The distinctive blue-green claystone layers found at John Day Fossil Beds are the Turtle Cove beds which contain a remarkable number of mammal fossils. In fact, the vast majority of localities and museum specimens from the John Day Basin are from the Turtle Cove fauna. During the Turtle Cove, the climate continued to cool and dry. Hardwood forests were sometimes flooded with ash and pumice from nearby volcanoes. Grasses were beginning to appear, but they were not yet a major part of the landscape. Given the span of time preserved in the Turtle Cove Member (about 5 million years), it is not surprising that the fauna is not homogenous; as the regional environment changed so did the mammals, and that evolutionary progression is depicted in the fauna.
Three-toed horses, mouse-deer, beavers, and oreodonts, - an unusual family of sheep-like animals – browsed on the still prevalent leafy plants. They were stalked by carnivores such as bear-dogs, nimravids - fierce cat-like animals, and the giant pig-like entelodonts. Paleosol (fossilized soil) evidence from this interval indicates a change in habitats, with forests opening up as the global climate became cooler and drier. The co-occurrence of Oregon’s first burrowing animals, running mammals, and clearly arboreal species (tree squirrels like Protosciurus and Miosciurus, the primate Ekgmowechashala) supports the reconstruction of Turtle Cove as a heterogeneous environment, with a mixture of wooded forests and open areas.