The Cenozoic Era began about 66 million years ago, immediately after the mass extinction that ended the age of nonavian dinosaurs. It is often called the Age of Mammals because mammals diversified into many large and small forms after many ecological niches became open. Birds, which are living dinosaurs, also continued to evolve and spread across land, sea, and air.
Studying the Cenozoic helps scientists connect fossils, climate change, plate movement, and the origins of modern ecosystems.
Key Facts
- Cenozoic time span: 66 million years ago to the present.
- Major periods: Paleogene, Neogene, and Quaternary.
- K-Pg extinction date: about 66 million years ago.
- Relative dating uses fossil order and rock layers, while radiometric dating estimates numerical ages.
- Half-life model: remaining parent isotope = original amount x (1/2)^n, where n is the number of half-lives.
- Modern humans, Homo sapiens, appeared about 300,000 years ago, which is only about 0.45 percent of the Cenozoic Era.
Vocabulary
- Cenozoic Era
- The geologic era from 66 million years ago to today, marked by the diversification of mammals, birds, flowering plants, and modern ecosystems.
- K-Pg Extinction
- The mass extinction event about 66 million years ago that eliminated nonavian dinosaurs and many other species.
- Fossil
- A preserved remain, trace, or impression of an organism that lived in the past.
- Radiometric Dating
- A method for estimating the age of rocks or fossils by measuring the decay of radioactive isotopes.
- Pleistocene
- A Quaternary epoch from about 2.58 million to 11,700 years ago known for repeated ice ages and large mammals such as mammoths.
Common Mistakes to Avoid
- Saying all dinosaurs went extinct at the K-Pg boundary is wrong because birds are living dinosaurs that survived and diversified during the Cenozoic.
- Placing early humans near the start of the Cenozoic is wrong because Homo sapiens appeared only about 300,000 years ago, very late in the era.
- Assuming fossils directly show every species that lived is wrong because fossilization is rare and the fossil record is incomplete.
- Confusing climate change with one constant cooling trend is wrong because the Cenozoic included warming events, long-term cooling, ice ages, and regional climate shifts.
Practice Questions
- 1 The Cenozoic Era began 66 million years ago. If the Quaternary Period began 2.58 million years ago, how many million years passed between the start of the Cenozoic and the start of the Quaternary?
- 2 A volcanic ash layer contains a radioactive isotope with a half-life of 1.25 million years. If 25 percent of the original parent isotope remains, how old is the ash layer?
- 3 Explain why the Cenozoic Era can be called the Age of Mammals even though birds, reptiles, plants, insects, and marine organisms also evolved during this time.