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 it was the longest part. The name comes from the Latin word for chalk, which is found often in rocks from this time.

The Cretaceous had a warm climate, which caused sea levels to rise and created many shallow inland seas. These seas were home to many sea creatures that are now gone, like marine reptiles and ammonites. On land, dinosaurs were the main animals.

Many groups of animals and plants that we see today first appeared during the Cretaceous. Early mammals and birds showed up, and flowering plants began to spread and become common. This time was important for the start of many modern life forms.

The Cretaceous ended with a big mass extinction event, where many groups like non-avian dinosaurs and pterosaurs disappeared. Many scientists think this happened because of a huge space rock that hit Earth, creating the Chicxulub crater in the Gulf of Mexico. This event marks the end of the Mesozoic and the start of a new time in Earth's history.

Etymology and history

The Cretaceous period was named in 1822 by a scientist named Jean d'Omalius d'Halloy. He looked at rock layers near Paris and found lots of chalk, a soft, white rock made from tiny sea creatures. The name "Cretaceous" comes from a Latin word for chalk.

Later scientists divided the Cretaceous into stages to organize the time better. One scientist added more stages to make things clearer.

Geology

The Cretaceous period was a time when Earth had two main parts called the Early and Late Cretaceous. Scientists sometimes split it into three smaller parts, but today we mostly use twelve steps to describe changes during this time.

One big change at the end of the Cretaceous was when a large object hit Earth. This caused many plants and animals to disappear, including most dinosaurs. However, some animals like crocodiles and small mammals survived. The rocks and chalk we see today in places like England's white cliffs were formed during this period, showing us how different the world looked millions of years ago.

Paleogeography

During the Cretaceous, the supercontinent Pangaea broke apart into the continents we have today, though they looked different then. As the Atlantic Ocean grew wider, big mountain ranges formed along the edge of what is now North America.

Gondwana, a large landmass that included South America, Antarctica, and Australia, also split apart. This created new oceans, including the South Atlantic and Indian Oceans. These changes made sea levels rise around the world. In North America, a huge inland sea called the Western Interior Seaway split the land into two parts for much of this time.

The Cretaceous period is famous for the chalk that formed then. Tiny sea creatures made chalk, which now makes up many rock layers we see today. Important rock formations from this time can be found in places like Kansas, Europe, China, and India.

Climate

The Cretaceous period had three main climate phases. It began with a warm and dry time, then changed to a warm and wet period, and ended with cooler and drier conditions.

The climate changed because of Earth’s natural cycles and the movement of ocean currents. During some parts of the Cretaceous, the planet was very warm, with high temperatures even at the poles. This warmth allowed tropical plants and animals to live in places that are now very cold, like near the Arctic and Antarctic. The warm climate also caused big storms, which affected forests and plants.

Flora

During the time of the dinosaurs, plants on Earth looked very different from today. Before flowering plants existed, the land was mostly covered with plants like conifers and cycads. Flowering plants, also called angiosperms, began to appear around 134 million years ago. The first signs were tiny pollen grains found in places like Israel and Italy.

These new plants spread quickly during the Cretaceous period. By the end of this time, flowering plants had become the most common plants on land. They overtook groups like conifers. We also see the first signs of grasses and large flowering trees during this period. Ferns, which are still around today, also became more diverse.

Terrestrial fauna

During the Cretaceous period, many interesting animals lived on land. Small mammals, such as the cimolodont multituberculates, were common. They were sometimes even more numerous than dinosaurs. True marsupials and placentals did not appear until the very end of this time, but other early mammals were beginning to diversify.

The biggest land animals were archosaurian reptiles, especially dinosaurs. They were more varied than ever. Early versions of today’s birds were also around, spreading across all continents. Pterosaurs, flying reptiles, were common early on but became rarer later.

Important fossil sites, like the Liaoning area in China, show us well-preserved remains of small dinosaurs, birds, and mammals from the Early Cretaceous, including dinosaurs with hair-like feathers.

Insects also became more diverse during this time. The oldest known ants, termites, and early forms of butterflies and moths appeared. Some notable animals from the Cretaceous include:

• Tyrannosaurus rex, one of the largest land predators of all time, lived during the Late Cretaceous
• Up to 2 m long and 0.5 m high at the hip, Velociraptor was feathered and roamed the Late Cretaceous
• Triceratops, one of the most recognizable genera of the Cretaceous
• The azhdarchid Quetzalcoatlus, one of the largest animals to ever fly, lived during the Late Cretaceous
• Confuciusornis, a genus of crow-sized birds from the Early Cretaceous
• Ichthyornis was a toothed, seabird-like ornithuran from the Late Cretaceous

Marine fauna

During the Cretaceous, many interesting animals lived in the oceans. You could find rays, modern sharks, and teleosts swimming around. Marine reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs also lived in these seas. Sea turtles, including the leatherback sea turtle, were around too. There were even special flightless birds called Hesperornithiformes that dove into the water to hunt.

Other sea creatures included Baculites, a type of ammonite with a straight shell, and reef-building rudist clams. Inoceramids, a kind of bivalve, were common and helped scientists understand changes in the environment. Predatory snails with drilling habits were also widespread. Tiny organisms called globotruncanid foraminifera and echinoderms like sea urchins and starfish thrived. Diatoms, which are a type of algae with glass-like shells, first appeared in the oceans during this time.