How the West Was Swum

Natural History, June, 2001 by Richard L. Orndorff, Robert W. Wieder, Harry F. Filkorn

At Nevada's Berlin-Ichthyosaur State Park, fossils of giant marine predators point to the region's watery past.

Berlin-Ichthyosaur State Park sits on the western flank of Nevada's Shoshone Mountains, about 7,000 feet above sea level. Today a seemingly endless expanse of sagebrush covers the relatively featureless landscape of the valleys, while the rocky slopes of the mountains support scattered sagebrush and stands of juniper and pinon trees. This sparsely populated part of Nevada has changed little since the first settlers arrived more than one hundred years ago. But it has changed much since the largest marine predators of the Triassic Period lived here, more than 200 million years ago.

Extinct reptiles that plied ancient oceans, ichthyosaurs had highly streamlined bodies resembling those of some of today's fastest fish, such as swordfish, marlin, and tuna. Despite their fishlike exteriors, ichthyosaurs had to surface to breathe air and they gave birth to live young. Their elongate mouth and strong jaws held rows of pointed conical teeth, similar in shape to those of modern toothed whales. A circular set of overlapping bony plates internally reinforced the disproportionately large eyes of some ichthyosaurs and compensated for changes in water pressure when the animals dived or surfaced. This feature enabled them to consistently maintain their highly developed sense of vision at all swimming depths.

Fossil ichthyosaurs are common in the sedimentary rocks that make up Nevada's Luning Formation. About 230 million years ago, when these sediments were deposited, North America was part of the northern supercontinent known as Pangaea, and the Panthalassa Ocean covered much of what is now the western United States. Outcrops, or protruding layers of rock, of the Luning Formation are scattered throughout the mountain ranges of central Nevada, and paleontologists have found ichthyosaur remains in the West Humboldt Range and the New Pass Range, as well as in Union Canyon in the Shoshone Mountains, the site of Berlin-Ichthyosaur State Park.

The first ichthyosaur bones discovered in Union Canyon were excavated by gold prospectors from the mining town of Berlin (now a well-preserved ghost town and part of the state park). While the miners saw the bones as novelties and sometimes used them to decorate their cabins, the fossils did not become known to the scientific community and recognized as the remains of ancient reptiles until 1928. The first field expedition to Union Canyon was launched by Berkeley paleontologist Charles L. Camp and his colleague Samuel P. Welles in 1954. Today a stroll through Union Canyon takes visitors by Camp's cabin, where he and other scientists worked diligently for years to reconstruct the ichthyosaurs' skeletons and solve the puzzle of their presence here. Union Canyon has yielded at least thirty-seven mostly complete ichthyosaur skeletons. In 1966 an A-frame shelter was built over the main quarry to protect some of the exposed fossils and allow visitors to see them. The skeletons of nine individuals, their bones still embedded in rock, are on view. While Camp considered the Union Canyon fossils to be of three different species of ichthyosaurs, all the specimens are now thought to be the renlains of a single species, Shonisaurus popularis, named for the surrounding mountain range.

Thanks to the abundance of Shonisaurus specimens collected in the region, scientists know more about the skeleton of this species than about any other Late Triassic ichthyosaur. Fifty or more feet long and weighing an estimated forty tons, Shonisaurus was one of the largest creatures of its time--about the size of a modern sperm whale and twice the size of a killer whale. Larger individuals had six-foot-long front fins, twenty-five-foot-long tails, and ten-foot-long skulls with elongate jaws, filled with conical teeth. In contrast, the well-preserved ichthyosaurs found in shale quarries in Holzmaden, Germany, which date from the Jurassic Period, were only the size of today's dolphins.

While fossil bones tell us about the body plans of extinct creatures, the composition of the surrounding rock can give clues to the environments they inhabited and help answer such questions as how so many large ocean-going predators ended up in the Shoshone Mountains. At the time the Union Canyon ichthyosaurs lived, the region was a tropical sea, situated along the west coast of what is now North America. The bedrock in the area indicates that the ichthyosaur bones were deposited in a deep ocean shelf environment. One of the most convincing pieces of evidence is the fine-grained sedimentary rock that encloses the fossils. As rivers carrying sediment spill into the ocean, freshwater mixes with standing marine water, and the momentum of the flow decreases; as a result, coarse sediment drops to the bottom near the shore. Fine sediment remains in suspension much longer and travels far out to sea, where it settles slowly to the bottom. The sediment layers in the Union Canyon rocks suggest they accumulated in the deeper areas of the continental shelf.