Dinosaur Fossils in the North American Deserts
Dinosaurs have a broad public and scientific appeal. They are portrayed as gigantic and fearsome beasts who ruled the Earth longer than any other land animals – about 150 million years – and then suddenly and dramatically disappeared.
In fact, the word "dinosaur" was originally coined by Richard Owen in 1842 to mean "fearfully-great lizard." Over time, the term became simplified to "terrible lizard," but dinosaurs were neither terrible nor were they lizards. Indeed, many were quite small, especially when they first appear in the fossil record of about 225 million years ago.
If you're hunting for dinosaurs, you couldn't pick a better place than the North American deserts. Here in the arid regions of the Southwest, a sequence of geologic and climatic conditions have preserved, and then unearthed the remains of many fantastic reptiles from our ancient past. While many sites are concentrated in the famous "Dinosaur Diamond" – a 550-mile route following several highways through western Colorado and eastern Utah – all of the desert states contain good hunting grounds for dinosaur fossils.
To be a successful dinosaur hunter, you must also be a time traveler, which requires an understanding of the Geologic Time Scale and the theory of plate tectonics. Scientists who study dinosaurs and excavate their remains are called paleontologists. Paleontology is a combination of geology and biology; it is concerned with all fossils and covers the last 3.5 billion years of Earth's history.
The Geologic Time scale divides Earth's history into eras, which are further subdivided into periods, all based on extinctions of biological life forms. The mass extinction of dinosaurs, in addition to many other animal and plant species about 65 million years ago (MYA), marks the end of the Mesozoic era (the Age of Dinosaurs) and the beginning of the Cenozoic era (the Age of Mammals).
- Triassic Period (245 - 208 MYA)
- Jurassic Period (208 - 146 MYA)
- Cretaceous Period (146 - 65 MYA)
Geologists tell us that at the beginning of the Mesozoic era 245 million years ago, all of the world's land masses formed one large continent called Pangea. The portion of this continent which eventually became the North American deserts, was at the time a moist, tropical lowland located on the equator.
About 225 million years ago (in the upper Triassic period), the first small dinosaurs, descendants of theropods, made their appearance in Pangea, just as the continents began to separate. During the next 150 million years, proto-North America drifted continually northward from the equator, undergoing numerous, dramatic geologic and climatic changes, until by the end of the Mesozoic era, it resembled more closely its present size, shape and location.
During this time, various species of dinosaurs evolved, proliferated and faded from the geologic record. [For a detailed description see Desert Dinosaurs.] Under ideal circumstances, many dinosaur remains became preserved through the sequence of geologic processes of sedimentation, erosion, accretion and orogeny (mountain building).
In a region straddling northern Colorado and Utah, such ideal circumstances occurred at Dinosaur National Monument, which now preserves and exhibits a deposit of dinosaur remains from the Jurassic Morrison Formation.
The bones in this "quarry" came from individual dinosaurs who died of ordinary causes near an ancient river 150 million years ago. Their carcasses were washed downstream and came to rest on a sandbar. As soft parts such as marrow decayed, silica filled the hollow spaces, fossilizing bones and teeth.
During the following millions of years, tons of river sand, eroded from the ancient Rocky Mountains, accumulated on top of the bones, and as climates changed, layers of volcanic ash and marine mud sediment followed, covering the fossils to a depth of a mile or more. Subsequent tectonic compression of the region caused an upheaval of folds and mountains, and when erosion slowly washed away the mile-deep accumulated sediments, the dinosaur boneyard of the ancient sandbar was revealed.
Geologists call this particular layer of sedimentation the Morrison Formation, which extends, and reveals dinosaur remains, across a large region of the North American deserts in Utah, Colorado, South Dakota, New Mexico and Arizona. Exposure of younger and older formations, at numerous other desert locations, also reveals dinosaurs fossils, dinosaur skin and dinosaur eggs. Dinosaur trace fossils (which show the presence or activities of dinosaurs) like coprolites, gastroliths, toothmarks and trackways also abound in the desert of the American Southwest.
Dinosaurs reproduced through eggs laid on land, as do most modern reptiles and birds, rather than giving live birth (as do most mammals). Dinosaur eggs appear in the geologic record individually or arranged in an assemblage as part of a clutch, indicating the presence of nests. Dinosaur eggs are preserved as oblate structures that show distinctive shell structures. In some cases, dinosaur eggs also preserve parts of embryonic dinosaurs, which helps to correlate a dinosaur egg with a species of dinosaur.
Coprolites are fossilized feces, and dinosaurs were no different from other animals in leaving such deposits after digesting their meals. Coprolites can show either body fossils of plant material (indicating an herbivorous diet) or bones (indicating a carnivorous diet). Coprolites also provide information about habitats and the presence of dinosaurs in areas otherwise lacking dinosaur body fossils or other trace fossils (such as tracks). Preservation of coprolites is dependent on their original organic content, water content, where they were deposited, and method of burial.
Some modern birds swallow stones, which then reside in their gizzards and aid in digestion of food. Dinosaurs swallowed these gastroliths, also called "gizzard stones," for the same purposes and many large polished stones are associated with some dinosaur remains.
Dinosaurs who fed on other dinosaurs left distinctive tooth marks on bones, indicating feeding habits for some types of dinosaurs. In some cases, these tooth marks perfectly match the "dental records" of teeth from known dinosaurs and are preserved in bones of an identifiable species of dinosaur.
Dinosaurs were social animals who traveled in groups. There are many locations in the North American deserts where dinosaur footprints have been preserved by the same geologic processes responsible for fossilization. Excellent examples of quadrupedal ornithopod prints are preserved at Dinosaur Ridge, just west of Denver, Colorado.
If you're ready to go dinosaur hunting, don your safari hat, put on your paleontologist vest, make sure you have plenty of water, plenty of gas, a reliable vehicle, sturdy footwear and a sense of adventure.
Let's head to Desert Dinosaur Hunting Sites!
You might also like:
- Desert Dinosaurs
- Desert Dinosaur Hunting Sites
- Dinosaur National Monument
- Utah Field House of Natural History State Park
- Cleveland-Lloyd Dinosaur Quarry
- Dinosaurs & Windmills near Palm Springs
- Fossils - Dinosaurs
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