The Cambrian Period (543 to 505 million years ago) is the first time period of the Paleozoic Era. It is preceded by the Precambrian. The Cambrian Period opened in the aftermath of a mass extinction. During the last 100 million years of the Precambrian, a unique fauna had developed. Refered to as the Ediacaran biota, these creatures are the earliest known multicellular creatures to have inhabited Earth. But for reasons that remain unclear, they suddenly disappeared and their extinction marks the end of the Precambrian and the dawn of the Cambrian.
During the Cambrian, life on Earth evolved dramatically despite the decline of most Ediacaran species. A few organisms survived the Ediacaran mass extinction and persisted into the Cambrian Period. The lifeforms that remained changed dramatically over the coming millenia. Early in the Cambrian Period the first bivalves and arthropods appeared followed by the first molluscks, brachiopods, and triolobites.
During the Cambrian Period, all but one of the major groups of animals evolved during a period of 40 million years. This sudden burst of evolutionary innovation, known as the Cambrian Explosion, lasted from about 570 to 530 million years ago. The only animal phylum to evolve after the Cambrian Explosion was the phylum Bryozoa, which appeared during the early Ordovician.
Numerous Cambrian fossil beds have been discovered around the world but there is one that stands apart from the rest. The Burgess Shale is among the best-preserved and species-rich fossil beds in the world. The Burges Shale is nestled amongst in the Canadian Rockies in Yoho National Park in British Columbia. Discovered in 1909 by paleontologist Charles Walcott, the Burgess Shale contains a wealth of marine fossils that date back to the Cambrian Explosion. At that time all life was aquatic, the terrestrial habitats were still barren.