By Doug Nichols
As dinosaur lovers of all ages in our state know, the Colorado State Fossil is the stegosaur (Stegosaurus stenops). This familiar herbivorous dinosaur with rows of plates along its back was officially adopted by executive order of Gov. Richard Lamm in 1982, following a two-year campaign by schoolchildren.
Stegosaurs roamed Colorado during the Late Jurassic Period of geological time, about 145-150 million years ago. They were large animals, weighing an estimated five tons and measuring 7-9 meters (23-30 feet) when fully grown. They dined on mosses, ferns, horsetails, and cycads. There were three closely related species of stegosaur: Stegosaurus armatus (the largest), S. longispinus, and S. stenops. All three are known from the Morrison Formation, a geological unit named for the town of Morrison, Colorado. When stegosaurs lived in what is now Colorado, it was a lowland plain dotted with streams and lakes. In Late Jurassic time, the present Rocky Mountains were yet to rise. The contemporaries of Stegosaurus included the long-necked giants Diplodocus and Apatosaurus (the latter sometimes still known by the obsolete synonym Brontosaurus) and the fearsome predators Allosaurus and Ceratosaurus.
The first known stegosaur skeleton was discovered near Morrison in 1877. Stegosaurus stenops was first collected at Garden Park, north of Cañon City, Colorado, in 1886. Remains of about 80 individuals of all three species have been collected since (S. stenops itself is known from at least 50 partial skeletons of adults and juveniles, one complete skull, and four partial skulls), but only six skeletons of stegosaurs are on public display in the U.S. One of them, a particularly complete skeleton of our State Fossil, is at the Denver Museum of Nature & Science. It was discovered by a teacher and students from Cañon City High School and was collected by museum paleontologists, with an assist from an army helicopter used to lift the fossil from the ground.
Colorado is home to many other interesting fossil species. Last month’s column mentioned plant and insect fossils from Florissant Fossil Beds National Monument, near Colorado Springs. Fossil remains of an entire tropical rainforest were discovered along Interstate 25 near Castle Rock, Colorado, a few years ago. This flora, which dates from the Paleocene Epoch, about 64 million years ago, includes more than 100 species, most of them new to science and still under study at the Denver Museum of Nature & Science. The large sizes, smooth-edged shapes, and characteristic drip-tips of many of these leaves testify to the tropical temperatures and high rainfall conditions in which they grew.
Of course, today that area is semi-arid, with snow in winter (talk about climate change!). Spectacular ammonites, extinct relatives of the living chambered nautilus, are found near Kremmling, in northwestern Colorado. They were like squids with disc-shaped shells, and they darted about in a vast seaway that covered the region in Late Cretaceous time, some 73 million years ago. These are just a few examples of Colorado’s fascinating fossil heritage, of which there are far too many to discuss right now.
Doug Nichols is a Scientist Emeritus with the U.S. Geological Survey and a Research Associate with the Denver Museum of Nature & Science. He is a resident of Berthoud. Send comments to email@example.com.