The Carpolestes, which weighed about 4 ounces (100 grams), had a long tail, and a body about 14 inches (35 centimeters) long, shared some, but not all of the characteristics of modern primates, and thus can be viewed as a transitional animal. It had very primate like teeth that were highly specialized for eating flowers, seeds, and fruit. The opposable big toe gave it a grasping ability that indicates it spent most of its time climbing trees.
Carpolestes also had a nail on its big toe, but its eyes were not forward facing, and it did not have the bone structure that would allow for specialized leaping, like some of the earliest primates.
Bloch and his co-author Doug Boyer conclude that Carpolestes spent most of its time clinging to tree branches and eating fruit, rather than spotting prey or leaping for its dinner. Boyer has been working with Bloch under a National Science Foundation grant to study plesiadapiform skeletons from Wyoming and the origin of primates.
The authors speculate that as the diversity of fruits, flowers, leaf buds, and nectar increased in the Paleocene, 65 to 55 million years ago, Carpolestes took to the trees to exploit a new food source and to avoid competition with early rodents.
A small group of scientists, led by Robert D. Martin at the Field Museum in Chicago, has argued for a primate origin date of 85 million years ago, based on statistical modeling. There is no fossil evidence to support the contention, and most paleontologists rely on the fossil record to piece together the story of evolution.
Clarks Fork Basin, where the fossil was found, is particularly rich and has yielded many nearly complete and almost fully articulated Eocene (55 to 34 million years ago) fossils. The researchers are expanding their search to include Bighorn Basin in Wyoming, and Crazy Mountain Basin in Montana. The quarries are expected to produce fossils that will further scientific understanding of the evolution of primates.
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