undifferentiated rock units

Paleontology and geology

In the Cambrian, the seas flooded over the edge of the early North American continent (Laurentia), and only a small area in northeastern British Columbia was exposed above sea level. Most of the province was part of the shallow continental shelf, which passed into the deeper continental slope and ocean basin to the west. Algae and archeocyathids built up reefs in the warm, tropical waters, which were home to brachiopods, gastropods, bivalves, echinoderms, conodonts, trilobites, and other arthropods. Fossils of some of these animals can be found in the sandstones, shales, and limestones in eastern parts of the province. But the most famous Cambrian fossil locality, the Burgess Shale in the southeast, also records unusual soft-bodied creatures that rarely fossilize. There animals include priapulid, polychaete, and velvet worms (Ottoia, Burgessochaeta, Aysheaia), sponges (Vauxia), sea pens (Thaumaptilon), various arthropods (Canadaspis, Marrella), an early chordate (Pikaia), and many other creatures, some of which remain enigmatic (e.g., Hallucigenia, Opabinia, and Anomalocaris). These animals lived on the muddy bottom below a large reef, and they were fossilized when an underwater avalanche carried them down into deeper waters. The fine sediments and low oxygen levels prevented bacteria from consuming the buried animals, and preserved small details and delicate structures.