From about 570 to 530 million years ago, an evolutionary burst of life forms occurred, often referred to as the “Cambrian Explosion.” This marks an important point in the history of life on earth, as most of the major lineages of animals got their starts during the Cambrian Period and have been evolving ever since. If we wound the clock back a little more than half a billion years to the Cambrian, we would find that life then was different from life today:

• All life was aquatic.
• Most life was relatively small.
• Many animals had unusual body layouts.

Many Cambrian animals seem bizarre at first glance, but are actually members of groups that are still around today — such as the arthropods.

A glimpse at life along the Cambrian sea floor

In this investigation, you’ll study the fossils and reconstructions of five Cambrian animals and determine which were arthropods and which were not. As you’ve learned, arthropods inherited a particular set of characters from their common ancestor. These characteristics are often preserved in fossils.

Let’s begin our investigation with Sanctacaris.