Cambrian 

During the Cambrian (550 to 500 million years ago), evolution of multicelled animals was so rapid and resulted in such a proliferation of forms, that archaeologists refer to it as an "explosion" of life. Where the Rocky Mountains are now was then a warm, coastal marine ecosystem at the western edge of what would become North America. Gould (1989), in Wonderful Life: the Burgess Shale and the nature of history (see references, below), describes the marine invertebrate community that lived there, and can be seen today in the fossils of the Burgess Shale near Field, B.C.

There, excellent preservation of soft-bodied organisms, such as jellyfish, sea-cucumbers and worms, lets us look directly at animals unlike anything living today. There were representatives of familiar groups like mollusks and arthropods, but there are also strange ones that defy classification into today’s compendium of living things. Two examples are pictured at left: the species Opabinia sp. and Marrella sp. Many species, genera, families - indeed, whole phyla - did not survive later mass extinctions. Rocks of the same era on Mount Stephen in Yoho National Park, and near Cranbrook and Fort Steele, also have Cambrian fossils from this early sea, but only those with hard exoskeletons - like clam shells and the "shells" of crustaceans - were preserved. In those rocks is recorded a great variety of trilobites, an extinct group looking somewhat like horseshoe crabs, but unrelated.