CHEEK TOOTH ADAPTATIONS IN CRICETIDS, MICROTOID CRICETIDS AND ARVICOLIDS

During the Late Miocene various lineages of cricetids developed similar dental adaptations toward increasing hypsodonty. The acquisition of hypsodont molars was a remarkable advantage, because high-crowned cheek teeth are more adapted to wear and therefore are more suitable for a diet of abrasive grasses (e.g., Gramineae), which were gaining prominence on a worldwide scale at this time (Fortelius et al. 2003). A second adaptation of certain cricetid rodents, the development of a more complex crown pattern of the cheek teeth, began slightly before hypsodonty was achieved. It also evolved progressively and had the same advantage, increasing the amount of enamel available for chewing and ensuring that a chewing surface with enamel ridges (cutting edges) persisted throughout life.

Complexity of tooth crown pattern was in the form of transverse infolding of the enamel walls of the sides of the teeth in these muroids, and this produced a series of transverse ridges, or lophs. This lophodont condition was ideal for chewing by moving the teeth horizontally across each other. This type of chewing is comparable to sharpening an axe by holding a file parallel to the cutting edge and drawing it parallel to the axe head; all grinding points come into contact with the cutting edge of the axe, but for a very short stroke. In combination with increased hypsodonty, the lophs could be worn flat, producing a series of enamel edges at the site of infolded enamel walls of the lophs. A Morpho-functional model of evolution towards longitudinal masticatory movement from Cricetids to Arvicolids is demonstrated by Lazzari et al. (2008). Propalinal chewing, moving the teeth forward and backward over each other, greatly increases the number of enamel-to-enamel contacts with one stroke, maximizing return for muscle effort in chewing; it also evolved in many other types of mammals, e.g., elephants.

Evidence indicates that development of hypsodonty in cricetid molars happened simultaneously in several lineages and in different areas of Eurasia and North America in response to a grazing habit (e.g., Fejfar and Heinrich 1983; Repenning 1987; Repenning et al. 1990; Fejfar and Repenning 1992; Fejfar 1999; Kälin 1999; Bell 2000).

Lineages have yet to be traced through the labyrinth of varied combinations of specialization by molar hypsodonty, complexity, and chewing methods. The fossil record of microtoid cricetids suggests that dental complexity followed two patterns: first, initial opposition of the cusps to form complete cross-lophs on the teeth; and second, initial alternation of the cusps to produce alternating hemi-lophs on the molars. The latter apparently led to the particular arvicolid condition characterised by the increasing development of alternating prismatic columnar cusps on high-crowned molars. However, the fossil record reveals that some microtoid cricetids gradually changed well-developed lophodont molars to cheek teeth with alternating triangular hemi-lophs, indicating an independent evolutionary pathway to the arvicolid condition.

In any case, alternating prismatic triangles seem to be the optimum condition in hypsodonty and complexity of molar structure, presumably because they reduce by half the force needed to perform propalinal chewing at any specific moment. Nevertheless, efficient propalinal chewing required some modification of jaw musculature, and this appears to be another typical arvicolid specialization, judging from its general absence in microtoid cricetids (Repenning 1968).

In this paper, selected true early arvicolids and microtoid cricetids are reviewed and possible relationships between microtoid cricetids and their ancestors are discussed briefly. The molars of the considered rodent taxa are arranged morphologically (typologically) in eight tooth groups (A-H), ranging from brachyodont-bunodont to hypsodont-prismatic cheek teeth (Figure 1, Figure 7). It is beyond the scope of this paper to provide a taxonomic classification, which can only follow clarification of many uncertainties rendered in the following discussions.