A fossil-rich deposit of shale and slate dating from the mid-Cambrian period (about 505 million years ago) and located in the Burgess Pass in the Rocky Mountains of British Columbia, Canada. First excavated in 1909, it has since yielded one of the world’s oldest assemblages of fossilized marine invertebrates and early vertebrate animals, as well as algae and sponges. The species described include brachiopods, crustaceans, trilobites, and other arthropods, plus a remarkably well-preserved array of fossil worms and other soft-bodied animals. Many forms are of uncertain classification and have been assigned to new phyla. For example, Wiwaxia resembles a mollusc but the body is covered with scalelike elements and has two rows of bladelike spines projecting from the dorsal surface. Nectocaris has a kite-shaped body with wide lateral fins and a nozzlelike anterior funnel; two long slender tentacles extend from the head, which also has a pair of stalked eyes. It is now regarded as an early mollusc with affinity to the cephalopods. Another curious design is Opabinia, which has a segmented body bearing gills and paired flaplike appendages and a head with five eyes and a long flexible hoselike proboscis. The Burgess shale also contains specimens of Pikaia. Originally thought to be an annelid worm, it is now interpreted as a small eel-like chordate with a putative notochord and segmented muscle blocks characteristic of modern lancelets. Some of the organisms found in the Burgess shale have also been unearthed in deposits of the lower Cambrian, notably at the Sirius Passet site in Greenland and the Maotianshan shales in Chengjiang County, China (see chengjiang fossils). The great diversity of forms in deposits from this period is seen by some biologists as evidence for a burst of rapid evolution (see cambrian explosion).