K–Pg extinction event

K–Pg extinction event
Name	K–Pg extinction event
Caption	Artist's impression of the Chicxulub crater impact
Date	Approximately 66 million years ago
Location	Global
Type	Mass extinction
Cause	Asteroid impact, volcanism
Affected	Non-avian dinosaurs, pterosaurs, ammonites, many marine reptiles
Preceded by	Cretaceous
Followed by	Paleogene

K–Pg extinction event. The K–Pg extinction event, formerly known as the K–T extinction, was a sudden mass extinction approximately 66 million years ago that marks the boundary between the Cretaceous and Paleogene periods. This catastrophic event resulted in the demise of an estimated 75% of all plant and animal species on Earth, most famously ending the long reign of the Non-avian dinosaurs. The event's cause is widely attributed to the impact of a massive asteroid or comet, with the Chicxulub crater on the Yucatán Peninsula in Mexico considered the primary impact site, though contributing factors like the massive volcanic eruptions of the Deccan Traps in India are also studied.

Overview

The event defines the boundary between the Mesozoic Era and the Cenozoic Era, fundamentally reshaping the course of evolutionary history on Earth. Its timing is precisely dated through radiometric analysis of iridium-rich clay layers found globally. The aftermath of the extinction created ecological niches that were rapidly filled by surviving groups, most notably mammals and birds, allowing them to diversify and become the dominant terrestrial vertebrates. This transition is documented in the fossil records of formations like the Hell Creek Formation in North America.

Causes

The leading hypothesis for the primary cause is the impact of a large extraterrestrial body, estimated to be over 10 kilometers in diameter. Evidence for this includes the global distribution of the iridium anomaly, shocked quartz grains, and tektites found in boundary sediments from locations like Gubbio and the Ratón Basin. The Chicxulub crater, discovered by geophysicists like Walter Alvarez and his father Luis Walter Alvarez, provides a smoking gun for this theory. Concurrently, the immense flood basalt eruptions of the Deccan Traps, which released vast quantities of CO₂ and SO₂, are argued by some scientists, such as Gerta Keller, to have caused prolonged climate instability that stressed ecosystems before the impact.

Impact evidence

A global layer of clay at the Cretaceous–Paleogene boundary contains highly concentrated levels of iridium, an element rare on Earth's surface but common in asteroids. This layer, first identified by the team of Walter Alvarez, is found from the Denver Basin to the cliffs of Stevns Klint in Denmark. Additional physical evidence includes microtektites, spherical glass beads formed from molten rock, and shocked quartz crystals exhibiting planar deformation features, which are diagnostic of extreme high-pressure events. The discovery of the Chicxulub crater by teams including Alan Hildebrand confirmed a crater of the correct age and size to have caused global catastrophe.

Extinction patterns

The extinction was highly selective, disproportionately affecting larger-bodied animals and organisms with specialized diets or limited geographic ranges. Non-avian dinosaurs, pterosaurs, mosasaurs, plesiosaurs, and ammonites were driven to extinction. In contrast, smaller vertebrates like mammals, lizards, turtles, crocodilians, and many birds survived. Marine ecosystems suffered greatly, with groups like the rudist bivalves and most planktonic foraminifera disappearing, while freshwater communities exhibited higher survival rates.

Recovery and aftermath

In the immediate aftermath, Earth experienced a prolonged impact winter, with darkness and cooling from atmospheric debris, followed by potential greenhouse warming. The recovery of life, known as the Paleocene ecological recovery, was gradual. Surviving small mammals, such as early members of the Purgatoriidae, began to radiate into vacant ecological roles. The Paleocene–Eocene Thermal Maximum later marked a period of significant mammalian diversification. The event set the stage for the rise of modern ecosystems, ultimately leading to the evolution of primates and, much later, Homo sapiens.

In popular culture

The K–Pg extinction event is a frequent subject in documentaries, films, and literature, often dramatizing the impact and the demise of the dinosaurs. It is central to the narrative of films like *Jurassic Park* and its sequels, which explore the cloning of extinct species. The event is featured in documentary series such as the BBC's Walking with Dinosaurs and National Geographic specials. The discovery of the impact theory was popularized in books like Walter Alvarez's *T. rex and the Crater of Doom*.

Category:Extinction events Category:Cretaceous Category:Paleogene Category:Impact events