Burgess Shale Fossils and the Cambrian Explosion

I took the dogs down to Galveston Island last month to romp in the Gulf of Mexico’s warm waters. While there I was fascinated by the jellyfish washing up on the beach. They number beyond counting, to be sure; but the delicate bodies quickly disintegrate, even in Galveston’s gentle surf. It occurred to me that some distant paleontologist may never know that those blobs of jelly, even the majestic Portuguese Man o’ War, ever existed. The reason is really quite simple: the fossil record is unkind to soft-bodied creatures, and to the soft parts of creatures with skeletons. Except in very, very rare cases, all that survives when an animal dies and is buried is hard parts: bones and teeth sometimes, but mostly shells.

I said “except in very, very rare cases”: one such case is the Burgess Shale of British Columbia, Canada. The fine-grained sediments of this geological formation are some 530 million years old, yet hold within solid rock a record of thousands of long-extinct animals. Some of those species have never been seen anywhere else. The fossils include more than just the animals’ hard parts: because of the highly unusual conditions of their deposition, many soft-bodied organisms were also preserved as a sort of coal-like film on rock surfaces. Both the age of the Burgess and the fauna present make it one of the most important fossil sites of any age, one result of which that UNESCO designated the Burgess Shale as a World Heritage Site in 1981. Access to exposures of the formation is now restricted and fossil collection requires a research permit.

As is the practice among sedimentary geologists, the Burgess Shale is named for a site at which it is well exposed. This “type locality” is Burgess Pass in the Canadian Rockies. The formation is of Middle Cambrian age, so was deposited near the end of the great Cambrian Explosion, a 20-million-year period of explosive evolution across several phyla. The rock unit is some 150m (500 ft) thick, and records deposition in a deep, quiet basin. Just a few miles away sat a great reef with a precipitous cliff-like wall hundreds of feet high on the side facing the Burgess basin. Because of its height and steepness, great landslides of muddy, sediment-laden seawater would sporadically roar down the face to spill out over the basin floor. The roiling waters trapped animals living on the reef’s surface, and washed their bodies into the basin. A rain of sediment and dirty water quickly buried the remains, after which life returned to normal – at least until the next great landslide decades or hundreds of years later. Because of that rapid burial in otherwise quiet water, soft parts were often preserved; unlike the jellyfish I saw being beaten about by the waves on Galveston Island.

Since the discovery of the Burgess by Charles Walcott (of the Smithsonian Institution, USA) a century ago in 1909, more than 60,000 individual fossils have been described. The two quarries at the site (one of which was opened by Walcott himself) have yielded sponges, arthropods, algae, worms, trilobites, mollusks, even bacteria; as well as enigmatic specimens whose lives and functions remain as mysterious today as the day they were excavated. Not only are there unique species among the fossils found in the Burgess beds, but the state of preservation of even more common skeletal fossils is said to range from superb to spectacular. In a sort of “life imitates art,” researchers have even identified partially-digested stomach contents within the fossils of soft-bodied predators. No word on whether anyone has decided to film “Cambrian Park,” at least so far.

The Burgess Shale’s chief significance, however, is the snapshot of the Cambrian Explosion recorded within its layers. Though there are other sites around the world where soft-bodied Cambrian faunal fossils were preserved, there is none that approaches the diversity and quality of preservation seen in the Burgess. While some of the earliest ancestors of modern animals – sponges, arthropods, early chordates (yep – humanity’s most distant forebears) – can be found within its layers, the Burgess also yields the remains of puzzling creatures that reached an evolutionary dead end some time in the last 500 million years. For paleontologists and evolutionary biologists alike, the Burgess Shale and the fossilized remains it contains form a fascinating collection of subtle clues about the distant past that is without price.

For more information on the Burgess Shale, see the Smithsonian Museum’s Burgess Shale site (http://paleobiology.si.edu/burgess/index.html) and University of California at Berkeley site (http://www.ucmp.berkeley.edu/cambrian/burgess.html). Photographs of Burgess fauna can be found at a University of Calgary site (http://www.geo.ucalgary.ca/~macrae/Burgess_Shale/) and at the Canadian National Park system’s Hooper Museum (http://park.org/Canada/Museum/burgessshale/faunaandflora.html). A set of highly detailed drawings (sometimes a better representation than photos) can be found at the Smithsonian site (http://paleobiology.si.edu/burgess/burgessSpecimens.html)