OverviewThe Morrison Formation is a rock unit from the Late Jurassic Period (about 163.5 to 145 million years ago). Named for the town in Colorado where the layer was first described, the Morrison Formation extends throughout the Western United States and contains fossils of numerous dinosaur species that lived during that period of time. In this area, the Morrison Formation preserves the bones of dinosaurs such as Camarasaurus, Apatosaurus, Stegosaurus, Allosaurus, and others. While many rock units from the Mesozoic Era (252 to 66 million years ago) are exposed in Dinosaur National Monument, it is the Morrison Formation that contains the majority of the dinosaur fossils found here. Although the MorrisonFormation was deposited in a wide variety of environments, it is the river channels that are most likely to contain dinosaur bones.
GeologyThe Morrison Formation is divided into several "members," or subunits that share similar characteristics that distinguish them from surrounding rocks and each other. The Carnegie Quarry was found in the Brushy Basin member of the Morrison Formation, which was home to a large flat floodplain during the Late Jurassic Period. Most of the Brushy Basin member is comprised of mudstones that range from reddish, to greenish gray, and even maroon purple in color. These mudstones contain large amounts of bentonite, a kind of clay that contains volcanic ash. Bentonite expands and contracts with moisture, turning slick when wet and popcorn-textured when dry. The ash component of the bentonite came from volcanoes that are thought to have existed to the south and west of the Morrison Floodplain. It's thanks to this volcanic ash and the crystals preserved inside it that scientists were able to date the Quarry fossils to the Late Jurassic period using radiometric dating. The mudstones themselves represent the flatter parts of the floodplain, while the sandstones and limestones came from rivers, braided streams, and small lakes that dotted the landscape. Almost all of the fossils from the Brushy Basin member are found in sandstones, including the bones of the Quarry Exhibit Hall. This is because the water continuously depositing sediments quickly buries bones before they have the chance to decay. The water also brings bacteria and dissolved minerals into the bones' pores, which solidify and fossilize the bones.
The Morrison Ecosystem
Climate & GeographyAt the time the Quarry dinosaurs were alive in the Late Jurassic, the world's continents had already begun to split apart from the supercontinent Pangea. North America was located further south than it is today. It would still be many millions of years before the Rocky Mountains began to rise. Based on analysis of oxygen isotopes in marine fossils, the global climate in the Late Jurassic is thought to have been much warmer and more humid than it is now. Fossilized plant species indicate that most of the Morrison ecosystem was home to lush forest and shrubland. The landscape was cut through by multiple large rivers that left limestone and sandstone deposits with ripple marks indicating the directon of water flow. The plane that would become the Quarry Wall was still horizontal at the time. Its sandstone was once sandy bottom of a river channel. Fossils indicate that the area experienced at least three severe droughts, followed by localized flooding events when the rains returned.
Morrison PlantsThe Morrison ecosystem was home to a wide variety of plants. Because fossilized grass and flowers have never been found in any of the world's rock layers dating to before the Cretaceous period, neither are thought to have existed in the Late Jurassic. Instead, the Morrison Formation has yielded numerous fossilized ferns, cycads, ginkos, horsetails, and large conifer trees. As big as the dinosaurs of the Morrison were, they would have been dwarfed by the massive conifers that dominated their ecosystem. Many of these trees are Brachyphyllum (short leafed plants) in the family Araucariaceae (very tall evergreen trees). By measuring their diameters and comparing them to what we know of their modern-day relatives, scientists have determined that some of these conifers may have been similar in size to today's tallest redwoods. This lush forest environment supported a wide variety of ancient species, from massive sauropods that likely consumed multiple tons of plant matter each day, all the way down to tiny omnivorous mammals no bigger than a mouse.
When exacavating the Morrison Formation in this area, paleontologists found a wide array of animals fossilized in the rock. Of course, the dinosaurs are the most popular among park visitors. The largest of these were the long-necked, plant-eating sauropods, Apatosaurus, Barosaurus, Camarasaurus, and Diplodocus. The smaller plant-eating dinosaurs included the ornithischians, Stegosaurus, Camptosaurus, and Dryosaurus. The habitat also contained large dinosaurian predators. The most common of these was Allosaurus. Less common were the larger Torvosaurus, and the smaller, horn-nosed Ceratosaurus.
With dinosaurs dominating the large-bodied roles of the Morrison ecosystem, smaller niches were naturally filled by smaller animals. During the Late Jurassic, mammals rarely grew larger than a modern-day badger. Most were small rodent-like creatures, such as Glirodon grandis. Fossilized crocodilians and turtles provide still more evidence that this was a river habitat. The shells and body parts of Dinochelys whitei and Glyptops plicatulus provide evidence for the presence of turtles. Small terrestrial crocodilians, like Hoplosuchus kayi, were common in the Mesozoic. Other reptiles included pterosaurs and the lizard-like Opisthias rarus. Ancient species of frogs and salamanders were among the habitat's amphibians, including Iridotriton hechti and Rhadinosteus parvus. Finally, boreholes in fossilized organic matter from the Morrison Formation of this area indicates the presence of insects, including termites.
What Happened to the Quarry Dinosaurs?
Evidence of DroughtThe dinosaurs in the Quarry Exhibit Hall lived in the Late Jurassic period, long before the end-Cretaceous extinction event. While dinosaurs are the most popular fossils at the Quarry, it's actually clams that are much more common. The clams belong to the group Unionidae, which still has many living members in freshwater rivers around the world today. Their presence is a clear indication that the Quarry sandstone was home to a freshwater river channel in the Late Jurassic. We know from modern river ecology that unionid clams require a permanent source of moving water to survive. When water is present, clams are sometimes buried alive in the river sediment. These are usually found with closed shells. Conversely, unionid clams that die when water isn't present are typically found with their shells open. During excavations at the Quarry, paleontologists found at least three events where large numbers of clams were fossilized with open shells. This suggests that their river habitat experienced at least three severe droughts where large swaths of the river dried up, causing the clams to die.
Death and BurialAs for the dinosaurs, the Wall of Bones reveals a variety of species and ages among the bones. Sauropods (long-necked plant eating dinosaurs) are the most common, but three species of theropods (two-legged meat eating dinosaurs) were also found. Many of the bones in the Quarry sandstone are disarticulated (separated from one another), indicating that they would have had some time for their flesh to decay before burial. Some of the bones even bear signs of scavenging, such as tooth marks. All of this seems to indicate that the land-dwelling animals were just as affected by droughts as the clams. Based on observations of modern wetland ecosystems, paleontologists think the dinosaurs spent a lot of time around the river, especially during droughts. Without rain, the plants would have died in large numbers or had greatly reduced nutritional value. Each day, the dinosaurs would have had to travel farther for this low-quality food, and still return to the river for water. For some, this lifestyle was unsustainable, and they died.
Studies of wetland ecosystems today have shown that large animals, such as elephants, tend to die along the shore or in the dried-up sections of riverbeds during droughts. When the rains return, these rivers often overflow their banks, restoring clam populations, depositing sediments, and carving new channels. Nearby bones and debris are collected during this process and swept downstream. These tend to settle along bends or sandbars, where the sediment is higher. Protruding elements such as vertebrae anchor the bones in the sediment, altering the river's scouring patterns and collecting debris. The upstream-facing ends of bones tend to sink as the water passes around them, sweeping sediments out from under that end of the bone. In effect, the bones set up the conditions for their own burial. Identical dispersal patterns also appear in the Quarry's fossil wall. For these reasons, it is thought that the current Quarry Wall was once a bend or sandbar in an ancient riverbed where debris and dinosaur bones collected. Like many animals today, the dinosaurs likely died along the shore or in dried-up sections of their river during droughts. Because some of the bones are articulated (connected as they would have been in life) and some are disarticulated, it's thought that individual dinosaurs were in various states of decay when they were buried, suggesting that they died at different times rather than in one catastrophic event. After the drought ended and the rains returned, the river would have overflowed with water again, sweeping the bones downstream. With the river flowing and continuously depositing sediment, this would have allowed the bones and debris to be buried quickly and begin the process of fossilization.
Fossilization and Geologic UpliftAfter the bones became quickly buried in the river sediments, the fossilization process could begin. The modern definition of "fossil" covers any evidence of ancient life that gets preserved. "Fossilization" is an umbrella term that covers many different processes, from petrification, to mummification, carbonization, and so on. The dinosaurs at the Carnegie Quarry underwent a type of fossilization called permineralization. As the river flowed, its slightly acidic water continuously dissolved minerals from the sediment. Some of the water flowed down into the sand through a process called percolation. Buried in the sand were the dinosaur bones, which like all bones, were porous (full of tiny holes). The bones acted like a sponge, absorbing the water along with any sediments, bacteria, and dissolved minerals it contained. Some of these elements were deposited onto and within the bones' pores as the water flowed through. As the pores filled with minerals, bacteria ate away at the bones, producing calcium carbonate as a byproduct. The calcium carbonate formed a hard shell that entombed the bacteria and permeated the organic bone cells. With time, the bones became fully permeated with minerals, hence the name "permineralized."
Over millions of years, the continents moved and the landscape changed, hosting many different environments, including forests, inland seas, and others. Each of these environments left their own sediments behind, burying the ancient riverbed ever deeper. The weight and pressure of the uppermost layers slowly turned the sediments of lower layers into different kinds of sedimentary rock. Any bones or other evidence of ancient life that had fossilized in those sediments were preserved within their respective rock layers. Long after the Morrison Formation was buried, all the layers were bent upward into an anticline (hill-shape) when the Rockies rose. Over time, erosion took away the top of the anticline's hill, exposing the bent rock layers. This included the Morrison layer and the dinosaur fossils within it.
DiscoveryWhen paleontologist, Earl Douglass, was asked by his employers at the Carnegie Museum to find dinosaur fossils in 1909, the Morrison Formation was already known to be fossil-rich in other areas where it was exposed. Having worked in this region before, Douglass knew the layer was exposed here also. He made the choice to return in search of dinosaurs. After weeks of finding only small fragments, Douglass found eight vertebrae of an Apatosaurus (a large, long-necked saurpod) eroding out of the hillside. Although the original specimen was extracted and sent back to the Carnegie Museum, the hillside where it was found is where the Quarry Exhibit Hall stands today. During their excavations of the Quarry, paleontologists found several new species. Some well preserved specimens of already-known dinosaurs provided groundbreaking new insight into how these animals lived and looked in life. Today, paleontologists across the North American West are still studying the Morrison Formation and its fossils to learn more about this unique period in geologic time.
Last updated: May 1, 2023