2.21: Cretaceous Period (145 to 66 million years)

Cretaceous Period (145 to 66 million years)

During the Cretaceous Period the Earth was relatively warm compared to the world today. There were no glaciers on the planet and sea level was as much as 200 feet higher that today. Fossils of warm-water organisms are found in rocks that are arctic regions today. The dinosaurs that survived into the Cretaceous Period diversified and evolved into many unusual forms. Large marine reptiles called Mosasaurs were the dominant organism in the ocean. Sediments deposited in shallow sea flooding onto the continents had an abundance of ammonites—squid-like organisms that had calcareous shells similar to modern nautilus species. Cretaceous gets its name for Creta—Latin for the word chalk. The shallow warm seas of the Cretaceous Period were locations where the calcareous skeletal remains of planktonic organisms called coccoliths accumulated, forming great accumulations of chalk, such as exposed in the Great White Cliffs of Dover, England. In many places in the equatorial realm oyster-like organism called rudists formed great reefs. Flowering plants also first appear in the fossil record, birds existed in Cretaceous time but were insignificant compared to flying non-avian pterosaurs. Small mammals first appear in abundance in the Cretaceous Period, but they were still generally insignificant compared with more dominant reptile and dinosaur species that existed around them.

During Late Cretaceous time, a large mountain range and volcanic arc developed along the western margin of North America as the Atlantic Ocean basin began to rapidly expand. The rising mountains in the west forced an isostatic down warping of the central part of the North American continent, allowing the shallow Western Interior Seaway to flood across much of the region extending from Arctic Ocean in Alaska and Canada to the Texas Gulf Coast region (Figure 2.56).

Sedimentary rocks of the Zuni Sequence are well preserved and throughout the Colorado Plateau region. During the late Jurassic Period a great coastal sand desert covered much of the western part of the continental United States. As Pangaea broke apart, a great volcanic arc system began to form along the western margin of North America (called the Cordilleran Range). At the same time, shallow seaways began to expand across the central North America Seaways eventually merging to form the ancient Western Interior Seaway. This ancient seaway extended from Texas to Alaska by Cretaceous time and covered what are now the Great Plains and Rocky Mountain regions of the United States and Canada (Figure 2.54)

Figure 2.53. Jurassic- and Cretaceous-age rock formations in Capitol Reef National Park in central Utah (sedimentary layers of the Zuni Sequence).

Figure 2.51. Late Cretaceous ammonites of the Western Interior Seaway - an ancient seaway that existed in the Great Plains and Rocky Mountain region during Cretaceous Time.	Figure 2.52. Triceratops, a Late Cretaceous herbivore dinosaur. Chicago Field Museum.	Figure 2.54. Parasaurolophus- a Late Cretaceous dinosaur with a crested skull.	Figure 2.55. Dinosaur Sue, a famous Tyrannosaurus rex fossil on display at the Chicago Field Museum. T. rex was a large carnivorous dinosaur of the Late Cretaceous Period.