Living primates exhibit a dazzling diversity of forms--from the saucer-eyed bush babies of sub-Saharan Africa to Borneo's proboscis monkey (the Pinocchio of primates) to humans, the cosmopolitan bipeds. They are united, however, in having large brains, forward-facing eyes, nails instead of claws, an ability to grasp and an ability to leap. For almost three decades, evolutionary biologists have puzzled over how modern primates came to possess this distinctive suite of characteristics. Some workers reasoned that these features evolved to permit predation on insects, others proposed that they enabled the procurement of fruit from the tips of tree branches, and still others envisioned these traits as adaptations to a mode of locomotion combining grasping and leaping. But the scrappy fossil record of early primates--mostly teeth and isolated skeletal bones--left researchers hard put to test these hypotheses.
A spectacular find from the badlands of Wyoming is bringing some answers to light. Paleontologists recently uncovered a nearly complete 55-million-year-old skeleton of a mouse-size creature known as Carpolestes simpsoni. Like modern primates (or euprimates, as they are termed), it has long fingers and toes, as well as nails on its opposable digits--good for grasping spindly tree limbs. But unlike euprimates, this animal exhibits laterally positioned eyes and legs built for climbing, not leaping. Previously some scholars had placed carpolestids and their kin--a group known as the plesiadapiforms--in a category of gliding mammals called dermopterans. But the anatomy evident in the new specimen signifies to discoverers Jonathan I. Bloch, now at the South Dakota School of Mines and Technology, and Doug M. Boyer of the University of Michigan at Ann Arbor that Carpolestes and its fellow plesiadapiforms were in fact
This article was originally published with the title Out on a Limb.