The most significant appearance of major animal groups in the fossil record is often associated with the Cambrian Period, which occurred approximately 541 to 485 million years ago. This period is known as the "Cambrian Explosion." During this time, there was a remarkable diversification of multicellular life forms, including the appearance of various major animal groups, many of which are the ancestors of modern animal phyla.
The Cambrian Explosion is considered a pivotal event in the history of life on Earth because it marked the rapid emergence of diverse and complex organisms, including arthropods, mollusks, chordates, and other major animal groups. These fossils provide critical insights into the early evolution and development of animal life on our planet.