In what period does the common ancestor of the Crustaceus species of Montanian period appear?

The common ancestor of Crustacean species is believed to have emerged during the Cambrian period, approximately 500 million years ago. This period is characterized by a significant diversification of marine life, including the early evolution of arthropods, which later gave rise to modern crustaceans. The Montanian period, part of the Late Cretaceous, occurred much later and is not when these ancestors appeared.