Cretaceous Period

Introduction

The Cretaceous period is the third and final geological time interval of the Mesozoic era, marking the end of the age of the dinosaurs that dominated the world for 160 million years. The Cretaceous saw some of the most widely recognized dinosaurs (e.g., Tyrannosaurus rex, Triceratops, Velociraptor) come and go, while the continental landmasses of the world assumed shapes and positions resembling those of modern times. The Cretaceous period, along with being remembered for the dinosauars that roamed it, is remembered for its catastrophic and dramatic ending that killed off these giant animals. Remnants of that long-ago time have survived through the intervening 66 million years since the end of the Cretaceous. The modern world is filled with dinosaurs known as birds, as well as some other species that lived during the Cretaceous, such as the Coelacanth fish and Araucaria and Ginkgo biloba trees.

Time Defined

The Cretaceous period extended from 146 to 66 million years ago when the Chicxulub meteor impact devastated the world. Unlike the Triassic and Jurassic periods that precede it, the Cretaceous is divided into only two epochs, Early (146–100.5 million years ago) and Late (100.5–66 million years ago). These epochs are each further subdivided into six recognized ages that vary in length from about 2.5 million years to 13 million years. The end of the Cretaceous is sharply defined in the geological and fossil records by the appearance of the Cretaceous–Tertiary (K–T) Boundary, an iridium-rich layer of ash and debris blown around the world as a result of the Chicxulub meteor strike.

Research History

The very event that ended the Cretaceous may have also guaranteed it to be the most well-known period of the Mesozoic. The suddenness with which vast numbers of creatures were killed would have injected a large number of specimens into the fossil record at once rather than singular events over a long period of time. Because they are the most recent, Cretaceous strata are also the most readily accessible of the Mesozoic era and are often identified at the surface in areas that have been eroded. Prime examples of this effect are the western badlands of North America, in the Canadian province of Alberta and the states of Montana, Wyoming, Colorado, Utah, New Mexico, and Texas in the United States, and the Gobi Desert in Mongolia.

Beginning in the late nineteenth century, Edward Drinker Cope and Othniel Charles Marsh worked feverishly to recover Cretaceous fossils in the American West. The Alberta Badlands, near Drumheller, home of the Tyrell Museum, have become one of the richest sources of late Cretaceous fossils known. In the 1920s, Roy Chapman Andrews led the first paleontological expeditions into the Gobi Desert of Mongolia. There, he located the rich Cretaceous fossil beds at the region known as the “Flaming Cliffs,” which yielded the first identified caches of fossilized dinosaur eggs, proving that at least some dinosaurs were oviparous (egg-laying). The fossilized remains suggested that the creatures found with the eggs had been robbing the nests of other dinosaurs found nearby, leading Andrews to name the creatures Oviraptor; however, it has since been realized that the creatures had been guarding their own nests rather than raiding others.

Paleontologists continue to conduct field research in Cretaceous deposits around the world, where they have found many previously unknown species. The most famous of Cretaceous fossils discovered to date (which attests to the suddenness with which death overtook some creatures) was found in Mongolia, where the remains of a Protoceratops and a Velociraptor that had fallen in deadly combat were found fossilized together.

Major Events

During the Cretaceous period, the changing distribution of the continental landmasses brought about great changes in the movement of oceanic currents and environmental conditions in different parts of the world. The terrestrial climate became generally wetter, eliminating the conditions that had been conducive to massive forests of gymnosperms (plants that produce exposed seeds). This, in turn, allowed the development of angiosperms (flowering plants). With seeds better able to remain viable through variable conditions, the angiosperms subsequently displaced gymnosperms as the dominant plant species.

The Cretaceous period lasted for about 80 million years, making it the longest single period in the planet's geological history during which a specific class of creatures dominated the environment. The most important event of the time, though, was the drastic event that ended the period. No other event has had so profound an effect on the course of evolution. The Chicxulub meteor impact is now widely believed to have eradicated as much as 50 percent of all living creatures of the time, spawned earthquakes and tsunamis around the world, and caused a flash conflagration that burned most of the world's plant life. In the aftermath, dinosaurs were displaced, allowing mammals to become the dominant land creatures of the Cenozoic era.

Life in the Period

Cretaceous fauna and flora are perhaps the best known of all Mesozoic life forms, due in large part to the relative availability of the corresponding fossils. The wetter climate of the Cretaceous period produced varied forests of coarse ferns and cycad-like trees, as well as many species of trees that are well known in the present day. Cretaceous forests included conifers such as Araucaria, Abietinea, and Sequoia, and deciduous trees such as the ginkgo, birch, oak, poplar, plane, willow, laurel, and eucalyptus. The undergrowth in such forests is believed to have been dominated by small mammalian species, such as Zalambdalestes. It is thought that the burrowing habits of many such creatures may have been an essential factor in their survival through the post–Chicxulub conflagration.

Shallow and relatively warm Cretaceous seas teemed with vast numbers of fish, such as Portheus, upon which such marine reptiles as Mosasaurus would feed, though these seem to have become extinct sometime late during the Cretaceous period. Other aquatic creatures included the great turtle Archelon and the ammonites, both of which also disappeared from the fossil record before the end of the period. Flying creatures, such as Pteranodon and the gigantic Quetzalcoatlus, dominated the skies, while large numbers of flying and nonflying insects were abundant closer to the earth. Several types of pollinators, including bees, ants, and wasps, developed at this time. The Cretaceous period also saw the rise and demise of some of the most widely recognized dinosaurs, including Tyrannosaurus rex, Triceratops, and Velociraptor.

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