4 - Why cheirogaleids are bad models for primate ancestors: a phylogenetic reconstruction

Summary

Introduction

Primate origins and dietary evolution

The provenance, timing, and environmental circumstances of the origin of primates and their subsequent dispersal are among the most heavily contested subjects in primate evolution. Several authors have contributed to the development of adaptive theories of primate origins (Jones, 1916; Szalay, 1968, 1972; Cartmill, 1974, 1992; Szalay and Dagosto, 1980, 1988; Sussman, 1991), proposing different models to explain the evolution of the unique combination of characteristics associated with the “adaptive shift” that marked the divergence of the primate lineage. In most recent models, diet plays the central role – not too surprisingly, as dietary evolution is one of the cornerstones for explaining the emergence of most mammalian lineages. The only recent model that does not prioritize dietary adaption is that of Szalay and Dagosto (1980, 1988), who proposed that grasping extremities and nails on the digits evolved together with leaping adaptations to facilitate grasp-leaping locomotion. All other models construe the defining primate characteristics as adaptations for food acquisition, usually for a single “ancestral diet,” despite the diversity and versatility of modern primate dietary adaptations. Two opposing models of primate dietary evolution enjoy majority support today, both of which owe more to hypothetical scenarios of evolution than they do to fossil specimens. Diet is not simple to read from the fossil record, but the fact that teeth are preserved more frequently than any other body parts, and reflect dietary composition at least in part, provides some insight into ancient primate diets. Additionally, diet coevolves with body size and locomotion, and these additional characteristics can inform our interpretations of fossil diets. Fossil data largely confirm that primate dietary diversity evolved early. Eocene strepsirrhines were clearly insectivorous, frugivorous, both insectivorous and frugivorous, and folivorous, all of which required specialist adaptations (Kirk and Simons, 2001).

Unfortunately, the fossil record for primates – and particularly for strepsirrhines – is largely incomplete. According to Soligo and Martin (2007), about 25 Ma are missing from the record, and the gaps include some key periods in primate evolution. We know very little of the evolution of primates prior to the Eocene (Silcox et al., 2007), and less about the transition between the first “primates of modern aspect” (Euprimates), represented by the extinct Eocene adapiforms and omomyoids, to crown lineages that seem to appear suddenly in the Neogene.