Triassic

In Triassic times (250-210 million years ago), the future Rocky Mountains were still a shallow marine sea at the North American margin but, in tropical oceans far away to the south, marine corals were depositing sand that would later become limestone - absent in the North American margin because corals do not live in these colder seas (Tozer, in Ludvigsen, 1996). These tropical "terranes" would eventually shift northwards and collide with the North American Margin. Ammonoids (an extinct group related to squids and octopus), clams and other mollusks in these newly arrived terranes are different species, typical of warm-water, than those cold-water species whose fossils are found in rocks that were deposited here and stayed here. By this time, insects ruled the land and the first adventurous amphibians were adapting to air.