Region 2: The Columbia Plateau

During the Jurassic and part of the Cretaceous, ammonoid cephalopods and bivalves were abundant and diverse in the shallow sea that covered this region (Figure 3.13).

Figure 3.13: Cretaceous marine mollusks. A) Ammonite, Canadoceras newberryanum, Cretaceous of California, about 15 centimeters (6 inches) in diameter. B) Inoceramid bivalve, Cretaceous of Oregon, about 12 centimeters (5 inches). C. Bivalve, Trigonia, Cretaceous of California, about 6 centimeters (2 inches).

Several sites in Oregon and Washington preserve abundant Cenozoic terrestrial fossils, especially plants and mammals, as a result of widespread volcanic activity in the area. During the Neogene, the Columbia Plateau was covered in large volcanic outflows of flood basalt. These outflows are associated with the same hot spot that now heats Yellowstone National Park. Some of these lava flows overran forests, leaving behind empty molds of trees. At some localities (such as Ginkgo Petrified Forest State Park in Washington) petrified (permineralized) wood has been preserved either by burial within lake sediments or in volcanic mudflows (Figure 3.14). Tree species thus preserved include swamp cypresses, hemlock, spruce, oak, and ginkgo.

The Eocene Clarno Formation, exposed at several sites in central Oregon, consists of a series of volcanic ash deposits, which quickly buried plants and animals and protected them from decomposition. The resulting spectacular fossils include hundreds of kinds of leaves and seeds, as well as insects, mammals, and other animals. The famous Clarno “nut beds” exposed in Wheeler County in north-central Oregon have yielded more than 170 species of fossil seeds (Figure 3.15). Clarno fossil leaves include palms, bananas, and many other flowering tree species (Figure 3.16). These fossils indicate that between 50 and 44 million years ago, the climate of what is now the Pacific Northwest was warm and humid.

Figure 3.14: A petrified log at the Ginkgo Petrified Forest State Park, central Washington. About 0.6 meters (2 feet) in diameter.

Figure 3.15: Fossil seeds from the Clarno “nut beds”: A) Walnut (Juglans), about 2 centimeters (0.8 inches). B) Oak acorn (Quercus), about 2.5 centimeters (1 inch).

The Oligocene John Day Formation is another series of volcanic ash layers rich in fossil plants and animals, which formed between 35 and 25 million years ago. The climate was cooler, but species diversity is still high. Fossils found in the John Day include more than 60 species of plants—including the “dawn redwood” Metasequoia (Figure 3.17), a relative of sequoias that was believed to have gone extinct in the Miocene, until living specimens were discovered in China. Metasequoia differs from Sequoia (the giant redwood) in that it is deciduous. Living Metasequoia leaves are identical to late Cretaceous fossils, indicating that this species has retained much the same form for over 65 million years. The John Day Formation also contains more than 100 species of mammals, including oreodonts, saber-toothed cats, horses, camels, and rodents (Figure 3.18).

Figure 3.16: Fossil leaves of broadleaf (flowering) trees, from the Clarno Formation. Leaves are about 5 centimeters (2 inches) long.

Figure 3.17: The “dawn redwood,” Metasequoia (top) with an unidentified broadleaf angiosperm. Slab is about 5 centimeters (2 inches) wide.

Figure 3.18: Mammal fossils from the John Day beds of Oregon. A) A hornless rhinoceros, Teletoceras; skull is roughly 20 centimeters (10 inches) long. B) A small sheep-like herbivorous mammal called an oreodont, Eporedon; skull is about 12 centimeters (5 inches) long.

Following the eruption of the Columbia River Basalt flows, between 17 and 12 million years ago, further ashfalls from eruptions in the Cascades formed the Mascall Formation, which includes another diverse assemblage of mammals (including horses, camels, rhinoceroses, bears, pronghorn, deer, weasels, raccoons, and cats). The Mascall’s plant fossils, including oak, sycamore, maple, ginkgo, and elms, reflect the region’s cooling climate during this time period.

The flood basalts also contain one of the world’s most unusual fossils—the “Blue Lake Rhino,” in Grant County, Washington (Figure 3.19). It is an external mold of a rhinoceros, which lived around 14.5 million years ago. It apparently formed when lava flowed into the water, forming pillows. These were still hot enough to be soft but not hot enough to completely burn away the body.

The Clarno, John Day, and Mascall formations are collectively known as the John Day Fossil Beds. They span over 40 million years and can be best seen in and around John Day Fossil Beds National Monument in Wheeler and Grant Counties in north-central Oregon.

Large mammals, including woolly mammoths and camels dating from the Quaternary, have been found in a variety of locations representing ancient riverbanks and lakes. The oldest known mastodon (another relative of modern elephants) in North America comes from the Ringold Formation at White Bluffs in south-central Washington (Figure 3.20).

Figure 3.19: The “Blue Lake Rhino,” probably Diceratherium. A) External cast made by applying plaster-soaked burlap to the sides of the hollow natural mold of the body, and then removing it in sections. Total length about 2.4 meters (7 feet). B) What the carcass may have looked like before it was covered with lava. C) Reconstruction of the living animal.

Mastodons and Mammoths

People frequently confuse these two kinds of ancient elephants (or, more technically, proboscideans). Both were common during the Pleistocene, but they had different ecological niches and are usually found separately. Mammoths are from the same line of proboscideans that gave rise to African and Asian elephants; mastodons are from a separate line of proboscideans that branched off from the modern elephant line in the Miocene. Mastodons have a shorter, stockier build and longer body; mammoths are taller and thinner, with a rather high “domed” skull. In skeletal details, the quickest way to tell the difference is with the teeth: mastodons have teeth with conical ridges, a bit like the bottom of an egg carton; mammoths, in contrast, have teeth with numerous parallel rows of ridges. The teeth are indicative of the two species’ ecological differences. Mastodons preferred to bite off soft twigs and leaves, while mammoths preferred tough siliceous grasses. Thus, mastodon teeth are more suitable for cutting, while mammoth teeth are more suitable for grinding.

A mastodon tooth, suitable for chewing twigs and tree leaves. About 20 centimeters (8 - 9 inches) long.

A mammoth tooth, suitable for grinding grass and softer vegetation. About 25 centimeters (almost 1 foot) long.

Figure 3.20: A Pleistocene mastodon, Mammut americanum.