Cretaceous Period

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The Cretaceous period is the third of the three divisions of the Mesozoic era of the geologic time scale. The period lasted 79 million years, from 144 million to 65 million years ago. The Cretaceous is named for chalk beds found in England.

Era	Period	Epoch	Million Years Before Present
Mesozoic	Cretaceous		144
	Jurassic		208
	Triassic		245

Cretaceous and surrounding time periods.

Laurasia and Gondwanaland, the northern and southern landmasses that resulted from the initial breakup of the supercontinent Pangea, continued to separate from each other during the Cretaceous period. These landmasses also began to fragment within themselves to form our modern continents. Throughout most of the Cretaceous, North America was divided by a vast inland sea that extended from the Gulf of Mexico to Canada.

Early in the Cretaceous, the climate was warm and semitropical, very much like at the end of the Jurassic period (144 million years ago). However, during the second forty million years the climate became colder at the polar regions and warmer at the equator, setting in motion ecological changes that affected the evolution of plants and animals.

Shallow oceans supported abundant marine life, including new forms of oysters, diatoms, and algae as well as fish and sharks, corals, echinoderms, ammonoids, and mollusks. The edges of these shallow seas provided important habitat for mammals, turtles, crocodiles, fish, lizards, and many invertebrates.

Dramatic changes occurred in plant life during the Cretaceous. Pollinating insects such as bees and butterflies allowed the emerging flowering plants—the angiosperms —an advantage over seed-bearing plants that relied on the wind or a chance encounter with an animal to disperse their seeds. Today, nearly 90 percent of plants on Earth are angiosperms, signifying a remarkable evolutionary success story. Forests of oak and willow, cypress, magnolia, palms, and sycamore slowly replaced the cycad forests—palm-likeplants with a barrel shaped trunk and many long leaves growing from the top. These new plant communities provided new sources of food and habitat for many kinds of animals.

During the Cretaceous, the dinosaurs reached the height of their evolutionary success. Fossil bones found in Africa, the Gobi Desert, South America, China, Mongolia, and North America suggest that many new species emerged while earlier dinosaurs went extinct. The carnivorous animals such as Albertasaurus and Tyrannosaurus remained the top predators as they roamed and hunted their prey. Hadrosaurs, (duck-billed dinosaurs), Ankylosaurs (armored dinosaurs), and Ceratopsians (horned dinosaurs) replaced the giant Jurassic sauropods as the main herbivores. Triceratops fossils by the hundreds have been found, suggesting that these cows of the Cretaceous traveled in huge herds across the plains. Both herbivore and carnivore nests found in Montana, Mongolia, China, and South America suggest that many Cretaceous dinosaurs nested in colonies and possibly even cared for their young after they hatched.

Mammals became more abundant during the Cretaceous. One group, the multituberculates, were a successful group of early mammals. By the end of the Cretaceous, when the dinosaurs were becoming extinct, the mammals survived and became a very successful group of animals.

A worldwide mass extinction occurred at the end of the Cretaceous period. This extinction killed off nearly 50 percent of Earth's existing species, including all of the remaining terrestrial dinosaurs other than birds, all marine and flying reptiles, and the ammonoids and other invertebrate and microscopic marine organisms. Many groups, however, including most plant species, birds, lizards and snakes, crocodiles and turtles, fish and sharks, many invertebrates, and the mammals survived into modern times.

The causes of this mass extinction are still not completely understood and have led to many lively debates among scientists. One theory suggeststhat massive volcanic eruptions ejected enormous amounts of ash and harmful gases into the atmosphere, creating dark and cold conditions inhospitable to some plants and animals. One hypothesis suggests that a large asteroid hit the Earth, contributing to devastating climate changes. Still others, however, counter with the argument that the global climate was already in the process of cooling off, perhaps in part because of the drying up of North America's inland sea, and that this slow cooling may have led to more gradual changes in plant and animal life. Another proposition is that multiple factors, rather than one single catastrophic event, was responsible for the mass extinction.

Geological Time Scale; K/T Boundary.

Lane, Gary, A., and William Ausich. Life of the Past. 4th ed. Englewood, NJ: Prentice Hall, 1999.

Russell, D. A. An Odyssey in Time: The Dinosaurs of North America. Chicago: North-wood Press, 1989.

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