10-6 Dinosaurs

The Morrison Formation contains many Late Jurassic dinosaurs, such those in Jurassic Park movies. The greatest fossil deposit in the Morrison formation is at Dinosaur National Monument (DNM) in the far northern part of the Colorado Plateau. Uneven uplift has exposed geologic strata at the Carnegie Quarry, which is at a high elevation at the west end of Dinosaur National Monument (DNM). The Cedar Mountain formation overlies the Morrison Formation and documents dinosaur evolution during the Cretaceous Period. The Hell Creek Formation in North Dakota has Tyrannosaurus Rex and other End Cretaceous dinosaur fossils.

In addition to the Carnegie Quarry, Dinosaur National Monument also includes strata that are exposed in the Green and Yampa river canyons (Figure 10‑46). Meet some thrill seekers in the following video as they tour the Carnegie Quarry and then view the DNM from the river. 

The following videos are not as exciting as the previous video.

The Morrison Formation is above the Chinle Formation in the stratigraphic column (Figure 10‑47), but some of the layers in between have different names in Figure 10‑34 and Figure 10‑47. There are a few reasons for different names in different locations. Because it is at the northern end of the plateau, the deposition environment at the time may have been different from the central and southern part of the Colorado Plateau region. Others formations might extend across the plateau but have different names because different people discovered them in different locations. Names might also vary because a series of formations can comprise a larger formation. For example, the Glen Canyon Sandstone (Figure 10‑47) contains the Navajo and Wingate Sandstones (Figure 10‑34 and Figure 10‑47).

The Laramide Orogeny was not an entirely uniform uplift of the plateau. Uneven uplift can form mounds, which then erode form an anticline in which layers are exposed at the ground surface. This process exposed the Morrison and Cedar Mountain Formations (Figure 10‑48 and Figure 10‑49) at Dinosaur National Monument at the famous Carnegie Quarry. 

The San Rafael Swell is the name of the anticline that formed due to the localized uplift at Dinosaur National Monument. The tilted layers are evident in the canyon in Figure 10‑48. They are also evident at the level of the plateau as tilted layers (San Rafael Reef) are exposed at the surface. The Morrison Formation was deposited when the area consisted of rivers and lakes. A flood event deposited many dinosaur bones at the location of the Carnegie Quarry.

The dominant carnivore in the Late Jurassic Morrison Formation (146 Ma) was Allosaurus (Figure 10‑50), a saurischian theropod, commonly known as Big Al. Brontosaurus, Apatosaurus and other large saurischian sauropods are also in DNM. Apatosaurus was an enormous sauropod found in the Morrison Formation at several locations across the Colorado Plateau (Figure 10‑51).

The Cedar Mountain Formation overlies the Morrison Formation at DNM (Figure 10‑52).

The Cedar Mountain Formation represents the mid Cretaceous Period (Figure 10-53). There is a missing period above the Morrison Formation, which occurs where layers were washed away by erosion. The lower limit is the Yellow Cat Member, and the upper limit of the Cedar Mountain Formation is the Mussentuchit Member.  

The Morrison formation holds many of the most famous dinosaurs of the Jurassic Period, including Diplodocus, Stegosaurus, and Allosaurus (Figure 10‑54).

The story of dinosaur evolution continues up through the layers of the Cedar Mountain Formation. The dinosaurs in the Morrison Formation were generally not present in the Cedar Mountain Formation. The earliest member of the Cedar Mountain formation is the Yellow Cat Member (126 Ma). Fauna in this phase of the Cedar Mountain Formation are in Figure 10‑55. 

Dinosaurs continued to evolve until the End Cretaceous Extinction 66 Ma. Fossils were rare in the Late Cretaceous in the Cedar Mountain Formation, but there were abundant fossils in the Hell Creek formation in the Dakotas and Montana (Figure 10‑56). As with the lack of Morrison dinosaurs in the Cedar Mountain Formation, there was an almost entirely new group of dinosaurs in the Hell Creek Formation.