Cretaceous–Paleogene extinction event

The Cretaceous–Paleogene extinction event happened about 66 million years ago and changed life on Earth forever. It caused many plant and animal species to disappear, including all the big, non-avian dinosaurs. This event marked the end of a time called the Cretaceous period and the start of a new era, the Cenozoic Era.

Scientists believe a huge asteroid, about 10 to 15 kilometers wide, crashed into Earth, creating a massive crater called Chicxulub in what is now the Gulf of Mexico. This impact threw dust and particles into the air, causing darkness and cold that hurt plants and animals. The asteroid also likely made the oceans more acidic, which harmed many sea creatures.

Many different species, from dinosaurs to marine animals like plesiosaurs and ammonites, went extinct. But this event also opened doors for new life. Mammals began to evolve into many new forms, including the first horses, whales, bats, and primates. Birds, which were the only dinosaurs to survive, also evolved into many species we see today.

Extinction patterns

The K–Pg extinction event happened all over the world and happened quickly. It caused many species to disappear.

The event affected all the continents at the same time. For example, non-avian dinosaurs lived in Maastrichtian in North America, Europe, Asia, Africa, South America, and Antarctica, but they are not found after the Cenozoic era anywhere. Fossil pollen shows that plants were badly damaged in places as far apart as New Mexico, Alaska, China, and New Zealand. However, areas far from the poles were affected more than areas near the poles.

Even though many species disappeared, some survived better than others. Species that needed photosynthesis were hurt because dust blocked sunlight and reduced the solar energy reaching the ground. This caused many plants to die. Omnivores, insectivores, and carrion-eaters survived because they could eat whatever food was around. Mammals and birds that only ate plants or only ate meat did not survive. The survivors ate insects, worms, and snails, which fed on dead plants and animals called detritus.

In streams and lakes, only a few large animals like crocodyliforms and champsosaurs disappeared. These places get food from dead material washed in from the land, so they were less affected. Modern crocodiles can survive without food for a long time, and their young are small and eat small animals and dead things.

In the oceans, animals that lived in the water were affected more than animals that lived on the sea floor. Animals in the water depend on tiny plants called phytoplankton for food. When sunlight was blocked, these tiny plants died, hurting the animals that ate them. Animals on the ocean floor could eat dead material, so they were less affected. Many important ocean animals like coccolithophores almost disappeared, but some that could move and eat other things helped the ocean recover. Many coccolithophorids, mollusks (including ammonites, rudists, freshwater snails, and mussels), and animals that ate them disappeared or were greatly reduced. For example, ammonites were the main food for mosasaurs, large ocean reptiles that also disappeared.

The K–Pg extinction changed life on Earth. With many large groups gone, new organisms could grow and change. This led to many new species appearing in the Paleogene Period. It took a long time for the world to recover, even though many places had empty homes for animals and plants. Evidence suggests recovery happened faster in the Southern Hemisphere than in the Northern Hemisphere.

Even though many animals died, only a few places show clear signs of this happening at the exact time of the extinction. One place is the Tanis site in the Hell Creek Formation in North Dakota, United States. It has many well-preserved fossils that seem to have been buried quickly by a huge flood, probably from the impact. Another place is the Hornerstown Formation in New Jersey, United States, which has a layer at the K–Pg boundary called the Main Fossiliferous Layer (MFL). This layer has many animal bones that were likely buried by a flood caused by the impact.

Dating

Scientists have worked to find out exactly when many plants and animals died out. Some studies of fossil leaves suggest it happened in early June. Other studies of special kinds of fish bones suggest it was during the spring. Recent research on fish bones from a place called Tanis in North Dakota also points to spring. These studies help us learn more about when this big change happened long ago.

Duration

Scientists are not sure how quickly this big event happened. Some think it was very fast, while others think it took longer. It is hard to know for sure because we do not have all the information from fossils.

Studies of rock layers suggest the extinction may have happened in less than 10,000 years. One place in Colorado shows that it took about 1,000 years for plants to start growing again after the event. This helps us understand how quickly life changed during this time.

Causes

Chicxulub impact

Main articles: Cretaceous–Paleogene boundary, Alvarez hypothesis, and Chicxulub crater

In 1980, scientists found that rocks from the time of the Cretaceous–Paleogene boundary have a lot of iridium, an element rare on Earth but common in space rocks. This made them think an asteroid hit Earth around that time.

More studies found clues like tiny glass beads and special minerals in the rocks, supporting the idea of a big impact. The Chicxulub crater in Mexico was identified as where the asteroid struck.

The impact released huge energy, causing giant waves and fires. It also made a dust cloud that blocked sunlight for years. This made it hard for plants to grow and affected many animals and plants, leading to many going extinct.

Multiple impact event

Besides the Chicxulub crater, there are other places on Earth that might have been hit by space rocks around the same time. These include the Boltysh crater in Ukraine and the Silverpit crater in the North Sea.

Deccan Traps

Main article: Deccan Traps

The Deccan Traps are large areas of volcanic rock in India. They formed when many volcanoes erupted around the same time as the mass extinction. These eruptions could have put dust and gases in the air, blocking sunlight and changing the climate.

The Deccan Traps might have helped cause the extinction by making it hard for many species to survive. Some scientists think the asteroid impact might have caused more volcanic activity in the Deccan Traps.

Maastrichtian sea-level regression

Sea levels fell near the end of the Cretaceous period. This could have reduced homes for many sea animals. This change might have helped cause the extinction by changing where animals could live and affecting the climate.

Multiple causes

Some scientists think several things together caused the mass extinction. These include the asteroid impact, volcanic eruptions from the Deccan Traps, and changes in sea levels. Together, these events made it very hard for many species to survive.