In the study of mammalian adaptation to the environment, teeth are of primary importance due to their role as one of the direct interaction points between an individual and its ecological surroundings. Here, molar shape and function are investigated through traditional multivariate statistics and Thin-Plate Splines deformations to compare the relative location of lower first molar occlusal structures (protoconid, metaconid, hypoconid, entoconid, cristid obliqua, and protolophid) in modern lemurs, lorises, tarsiers, and a non-primate outgroup taxon (Tupaia). Results suggest that shape is based both on tooth size and dietary patterns. Small teeth tend to be short (anteroposteriorally) with wide talonids, whereas larger teeth are generally characterized as being long and narrow. In considering non-size related shape trends, frugivorous and graminivorous taxa generally exhibit a relatively buccal intersection of the cristid obliqua with the base of the protolophid, and a relatively “perpendicular” position of the protolophid in relation to the anteroposterior axis of the tooth (defined as the axis connecting the protolophid and hypoconid). Morphological trends of folivores include a central (midline) position of the cristid obliqua-protolophid base intersection and an oblique angle of the protolophid. Insectivorous taxa (primate and non-primate) generally exhibit a central placement of the cristid obliqua-protolophid base intersection (as in folivores), along with a relatively perpendicular angle of the protolophid (as in frugivores). Omnivorous taxa exhibit shape patterns that are intermediate between these three former groups. This study provides a comparative baseline for the interpretation of morphological trends in fossil primate groups, particularly the Adapiformes. Anat Rec, 292:701–719, 2009. © 2009 Wiley-Liss, Inc.