The 16-13 million year old Mascall assemblage caps the Picture Gorge Basalts with a sequence of ashy layers and paleosols (fossilized soils). It records a gradual yet influential climate change event that took place over millions of years, the mid-Miocene climatic optimum. Paleosol evidence from the Mascall assemblage indicates a period where forests returned, outcompeting the sagebrush steppe of the mid-Miocene.
Dominant Fossils Found in this Assemblage:
Mylagaulus was a horned rodent with long claws that would have looked similar to a gopher. These rodents used their claws to create burrows. They gnawed on bones of dead animals for a source of calcium. It is thought that these horns were used a self-defense mechanism; as Mylagaulus left its burrow any predator sticking is nose down the hole would get a pointed attack on its nose. Although a skull from Mylagaulus has yet to be found in Oregon, it is inferred that this taxa had horns like other specimens show from different parts of the country.
Tephrocyon was a ground dwelling carnivorous dog that had teeth very well suited to crushing bones. Tephrocyon is part of a subfamily of canines known as “bone crushing dogs” or Borophaginae. It had a large powerful jaw that would have easily crushed smaller prey.
Dromomeryx was an artiodactyl (even toed ungulate) which was similar in size to a modern white tail deer. Its ossicones grew up and forward of their eyes, similar in style to a gazelles. Dromomeryx was a browser as isotopic data from its teeth and tooth structure suggest that it ate leaves and other foliage above ground level.
Merychippus was a perissodactyl (odd toed ungulate). This horse records one of the biggest evolutionary steps in the lineage of horses. Since grasses were lower to the ground dust and other particles had tendencies to wear down teeth rather quickly. The teeth of Merychippus had permanently formed tall crests with higher crowns reinforced with cementum which allowed it to become a true grazer and not accrue extreme wear on its teeth. This trait would be carried on through each successive species, allowing generations of horses to keep grazing in the expanding steppe landscape.
Gomphotherium in Latin means “welded beast.” It is an extinct genus of ancestral elephant (proboscidean) that immigrated to North America from other continents. Gomphotherium looked much like a modern elephant; however it had four tusks - two on the upper jaw and two on the lower jaw. It was a browser that had teeth adapted for grinding plant material to a pulp.
For a complete list of fossils in the Mascall Assemblage, email us.
Last updated: January 30, 2023