Most major animal groups first appeared during the Cambrian Period (544 to 505 million years ago) during the Paleozoic era.
During the Cambrian period hard shelled animals appeared in great numbers for the first time, significantly because shallow seas flooded the continents. It is truly an astonishing period in evolution of life on earth. Most major groups of animals first appear in the fossil record, an event popularly and scientifically called the Cambrian Explosion.
