily Muroidea
Muroidea
Muroidea is a large superfamily of rodents. It includes hamsters, gerbils, true mice and rats, and many other relatives. They occupy a vast variety of habitats on every continent except Antarctica. Some authorities have placed all members of this group into a single family, Muridae, due to...
, which includes mice, rats, voles, hamsters, bamboo rats, and many other species, generally have three molars
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
in each quadrant of the jaws. A few of the oldest species retain the fourth 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...
, and some living species have lost the third and even the second molars. Features of the molar crown are often used in muroid taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
, and many different systems have been proposed to name these features.
Description
Muroids are most closely related to the DipodidaeDipodidae
The Dipodidae, or dipodids, are a family of rodents found across the northern hemisphere. This family includes over 50 species among the 16 genera....
, a smaller group of rodents that includes the jerboa
Jerboa
The jerboa form the bulk of the membership of the family Dipodidae. Jerboas are hopping desert rodents found throughout Asia and Northern Africa. They tend to be found in hot deserts....
s, birch mice
Birch mouse
Birch mice are small jumping desert rodents that resemble mice with a long tufted tail and very long hind legs, allowing for remarkable leaps. All variants possess a long tail of of length and weigh about . The head length lies between...
, and jumping mice. Jerboas have a dental formula of , including incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s in the upper and lower jaws, three molars in the upper and lower jaw, and in most species a small 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...
(the fourth upper premolar, P4) in the upper jaw only. In contrast, all muroids lack the P4, but some species of Pappocricetodon from the 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, one of the earliest known muroids, do have a P4. Some have suggested that the first molar in muroids is in fact a retained deciduous premolar, but this hypothesis has been discredited. Several species have lost the M3 and/or m3, and four species of the New Guinea murine
Murinae
The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. This subfamily is larger than all mammal families except the Cricetidae and Muridae, and is larger than all mammal orders except the bats and the remainder of the...
genus Pseudohydromys
Pseudohydromys
Pseudohydromys is a genus of rodent in the family Muridae.It contains the following species:* Bishop moss-mouse * Huon small-toothed moss-mouse...
have also lost the M2 and m2, so that they only have four minute molars.
Nomenclature
In 1977, Reig noted that eleven distinct nomenclatures had been proposed for the features of "cricetid" (generalized muroid) molars, by Schaub, Viret, Winge, Wood and Wilson, Hershkovitz, James, Vandebroek, Fahlbusch, Alker, Vorontzov, and Mein and Freudenthal. He concluded that none of these were satisfactory and proceeded to propose another, "unifying" nomenclature. Additional nomenclatures have been proposed for the MurinaeMurinae
The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. This subfamily is larger than all mammal families except the Cricetidae and Muridae, and is larger than all mammal orders except the bats and the remainder of the...
and for other, smaller groups.
OntogenyOntogenyOntogeny is the origin and the development of an organism – for example: from the fertilized egg to mature form. It covers in essence, the study of an organism's lifespan...
Muroids are often considered adult when the third molar has erupted. After the molars erupt, wear progressively obliterates the distinct features of the molar crown.Literature cited
- Carleton, M.D. and Musser, G.G. 1984. Muroid rodents. Pp. 289–379 in Anderson. S. and Jones, J.K., Jr. (eds.). Orders and families of Recent mammals of the world. John Wiley and Sons, New York, 686 pp.
- Carleton, M.D. and Musser, G.G. 2005. Order Rodentia. Pp. 745–752 in Wilson, D.E. and Reeder, D.M. (eds.). Mammal Species of the World: a taxonomic and geographic reference. 3rd ed. Baltimore: The Johns Hopkins University Press, 2 vols., 2142 pp. ISBN 978-0-8018-8221-0
- Ellerman, J.R. 1940. The families and genera of living rodents. Volume I. Rodents other than Muridae. London: Printed by order of the Trustees of the British Museum, 689 pp.
- Emry, R.J. 2007. The middle Eocene North American myomorph rodent Elymys, her Asian sister Aksyiromys, and other Eocene myomorphs (subscription required). Bulletin of Carnegie Museum of Natural History 39:141–150.
- Helgen, K.M. and Helgen, L.E. 2009. Biodiversity and biogeography of the moss-mice of New Guinea: a taxonomic revision of Pseudohydromys (Muridae: Murinae). Bulletin of the American Museum of Natural History 331:230–313.
- Musser, G.G. and Newcomb, C. 1983. Malaysian murids and the giant rat of Sumatra. Bulletin of the American Museum of Natural History 174:327-598.
- Reig, O.A. 1977. A proposed unified nomenclature for the enamelled components of the molar teeth of the Cricetidae (Rodentia) (subscription only). Journal of Zoology, London 181:227–241.
- Voss, R.S. 1991. An introduction to the neotropical muroid rodent genus Zygodontomys. Bulletin of the American Museum of Natural History 210:1–113.
- Zijlstra, J.S., Hoek Ostende, L.W. van den and Due, R.A. 2008. Verhoeven's giant rat of Flores (Papagomys theodorverhoeveni, Muridae) extinct after all? Contributions to Zoology 77(1):25–31.