Ordovician

A hundred million years later, in the Ordovician (500 to 440 million years ago), much of the Columbia Basin was still under shallow marine waters but populated by the progeny of the species that had survived a mass extinction at the end of the Cambrian. They left spectacular fossils of stromatoporoid (sponge-like) colonies in Top of the World Provincial Park. Also living in Ordovician seas near Golden were many species of an ancient group of elongate, jawless fishes, perhaps akin to modern hagfishes and lampreys. We know them only from their teeth, termed conodonts, which occur in rocks of Cambrian to Triassic age.