Region 1: The Basin and Range
During the early Paleozoic, the area that is now the Basin and Range was a passive continental margin with no tectonic activity, similar to the east coast of the US today. A warm shallow sea flooded what is now Nevada, and trilobites were abundant and diverse (see Figure 3.21, see box p. 101). During the Cambrian, reefs were built by an extinct group of sponge-like organisms known as archaeocyathids (see box p. 86). During the Ordovician and Silurian, these were replaced by reefs built by rugose and tabulate corals (Figure 3.2, see box p. 87) along with brachiopods (Figure 3.3, see box p. 88) and bryozoans (colonial filter-feeding animals that build calcium carbonate skeletons, Figure 3.4). One kind of brachiopod found in Mississippian rocks in northern California and Oregon is among the largest brachiopods in the world: Titanaria (Figure 3.5) reached shell widths (along the hinge line) of more than 35 centimeters (14 inches). In deeper waters, planktonic graptolites were common.
The Triassic and Jurassic rocks of Oregon testify to major global changes in marine life, especially when compared to the Paleozoic. The once-abundant brachiopods on the seafloor are gone, replaced by faunas composed primarily of burrowing bivalves and gastropods. Tabulate and rugose corals were replaced by scleractinian corals, which were building reefs in this region by the mid-Triassic (see Figure 3.2D).
Archaeocyathids
Archaeocyathids were the first important animal reef builders, originating in the early Cambrian. These vase-shaped organisms had carbonate skeletons and are generally believed to be sponges. They went extinct in the late Cambrian, but were very diverse. Archeocyathids are often easiest to recognize in limestones by their distinctive cross sections.
Archaeocyathids are found in early Cambrian rocks in northern California and southern Oregon. Their vase-shaped calcite skeletons commonly reached lengths of 5-20 centimeters (2-8 inches).
Ammonoid cephalopods (Figure 3.6) also diversified during the Triassic, and their fossils can be found in Oregon, California, and Nevada. Swimming with (and probably feeding on) these ammonoids were marine reptiles called ichthyosaurs (Figure 3.7, see box p. 92). Shonisaurus popularis (Figure 3.7B) was an ichthyosaur first discovered in Berlin, Nevada in the mid-1800s. It lived around 217 million years ago, while Nevada was still covered by the ocean. In contrast to Jurassic and Cretaceous ichthyosaurs, Shonisaurus (along with other early ichthyosaurs) is thought to have lacked dorsal fins. Shonisaurus was over 15 meters (50 feet) long and was the largest known ichthyosaur until 2004, when an even larger species was discovered in British Columbia.
Corals
Corals are sessile relatives of jellyfish and sea anemones. They possess stinging tentacles, which they use to feed on small planktonic prey. Each group of coral possesses distinctly shaped “cups” that hold individual animals, or polyps.
Rugose corals were both colonial and solitary (solitary forms are often called “horn corals”). Tabulate corals were exclusively colonial and produced a variety of shapes, including domed and chainlike forms. These corals receive their name from the table-like horizontal partitions within their chambers. Both rugose and tabulate corals went extinct at the end of the Permian. Modern corals, or scleractinians, appeared in the Triassic, and include both solitary and colonial species. Many modern scleractinian corals have photosynthetic symbiotic algae called zooxanthellae in their tissues. These algae provide nutrition to the coral polyps, helping them to grow more rapidly.
Nevada’s shallow seas persisted into the Jurassic and part of the Cretaceous, and remained inhabited by ammonoids and bivalve mollusks (Figures 3.8 and 3.13). In the northernmost part of the Basin and Range (northern California and southern Oregon), bivalves called rudists (Figure 3.9) frequently formed reef-like mounds. Rudists became extinct with the dinosaurs and many other species at the end of the Cretaceous.
Figure 3.2: Corals. A) Solitary rugose (“horn”) coral, Campophyllum, up to 20 centimeters (8 inches) long, Devonian. B) Colonial rugose coral, Lithostrotion, Carboniferous. C) Tabulate coral, Favosites, Devonian. D) Colonial scleractinian coral, Kompsasteria, Triassic, Alaska.
The sea retreated during the late Cretaceous, and the Basin and Range became entirely terrestrial. During the Cenozoic era, the region was home to diverse and abundant mammals such as camels, mammoths, and rhinoceroses. Freshwater lakes dotted the area and were inhabited by fish such as sticklebacks, Nevada killifish, and topminnows.
Brachiopods
Brachiopods are filter-feeding animals that have two shells and are superficially similar to bivalves (such as clams). Instead of being mirror images between shells (symmetrical like your hands), brachiopod shells are mirror images across each shell (symmetrical like your face). Internally, brachiopods are substantially different from bivalves, with a lophophore (filter-feeding organ made of thousands of tiny tentacles) and a small and simple gut and other organs. Bivalves, in contrast, have a fleshier body and collect their food with large gills.
The difference between the shells of a typical brachiopod (left) and a typical bivalve mollusk (right). Most brachiopods have a plane of symmetry across the valves (shells), while most bivalves have a plane of symmetry between the valves.
Figure 3.3: Paleozoic brachiopods. A) Strophatrypa, Silurian of Alaska, about 1 centimeter (inches 0.4 inches). B) Warrenella, Devonian of Oregon, about 2 centimeters (1 inch). C) Retzia compressa, Carboniferous of California, about 3 centimeters (1.2 inches). D) Kirkidium alaskense, Silurian of southeastern Alaska, about 8 centimeters (3 inches).
Figure 3.4: A branching bryozoan colony. Some bryozoans had skeletons resembling branching corals, but with smaller cavities for the individual organisms. About 2 - 5 centimeters (1 - 2 inches) tall.
Figure 3.5: Specimen of the giant brachiopod Titanaria, from the Mississippian of Oregon. The specimen is slightly broken; the dotted line shows its reconstructed size and shape. This specimen, from the collections of the National Museum of Natural History, Smithsonian Institution, in Washington, DC, is one of the largest brachiopod fossils in the world.
Graptolites
Graptolites (meaning “rock writing”) are an extinct group of colonial, free-floating organisms. They lived from the Cambrian to the Carboniferous, and were relatives of modern hemichordates such as acorn worms. Graptolites are frequently preserved as thin black sawblade-like streaks across black shale; tiny cups along these structures held individual animals. Graptolites are often useful as index fossils.
Rock with many fragments of Climacograptus colonies.
Restoration of what graptolite colonies may have looked like when they were alive, floating in the water.
Ammonoids
Ammonoids are a major group of cephalopods that lived from the Devonian to the end of the Cretaceous. Both nautiloids (the group that today contains the chambered nautilus) and ammonoids have chambered shells subdivided by walls, or septa (plural of septum). These shells are frequently, but not always, coiled. The term “ammonoid” refers to the larger group of these extinct cephalopods, distinguished by complex folded septa. Within ammonoids, “ammonites” is a smaller sub-group, distinguished by the extremely complex form of their septa. Ammonites were restricted to the Jurassic and Cretaceous periods. The form of the septa in nautiloids and ammonoids is not visible in a complete shell; it is most often seen in the trace of the intersection between the septum and the external shell. This trace is called a suture. Sutures are usually visible in fossils when sediment has filled the chambers of a shell, and the external shell has been broken or eroded away.
Ammonite shell break-away cross section; surface plane of a septum and sediment-filled chamber.
Figure 3.6: Triassic ammonite, Harpoceras, about 15 centimeters (6 inches) in diameter.
Ichthyosaurs
Ichthyosaurs are an extinct group of Mesozoic marine reptiles that evolved in the Triassic and went extinct in the late Cretaceous. They superficially resembled dolphins with long, toothed snouts, dorsal fins, and vertical tail fins. These animals were descended from land-dwelling reptiles, and evolved dorsal fins and vertical tails independently from other animal groups. These structures were not bony and are only known due to a few exceptionally preserved specimens that show outlines of the entire animal. Ichthyosaurs are known to have given live birth, and some may have been warm-blooded.
Figure 3.7: Ichthyosaurs. A) A typical ichthyosaur, as it might have looked in life. B) Shonisaurus, the largest known ichthyosaur, from the Triassic of Nevada. Painting at Ichthyosaur State Park, Nevada.
Figure 3.8: Jurassic marine mollusks. A) Ammonite, Phylloceras, about 15 centimeters (6 inches) in diameter. B) Bivalve, Buchia piochii, about 5 centimeters (2 inches) in diameter. C. Reconstruction of a typical belemnite as it appeared alive. D) Belemnite internal shell; most are 5 - 10 centimeters (2.5 - 5 inches) long.
Figure 3.9: Rudists were unusual cone- or cylinder-shaped bivalves that clustered together in reef-like structures and went extinct at the end of the Mesozoic era. They ranged in size from a few centmeters to more than 50 centmeters (1.5 feet) tall.
During the last glacial period (Pleistocene), southern Oregon was home to many species of large and now-extinct mammals, including Arctodus, the giant short-faced bear, which was 1.8 meters (6 feet) tall at the shoulder with 25-centimeter-long (10-inch-long) paws (Figure 3.10). Alongside these massive mammals lived Teratornis, a giant vulture with a wingspan of up to 3.8 meters (12.5 feet)—the modern California condor, in comparison, has only a 2.7-meter (9-foot) wingspan (Figures 3.11 and 3.12).
Figure 3.10: Reconstruction of the giant short-faced bear Arctodus, compared to a six-foot human.
Figure 3.11: Skeleton of Teratornis, a giant predatory/scavenging bird similar to a modern condor.
Figure 3.12: Teratornis size comparisons.
