Geologic Time Scale: Periods, Fossils and Earth's History
Explore Earth's history one period at a time. This geologic time hub connects detailed Fossilguy guides to ancient climates, changing continents, fossil ecosystems, evolutionary milestones, and the mass extinctions that reshaped life.
Understanding Geologic Time
The geologic time scale is the calendar of Earth's history. Geologists divide roughly 4.6 billion years into eons, eras, periods, epochs, and ages. These divisions allow scientists to organize the rock record and describe when major changes in climate, geography, oceans, and life occurred.
The boundaries between time intervals are based on evidence preserved in rocks. Some mark major appearances or extinctions in the fossil record, while others record changes in climate, ocean chemistry, magnetic polarity, or sedimentation. Radiometric dating places numerical ages on these events, but the physical rock record remains the foundation of the scale.
This guide begins with the Cryogenian Period, when enormous glaciers may have covered much of the planet, and continues through the Ediacaran rise of complex life and the fossil-rich Phanerozoic Eon. From there, it follows the Paleozoic seas and forests, the Mesozoic world of reptiles and dinosaurs, and the Cenozoic expansion of mammals, birds, grasslands, and humans.
How to use this page: Select any completed period guide for a detailed overview of its climate, geography, fossils, evolutionary events, major fossil sites, and collecting information. Additional period guides and thumbnail images will be added as they are completed.
For places where fossils can be collected or viewed, visit the Fossilguy Fossil Hunting Locations guide.
Late Precambrian
The late Precambrian records a dramatic shift from a planet repeatedly affected by global glaciation to one inhabited by large, complex organisms. These periods set the stage for the animal diversification of the Cambrian.
Paleozoic Era
The Paleozoic began with the rapid diversification of marine animals and ended with the largest mass extinction in Earth history. During the era, fishes diversified, plants and animals colonized land, forests spread, and the first amniotes appeared.
Mesozoic Era
The Mesozoic followed the end-Permian crisis. Dinosaurs, marine reptiles, pterosaurs, mammals, and birds evolved as Pangaea broke apart and modern ocean basins began to form.
Cenozoic Era
The Cenozoic began after the extinction of non-avian dinosaurs. Mammals and birds diversified, grasslands expanded, global climates cooled, repeated ice ages developed, and humans eventually appeared.
Explore More Fossilguy Resources
Geologic time is easiest to understand when the periods are connected to real fossils and rock layers. Continue exploring through the Fossil Gallery, browse places to collect or view fossils in the Fossil Hunting Guide, or learn how fossils form in What Is a Fossil?
