The John Day Fossil Beds National Monument in central Oregon is one of the world’s richest fossil sites, preserving over 40 million years of plant and animal evolution. Its three units—Painted Hills, Sheep Rock, and Clarno—display colorful layers of ash and clay that reveal the story of changing ecosystems, from subtropical forests with crocodiles and palm trees to open grasslands where early horses and saber-toothed cats once roamed. I visited the Sheep Rock Unit, the monument’s largest and most diverse section, renowned for its dramatic blue-green claystone hills and striking rock formations. Here, fossil-rich deposits record more than 30 million years of evolutionary history, showcasing ancient environments where oreodonts, nimravids, and three-toed horses once thrived. Visitors can explore the Thomas Condon Paleontology Center, where fossils are displayed and studied, or hike interpretive trails like Blue Basin and Island in Time, which wind through vividly colored badlands sculpted by volcanic ash and erosion.
The environment 40 million years ago
Plam Frond
Amynodonts, sometimes called ‘Marsh Rhinos,’ are among the most common mammals found in the quarry. These semi-aquatic relatives of modern rhinoceroses likely spent much of their lives in or near the water, feeding on lush vegetation along rivers and wetlands
Around 33 million years ago, Earth entered one of its long cooling trends. Central Oregon, once a lush subtropical landscape, was covered in wet forests, lakes, and swamps—much like the warm, humid environments of today’s southeastern United States.
Many of the trees in the ancient forest are related
to modern alders, elms, maples and oaks.
Around 29 million years ago, a hungry Pogonodon—sometimes called the ‘false saber-toothed tiger’—lurks in the shadows, waiting for the perfect moment to ambush its prey.
Entelodonts were massive omnivores—standing up to six feet at the shoulder—and were distant relatives of pigs and hippos.
About 15 million years ago, the Mascall Formation records a time when forests retreated and grasslands spread across the land. This savanna-like landscape featured broad floodplains dotted with lakes. In these new environments of grass and scattered woodland, swift, fleet-footed animals began to emerge.
Seven million years ago, a rich grassland ecosystem was preserved within the Rattlesnake Formation. During this time, the region was suddenly engulfed by a massive wall of fiery volcanic debris. In the aftermath of this cataclysm, shrubs and grasses reclaimed the semi-arid landscape, while forests grew along lakes, rivers, and higher elevations. This prairie environment supported great herds of grazers, including now-extinct horses, elephants, rhinos, camels, pronghorns, and deer.
