Morphological indicators of foot posture in mammals: a statistical and biomechanical analysis

Mammalian foot posture has previously been described through the three-tiered hierarchy of plantigrady, digitigrady and unguligrady. This broadly applicable terminology has been used to categorize both living and fossil taxa, despite the fact that these postural grades, as traditionally used, do not rely on strict mechanical, kinematic or morphologic definitions. This paper redefines these terms: hind foot posture can be more accurately categorized if these definitions are based on the primary joint of flexure in the standing position. Once postures are so defined, a number of morphological features in taxa can be correlated with the use of particular postures. Bivariate and multivariate statistical analyses of morphological measurements from a large (n= 61) group of plantigrade and digitigrade eutherian mammals indicate that foot posture can be reliably detected from a relatively small number of morphological measurements on a few limb and tarsal bones (femur, astragalus, calcaneum and metatarsals). The features important in distinguishing postural grades fit expectations from biomechanical theory, and should prove useful for the prediction of foot posture in fossil taxa, particularly when complete specimens are not known.