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Wyoming's Garden of Eden
Natural History, April, 2001 by Kenneth D. Rose
The rich fossil record from the early Eocene Bighorn Basin includes the remains of the most ancient primates, hoofed animals, and carnivores.
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Today Wyoming's Bighorn Basin, home to pronghorns and prairie dogs, coyotes and rattlesnakes, is nearly a desert. But close your eyes and imagine a scene in this same region 54 million years ago, after a time of rapid and dramatic warming. The ancient Bighorn is a subtropical lowland floodplain, lush with vegetation and teeming with animal life. Many of the mammals are new immigrants to the basin, and some look, if not modern, then strangely familiar. It is just after daybreak, and a group of primates forages peacefully among the trees bordering a pond. Resembling lemurs, they leap from branch to branch in search of fruit and nuts. Nearby, raccoonlike animals climb through the trees; occasionally one stops to groom itself, using its front teeth. On the forest floor, two rabbit-sized, hoofed animals hop through the low underbrush. Browsing on herbs and shoots, they are apparently oblivious to a herd of dawn horses, no bigger than beagles, running by. Suddenly the serenity is broken as a giant bird, recalling the mythological roc, barges through the brush, creating a momentary panic among the dawn horses. As they scatter, they are startled by a bear-sized Pachyaena with a monstrous head and threatening jaws. This time, however, the Pachyaena merely wants to scavenge a carcass. Some distance away, a pair of tapirlike animals ambles through a swamp while a hippolike Coryphodon and its baby stand in the still waters and feed on aquatic plants.
For those of us who study Eocene mammals of the Bighorn Basin, such scenarios are easy to envision, for the life and death of the region's ancient inhabitants are recorded in the fossils that abound in the badlands. Over the past 120 years, paleontologists working there have recovered remains of more than 200 species of mammals. These fossils not only tell us what early denizens of the basin looked like but also provide information about what they ate, how they moved, how they were related to one another, and what habitats they lived in. Tooth marks on bones reveal attacks or scavenging by ancient carnivores; some bones are even broken in ways that suggest an individual was killed by an owl. Bones that lay exposed long after the animals died often have characteristic gnaw marks left by rodents, or cracks due to drying.
Each summer for the past twenty years, often in collaboration with Tom Bown of the U.S. Geological Survey, I have led a group of students and associates to the Bighorn Basin to collect fossils of early Eocene vertebrates. Of particular importance to paleontologists, the early Eocene--from about 55 to 52 million years ago--is the interval when many of the modern orders of mammals, including our own, first appeared. The more than 50,000 specimens we have amassed from the 2,000- to 3,000-foot-thick sediments of the basin's Eocene outcrops now reside at the Smithsonian's National Museum of Natural History. Our collection, together with the others unearthed from the Bighorn Basin, constitutes the world's richest and most diverse record of the vertebrate life that existed during the warmest climatic interval since the demise of the dinosaurs. These specimens also provide evidence--some of the most detailed in the entire vertebrate fossil record--of how certain animals evolved, in many cases showing a continuous, gradual transformation from one species to another through time.
Like most Eocene fossils, those in the Bighorn Basin are almost always fragmentary, consisting mainly of isolated bones, jaw fragments, and teeth. While teeth can tell us a lot, other parts of the skeleton are essential if we are to understand many other aspects of these mammals' lifestyles and relationships. One of the main goals of our expeditions has been to find mammal fossils other than teeth and jaws. By concentrating on ancient soils whose color and other sedimentary features indicate that they are likely to preserve skeletal material, we have been rewarded with hundreds of partial skeletons of nearly fifty kinds of mammals. These include the most ancient primates, hoofed animals (ungulates), carnivores, and rodents, as well as many extinct groups. We can now re-create the mammalian life of the Eocene in considerable detail.
Impressive numbers of the lemurlike Cantius inhabited the newly formed basins and ranges of the Rocky Mountain region during the early Eocene. Thousands of its jaws and hundreds of isolated bones have been found in the Bighorn Basin. So far, however, only three partial skeletons have been unearthed. From these we know that Cantius was one of the earliest of the primates, the group that now includes monkeys, apes, and humans. It was an agile climber, with long hind limbs for leaping, nails on its digits, and a grasping big toe. Compared with other early Eocene mammals, Cantius also had a large brain and relatively large eyes. In all these characteristics, Cantius resembles modern primates and differs from the so-called archaic primates that were common in the Paleocene, the 10-million-year period after the dinosaur extinction and just before the Eocene.
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