The Mesozoic (approximately 0.25–0.066 billion years ago) is called the Age of Reptiles, during which dinosaurs, pterosaurs, and plesiosaurs flourished. The ancestors of mammals and birds also appeared during the Mesozoic. The era ended with a mass extinction caused by an asteroid impact at the end of the Cretaceous.
Basic Knowledge
The Mesozoic began after the end-Permian mass extinction. During the Triassic (0.252–0.201 billion years ago), the supercontinent Pangaea began to break apart, and the common ancestors of dinosaurs and mammals appeared. During the Jurassic (0.201–0.145 billion years ago), dinosaurs diversified and became larger, and Archaeopteryx (Archaeopteryx) appeared, marking the origin of birds.
During the Cretaceous (0.145–0.066 billion years ago), angiosperms rapidly diversified, and large carnivorous dinosaurs such as Tyrannosaurus appeared. Earth was approximately 6–10℃ warmer than today, with no polar ice and high sea levels, a condition known as a greenhouse climate.
The K–Pg mass extinction at the end of the Cretaceous (approximately 66 million years ago) is thought to have been caused by the “impact winter” (sunlight blocked by dust, cooling, and acid rain) resulting from the impact of the approximately 10–15 km-wide Chicxulub asteroid off the Yucatán Peninsula, combined with volcanic activity at the Deccan Traps. Dinosaurs (except birds), pterosaurs, ammonites, and other organisms became extinct. An iridium anomaly (concentration of a metal from outer space) has been detected worldwide in strata at the K–Pg boundary.
Explain why the iridium anomaly at the K–Pg boundary is considered evidence of an asteroid impact.
Answer: Iridium is extremely rare on Earth’s surface but abundant in meteorites. The detection of high concentrations of iridium in strata at the K–Pg boundary worldwide indicates that a large asteroid struck Earth and that its material fell and accumulated globally.
- Three Mesozoic periods: Triassic → Jurassic → Cretaceous
- Breakup of Pangaea began during the Triassic–Jurassic
- Archaeopteryx: Jurassic; origin of birds
- Diversification of angiosperms: Cretaceous
- Cretaceous climate: greenhouse climate; no polar ice
- K–Pg mass extinction: 66 million years ago; Chicxulub asteroid impact
- Iridium anomaly at the K–Pg boundary is evidence of an asteroid impact
Points to Note
① “Dinosaur extinction” more precisely means the extinction of non-avian dinosaurs; birds still exist today as descendants of dinosaurs. ② The “K” in the K–Pg boundary comes from Kreide, the German word for Cretaceous, and “Pg” stands for Paleogene. ③ Archaeopteryx was not the “first bird”; it is now regarded as a transitional form toward birds.
Practice
- State the three periods of the Mesozoic in order from oldest to youngest.
- What is considered the main cause of the mass extinction at the end of the Cretaceous?
- What does the iridium anomaly at the K–Pg boundary indicate?