As the name implies, the fossils documenting the Cambrian explosion appear within a relatively narrow slice of geologic time. Until the early 1990s, most paleontologists thought the Cambrian period began 570 million and ended 510 million years ago, with the Cambrian explosion of novel animal forms occurring within a 20- to 40-million-year window during the lower Cambrian period. Two developments have led paleontologists and geochronologists to revise those estimates downward.
