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Muroid molar
Rodents 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.
==Description==
Muroids are most closely related to the Dipodidae, a smaller group of rodents that includes the jerboas, birch mice, and jumping mice.〔Carleton and Musser, 2005, p. 749〕 Jerboas have a dental formula of , including incisors 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.〔Ellerman, 1940, p. 561〕 In contrast, all muroids lack the P4,〔Carleton and Musser, 1984, p. 292〕 but some species of ''Pappocricetodon'' from the Eocene of Asia, one of the earliest known muroids, do have a P4.〔Emry, 2007, p. 147〕 Some have suggested that the first molar in muroids is in fact a retained deciduous premolar, but this hypothesis has been discredited.〔Reig, 1977, p. 228〕 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.〔Helgen and Helgen, 2009, p. 240〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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