Muroid molar


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.

Description

Muroids are most closely related to the Dipodidae, a smaller group of rodents that includes the jerboas, birch mice, and jumping mice. 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 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.

Nomenclature

In 1977, Reig noted that eleven distinct nomenclatures had been proposed for the features of "cricetid" 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 Murinae 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.

Literature cited