Pikaia has a head and flexible cord running the length of its body. It was related to the ancestor of all backboned animals. This fossil of Pikaia gracilens was collected from the Burgess Shale site by Dr. Charles Walcott.
DIVERSITY BURSTS FORTH
Cambrian seas swarmed with an amazing assortment of new animals living in novel ways. Nearly all of today’s major groups of animals arose in this ancient explosion of evolution. What set it off?
Rising oxygen levels, mild temperatures, and higher sea levels made Earth more hospitable to ocean life. Some animals began living higher in the water, while others burrowed in the sea floor. Some hunted other animals, and many evolved new defenses.