WYOMING RAINFOREST

Early–Middle Eocene

Bridgerian Land Mammal Age, 50.3 to 46.2 million years ago

 

During the early and middle parts of the Eocene Epoch (approximately 46 to 50 million years ago), North America was a much warmer, wetter continent than it is now. Humid rainforests were common across much of the western United States, which was home to primates, alligators, and other animals unknown in the region today. Drier, forested environments existed in the shadow of the uplifting Rocky Mountain ranges, adding to the diversity of the continent’s ecosystems.

As mammals evolved to fill many of the ecological niches left open by the extinction of dinosaurs 66 million years ago, they began to reach larger body sizes. The first truly giant mammals, the rhinoceros-sized uintatheres, evolved at this time in Asia and then migrated into North America across the Bering land bridge. These wide-hipped herbivores—they had elephant-sized pelves on rhino-sized bodies—were only distantly related to modern hoofed mammals, and they did not survive the Eocene. They probably used their ornately horned heads and elongated canines for display and combat with others of their species.

Most other mammals of the time were much smaller than uintatheres but interesting in their own ways. Together they made a curious mixture of old, new, and odd. Certain species of now-extinct groups might have borne superficial resemblances to cats, dogs, or other living forms—and occupied similar roles in their ecosystems. But others, such as tillodonts, taeniodonts, and palaeanodonts, were genuinely bizarre-looking members of this archaic fauna, whose habits are not always easily interpreted. Some early members of modern animal lineages were present, but many of these (such as primates and tapirs) no longer live in the U.S. Indeed, a journey through the rainforests of Eocene Wyoming, where ancient primates sported through broad-leaved palm trees above crocodile-haunted rivers, would have seemed more like a hike through Central America.

The early-middle Eocene mural in The Age of Mammals in North America exhibit at the Smithsonian’s National Museum of Natural History, 1960s.

Evidence for the Ancient World

Two geologic formations in particular have provided the bulk of fossils from this time in North America. Spectacular early Eocene fossils have been found in the Green River Formation, a thick rock layer that was formed by three large ancient lakes that once spanned adjoining parts of Utah, Colorado, and Wyoming. The fine-grained sediments in these beds allowed for exquisite preservation of fossils of animals that lived in and near the lakes, including mammals, reptiles, birds, and fishes. More terrestrial fossils have come from the Bridger Formation, which formed from the deposition of stream and floodplain sediments adjacent to these big lakes. Plants of the early-middle Eocene are also well known, thanks to fossils from many sites in both formations. Together, the two strata offer an unusually good understanding of both the plants and the animals of this time.

Historically, these beds were first explored along the route of the First Transcontinental Railroad, which, after 1869, provided access to early paleontologists traveling west. From the Green River and Fort Bridger, in Wyoming Territory, they could access the extensive fossil-filled badlands. This region became another battlefield in the “Bone Wars” between Edward Cope and Othniel Marsh (see this page and this page), and their successes lured many others west in subsequent years, including Henry Fairfield Osborn (from the AMNH) and William Berryman Scott (from Princeton), who collected and studied Bridger Formation mammals in detail.

One valley, 15 miles west of Kemmerer, Wyoming, has yielded an almost endless supply of fossil fishes—often alongside fossils of plants and other animals—on creamy white slabs of marl that formed in ancient Fossil Lake. In 1972, Fossil Butte National Monument was set aside to protect this exceptional set of Green River Formation outcrops.

The Creation and Context of the Mural

The early–middle Eocene mural was the first that Matternes started and the first that he finished. He began working on it in 1957 under the direction of curator C. Lewis Gazin, who provided a list of mammal species to be included and general direction about the content of the scene itself. The focus was the mammals of the Bridgerian Land Mammal Age, specifically the fossils from the Bridger Formation, and the mural depicts a terrestrial community that lived alongside a body of water (which appears only in the lower right corner of the image).

Smithsonian preparator Norman Boss mounting the skeleton of the lizard Saniwa, 1933.

The early-middle Eocene mural as it is currently displayed at the New Mexico Museum of Natural History and Science.

At the NMNH, the mural was exhibited alongside a broad display of fossil mammals of the time, selected from the Smithsonian’s collections. As with the late Eocene (then called the Oligocene) exhibit, an array of fossil skulls was displayed to show the variety of sizes and diets among these largely archaic mammals. Reptiles were also included, specifically skeletons of a large crocodile and the lizard Saniwa. Matternes finished the mural in 1960.

The museum’s decision to focus on the Bridger Formation ecosystem in particular meant that few Green River fossils were included in the exhibit. Later, curators decided to redevelop the display to focus on Green River specimens, including birds, fishes, and a huge palm frond from an Eocene lake bed. The museum hired Robert Hynes to create a new mural depicting the ancient lake shoreline. Matternes’s early–middle Eocene masterpiece was loaned to the New Mexico Museum of Natural History and Science in Albuquerque, where it is still on display.