Patterns of integration and modularity in the primate skeleton: a review

The question of how complex morphologies evolve, given constraints imposed by genetic, developmental and functional factors, has been a topic of inquiry for many decades. In the mid-twentieth century the study of morphological trait covariation, and the implications of this for evolutionary diversification, was developed under the general concept of "morphological integration". Given the polygenic inheritance model underlying quantitative skeletal traits, and the existence of differential pleiotropic effects, it is assumed that variation in the genotype to phenotype map will lead to the emergence of semi-autonomous "modules" that share relatively stronger covariance (integration) among traits within them. Understanding these potential patterns of modularity in the primate skeleton is important for clarifying the seeming inconsistencies presented by "mosaic" morphologies found in fossil taxa, as well as providing hypothetical units of morphological evolution that can be compared across the primate order. A review of the primate skeletal integration and modularity literature was conducted with the aim of assessing (i) the general nature of primate skeletal integration patterns, and (ii) the extent to which any identified modularity patterns are ubiquitous across primates. The vast literature on cranial integration reveals some consistency in suggesting that the face and the neurocranium (and in some cases, the basicranium and vault) form distinct modules, but the intensity of this modular pattern varies across taxa. The much more modest postcranial integration literature suggests that apes show overall reduced covariation among skeletal regions compared with other anthropoid taxa, but the extent to which any identified modularity patterns hold true across primates is still very unclear. While much has been learned about primate skeletal integration in the past two decades, we still need more studies that establish benchmarks as to what constitutes an integrated modular structure, and that empirically test these potential modules across a wider range of primate taxa.