Figure 1.
The four morpho-functional grades hypothesized in Muroidea by Butler [9] on first upper and lower molars.
First column: grades with cusp interlocking during occlusion. Second column: grades without cusp interlocking during occlusion. First line: grades displaying oblique chewing movements. Second line: grades displaying longitudinal chewing movements. Grey tinted areas on teeth delimitate wear facets while white lines display the orientation of microscratches. Arrows indicate the spatial orientation of the tooth. Full arrows indicate the occurrence of spatial components of the chewing movement in the related direction. Dotted arrows indicate no spatial component of movement in the related direction. Presence of a dorsal component of the chewing movement implies cusp interlocking. Absence of a lingual component of chewing movement implies propalinality. A. Grade B: oblique chewing and cusp interlocking associated with cuspidate tooth crown and cricetine dental plan [9]. B. Grade C: oblique chewing and non cusp interlocking associated with flattened tooth crown and cricetine dental plan [9]. C. Grade M: longitudinal chewing and cusp interlocking associated with cuspidate tooth crown and murine dental plan [9], [11]. Grade O also corresponds to this association. D. Grade D: longitudinal chewing and non cusp interlocking associated with flattened tooth crown and cricetine dental plan [9].
Table 1.
Material. For each taxa, subfamily, collection, geographical origin (with locality) and geological age are indicated.
Figure 2.
Topographic descriptors of the muroid molar tooth crown shape.
A: Dental plans in Muroidea. The cricetine dental plan refers to first upper molar teeth which display the six following cusps: LaA (labial anterocone), LiA (lingual anterocone), Pa (paracone), Pr (protocone), M (metacone) and H (hypocone). The intermediary dental plan refers to the occurrence of one single supplementary lingual cusp (in red). The murine dental plan refers to the occurrence of two or more supplementary lingual cusps (in red and in yellow). The black arrow indicates the mesial and lingual sides of the tooth. B: Hypsodonty Index (H). l: length of the tooth crown. h: high of the tooth crown. H = h/l. C: Average orientation (O) of the lowest slope of the four main cusps of the muroid first upper molar. The orientations of the lowest slopes are observed thanks to a slope colour map with superimposed topographic contour lines (computed with Surfer for Windows). OPa: lowest slope orientation of the paracone. OPr: lowest slope orientation of the protocone. OM: lowest slope orientation of the metacone. OH: lowest slope orientation of the hypocone. O = (OPa+OPr+OM+OH)/4. D: Crown flattening index (K). K refers to the global shape of the crown topography (red line) and is calculated as the kurtosis of the distribution of the crown slope values provided for each node (black point) of the computed slope maps (3D slope colour map with superimposed topographic contour lines, computed with Surfer for Windows).
Figure 3.
Topographic investigation of tooth crown planation in mammals.
A: Topography of the left first upper molars of Atavocricetodon huberi and Gerbillus dasyurus, and the associated histograms of slope value distribution on the tooth crown, with values of Kurtosis K calculated for both taxa (3D slope map with superimposed contour lines computed with Surfer for Windows). B: Box plots showing K value distribution of Muroidea displaying cusp interlocking (n = 23, mean = −0,415) and Muroidea displaying no cusp interlocking (n = 4, mean = −1,08). Mean K values are significantly different between both groups (Student t test: P<0,001).
Table 2.
Values of functional and topographic crown descriptors for the left first upper molar of each taxa.
Table 3.
Correlation matrix between the morpho-functional descriptors CD, K, O and H.
Figure 4.
Discrimination of the masticatory grades in Muroidea (27 taxa).
PCA performed on morpho-functional parameters CD, K, O and H. PC1 (63% of variance) is strongly supported by the direction of chewing (CD) and by the average orientation of main cusps lowest slopes (O). PC2 (33% of variance) is strongly supported by crown flattening (K). The green area indicates grade B, red area grade M and blue area grade D. Grades C and O are situated in intermediary positions. Cricetine dental plans are shown by circles, intermediary plans are indicated by pentagons and murine plans are represented by squares. Full points indicate cusp interlocking (discontinuous wear facets), while empty points indicate no cusp interlocking (continuous wear facets).
Table 4.
Microwear patterns of various muroid rodents.
Figure 5.
Morpho-functional chewing grades and muroid phylogeny.
This phylogeny is adapted from complementary results from the three most recent molecular phylogenies [13]–[15] and some palaeontological hypotheses [22]–[25]. The phylogenetic position of Platacanthomyidae has not been investigated yet with molecular data. We inferred the chewing grade of taxa whose direction of chewing has already been published or was measured in this study (See Table 4). B: grade B; C: grade C; D: grade D; O: grade O; M: grade M; †: fossil taxa. 1: Platacanthomyidae; 2: Spalacidae; 3: Calomyscidae; 4: Nesomyidae. 5: Muridae; 6: Cricetidae.
Figure 6.
Convergent morpho-functional evolution revealed by microwear pattern and topographic slopes crown maps in three muroid lineages.
Cricetidae (A. Democricetodon sp., B. Rotundomys motisrotundi, C. Microtus duodecimcostatus), Gerbillinae (D. Myocricetodon irhoudi, E. Myocricetodon ouedi and F. Gerbillus dasyurus) and Nesomyidae (G. Mystromys sp., H. Dendromus sp. 2 and I. Cricetomys sp.). The morpho-functional grade (B, C, O, M or D in bold) is inferred from crossed quantitative interpretations of crown topography and microwear pattern on left M1. In the upper left quarter is a picture of a wear facet for each species. The white arrow indicates the mean direction of microscratches corresponding to the direction of chewing. White scale bar: 100 µm. A colour slope map displaying the orientation of the cusps lowest slopes is presented on the right half of the diagram for each species. Black scale bar: 500 µm. The histogram of distribution of crown slopes is presented in the lower left quarter. Unimodal histograms (Kurtosis superior to −1) indicate cuspidate crowns, with predominant intermediary slope values associated with round cusps. Bimodal histograms (Kurtosis inferior to −1) indicate flattened crowns, with predominant extreme slope values associated with angular cusps.
Figure 7.
The five morpho-functional grades recognized in the present work in muroid left M1 along with their potential evolutionary relationships.
For each grade, a computed 3D rendering with superimposed colour topographic map is presented on the left. The colour code refers to the elevation while the grey-level code refers to the slope value. Scale bar: 500 µm. The topologic map presented on the right indicates the main cusp tip (crosses) or their flattened surface (grey areas), the outline of the four main cusps (dotted lines), the orientation of their lowest slope (black arrow) and the direction of chewing (grey arrow). A. Grade B: oblique masticatory movements and cusp interlocking associated with cuspidate crown, oblique cusp lowest slope and cricetine or intermediary dental plan. B. Grade C: oblique masticatory movements and non cusp interlocking associated with flattened crown, oblique cusp lowest slope and cricetine dental plan. C. Grade O: longitudinal masticatory movements and cusp interlocking associated with cuspidate crown, oblique cusp lowest slope, and cricetine plan. D. Grade M: longitudinal masticatory movements and cusp interlocking associated with cuspidate crown, longitudinal cusp lowest slope, and murine or intermediary dental plans. E. Grade D: longitudinal masticatory movements and non-cusp interlocking associated with flattened crown, longitudinal cusp lowest slope, and murine or cricetine dental plan. During muroid radiation, Grades C, O, and M emerged from grade B and independently gave rise to grade D. The example chosen to illustrate the grades are not phylogenetically related.