Region 5: The Pacific Border

The Pacific Border includes terranes and former island arcs that accreted onto the West Coast, along with sediments deposited after this merger. Nearly every newly-exposed hillside or roadcut in this region exposes fossiliferous sediment, even in developed areas.

Fossil-bearing rocks of both Jurassic and Cretaceous age can be found in this region in northwestern California and southwestern Oregon. Jurassic marine fossils include abundant clams such as Buchia (see Figure 3.8B) and ammonoids (see Figure 3.8A). During the Cretaceous, sea levels were higher, and the Pacific shoreline was much further inland. The shore was lined with palms, and the waters were filled with bivalves such as Inoceramus (see Figure 3.13B) and Trigonia (see Figure 3.14C). Recognizable relatives of many extant bivalves, such as oysters, also became common during the Cretaceous. Ammonoid cephalopods, including the straight-shelled Baculites (Figure 3.25), were extremely diverse and can be found in many Cretaceous rocks. Marine reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs are also found through much of coastal California.

Although there were presumably many dinosaur species on land, only a few dinosaur fossils have been found in this region. These include the bones of hadrosaurs and the armor-plated ankylosaur Aletopelta (Figure 3.26). One specimen of Aletopelta found in California evidently floated out to sea, where its armor plates and spines were encrusted by bivalves!

Figure 3.25: A) Broken specimen of Baculites, a straight-shelled ammonite from the Cretaceous, showing internal suture lines. Usually around 3-4 centimeters (2 inches) in diameter. B) Reconstruction.

Figure 3.26: Reconstruction of the Cretaceous ankylosaur Aletopelta from California, about 6 meters (20 feet) long.

The Paleogene marine fossils of the Pacific Border strongly resemble the hard-shelled organisms living in the Pacific today, although some may seem geographically out of place (an important piece of evidence for environmental change through geological time). Gastropods and bivalves are the most common marine fossils of the Paleogene and include clams, oysters, whelks (Buccinidae), moon snails (Naticidae), and tower snails (Turritellidae) (Figure 3.27). Crabs are sometimes common, although it is rare to find fossils of whole individuals, as these organisms typically break apart after death. However, some locations preserve crabs (and other fossils) within concretions (Figure 3.28). Concretions are hard, layered nodules, often with a different chemical makeup from the surrounding rock. They form when minerals precipitate (crystalize) around a nucleus within the sediment. While concretions are not fossils themselves, they may contain fossils—even trace fossils, as many organisms line their burrows with mucus, and the decay of that mucus may begin the formation of a concretion.

Figure 3.27: Eocene marine mollusks from Washington, Oregon, and California. A) Bivalve, Nemocardium, about 3 centimeters (1.3 inches). B) Gastropod, Natica, about 2 centimeters (1 inch). C) Gastropod, Turritella, about 3 centimeters (1.3 inches).

Figure 3.28: The crab Zanthopsis vulgaris, preserved in a concretion from Oligocene strata in Vernonia, Oregon. Specimen is about 10 centimeters (4 inches) wide.

Figure 3.29: Neogene (Miocene - Pliocene) fossil mollusks of the coast of Oregon and California. A) Turritella, about 3 centimeters (1.3 inches). B) Polinices, about 2 centimeters (1 inch). C) Flabellipecten, about 4 centimeters (1.5 inches).

The general character of fossils from the Neogene is similar to that of the Paleogene, with gastropods such as moon snails, whelks, and tower snails remaining important components of the marine fauna (Figure 3.29). Mussels, clams, scallops, and oysters are all common, and can often be found in the rocks exposed at beach cliffs along much of the Oregon and California coasts.

Prior to 100,000 years ago, much more of the coast was submerged, and the region was ultimately exposed when the expansion of glaciers caused a regression in sea level. The coasts of California, Oregon, and Washington and the central valley of California contain numerous Pleistocene-age fossil deposits. Bivalves and gastropods are the most common marine invertebrate fossils from this time, particularly scallops (Pecten) and oysters (Ostrea), but also clams such as Saxidomus, Mya, and Clinocardium and snails such as Polinices, Neptunea, Turricula, and Cancellaria.

During the Pleistocene, large terrestrial mammals were common, including mammoths, woolly rhinos, horses, camels, bison, saber-toothed cats, and dire wolves. The famous La Brea Tar Pits in downtown Los Angeles provide a spectacular window into the region’s Pleistocene mammal communities. The tar pits formed around 40,000 years ago, when natural asphalt deposits began to seep up from cracks in the ground to form pools. As this asphalt seeped from the ground, it became covered with leaves and dust. Animals that wandered in became trapped, as did predators that arrived to eat the mired animals (Figures 3.30, 3.31). The oldest remains from La Brea have been dated to 38,000 years old, and the tar pits still continue to trap unsuspecting animals today.

As bones sink into the asphalt, it stains them dark brown or black, leading to the unique appearance of the fossils found here (Figure 3.32). Fossils of more than 600 species of animals and plants have been excavated from the asphalt here (Figure 3.33)—in addition to large mammals and birds, the tar pits have preserved a remarkable array of microfossils ranging from insects and leaves to pollen grains, seeds, and ancient dust.

Figure 3.30: A life-size model of a Columbian mammoth (Mammuthus columbi) shown stuck in the tar outside the Page Museum at the La Brea Tar Pits in downtown Los Angeles. Columbian mammoths were close relatives of woolly mammoths, and became extinct about 11,000 years ago. They ranged from the southern US to Nicaragua and Honduras, but not as far north as the woolly mammoth.

Figure 3.31: A 1911 illustration of several mammal species becoming mired in the tar pit—Smilodon fatalis and Canis dirus fight over a Mammuthus corpse.

Figure 3.32: Skeleton of the most common large mammal at the site, the dire wolf, Canis dirus.

Figure 3.33: Ongoing excavation of the asphalt deposit continues to uncover new fossils.

Smilodon fatalis, the saber-toothed cat, is among the most famous species represented in the La Brea Tar Pits, and is well known for its prominently elongated canine teeth (Figure 3.34). While these animals are sometimes referred to as “saber-toothed tigers,” they are not actually close relatives of tigers or any other living feline group. Elongated canines actually evolved separately in a number of cat-like lineages, including some groups more closely related to living marsupials than living cats. Smilodon belongs to the group known as “dirk-toothed” cats, which possess teeth with fine serrations. The elongated canines of Smilodon were also fairly thin and would have broken if they bit into bone, so these teeth would likely have been used to kill prey that was already subdued. The cats’ most common prey were likely bison and camels, and dire wolves were probably their direct competitors. Smilodon became extinct during the Quaternary extinction around 10,000 years ago, along with many of the large mammals it utilized for food. These extinctions have been related to both climate change and hunting by humans, although the relative importance of each of these factors is still debated.

Figure 3.34: The skull of Smilodon fatalis.