The distinctive 29 million-year-old Turtle Cove beds 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, - a strange family of sheep-sized creatures – 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 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 heterogenous environment, with a mixture of wooded forests and open areas.
Dominant Fossils Found in This Assemblage (Images 1-4, 12,13 cropped from frame):
5. Stylemys (first dry land tortoise – multiple species)
6. Nimravus brachyops (false saber-toothed cats)
7. Miohippus (three-toed horse)
9. Mesocyon (dog – multiple species)
10. Agriochoerus (bizarre clawed oreodont)
11. Cormocyon copei (dog)
14. Quercus (oak)
15. Herpetotherium merriami (marsupial)
16. Micropternodus morgana (burrowing, mole-like insectivore)
17. Celtis (hackberries)
18. Dyticonastis rensbergeri (worm lizard)
Filling the Role...
Eusmilus cerebralis was a feliform (cat like), very similar in body structure to a leopard with exception of having incredibly long upper canines akin to saber teeth. Even though Eusmilus was “cat like”, it shares no modern descendant. As Nimravids went extinct, felines evolved to fill the same ecological niches left by Nimravids. E. cerebralis also had the ability to open its jaw to an almost 90 degree angle, which would likely be an adaptation for lunging at the necks of prey like three-toed horses and oreodonts. To protect its saber teeth from any sheer stresses, Eusmilis had bony flanges that protruded from its mandible (lower jaw).
Agriochoerus was a sheep-sized clawed oreodont unlike anything alive today. They get their common name from the fact they have large curved claws on their fingers and toes, rather than hooves like other artiodactyls (even-toed, hoofed mammals). Their claws and limb anatomy suggest agriochoerids were able to climb trees, perhaps living like tree kangaroos today. These forest browsers lived all over North America for more than 20 million years, but vanished at the end of the Oligocene. Why they became extinct is not known, but the opening of habitats in the Oligocene and early Miocene may have removed the forests favored by Agriochoerus.
Miohippus was a small three-toed ancestor of modern horses that had longer legs and larger teeth than earlier horses. Longer legs allowed it to run quickly and evade predators in open environments. Its larger teeth were adapted to effectively grind tough vegetation. As climates continued to change, horses evolved better running and grinding adaptations, which can be seen in horses today.
Mesocyon is one of the most common dogs found in the John Day Formation. It was the earliest dog to reach the size of a coyote. Mesocyon has a short snout and more blade-like teeth than other earlier dog species, indicating a more predatory lifestyle. These features suggest that Mesocyon was a forest dwelling ambush predator, likely feeding on rabbits, rodents, mouse deer, and possibly small oreodonts and horses that thrived alongside Mesocyon in the forests of the Turtle Cove member.
Micropternodus morgani is a tiny relative of shrews and moles. Like moles today, M. morgani likely had a strong sense of smell and was a good burrower, feeding on worms and insects living in the soil. Small mammals like Micropternodus help create intricate stories of the fossil records paleoecological niches.