Event 12

543 million years ago

Burgess Shale Sea Floor
The Cambrian Explosion. This is a reconstruction of animals from the Burgess Shales, located in the Canadian Rockies in British Columbia.
Credit: Illustration by John Sibbick, courtesy of John Sibbick Ilustration. This illustration is used with permission. All rights reserved.

543 million years ago, there was a dramatic change in the diversity of animal life. Starting from simple worm-like forms, an astonishing range of more complex animals emerged, including all of the major groups (or ‘phyla’) of modern animals. This radiation occurred after the Precambrian “Snowball Earth” ice age, when the earth warmed, sea levels rose, and the supercontinent of Gondwanaland began to break up, creating many new marine habitats along shallow continental shelves. The earliest chordates, or animals with a notochord (a flexible rod that runs the length of the body and protects the nerve cord), include the two-inch fossil Pikaia, which could be a direct human ancestor.

Early Cambrian Trilobite
This is an image of an early Cambrian trilobite fossil from the Kinzers formation in eastern Pennsylvania. It is known as Olenellus getzi, and was about 15 centimeters long.
Credit: Photo by Bruce S. Lieberman, University of Kansas/ NSF. This image is used with permission.

These animal groups, with modern examples, include:

* molluscs (clams, oysters, scallops, mussels squid, octopus, nautilus, abalone, conch, snails)
*poriferans (sponges)
*cnidarians (jellyfish, sea anemones, corals)
*platyhelminths (flatworms)
*annelids (segmented worms: earthworms, leeches)
* nematodes (roundworms)
*arthropods (insects, spiders, lobsters, crabs, trilobites)
*echinoderms (starfish, crinoids, sand dollars, sea urchins)
*chordates (lancelets, vertebrates: fish, amphibians, reptiles, mammals, ourselves)



The Cambrian was first recognized in 1835 by British geologist Adam Sedgwick. The deposits are now dated to 545 to 495 million years ago in Cambria (the ancient name for Wales). Early fossils of animals with hard shells and external skeletons (such as trilobites) were recognized from these deposits. Major fossil localities that document the Cambrian explosion include Chengjiang in Yunnan Province, China, the Burgess Shales on the slopes of the British Columbian Rockies, and the Wheeler and Marjum Formations of Utah.


The phylum Chordata (chordates) includes the first animals with a notochord, the evolutionary precursor to the spinal cord and backbone (vertebrae). These early, worm-like chordates were ancestral to the vertebrates, including humans, but their body plans had not yet evolved arms or legs as would be seen in later member of the phylum. Had these early, tiny animals not survived the large predators of the Cambrian seas, we would not be here today.



This is a series of videos on the animals of the Cambrian Explosion based on information taken from the fossil records. https://www.lightproductionsvideo.com/Cambrian-Animals.html

See more links further down the page.

The Cambrian Explosion song in this video is written by John Palmer, an elementary school science teacher in Canada. The song is performed by Brighter Lights, Thicker Glasses, whose members are John Palmer on guitar/vocals, Michael Dunn on the dobro, and Brian Samuels on the cello. This video is used with permission from John Palmer.


A Wikipedia discussion of the Cambrian explosion.

A website by the University of California Museum of Vertebrate Zoology that discusses the Cambrian explosion, and includes links to pages about the stratigraphy, life forms, fossil localities, and tectonics of that era.

This is a PBS website that describes the Cambrian explosion and also contains a short animation clip of an artists reconstruction of Cambrian organisms based on fossil evidence.

The Smithsonian web site on the Cambrian World.


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