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 last part of the Mesozoic Era, and also the longest part. The name comes from the Latin word creta, meaning 'chalk', which is common in rocks from this time. It is often called K for short.

The Cretaceous had a warm climate. This caused sea levels to rise, creating many shallow inland seas. These seas had many types of life, such as now-extinct marine reptiles, ammonites, and rudists. On land, dinosaurs were the main animals. The world had little ice, and forests grew even near the poles.

Many groups of animals and plants that live today first appeared in the Cretaceous. This includes new types of mammals and birds. The first crown group birds lived near the end of this time. Teleost fish became more diverse, especially a group called Acanthomorpha. Flowering plants also appeared and became common. By the end of the Cretaceous, they were the main plants on Earth.

The Cretaceous ended with a big event called the Cretaceous–Paleogene extinction event. Many animals, including non-avian dinosaurs, pterosaurs, and large marine reptiles, disappeared. Many scientists think this happened because a huge space rock hit Earth. The end of the Cretaceous marks the start of a new time called the 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 found lots of chalk, a 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 understand its layers better. 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. 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 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 after this big change.

Paleogeography

During the Cretaceous, the supercontinent Pangaea finished breaking apart into the continents we know today. They were in different places than now. As the Atlantic Ocean grew wider, mountains formed in North America.

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

The Cretaceous is known for the chalk formed then. Lots of calcium in the oceans helped create thick layers of chalk and other sediments. This made the rock record from this period 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 began with a warm and dry time, then became warm and wet, and ended with cooler and drier conditions. The position of a major weather system called the Intertropical Convergence Zone was similar to today.

The climate cooled a little at the very start of the Cretaceous, but then it became warm 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 today, even at the poles, where reptiles could live. This warmth caused 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, big changes happened in plants on land. Before this time, plants like conifers and cycads were very common. Then, a new kind of plant called flowering plants or angiosperms started to grow. These plants later became the most common plants on Earth.

The first signs of flowering plants are tiny pollen grains found in places like Israel and Italy. Over time, these plants spread fast and became very common. By the end of the Cretaceous, flowering plants were everywhere, and other plants like conifers were less common. Even grasses, which are important today, first grew during this time.

Terrestrial fauna

During the Cretaceous period, many interesting animals lived on land. Small mammals were common, and some early mammals were starting to change and grow in different ways. The biggest land animals were reptiles, especially dinosaurs, which were more varied than ever. Birds, which came from dinosaurs, were also around and changing. Pterosaurs, flying reptiles, were common at first but became fewer 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 stayed in some southern parts of South America.

Choristodera

Choristoderes, a group of freshwater reptiles, were quite varied 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 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.

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 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.