Cretaceous

The Cretaceous (IPA: /krɪˈteɪʃəs/ krih-TAY-shəss) is the geological period that lasted from about 143.1 to 66 Ma (million years ago). It is the third and final period of the Mesozoic Era, as well as the longest. At around 77.1 million years, it is the ninth and longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, 'chalk', which is abundant in deposits from the latter half of the period. It is usually abbreviated K, for its German translation Kreide.

The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct marine reptiles, ammonites and rudists; while on land, dinosaurs continued to dominate. The world was largely ice-free – although there is some evidence of brief periods of glaciation during the cooler first half – and forests extended to the poles.

Many of the dominant taxonomic groups present in modern times can be ultimately traced back to origins in the Cretaceous. During this time, new groups of mammals and birds appeared, including the earliest relatives of placentals and marsupials (Eutheria and Metatheria respectively), with the earliest crown group birds appearing towards the end of the Cretaceous. Teleost fish, the most diverse group of modern vertebrates, continued to diversify during the Cretaceous with the appearance of their most diverse subgroup Acanthomorpha during this period. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm groups.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out; it is widely thought to have been caused by the impact of a large asteroid that formed the Chicxulub crater in the Gulf of Mexico. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic and Cenozoic Eras.

Etymology and history

The Cretaceous period was first identified in 1822 by a Belgian geologist named Jean d'Omalius d'Halloy. He studied rock layers in the Paris Basin and noticed lots of chalk, a type of rock made from tiny sea creatures. This is why the period is called "Cretaceous," which comes from a Latin word for chalk.

Later scientists split the Cretaceous into different stages to better understand its layers. One scientist, Alcide d'Orbigny, divided it into five main stages, and added a few more over time.

Geology

The Cretaceous period is split into two main parts: the Early Cretaceous and the Late Cretaceous. Scientists also sometimes use three smaller divisions called the Neocomian, Gallic, and Senonian. Today, we use twelve stages based on rock layers found in Europe to describe this time.

The Cretaceous began around 143.1 million years ago and ended 66 million years ago. Its beginning is not clearly marked by a special rock layer, but scientists estimate it to be about 145 million years ago. The end of the Cretaceous is clearly marked by a layer of iridium-rich soil found all over the world. This layer is linked to a huge space rock that crashed into Earth near what is now the Yucatán Peninsula in Mexico. This event caused many plants and animals to go extinct, including the dinosaurs. Life on Earth took a long time to recover and rebuild after this massive change.

Paleogeography

During the Cretaceous, the supercontinent Pangaea finished breaking apart into the continents we know today, though they were in different places than now. As the Atlantic Ocean grew wider, mountain-building continued in North America.

Gondwana, a large landmass, also broke apart during this time. South America, Antarctica, and Australia moved away from Africa, forming the South Atlantic and Indian Oceans. This rifting created underwater mountain chains and raised sea levels worldwide. A large sea even split North America into two landmasses for much of this period.

The Cretaceous is well-known for the chalk formed during this time. Lots of calcium in the oceans helped create thick layers of chalk and other sediments, making the rock record from this period very rich in fossils. Important fossil sites include Kansas’s Smoky Hill Chalk and the Hell Creek Formation in North America, as well as places in Europe and China.

Climate

The Cretaceous period had three main climate phases. It started with a warm and dry time, then shifted to a warm and wet period, and finally ended with cooler and drier conditions. The position of a major weather system called the Intertropical Convergence Zone was similar to where it is today.

The climate cooled a bit at the very start of the Cretaceous, but then warmed up again with several very hot periods. One especially warm time, called the Mid-Cretaceous Hothouse, lasted for millions of years. During this time, the Earth was much warmer than it is today, even at the poles, where reptiles could live. This warmth led to big changes in the oceans and weather patterns.

Later in the Cretaceous, the climate began to cool down, leading to a period known as the Late Cretaceous-Early Palaeogene Cool Interval. Even during this cooler time, there were a couple of brief warm spells, especially near the very end of the Cretaceous period.

Flora

During the Cretaceous period, major changes happened in plant life on land. Before this time, plants like conifers and cycads were very common. But then, a new kind of plant called flowering plants or angiosperms began to appear. These plants would eventually become the most common plants on Earth.

The earliest signs of flowering plants come from tiny pollen grains found in places like Israel and Italy. Over time, these plants spread quickly and became dominant. By the end of the Cretaceous, flowering plants were everywhere, and other plant groups like conifers were less common. Even grasses, which are important today, first appeared during this time.

Terrestrial fauna

During the Cretaceous period, many interesting animals lived on land. Small mammals were common, and some types of early mammals were starting to diversify. The biggest land animals, though, were reptiles, especially dinosaurs, which were more diverse than ever. Birds, the descendants of dinosaurs, were also around and evolving. Pterosaurs, which were flying reptiles, were common early on but became rarer later in the Cretaceous.

Some famous dinosaurs from this time include Tyrannosaurus rex, one of the largest land predators, and Velociraptor, a smaller but fast predator that had feathers. Triceratops was a large, well-known dinosaur with three horns. In the skies, Quetzalcoatlus was one of the largest flying animals ever. Birds like Confuciusornis and Ichthyornis also lived during this time.

Rhynchocephalians

Rhynchocephalians, a group of reptiles that today includes only the tuatara, were no longer found in North America, Europe, or northern South America by the end of the Cretaceous period. They seemed to disappear, perhaps because of competition with other lizards and mammals, but they remained in some southern parts of South America.

Choristodera

Choristoderes, a group of freshwater reptiles, were quite diverse in Asia during the early part of the Cretaceous. They included long-necked swimmers like Hyphalosaurus. Later in the Cretaceous, one type called Champsosaurus lived in western North America and even in the Arctic, where the climate was warmer.

Marine fauna

During the Cretaceous period, many interesting sea creatures lived in the oceans. Rays, modern sharks, and teleosts (a group of bony fish) became common in the seas. Marine reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs also swam in these waters. Sea turtles, including the ancestors of today’s leatherback sea turtle, were present too. There were also special flightless birds called Hesperornithiformes that dove into the water to hunt, similar to grebes.

Other sea life included Baculites, an ammonite with a straight shell, and reef-building rudist clams. Inoceramids, a type of bivalve, were also notable. Predatory snails with drilling habits were widespread, and tiny organisms called globotruncanid foraminifera and echinoderms such as sea urchins and starfish thrived. Diatoms, a type of microscopic algae, first appeared in the oceans during this time.