Cretaceous–Paleogene extinction event

The Cretaceous–Paleogene extinction event happened about 66 million years ago and changed life on Earth forever. It caused three-quarters of all 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 lots of dust and particles into the air, causing a long period of 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, for example, began to diversify and evolve into many new forms, including the first horses, whales, bats, and primates. Birds, which were the only dinosaurs to survive, also evolved into the many species we see today.

Extinction patterns

The K–Pg extinction event was global, rapid, and selective, eliminating a vast number of species. Based on marine fossils, it is estimated that 75% or more of all species became extinct.

The event appears to have affected all of the continents at the same time. Non-avian dinosaurs, for example, are seen in the Maastrichtian of North America, Europe, Asia, Africa, South America, and Antarctica, but are not found from the Cenozoic era anywhere in the world. Similarly, fossil pollen shows devastation of the plant communities in areas as far apart as New Mexico, Alaska, China, and New Zealand. Nevertheless, high latitudes appear to have been less strongly affected than low latitudes.

Despite the event's severity, there was significant variability in the rate of extinction between and within different clades. Species that depended on photosynthesis declined or became extinct as atmospheric particles blocked sunlight and reduced the solar energy reaching the ground. This plant extinction caused a major reshuffling of the dominant plant groups. Omnivores, insectivores, and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. Neither strictly herbivorous nor strictly carnivorous mammals seem to have survived. Rather, the surviving mammals and birds fed on insects, worms, and snails, which in turn fed on detritus (dead plant and animal matter).

In stream and lake ecosystems, few animal groups became extinct, including large forms like crocodyliforms and champsosaurs, because such ecosystems rely less directly on food from living plants, and more on detritus washed in from the land, protecting them from this extinction. Modern crocodilians can live as scavengers and survive for months without food, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous period.

Similar, but more complex, patterns have been found in the oceans. Extinction was more severe among animals living in the water column than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while animals on the ocean floor always or sometimes feed on detritus. The impact's blockage of sunlight devastated marine food webs which relied on photosynthetic plankton, and “the oceans may have very well returned to a single-celled state that the world had not seen in over half a billion years.” Consequently, coccolithophores—vital to the open ocean ecosystem during the late Cretaceous—were nearly eradicated; however, researchers have theorized that surviving mixotrophic coccolithophores, capable of movement and ingestion of prey particles in addition to photosynthesis, were critical to restoring the algal food web over time. Coccolithophorids and mollusks (including ammonites, rudists, freshwater snails, and mussels), and those organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal prey of mosasaurs, a group of giant marine reptiles that became extinct during the K-Pg event.

The K–Pg extinction had a profound effect on the evolution of life on Earth. The elimination of dominant Cretaceous groups allowed other organisms to take their place, causing a remarkable amount of species diversification during the Paleogene Period. After the K–Pg extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches. Evidence from the Salamanca Formation suggests that biotic recovery was more rapid in the Southern Hemisphere than in the Northern Hemisphere.

Despite the massive loss of life inferred to have occurred during the extinction, and a number of geologic formations worldwide that span the boundary, only a few fossil sites contain direct evidence of the mass mortality that occurred exactly at the K-Pg boundary. These include the Tanis site of the Hell Creek Formation in North Dakota, United States, which contains a high number of well-preserved fossils that appear to have been buried in a catastrophic flood event likely caused by the impact. Another important site is the Hornerstown Formation in New Jersey, United States, which has prominent layer at the K–Pg boundary known as the Main Fossiliferous Layer (MFL). It contains a mass accumulation of disarticulated vertebrate remains, likely deposited by a catastrophic impact‑related flood.

Dating

Scientists have tried to figure out the exact time of the big event when many plants and animals died out. One study looked at fossil leaves and thought it happened in early June. But later studies, looking at bones from special kinds of fish, suggested it was during the spring. More recent research, studying fish bones from a place called Tanis in North Dakota, also points to the event happening in spring. These studies help us learn more about when this big change happened long ago.

Duration

Scientists debate how quickly this big event happened. Some believe it was very fast, while others think it took a longer time. It's hard to know exactly how long it lasted because the fossil record is incomplete.

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

Causes

Chicxulub impact

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

In 1980, researchers discovered that layers of rock from the time of the Cretaceous–Paleogene boundary contain a lot of iridium, a rare element on Earth but common in space rocks. This led them to suggest that an asteroid hit Earth around that time.

Further studies found more clues, like tiny glass beads and special minerals in the rock layers, supporting the idea of a big impact. The Chicxulub crater in Mexico was identified as the place where this asteroid struck.

The impact released huge amounts of energy, causing giant waves, fires, and a cloud of dust that blocked sunlight for years. This made it hard for plants to grow and affected the whole food chain, leading to many animals and plants 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 that formed when many volcanoes erupted around the same time as the mass extinction. These eruptions could have released dust and gases into the air, blocking sunlight and changing the climate.

The Deccan Traps might have played a role in the extinction by making the environment harder for many species to survive. Some scientists think the asteroid impact might have triggered more volcanic activity in the Deccan Traps.

Maastrichtian sea-level regression

Sea levels dropped near the end of the Cretaceous period, which could have reduced habitats for many sea animals. This change might have contributed to the extinction by altering where animals could live and affecting the climate.

Multiple causes

Some scientists believe that several factors 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 difficult for many species to survive.