In Earth's history, five mass extinctions are known. Although mass extinctions cause catastrophic destruction, they also provided opportunities for evolution that promoted the diversification of new ecosystems.
Basic Knowledge
The five major mass extinctions, known as the "Big Five":
① End-Ordovician (about 440 million years ago): Cooling and sea-level fall. About 85% of marine species became extinct.
② End-Devonian (about 370 million years ago): Marine anoxic event. About 75% of marine species became extinct.
③ End-Permian (P-T boundary) (about 250 million years ago): The largest in history. About 96% of all species became extinct. Cause: CO₂ emissions, global warming, and ocean anoxia caused by large-scale volcanic activity in Siberia.
④ End-Triassic (about 200 million years ago): About 76% of species became extinct. This provided an opportunity for dinosaurs to begin rising to dominance.
⑤ End-Cretaceous (K-Pg boundary) (about 66 million years ago): Dinosaurs and ammonites became extinct. Causes: A massive meteorite impact on Mexico's Yucatán Peninsula (Chicxulub Crater) + volcanic activity. Mammals rapidly diversified after this extinction.
Today, it is also said that the sixth mass extinction, caused by human activities, is under way.
P-T boundary (Permian/Triassic boundary. About 250 million years ago. The largest mass extinction in history)
K-Pg boundary (Cretaceous/Paleogene boundary. About 66 million years ago. Dinosaur extinction)
Chicxulub Crater (The meteorite-impact site at the K-Pg boundary. Mexico's Yucatán Peninsula)
Iridium anomaly (Evidence that iridium of meteoritic origin is concentrated in the K-Pg boundary layer)
Marine anoxic event (A condition in which oxygen is depleted in the deep ocean. Involved in several mass extinctions)
Adaptive radiation (The evolution and diversification of many species into niches left vacant after a mass extinction)
Further Exploration (Background and Significance)
Evidence for dinosaur extinction began in the 1980s, when Alvarez and his son discovered unusually large amounts of iridium in the end-Cretaceous rock layer (the K-Pg boundary). Iridium is scarce on Earth's surface but abundant in meteorites of extraterrestrial origin. The Chicxulub Crater (about 180 km in diameter) was later discovered, establishing the meteorite-impact hypothesis.
Effects of the impact: Meteorite impact → large amounts of dust spread through the atmosphere → sunlight blocked (impact winter) → photosynthesis stops → food chains collapse → mass extinction. It is also thought that the damage was amplified by large-scale volcanic activity on India's Deccan Plateau occurring at the same time.
Mass extinctions temporarily greatly reduce biodiversity, but new ecosystems are built through subsequent adaptive radiation. After the extinction of dinosaurs, mammals diversified into thousands of species over about 66 million years. The perspective that "extinction is not an end but a beginning" is important.
- Know the five major mass extinctions (the Big Five)
- P-T boundary = largest extinction, 96%, Siberian volcanic activity
- K-Pg boundary = dinosaur extinction, meteorite impact
- Evidence = iridium anomaly + Chicxulub Crater
- Impact winter = dust blocks sunlight → photosynthesis stops
- After extinction = new diversity through adaptive radiation
- Today = the sixth mass extinction may be under way
Points of Caution (Easy to Confuse)
① The largest mass extinction was at the P-T boundary (the end of the Paleozoic Era). Do not confuse it with the K-Pg boundary, when dinosaurs became extinct. ② The iridium anomaly is evidence for the K-Pg boundary, not the P-T boundary. ③ Dinosaurs became extinct at the end of the Mesozoic Era (the end of the Cretaceous), not at the end of the Paleozoic Era. ④ Meteorite impact is used as an explanation specifically for the "K-Pg boundary."
Practice
- Give the name (abbreviation) of the boundary at which the largest mass extinction in history occurred.
- Give two pieces of evidence for the meteorite impact at the end of the Cretaceous.
- What is the term for the explosive evolution of diverse species after a mass extinction?