FOURIER-ANALYSIS APPLIED TO STEPHANOMYS (RODENTIA, MURIDAE) MOLARS - NONPROGRESSIVE EVOLUTIONARY PATTERN IN A GRADUAL LINEAGE

Size and shape are analyzed for Pliocene lineages of the rodent genus Stephanomys Schaub 1938. Previous phylogenetic studies were based main ly on size variation and descriptive comparisons, without any attempt to quantify shape changes. Hence, on the basis of regular size increas e, Stephanomys has been considered a prime example of phyletic gradual ism. In order to quantify morphological variation within the lineage, a method for analyzing complex outlines, the elliptic Fourier transfor m, was applied to tooth contour (upper and lower first molars). II was then possible to compare evolution in size, estimated by tooth area, as well as evolution of shape, represented by Fourier coefficients. Wh ile size seems to change gradually through time, morphology gives a ra ther discontinuous evolutionary pattern for both the upper and lower m olar. Such a discrepancy between the evolution of size and shape of a single structure suggests that different genetic determinisms and mech anical constraints may act on size and shape. Hence it may be misleadi ng to infer generalized evolutionary processes from either size or sha pe alone.