Dental microwear in relation to changes in the direction of mastication during the evolution of Myodonta (Rodentia, Mammalia)

Abstract

Observations of dental microwear are used to analyse the correlation between changes in molar tooth crown morphology and the direction of masticatory movement during the evolution of Myodonta (Rodentia, Mammalia). The studied sample includes 36 specimens representing both superfamilies of Myodonta (Muroidea and Dipodoidea) spanning 16 dipodoid and 9 muroid species. Microscopic scratches on occlusal surfaces resulting from contact between opposite teeth during mastication are analysed. Using these features, we determine the direction of masticatory movements. Microwear patterns display diverse orientations among Dipodoidea: oblique in Sicistinae, Euchoreutinae and Zapodinae, propalinal in Dipodinae and intermediary in Allactaginae. Similarly, Muroidea exhibit the following orientations: oblique in Cricetinae and propalinal in Arvicolinae, Cricetomyinae, Gerbillinae and Murinae. These various chewing types illustrate different evolutionary grades within the superfamilies. Acquisition of the antero-posterior masticatory movement in Dipodoidea is related to flattening of the molar occlusal surface. However, in some muroid subfamilies, this direction of mastication is associated with low-crowned and cuspidate molars (Cricetomyinae, Murinae).

Quantitative analysis of dental microwear in hadrosaurid dinosaurs, and the implications for hypotheses of jaw mechanics and feeding

“…This provides a less error-prone yet appropriately stringent statistical test, and we therefore used it for all subsequent testing of class mean orientations. Our analysis does not support the view advocated in previous analyses of microwear orientation data (16,17,27) that axial data (i.e., distributed through 0-180°) can be treated as linear data and subjected to linear statistical tests. This approach would have led us to wrongly reject the hypothesis that mean orientation does not differ between sites, and previous analyses of this type may have made similar errors.…”

“…A few authors have acknowledged that, strictly speaking, standard tests based on properties of linear distributions are not applicable to directional data (16,17), but we are unaware of any analysis that has applied directional statistical tests to microwear data. Rather, nonparametric linear statistical tests have been applied, either with or without explicit justification (16,17,27). To determine how best to test our null hypotheses, we applied 3 different tests (1 based on linear distributions; 2 specific to axial data) to a set of class 2 scratch orientation data sampled from 7 sites along a straight line transect from tip to base of tooth 2 ( Fig.…”

The evolution of jaw mechanism and dental function in heterodont crocodyliforms

“…Butler (1985) and Weijs (1994) regarded this anteromedially directed (oblique) powerstroke as the primitive condition. Considering this hypothesis, the phylogenetic context of myodonts based on molecular data (Charles et al 2007) unambiguously shows that proal jaw movement appeared multiple times among the different myodont clades.…”

“…This type of mandibular movement occured in sphenodontians already in the Late Jurassic and Cretaceous (Throckmorton et al 1981, Apesteguía andNovas 2003) but it is also present in the extant Sphenodon (Gorniak et al 1982, Schwenk 2000, in protoroid kangaroos (Sanson 1989, Tomo et al 2007) and in various groups of the Rodentia (Gans et al 1978, Charles et al 2007, for example rats (Weijs 1975) or golden hamsters (Gorniak 1977). Although the dentition of sphenodontians and rodents are completely different from each other and also from that of Malawisuchus, several characters are similar and comparison of the listed features can help in providing more details of this movement in crocodylians.…”

“…One of these groups is Myodonta that includes the Muroidea and Dipodoidea superfamilies with 1336 and 51 species, respectively (McKenna and Bell 1997). Study of the orientation of microwear scratches on the molars revealed several independent occurrences of proal mandibular movement during powerstroke among the different myodont groups (Charles et al 2007). On the other hand, a number of myodont clades (as various primitive rodents, Weijs 1994) can be characterized by an oblique pattern (i.e.…”