Continuous Tooth Replacement in a Mole-rat

Mammals are characterized by diphyodonty, only two generations of teeth, among other traits. However, a few mammals have evolved a form of continuous tooth replacement. For example, Manatees (Trichechus) add extra teeth at the back of the jaw as teeth near the front are worn away or lost. Such a conveyor-belt type of tooth replacement is exceedingly rare in mammals. The only other mammal know to exhibit continuous tooth replacement of this type is a metatherian, the pygmy-rock wallaby (Petrogale concinna). Elephants have a superficially similar type of tooth replacement, but they replace their teeth sequentially and do not form any extra teeth in the process.

As a new report in the Proceedings of the National Academy of Sciences, researchers have now added the silvery mole-rat (
Heliophobius argenteocinereus, Figure 1) to the short list of mammals with continuous tooth replacement (Gomes Rodrigues et al. 2011). These chisel tooth rodents not only possess a conveyor-like tooth replacement, but they are also hypsodont; have high crowned teeth (Figure 2).
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Figure 1. A silvery mole-rat (Heliophobius argenteocinereus). (From Wikipedia)

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Figure 2. Photos (A-C) and drawings of the teeth in Heliophoius argenteocinereus. (A-C) A lateral view of the upper and lower tooth rows with the mandible removed. (D) The pattern of dental replacement in the silvery mole-rat with the arrow indicating the direction of the movement of teeth. (From Gomes Rodrigues et al. 2011)

The researchers propose that the conveyor-like tooth replacement evolved in the silvery mole-rat because of their “hyper-chisel tooth digging behavior; they dig tunnels with their procumbent incisors thereby exposing their cheek teeth to extremely abrasive dirt, which causes their hypsodont cheek teeth to wear rapidly.

Manatees, the pygmy-rock wallaby, and silvery mole-rats undoubtedly evolved continuous tooth replacement convergently; they have different habitats, diets, and are distantly related phylogenetically. Nevertheless, all three groups share three dental traits: extra teeth (supernumerary teeth), delayed eruption, and dental mesial drift (Figure 3). The later character occurs when cheek teeth drift forward as a result of pressure from teeth erupting at the back of the toothrow.

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Figure 3. Phylogenetic tree of dental characteristics related to continuous tooth replacement in several mammals. Column one is the jugal dental formula. The remaining columns represent presence mesial drift (MD), supernumerary teeth (ST), delayed dental eruption (DE), hypsodonty (H), and in column six, replacement of a premolar (P), molar (M), or continuous dental replacement (CDR). (From Gomes Rodrigues et al. 2011)

The authors suggest that epithelial tissues involved in tooth formation may persist as a permanent dental lamina, thereby permitting continual replacement of teeth in
Heliophobius.

References
Gomes Rodrigues, H., Marangoni, P., Sumbera, R., Tafforeau, P., Wendelen, W., & Viriot, L. (2011). Continuous dental replacement in a hyper-chisel tooth digging rodent Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1109615108