Cretaceous Period, in geologic time, the last of the three periods of the Mesozoic Era. The Cretaceous began 145.0 million years ago and ended 66 million years ago; it followed the Jurassic Period and was succeeded by the Paleogene Period (the first of the two periods into which the Tertiary Period was divided). The Cretaceous is the longest period of the Phanerozoic Eon. Spanning 79 million years, it represents more time than has elapsed since the extinction of the dinosaurs, which occurred at the end of the period.

The name Cretaceous is derived from creta, Latin for “chalk,” and was first proposed by J.B.J. Omalius d’Halloy in 1822. D’Halloy had been commissioned to make a geologic map of France, and part of his task was to decide upon the geologic units to be represented by it. One of his units, the Terrain Crétacé, included chalks and underlying sands. Chalk is a soft, fine-grained type of limestone composed predominantly of the armourlike plates of coccolithophores, tiny floating algae that flourished during the Late Cretaceous.

The Cretaceous Period began with Earth’s land assembled essentially into two continents, Laurasia in the north and Gondwana in the south. These were almost completely separated by the equatorial Tethys seaway, and the various segments of Laurasia and Gondwana had already started to rift apart. North America had just begun pulling away from Eurasia during the Jurassic, and South America had started to split off from Africa, from which India, Australia, and Antarctica were also separating. When the Cretaceous Period ended, most of the present-day continents were separated from each other by expanses of water such as the North and South Atlantic Ocean. At the end of the period, India was adrift in the Indian Ocean, and Australia was still connected to Antarctica.

The climate was generally warmer and more humid than today, probably because of very active volcanism associated with unusually high rates of seafloor spreading. The polar regions were free of continental ice sheets, their land instead covered by forest. Dinosaurs roamed Antarctica, even with its long winter night.

The lengthy Cretaceous Period constitutes a major portion of the interval between ancient life-forms and those that dominate Earth today. Dinosaurs were the dominant group of land animals, especially “duck-billed” dinosaurs (hadrosaurs), such as Shantungosaurus, and horned forms, such as Triceratops. Giant marine reptiles such as ichthyosaurs, mosasaurs, and plesiosaurs were common in the seas, and flying reptiles (pterosaurs) dominated the sky. Flowering plants (angiosperms) arose close to the beginning of the Cretaceous and became more abundant as the period progressed. The Late Cretaceous was a time of great productivity in the world’s oceans, as borne out by the deposition of thick beds of chalk in western Europe, eastern Russia, southern Scandinavia, the Gulf Coast of North America, and western Australia. The Cretaceous ended with one of the greatest mass extinctions in the history of Earth, exterminating the dinosaurs, marine and flying reptiles, and many marine invertebrates.

The Cretaceous Period is biologically significant because it is a major part of the transition from the early life-forms of the Paleozoic Era to the advanced diversity of the current Cenozoic Era. For example, most if not all of the flowering plants (angiosperms) made their first appearance during the Cretaceous. Although dinosaurs were the dominant animals of the period, many modern animals, including the placental mammals, made their debut during the Cretaceous. Other groups—such as clams and snails, snakes and lizards, and most fishes—developed distinctively modern characteristics before the mass extinction marking the end of the period.

Marine life

The marine realm can be divided into two paleobiogeographic regions, the Tethyan and the boreal. This division is based on the occurrence of rudist-dominated organic reeflike structures. Rudists were large, rather unusual bivalves that had one valve shaped like a cylindrical vase and another that resembled a flattened cap. The rudists were generally dominant over the corals as framework builders. They rarely existed outside the Tethyan region, and the few varieties found elsewhere did not create reeflike structures. Rudist reeflike structures of Cretaceous age serve as reservoir rocks for petroleum in Mexico, Venezuela, and the Middle East.

Other organisms almost entirely restricted to the Tethys region were actaeonellid and nerineid snails, colonial corals, calcareous algae, larger bottom-dwelling (benthic) foraminiferans, and certain kinds of ammonites and echinoids. In contrast, belemnites were apparently confined to the colder boreal waters. Marine plankton took on a distinctly modern appearance by the end of the Cretaceous. The coccolithophores became so abundant in the Late Cretaceous that vast quantities accumulated to form the substance for which the Cretaceous Period was named—chalk.

Ammonites were numerous and were represented by a variety of forms ranging from the more-usual coiled types to straight forms. Some of the more-unusual ammonites, called heteromorphs, were shaped like fat corkscrews and hairpins.

Terrestrial life

Although the fossil record is irregular in quality and quantity for the Early Cretaceous, it is obvious that dinosaurs continued their lengthy dominance of the land. The Late Cretaceous record is much more complete, particularly in the case of North America and Asia. It is known, for instance, that during the Late Cretaceous many dinosaur types lived in relationships not unlike the present-day terrestrial mammal communities. Although the larger dinosaurs, such as the carnivorous Tyrannosaurus and the herbivorous Iguanodon, are the best-known, many smaller forms also lived in Cretaceous times. Triceratops, a large three-horned dinosaur, inhabited western North America during the Maastrichtian Age. The titanosaurs, a group of sauropods that included Argentinosaurus and Dreadnoughtus, emerged during the second half of the period and were the largest land animals that ever lived.

Various types of small mammals that are now extinct existed during the Triassic and Jurassic, but two important groups of modern mammals evolved during the Cretaceous. Placental mammals, which include most modern mammals (e.g., rodents, cats, whales, cows, and primates), evolved during the Late Cretaceous. Although almost all were smaller than present-day rabbits, the Cretaceous placentals were poised to take over terrestrial environments as soon as the dinosaurs vanished. Another mammal group, the marsupials, evolved during the Cretaceous as well. This group includes the native species of Australia, such as kangaroos and koalas, and the North American opossum.

In the air, the flying reptiles called pterosaurs dominated. One pterosaur, Quetzalcoatlus, from the latest Cretaceous of what is now Texas (U.S.), had a wingspan of about 15 metres (49 feet). Birds developed from a reptilian ancestor during the Jurassic and Cretaceous. Hesperornis was a Cretaceous genus of flightless diving bird that had large feet and sharp backward-directed teeth adapted for preying on fish.

The land plants of the Early Cretaceous were similar to those of the Jurassic. They included the cycads, ginkgoes, conifers, and ferns. The flowering plants (angiosperms) appeared in the Early Cretaceous, became common by the beginning of the middle of the Cretaceous, and came to represent the major component of the landscape by the mid-to-late Cretaceous. The oldest known fossil angiosperm is Montsechia vidalii, an aquatic plant that has been dated to 130 million to 125 million years ago. The angiosperm group included figs, magnolias, poplars, willows, sycamores, and herbaceous plants. With the advent of many new plant types, insects also diversified.

Cretaceous Era (Part one) : The pinnacle of the dinosaurs reign :croc-pog:

Prehistoric Planet series :grumpy-lizard:

Cretaceous Era (Part two) : pinnacle before the end :gator-bi:

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