The postcranial morphotype of primates

The goal of this chapter is to reconstruct aspects of the postcranial morphotype of the order Primates and to assess their significance for the positional behavior of the ancestor. What derived features of the limb skeleton are likely to have distinguished the last common ancestor of primates from more remote ancestors and what implications does this set of features have for the way of life of the ancestor of Primates? In pursuing this goal, the following questions are addressed: (1) What are the derived characters of the postcranium that characterize the most recent common ancestor of the primates? (2) What are the functional and biological role attributes of these characters individually? (3) Do the functional/biological role attributes of the traits as a whole constitute a cohesive story? Can they be explained by a single selective factor or a set of selective factors arising from a particular way of life? (4) If primate synapomorphies cannot be attributed to a single way of life, does the evidence suggest an order in which characters were added to the morphotype, and thus a plausible functional/behavioral sequence? These questions raise many issues, the primary one being, of course, what does one mean by the phrase "Origin of Primates?" This topic is dealt with further at the end of this chapter, but for immediate clarification, the intention is to explain the behavioral significance of the set of features that characterize the Most Recent Common Ancestor (MRCA; Figure 1) of crown group primates. Crown group primates are the anthropoids, lemurs, tarsiers, adapids, and omomyids. There remain few serious challenges to the hypothesis that this group of mammals shared a common ancestor relative to other extant and fossil mammals. The derived features of the postcranium that distinguish this ancestor from the outgroup are given in Table 1. A formal phylogenetic analysis of the features is not given here since it seems fairly certain from both morphological and molecular evidence that primates are part of the Euarchonta (Springer et al., this volume), and for most of the features listed in Table 1, primates differ from any of the most likely outgroups (Scandentia, Dermoptera, Plesiadapiformes, Rodentia, Lagomorpha), as well as from the majority of other mammals. The few exceptions are noted in the text. Question 2 entails having a philosophy for formulating and evaluating hypotheses about functional and biological role in fossil organisms. This is discussed in the next section. (See also Szalay, this volume.) Questions 3 and 4 ask if the set of traits can be reasonably considered to be a correlated complex-can a single niche, habitus, or way of life explain all or most of them? The most comprehensive "single niche" model for the Origin of Primates is the "nocturnal visual predation" model (NVP) developed by Cartmill (1972; 1974a; 1974b). It explains the grasping extremities, loss of claws, and optical convergence of primates as being related to a way of life involving visually directed predation in the small branch niche by nocturnal animals.