The Cretaceous Period (meaning "chalk," from the many chalk deposits of this age [the White Cliffs of Dover, to name one]) easily stands as one of the most popular times among the general populus. The period extends from about 141 million years ago until 65 million years ago. It was during this time that massive carnivorous dinosaurs such as Tyrannosaurus roamed, and flying pterosaurs ruled the sky. The Cretaceous was also punctuated and concluded by one of the greatest mass extinctions of all time. About half of all animal families did not make the transition to the Tertiary Period.
Classic Fossil-Bearing Sites
Shantung, China
Santana Formation, Brazil
Potomic Group, Maryland
Niobrara Chalk, Kansas
Belly River,
Alberta
Lance-Hell Creek Formations, Western U.S.
Eleven major impact structures are known from the Cretaceous, but none rival the terminal event in Cretaceous existence. Some 65 million years ago a massive asteroid, 10-20 km in diameter struck the Earth north of the Yucatan Peninsula in what is now the Gulf of Mexico. The resulting crater approaches 300 km in diameter, and is the second-largest craterform structure known.
The asteroid would have been travelling at a speed between 10 and 20 km per second when it entered Earth's atmosphere and a relatively oblique angle from the south. Both the air and any water would have provided negligible resistance, and the resulting impact with the Earth's surface would have kicked up billions of tons of substrate. The dust entered the atmosphere, presumably blocking out sunlight for an extended time (perhaps 6 months).
Models suggest that a global cooling would have resulted for about 10 years due to a 10-20% reduction in solar transmission through the atmosphere. This, in turn, would be followed by a warming trend for about 100 years due to a greenhouse effect from massive amounts of carbon dioxide released in the impact.
Special Events
Rise of the Rocky Mountains
During the Cretaceous, severe compressional forces were in play throughout Idaho, Wyoming, and Utah, causing masses of rock to be moved horizontally and folded upward. During the Middle Cretaceous, this uplift was further aided by plutonic and volcanic action on the West Coast.
Mass Extinction
The close of the Cretaceous also marks a time of mass extinction, certainly in part due to impact by an extraterrestrial body. This was the second-greatest extinction of all time, with about half of all animal families not making the transition to the Paleocene.
All of the dinosaurs became extinct, and marine reptiles and flying pterosaurs vanished as well. Among the invertebrates, the ammonoids and belemnoids were wiped out, and many families of clams and snails were eliminated. Corals lost two-thirds of their members.
A few groups, such as land plants, mammals, fishes, brachiopods, and amphibians, lost relatively few representatives.
Cretaceous Graphics
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| Cretaceous Sea |
Belemnites |
Placenticeras |
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| Placenticeras |
Helioceras |
Baculites |
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| Pteranodon |
Tyrannosaurus |
Baculites |
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| Placenticeras |
Helioceras |
Ammonites |
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| Triceratops |
Ammonite |
Tyrannosaurus |
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| Triceratops |
Fossil Leaves |
Sequoia Cones |
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| Fossil Leaf |
Cycad |
Birch Leaves |
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| Mosasaur |
Mosasaur Skull |
Mosasaur limb |
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| Triceratops |
Tyrannosaurus Skull |
Tyrannosaurus Teeth |
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| Tyrannosaurus Skull |
Protoceratops Hatching |
Oviraptor(?) Eggs |
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| Pteranodon |
Crinoids |
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