Upper (left) and lower (right) molar row of Megalomys desmarestii, showing commonly used nomenclature for the molars.
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.[1] Jerboas have a
dental formula of 1.0.0–1.31.0.000.3 × 2 = 16–18, 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.[2] In contrast, all muroids lack the P4,[3] but some species of Pappocricetodon from the
Eocene of Asia, one of the earliest known muroids, do have a P4.[4] Some have suggested that the first molar in muroids is in fact a retained deciduous premolar, but this hypothesis has been discredited.[5] Several species have lost the M3 and/or m3,[3] 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.[6]
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.[7] He concluded that none of these were satisfactory and proceeded to propose another, "unifying" nomenclature.[8] Additional nomenclatures have been proposed for the
Murinae[9] and for other, smaller groups.
Ontogeny of the molars in Megalomys desmarestii. From left to right: Left upper molar row of an older animal, left upper molar row of a younger animal, right upper molar row of the same young animal with the M3 not yet erupted, left lower molar row of the older animal, left lower molar row of the younger animal.
Muroids are often considered adult when the third molar has erupted.[10] After the molars erupt, wear progressively obliterates the distinct features of the molar crown.[11]
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.