Paleozoic era → Cambrian explosion (530 MA)

The "Cambrian Explosion" refers to the sudden appearance in the fossil record of complex animals with mineralized skeletal remains. It may represent the most important evolutionary event in the history of life on Earth.

The beginning of the explosion is generally placed about 530 million years ago, during the Cambrian Period at the start of the Palaeozoic Era. While the explosion was rapid in geological terms, it took place over millions of years. The explosion is particularly remarkable because all major animal body plans appeared during this time, changing the biosphere forever.

The rapid appearance of a wide variety of animals - particularly bilaterians - led to the development of radical new ecological interactions such as predation. Consequently, ecosystems became much more complex than those of the Ediacaran. As the number and variety of organisms increased, they occupied a variety of new marine environments and habitats. Cambrian seas teemed with animals of various sizes, shapes, and ecologies; some lived on or in the sea floor (a benthic lifestyle), while others actively swam in the water column (nektonic).

The fundamental ecological structure of modern marine communities was firmly established during the Cambrian. By the end of the Period, some animals had also made the first temporary forays onto land, soon to be followed by plants.

The early record of the Cambrian Explosion is based on fossils, principally the appearance of mineralized skeletons and complex trace fossils. The typically tiny skeletal elements from this time are called "small shelly fossils". These constitute a highly varied assortment of sclerites, spicules, tubes, and shells, suggestive of several different types of animals. Unfortunately, many of the fossils remain poorly understood and are difficult to classify within known taxonomic groups.