Geology & Fossils
Wyoming’s fossil heritage is much, much older than mammals. Algae that grew along seacoasts long ago became fossilized and were pushed skyward with rising mountains. Such “stromatolites” some 1.7 billion years old have been found in the Medicine Bow Mountains, near the Colorado border in arid south-central Wyoming.
Much younger Wyoming fossils include trilobites, brachiopods, corals, and sponges that lived during the Cambrian Period — a mere half billion year ago! Fossils of Ordovician fishes found in the Big Horn Mountains of north-central Wyoming are among the world’s oldest known vertebrates.
The Age of Dinosaurs
The Age of Dinosaurs is first represented in Wyoming by fossils of marine plesiosaurs, ichthyosaurs, and the squid-like belemnites they preyed on. Obviously, ancient seas ebbed across Wyoming in the distant past.
But fossils of Jurassic dinosaurs have been found in Wyoming, too. Many of these fossils come from the Morrison Formation, a blanket of rocks that stretches from Canada to New Mexico. Near the town of Medicine Bow, the Morrison Formation crops out as one of the world’s most famous dinosaur graveyards — Como Bluff. Como Bluff’s Bone Cabin dinosaur quarry was named for a cabin made of dinosaur bones!
The largest known dinosaurs were the long-necked sauropods, which were biggest and most diverse during the Jurassic Period. The best known sauropod was discovered at Como Bluff. This animal is Apatosaurus, more popularly known as Brontosaurus. Brontosaurus became even more famous after it became a symbol of Sinclair Oil Company.
The Jurassic Period was followed by the Cretaceous. One of the most fascinating dinosaurs from the early Cretaceous Period was a Wyoming desert-dweller that probably weighed less than 200 pounds. The second toe on its foot sported a claw as long as five inches that could be retracted — cocked might be a better word. Scientists speculate that it ran at its prey, grabbed it with its hands, and slashed it with this terrible claw. It was named Deinonychus, or “terrible claw.”
Some artists portray Deinonychus with feathers because it seems so similar to a bird, even though no feather imprints have been found. Deinonychus ’ discovery in 1964 helped change the popular image of dinosaurs as dull, slow brutes.
A dinosaur that lived later in the Cretaceous Period was chosen by Wyoming schoolchildren as their official state dinosaur. Like the bison on Wyoming’s state flag, this dinosaur had horns and probably lived in herds. In fact, the famous paleontologist who first discovered a Triceratops horn in Wyoming 1887 thought it was a giant, long-horned bison. The truth was revealed in 1897 when an entire skull was discovered.
That dinosaur, of course, is Triceratops, also adopted as the state fossil of neighboring South Dakota.
During the early Cretaceous Period, the climate apparently became more arid and few dinosaurs seem to have lived in Wyoming. Later in the Cretaceous, seas again approached, the climate became wetter, and coal forests once again flourished in Wyoming. But the Rocky Mountains were rising in the west, chasing the sea away yet again. Scientists believe great herds of Triceratops roamed Wyoming during the late Cretaceous, just before an enormous meteor slammed into Earth, ending the Age of Dinosaurs.
The Age of Mammals
Fossils of animals that lived after dinosaurs became extinct are most common in valleys in the Rocky Mountains. Here ancient animals were buried under sediments eroded from mountains. Wyoming’s Fossil Butte is one of the most famous Eocene Epoch fossil sites in the world. This is the legendary home of Wyoming’s official state fossil, a 50-million-year-old freshwater herring scientists named Knightia in honor of a Wyoming paleontologist.
Fossils of animals that lived after the Eocene are not so common in Wyoming, except for the Ice Age (Pleistocene Epoch) beasts that lie scattered across the state. Of course, many Ice Age animals, including the pronghorn and bison, still live in Wyoming.
Fossils in Our Lives
Wyoming’s most important fossils may be those that are buried deepest, its fossil fuels, as advertised by the word “oil” on the state seal. Mining is Wyoming’s most important industry and petroleum its major product, with thousands of oil and gas wells.
Coal production has also become very important in Wyoming. Vast fields of bituminous and subbituminous coal are mined at Rock Springs, Hanna, Kemmerer, Sheridan, Gilette, and Glenrock.
Symbols of Wyoming’s prehistoric past include its official state fossil and dinosaur and the oil wells that tower higher than tyrannosaurs over Wyoming’s sagebrush plains.
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