Cretaceous–Paleogene extinction event

Cretaceous–Paleogene extinction event. This was a sudden mass extinction event approximately 66 million years ago that marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. It is most famous for the demise of all non-avian dinosaurs, but also caused the extinction of numerous other organisms across marine and terrestrial ecosystems. The event is geologically defined by a thin, global layer of sediment known as the K–Pg boundary, which contains high concentrations of the element iridium and other evidence of a catastrophic asteroid impact.

Overview

The Cretaceous–Paleogene extinction event represents one of the five major mass extinctions in Earth's history, drastically reshaping the course of evolutionary biology. It occurred at the boundary between the Cretaceous and Paleogene periods, a transition visible in the geologic record worldwide. The event led to the extinction of an estimated 75% of plant and animal species, creating ecological niches that allowed surviving groups, particularly mammals and birds, to diversify rapidly in the subsequent Paleocene epoch. The discovery of the K–Pg boundary and its anomalous iridium layer by the team of Luis Alvarez and Walter Alvarez at the University of California, Berkeley provided the first definitive physical evidence linking the extinction to an extraterrestrial impact.

Causes

The predominant and most widely supported cause is the impact of a large asteroid or comet, estimated to be about 10 to 15 kilometers in diameter. The impact site, identified in the early 1990s, is the Chicxulub crater buried beneath the Yucatán Peninsula in Mexico. This Chicxulub impactor struck a carbonate platform rich in anhydrite, ejecting vast quantities of dust, sulfur aerosols, and debris into the atmosphere, triggering a cascade of environmental effects. A significant contributory factor was the intense volcanic activity of the Deccan Traps in India, which erupted over hundreds of thousands of years, releasing large volumes of carbon dioxide and sulfur dioxide that may have caused longer-term climate instability preceding the impact.

Impact evidence

The primary evidence for the impact hypothesis is the global distribution of the K–Pg boundary layer, which contains a spike in iridium, an element rare in Earth's crust but common in asteroids. This layer, first identified at sites like Gubbio, Italy, also contains shocked quartz, tektites, and microtektites, which are glassy spherules formed from molten rock ejected during the impact. Further evidence includes the presence of soot from global wildfires and the gigantic Chicxulub crater itself, which was identified through geophysical surveys by teams including Alan Hildebrand and confirmed by drilling projects like the International Ocean Discovery Program.

Affected biota

The extinction was highly selective, disproportionately affecting larger-bodied organisms and those with specialized diets or limited geographic ranges. Most famously, all non-avian dinosaurs perished, along with other major reptile groups like plesiosaurs, mosasaurs, and pterosaurs. In the oceans, major groups like the ammonites, rudist bivalves, and most species of planktonic foraminifera became extinct. Terrestrial ecosystems saw the collapse of many groups of plants, though ferns showed a rapid recovery spike. Notably, many lineages of birds, mammals, crocodilians, turtles, and lizards survived the event.

Recovery and radiation

In the immediate aftermath, ecosystems were dominated by disaster taxa like ferns and opportunistic generalists. The Paleocene epoch saw a gradual recovery of flora, with the rise of modern angiosperm families. The extinction of dominant dinosaurs and other large reptiles opened vast ecological space, leading to the adaptive radiation of mammals. Groups such as the ancestors of modern primates, rodents, and carnivorans began to diversify in size and ecological role, a process that continued explosively into the Eocene. Similarly, birds underwent a significant radiation, filling niches left vacant by pterosaurs and terrestrial theropods.

Cultural significance

The Cretaceous–Paleogene extinction event holds a profound place in both scientific and popular culture, symbolizing the concept of catastrophic change and the fragility of life. It is a central narrative in the study of evolution, illustrating how chance events can alter the trajectory of life on Earth. The event is frequently depicted in media, from documentaries produced by the BBC and National Geographic Society to major motion pictures like those from Universal Pictures and Walt Disney Pictures. It serves as a cautionary tale in discussions about modern extinction risks and planetary defense against near-Earth objects, studied by agencies like NASA. Category:Cretaceous Category:Extinction events Category:Paleogene