Glossary

The Chicxulub crater is a buried impact structure approximately 180 km in diameter located beneath the Yucatán Peninsula, Mexico, with its centre near the coastal town of Chicxulub Puerto. It was formed approximately 66 million years ago when an asteroid estimated at 10–15 km in diameter struck the Earth at a speed of roughly 20 km/s, releasing kinetic energy on the order of 72 teratonnes of TNT equivalent (approximately 300 zettajoules). The impact generated a transient cavity roughly 100 km wide and 30 km deep, which subsequently collapsed to form the final crater structure, including a prominent peak ring approximately 90 km in diameter. The impact is widely accepted as the primary cause of the Cretaceous–Paleogene (K–Pg) mass extinction, which eliminated approximately 75–80% of all species on Earth, including all non-avian dinosaurs. The collision injected vast quantities of dust, sulfate aerosols, and soot into the atmosphere, triggering a global impact winter that suppressed photosynthesis, disrupted food chains, and caused severe temperature fluctuations lasting years to decades. This event marks the boundary between the Mesozoic and Cenozoic eras, fundamentally reshaping the trajectory of life on Earth and enabling the subsequent radiation of mammals, birds, and flowering plants into ecological niches formerly occupied by dinosaurs and other Mesozoic fauna.

The Cretaceous–Paleogene (K-Pg) extinction event is a mass extinction that occurred approximately 66 million years ago at the boundary between the Cretaceous and Paleogene periods. It is the most recent of the geological 'Big Five' mass extinctions. The primary cause was the impact of an asteroid roughly 10 km in diameter that struck what is now the Yucatán Peninsula of Mexico, forming the approximately 180–200 km wide Chicxulub crater. The impact ejected vast quantities of dust, soot, and sulfate aerosols into the stratosphere, triggering an 'impact winter' that blocked sunlight, shut down photosynthesis, and collapsed food chains globally. Approximately 75% of all species on Earth perished, including all non-avian dinosaurs, pterosaurs, most marine reptiles, ammonites, and many groups of marine invertebrates. Simultaneously, the extinction created vast empty ecological niches that catalyzed the adaptive radiation of mammals and birds, ultimately establishing the ecological foundations of the Cenozoic Era.

**Extinction** is the complete and permanent disappearance of a biological species, occurring when no living individuals of that species remain anywhere on Earth. Species become extinct due to a range of environmental and evolutionary factors, including habitat fragmentation, climate change, natural disasters, overexploitation, interspecific competition, genetic inbreeding, and declining reproductive success. An estimated 99% of all species that have ever existed are now extinct. Under normal conditions, species disappear at a low, continuous rate of roughly one to five species per year across the entire fossil record, a process termed **background extinction**. Periodically, however, extinction rates spike dramatically during **mass extinction** events, in which a substantial proportion of Earth's biodiversity — typically 75% or more of species — is lost within a geologically brief interval. These catastrophic events, driven by asteroid impacts, large-scale volcanism, rapid climate shifts, and other global-scale perturbations, fundamentally restructure ecosystems and open ecological niches for surviving lineages, thereby shaping the trajectory of evolution.