Fossils Through Geologic Time

rangers holding large palm frond fossil
Fossil Butte National Monument, WY.

NPS photo.


The Geologic Time Scale is a way of organizing Earth's 4.5 billion-year history. The time scale is divided into four large periods of time—the Precambrian, Paleozoic Era, Mesozoic Era, and Cenozoic Era. National parks preserve fossils from each of these time blocks.

The Precambrian

The Precambrian (prior to 542 million years ago) was the "Age of Early Life." Soft-bodied creatures like worms and jellyfish lived in the world's oceans. The land remained barren. Common Precambrian fossils include mats of algae called stromatolites, microorganisms, and simple animals. These parks, and several others, preserve Precambrian fossils:

Paleozoic Era

During the Paleozoic Era (542 to 251 million years ago) Fish diversified and marine organisms were very abundant during the Paleozoic. Common Paleozoic fossils include trilobites and cephalopods such as squid, as well as insects and ferns. The greatest mass extinction in Earth's history ended this era. Paleozoic fossils have been discovered these parks and several others:

Mesozoic Era

The Mesozoic Era (251 to 65.5 million years ago) was the "Age of Reptiles." Dinosaurs, crocodiles, and pterosaurs ruled the land and air. As climate changed, sea levels rose world-wide and seas expanded across the center of North America. Large marine reptiles such as plesiosaurs, along with the coiled-shell ammonites, flourished in these seas. Common Mesozoic fossils include dinosaur bones and teeth, and diverse plant fossils. These parks, and several others, preserve Mesozoic fossils:

Cenozoic Era

The Cenozoic Era (65.5 million years ago through today) is the "Age of Mammals." Birds and mammals rose in prominence after the extinction of giant reptiles. Common Cenozoic fossils include cat-like carnivores and early horses, as well as ice age fossils like wooly mammoths. Caves can preserve the remains of ice-age animals that died in them or were transported there after death.

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    These parks, and and many more, preserve Cenozoic fossils:


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    Last updated: May 23, 2020


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