Phylogeny and the problem of adaptative significance: the case of the earliest primates

Methods by which adaptations, particularly in fossils, may be ascertained, are discussed in detail. A scheme for evaluating particular types of characters is offered which should facilitate both predicting value and testability of proposed adaptational hypotheses. It is suggested that studies by Kay and Cartmill which analyzed cranial and dental characters of early Paleogene plesiadapiform primates and marshalled evidence to suggest that these features are indicative of a terrestrial habitus are based on (a) nonphylogenetic and static assumptions, (b) misinterpretation of the form and mechanical attributes of the features analyzed, and (c) employment of irrelevant characters for the establishment of substrate preference. The diverse and now studied postcranial evidence, particularly the pedal and elbow morphology of selected groups of eutherians as well as Paleogene plesiadapiforms and euprimates, is reviewed. It appears that morphological features associated with inversion of the foot and increased radiohumeral mobility in Paleogene primates were present in the common ancestor of the Plesiadapiformes. Having uniquely similar, shared and derived, form-function complexes in the upper and lower ankle, astragalonavicular, and calcaneaocuboid joints, as well as in the known remains of radii and humeri, strongly suggests that these traits evolved for biological roles which are almost invariably associated with an arboreal environment.