Section III Adaptations and Evolution of the Postcranium

Definitions of the Order Primates always have made reference to traits of the postcranial skeleton, most notably the opposable hallux and the presence of nails instead of claws on the digits, and it always has been the received wisdom that something about an arboreal lifestyle has influenced the diagnostic limb features of primates. Several workers, especially Matt Cartmill, refined these early, vague ideas. As part of the Nocturnal Visual Predation (NVP) model he proposed a more specific relationship between hindlimb opposition and one aspect of primate-style arborealism, namely the need to balance and move slowly on small supports when stalking and capturing prey. By comparing primates with marsupials of similar habitus, Lemelin and Schmitt are able to demonstrate that additional prehensility enhancing features (pha-langeal proportions, metapoodial/phalangeal proportions) are correlated with superior ability to deal with a fine-branch substrate. Hamrick discusses the experimental and morphological evidence for the evolvability of the distal limb, explaining why the origin and adaptive radiation of primates is accompanied by high diversity in digit proportions.

There are also many behavioral aspects of primate locomotor behavior that distinguish them from most other arboreal mammals. The use of the diagonal sequence/diagonal couplets gait, emphasis on the hindlimb for support and propulsion, low stride frequencies but longer stride lengths, large angular excursions of the limbs, and the compliant gait are among them. Cartmill et al. and Lemelin and Schmitt address these behavioral differences concluding that the diagonal sequence gait is a solution to moving on small branches with a prehensile extremity. They stress that this would only be a successful strategy for an animal with relatively deliberate locomotor habits. Lemelin and Schmitt also show that Caluromys (a marsupial small-branch specialist) differs from terrestrial marsupials in sharing many of these behavioral traits, strongly suggesting that all of them are related to moving quadrupedally on small branches. Larson examines the morphological correlates of the highly protracted forelimb that results in the large angular excursion of the forelimb during quadrupedal walking. These are: a more obtuse spinoglenoid angle, a reduction in the anterior and superior projection of the greater tubercle possibly produced by an anterior shift of the humeral head. Smilodectes, the only early prosimian included in the study, appears more primitive in the humeral features than any extant primate.

In contrast to the slow moving ancestor envisioned by Cartmill and colleagues, Szalay and Dagosto, in their grasp-leaping model, propose a much more agile creature. In their view, leaping is a component of locomotor behavior equal in importance to grasping in defining the postcranial morphotype of Primates. Szalay offers a sharply critical account of previous reconstructions of the locomotor abilities of early primates and the philosophies underlying the logic of these reconstructions. Dagosto echoes these ideas, citing a number of derived leaping related features of the limb skeleton shared by all primates, including the presumably paraphyletic adapids and omomyids. She also points out the difficulty in attempting to explain all of the derived characters of a higher-level clade with a single adaptive hypothesis. A staged model for the acquisition of key locomotor related adaptations is proposed.

Bloch and Boyer describe the variation in postcranial bones and inferred locomotor adaptations present among plesiadapiform primates, identifying a variety of arboreal behavioral adaptations in this group. They find no evidence of gliding or phylogenetic links to Dermoptera among micromomyids, refuting Beard's Eudermoptera hypothesis. Believing the similarities of the hallucal-grasping complex in Carpolestes to be a synapomorphy with true primates, they posit that grasping was in place before anatomical adaptations for visual predation or leaping, thus contesting both the NVP and grasp-leaping hypotheses.

0 0

Post a comment