Earth's 4.6-billion-year history is systematically divided into geologic time based on changes in rocks, fossils, and life. Understanding these divisions makes it possible to grasp the general flow of Earth's history and the history of life.
Basic Knowledge
Geologic time is broadly divided into the Precambrian (about 4.6 billion to 540 million years ago) and the Phanerozoic Eon (540 million years ago to the present).
Precambrian: Accounts for about 88% of Earth's history. Few fossils remain because organisms with shells and skeletons had not yet appeared. In the later part, the Ediacaran biota (forerunners of multicellular organisms whose fossils remain) appeared.
The Phanerozoic Eon is divided into the Paleozoic, Mesozoic, and Cenozoic eras.
① Paleozoic Era (540–250 million years ago): Many animal phyla appeared explosively during the Cambrian Period (Cambrian Explosion). Organisms moved onto land during the Ordovician–Silurian periods. Large amphibians and fern forests appeared during the Carboniferous Period. At the end of the Permian Period, the largest mass extinction in history occurred (about 96% of species became extinct).
② Mesozoic Era (250–66 million years ago): Dinosaurs, ammonites, and gymnosperms flourished. A mass extinction occurred at the end of the Cretaceous Period.
③ Cenozoic Era (66 million years ago to the present): Mammals and angiosperms flourished. Humans appeared during the Quaternary Period.
Precambrian (About 4.6 billion–540 million years ago. About 88% of Earth's history. Few fossils)
Paleozoic Era (540–250 million years ago. Cambrian Explosion and the movement of life onto land)
Mesozoic Era (250–66 million years ago. Peak of dinosaurs and ammonites)
Cenozoic Era (66 million years ago to the present. Appearance of mammals, angiosperms, and humans)
Cambrian Explosion (The explosive appearance of many animal phyla about 540 million years ago)
Index fossil (A fossil indicating a particular period. Trilobites = Paleozoic, ammonites = Mesozoic, Naumann elephants = Cenozoic)
Further Study (Background and Significance)
Boundaries in geologic time often coincide with mass extinction events. Especially famous are the P-T boundary (the Paleozoic/Mesozoic boundary, 250 million years ago, when about 96% of species became extinct) and the K-Pg boundary (the Mesozoic/Cenozoic boundary, 66 million years ago, when dinosaurs became extinct).
Radioactive isotopes are used for radiometric dating to determine the absolute ages of geologic time. Methods include uranium–lead dating, potassium–argon dating, and carbon-14 dating. Because carbon-14 has a half-life of about 5730 years, carbon-14 dating is suitable for dating the remains of organisms up to several tens of thousands of years old.
In Japan, the geologic time name Chibanian was officially approved in 2020. It was named because a geological stratum in Ichihara City, Chiba Prefecture, shows evidence of a geomagnetic reversal about 770,000 years ago. It is the first geologic time name from Japan.
- Precambrian = 88% of Earth's history (4.6 billion–540 million years ago)
- Paleozoic = Cambrian Explosion, trilobites, and movement of life onto land
- Mesozoic = dinosaurs and ammonites
- Cenozoic = mammals and humans
- Index fossils: trilobites → Paleozoic, ammonites → Mesozoic
- P-T boundary = 96% extinction / K-Pg boundary = dinosaur extinction
- Chibanian = Japan's first geologic time name
Points to Note (Common Confusions)
① Do not confuse index fossils (time) with facies fossils (environment). ② Precambrian is an “eon,” not a “period.” ③ Dinosaurs lived throughout the Mesozoic Era (250–66 million years ago), not during the Cambrian Period (Paleozoic). ④ Mass extinctions occurred at the ends of both the Paleozoic and Mesozoic eras.
Practice
- Ammonite fossils are index fossils indicating strata from which era?
- Briefly explain what the Cambrian Explosion was.
- About what percentage of species became extinct during the mass extinction at the end of the Paleozoic (P-T boundary)?