Eocene Mammals

Fifty million years ago, the Western Interior of North America was blanketed with a tropical rainforest that resulted from a long period of global warming. Since the 1990s the Denver Museum of Nature & Science has been collecting fossil vertebrates from sites in Wyoming and Colorado that range in age from about 54 to 45 million years old. The Museum has worked in the Wind River and Green River basins of Wyoming and the Sand Wash Basin of Colorado to collect fossils that are 54 to 50 million years old, 50 to 46 million years old and 47 to 45 million years old in these areas respectively. More than 10,000 specimens have been discovered which give a rich picture of the changes in faunas through the warmest period of earth history over the past 65 million years since the extinction of the dinosaurs.

Colorado and Wyoming were teeming with wildlife that included the dawn horse, early lemur- and tarsier-like primates, squirrel-like rodents, early even-toed ungulates, tapirs and rhinoceroses, crocodiles and turtles, and a group of bizarre and unusual animals that have since gone extinct. Over the next several years an on-line resource will be developed which will provide in-depth information on the ecology, anatomy and natural history of these ancient creatures. The online guide will include keys to the identification of fossils as well as illustrations of what these animals looked like.

The tarsier-like omomyid primate, Shoshonius cooperi, was found in both Colorado and Wyoming between 51 and 50 million years ago. It was about the size of a house mouse and lived in the upper reaches of the canopy in the tropical rainforest. It is a close relative of the modern tarsier that today lives in Southeast Asia. Many different kinds of these small primates lived in Western North America during the peak time of Global Warming. Its likely that they fed on high energy insects, fruits and seeds.

Recent research has resulted in the description of a new hypercarnivorous creodont – Malfelis badwaterensis (“the bad cat from Badwater, Wyoming") which was the largest carnivore and potentially the largest mammal of its time. Badlands in Central Wyoming east of Casper. The Davis Ranch locality preserves an abundance of 50 million year old creatures including Shoshonius cooperi, crocodiles, the primate Notharctus, and over 80 species of different kinds of mammals, making it one of the richest fossil sites known. The fossils occur in both the red and gray mudstones of the Wind River Formation but are more common in the red beds. These red beds are the remnants of ancient soils where tropical forest once covered the landscape.

Research Updates