Muroid molar

Three molars in a bone, with narrowly connected cusps, labeled M1, M2, and M3 from the top down.
Three molars in a bone, with narrowly connected cusps, labeled m1, m2, and m3 from the top down.
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 1.0.0-1.31.0.0.3 × 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

Three molars in a bone. The cusps are broadly connected.
Three molars in a bone, with more narrowly connected cusps.
Two molars, inversed from the previous two, with the third still in the bone.
Three molars in a bone, with narrowly connected cusps.
Three molars in a bone, with broadly connected cusps.
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]

References

  1. ↑ Carleton and Musser, 2005, p. 749
  2. ↑ Ellerman, 1940, p. 561
  3. ↑ 3.0 3.1 Carleton and Musser, 1984, p. 292
  4. ↑ Emry, 2007, p. 147
  5. ↑ Reig, 1977, p. 228
  6. ↑ Helgen and Helgen, 2009, p. 240
  7. ↑ Reig, 1977, p. 232
  8. ↑ Reig, 1977, p. 235
  9. ↑ Musser and Newcomb, 1983, p. 332
  10. ↑ Voss, 1991, p. 16; Musser et al., 1998, p. 17
  11. ↑ Voss, 1991, p. 16; Zijlstra et al., 2008, p. 29

Literature cited

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