Geographic variation in cranial shape was examined in 20 samples of the echimyid rodent Thrichomys apereoides from northeastern, central, and southeastern Brazil. Canonical variate analysis and a dendrogram based on geometric shape descriptors revealed 2 groups of populations diagnosable by features of cranial shape and geographic continuity. The observed morphological discontinuity between the 2 groups of populations corresponds with a gap in sampling of T. apereoides, as represented by the museum series used. A positive, significant relationship between morphological and geographic distances was found among populations within each of the 2 groups but not between populations in the 2 groups. The indirect, statistical approach suggests that discontinuity in cranial shape in T. apereoides results from historical processes of differentiation rather than from an artifact caused by a gap in the available samples.
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