The Cretaceous Period: Age of Reptiles

Imagine, if you will, what the Earth might have been like 144 million years ago, at the end of the Mesozoic era. The Triassic period had come to an end and the Cretaceous period, spanning 79 million years, had just begun.

Ankylosaurus. Courtesy of Joe Tucciarone

Temperatures during the Cretaceous were generally warmer than those of today. Reptiles ruled the land and a variety of dinosaurs were abundant. Commonly found dinosaurs of this era include Edmontosaurus, Ankylosaurus, Tyrannosaurus and Triceratops. Mammals flourished and primitive, flowering plants, such as magnolia, ficus, credneria, sassafras and viburnum began to alter the landscape. Other commonly found plants of this era were palm trees, ferns, horsetails and conifer trees.

Early in the Cretaceous period, a global warming had just begun, seasonality was low, no polar ice was present, sea levels were high and the oceans were warm. Most of the land was at or around sea level and covered with tropical forests. Shallow seas surrounded the land mass.

Cretacous Earth
Click for more detail.
Map courtesy of Christopher R. Scotese and slightly modified by Norm Brown of UCSD

Seasonality increased as the Earth's tectonic plates began shifting during the mid-Cretaceous period, causing Pangea, the supercontinent, to separate into the seven separate continents we know today. This separation displaced ocean water, submerging the lowlands of the newly formed North American continent, which formed the Western Interior Seaway, a shallow, inland sea that stretched across what is now the Great Plains. This seaway covered an area from Canada to the Gulf of Mexico.

This tectonic activity, accompanied by volcanic activity, began the formation of the world's magnificent mountain ranges, including the Sierra Nevadas in California, the Rocky Mountains in the western United States and the European Alps. As volcanic activity and shifting of the land mass fluctuated, so did the size and depth of the immense Western Interior Seaway. The Western Interior Seaway hosted an abundance of ferocious reptiles and swimming dinosaurs, while the surrounding land was home to a diverse variety of prehistoric creatures and dinosaurs.

The sea level dropped toward the end of the Cretaceous period, exposing land on all continents, which began taking on their modern-day forms. As seasonality increased, the temperatures between the equator and the poles became more extreme. Eventually, severe climate changes and volcanic activity greatly altered our planet. What followed was a mass extinction that wiped out the dinosaurs and many prehistoric life forms.

While still debated, the primary cause for the mass extinction of the Cretaceous period 65 million years ago is believed to be a giant asteroid that collided with the Earth at 45,000 miles per hour. Other theories include high volcanism, severe climate changes due to continental drifting, disease, and an abundance of egg-eating mammals. Whether or not the mass extinction was the result of a cosmic disaster or a severe natural disaster, the Cretaceous period ended with a bang. As the Age of Reptiles came to an end, the Age of Mammals began.

The name Cretaceous is derived from the Latin word, Creta, which means chalk. Since an extensive amount of chalk deposits were left throughout Europe and parts of North America during this period, the name Cretaceous is most appropriate.

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