December 10. Eocene mammals

The Cenozoic really is the Age of Mammals. They rose almost meteorically after the extinction at the end of the Cretaceous eliminated the dinosaurs, which must have preyed on the mammals, but probably even more important, they occupied ecological niches that in their absence mammals could fill. And especially with the Paleocene-Eocene Thermal Maximum that we talked about the other day, mammals began to diversify dramatically, and to grow in size. 

3-foot-high Coryphodon reconstruction (source)

Coryphodon was a browsing animal that appeared in the Paleocene and thrived in the Eocene. It was common in North America, where its fossils have been found well-preserved in the Wasatch Formation of Wyoming and Utah, dating to about 57 to 46 million years ago.

The Wasatch Formation is more or less between the Paleocene Ft. Union formation and its coals, which we mentioned on Dec. 4, and the Green River Formation that we talked about yesterday, but they all overlap to some extent in both time and space. Coryphodon ranged into what is now the Canadian Arctic, Europe, and China too. At about a meter high, it was one of the largest land animals at the time.  

Its name, Coryphodon, means “pointed tooth.”  It used those teeth to crunch leaves and other plant matter in the swampy places where it lived a life that must have been similar to the modern hippopotamus, but it was not closely related to hippos.

Paleocastor burrow (source)

By Oligocene time, about 25 million years ago, mammals were clearly expanding into diverse ecological niches. Paleocastor – whose name means “ancient beaver” – lived in and around lakes and swamps in what are now Nebraska and South Dakota. They made corkscrew-shaped burrows as much as 3 meters deep, which were first identified as giant freshwater sponges, but eventually one was found with the ancient beaver within it, and the marks in the burrow were tied to the animal’s teeth used to excavate the hole.

Paleocastor was the size of a large rat, but beavers and many other mammals had a tendency toward gigantism over the course of the Cenozoic. Castoroides was a beaver that lived during Pleistocene time, the glacial period, and became extinct as recently as 10,000 years ago. Castoroides grew to as much as seven feet long.

Stylinodon, another Eocene critter, had a short, pug-like face. Its name means “ribbon tooth,” for a ribbon of enamel around its canine teeth. Strong claws have been interpreted to indicate that Stylinodon dug roots and tubers for its food, but the claws and forelimbs might have been used to pull down branches and strip them of fruits. The animal grew to as much as 180 pounds (80 kg). It’s been called “an aardvark with the head of a pig,” (Janis et al., 1998) but it really doesn’t have much in common with any modern mammal. It was extinct by the end of the Eocene, one of many variations that did not survive.

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We haven’t had any geological birthdays in a while, but today we have two. Israel C. Russell was born December 10, 1852, in Garrattsville, New York. He had a wide-ranging career with the U.S. Geological Survey and the University of Michigan, and he’s probably best known for his studies in Alaska and Mono Lake, California, as well as glaciers in the U.S. Fielding Bradford Meek was born December 10, 1817, in Madison, Indiana. His career as a paleontologist took him all over the United States, and he’s perhaps best known for his work in California and in the Badlands of the Dakotas.

—Richard I. Gibson

Paleocastor burrow photo from University of Nebraska and Agate Fossil Beds National Monument. Used under creative commons license. 

Coryphodon by Heinrich Harder (public domain)

Stylinodon

Evolution of Tertiary Mammals of North America: edited by Christine Marie Janis, Kathleen Marie Scott, Louis L. Jacobs, 1998, Cambridge University Press.