Burgess Shale and Ediacaran Faunas
Burgess Shale and Ediacaran Faunas
The Burgess Shale was one of the most famous and important fossil localities known at the end of the twentieth century. Charles Walcott, who at the time was secretary of the Smithsonian Institution, discovered this fossil-rich rock bed in 1909 while exploring the Canadian Rockies of British Columbia. The Burgess Shale, which is now a part of Yolo National Park, is famous for the wide diversity of fossils of soft-bodied marine animals that are embedded in it. These fossils are approximately 530 million years old, and represent an array of life-forms present during the early-middle years of the Cambrian period (545-495 million years ago). The area originally quarried by Walcott is surprisingly small given the array of unique animal forms found in it. His initial survey covered an area that is only 10 feet tall and 200 feet long, the length of one city block.
The fossils of the Burgess Shale are significant for a number of reasons. The quality of their preservation provided the first opportunity to examine, in astonishing detail, the morphology (form and structure) of early soft-bodied life forms. In addition, these fossils are an early record of the novel body plans that were created during the Cambrian Explosion (approximately 570-540 million years ago), a geologically abrupt time period during which multicellular forms organized in a variety of new ways. The Burgess Shale does not depict the Cambrian Explosion itself, but the aftermath. The fossils are impressive not only for the novelty of the body plans represented, but also for the diversity of body plans.
Fossils of soft-bodied forms are rare because the process that creates fossils works better at preserving bones and hard structures. Before the discovery of the Chingjiang fossils in Yunnan Province, China, in the late 1980s, the Burgess Shale fossils provided the only evidence of the early soft-bodied animals that appeared during the Cambrian Explosion. The quality of these fossils indicates that they were created under anoxic (low oxygen) conditions. Many millions of years ago, the site of the Burgess Shale was underwater and located near the equator. Soft-bodied marine animals were carried by strong currents from surrounding highly oxygenated areas to the site of the Burgess Shale and were buried in an underwater mudslide. The low oxygen content of these waters killed the animals and protected their remains from decay.
Classifying the Burgess Animals
Approximately 120 species are found in the Burgess Shale, including familiar forms as well as several species belonging to previously unknown phyla. Some of the fossilized species are members of groups (phylum) that still exist. These species can be categorized as members of the phyla Porifora (sponges), Annelida (segmented marine flatworms), Arthropoda (insects, crabs, and trilobites), and Echinodermata (sea urchins, sea fans, and sea lilies), and one species is the earliest representative of the phylum Chordata (which includes vertebrates). Most of these animals were scavengers, and a few were predators. Of these Burgess Shale animals, the aptly named Hallucigenia (phyla Annelida) is probably one of the most famous for its bizarre morphology. Seven pairs of stiltlike legs support its long, cylindrical body. It is hard to tell for certain which end is "head" and which end is "tail," but most scientists designate the head end by the bulbous projection that is prominent on one end of its body.
The previously unseen animal forms found within the Burgess Shale include a number of wormlike and segmented organisms, some of which were assigned to novel phyla (phyla Priapulida and Onychophora) while others remain "unclassified to this day." Opabinia was a five-eyed, 3-inch-long creature with a frontal "nozzle" that was presumably used in its search for worms and other fossorial (living in burrows) prey. Anomalocaris ("unusual shrimp") was a fierce, 2-foot-long predator with robust forelimbs for grasping its prey and a square-shaped mouth rimmed with multiple rows of sharp teeth. Because Anomalocaris existed in the Burgess Shale only as separate pieces, Walcott first reconstructed it as two animals: a bivalved (having two symmetrical, shelled parts joined by a hinge) arthropod and a jellyfish. Fossils of related species later found in China reached a length of up to 6 feet! Wiwaxia, a spike-covered, sluglike animal, was a bottom feeder that was protected from hungry predators by its scaly back.
In attempting to fit these new forms into the preexisting classification scheme, which included only phyla Porifora, Annelida, Arthropoda, Echinodermata, and Chordata, Walcott erroneously classified these animals as worms and arthropods. His categorization of the Burgess animals as ancestors of modern-day animals conformed to the idea that the diversity of life-forms arose in a manner resembling the shape of an inverted cone, with the large number of species that exist today arising from a small number of ancient organisms.
It was not until H. B. Wittington of Cambridge University examined the fossils forty years later that these forms were placed into unique phyla. Whittington's reclassification caused a major upheaval in the way people thought about the origin of animals. Instead of the popular view that a small number of general body plans originated during the Cambrian period and gave rise to all the animals seen today, Whittington contended that the body plans evident today represent only some of the novel forms that were created during the Cambrian period. He argued that many different body plans were created then, a number of which went out of existence while the remainder continued and gave rise to the forms we see today. This premise formed the basis for his reclassification of the Burgess animals.
Before the Cambrian Explosion and the associated appearance of new animal forms, there existed the Ediacaran Fauna (also known as Vendian Biota), a group of multicellular organisms with relatively simple body plans. Geologist Reginald Sprigg first discovered the fossil traces of these organisms in 1946 while exploring the Ediacara Hills of Australia. Since the initial discovery of the Australian fossils, additional Ediacaran fossils have been found on every continent except Antarctica. The age of the rocks containing these fossils range from 600 million to 544 million years old. Before the discovery of the Ediacarans, it was believed that animals did not exist before the Cambrian Period (before 545 million years ago).
In contrast to the Burgess Shale fossils, most of the Ediacaran fossils are burrows and trace fossils—casts and molds of the organisms they depict. The fossil traces of these simple animals can be broadly divided into those that are radially symmetric and those that are segmented. The radially symmetric traces are believed to have been formed by polyplike and disk-shaped organisms. The more complex, segmented forms are traces of tubelike units. The shape of these soft-bodied forms was preserved during rapid burial under sand on the ancient marine floor bed.
The classification of the Ediacarans as animals remains controversial. The superficial similarity that some of the Ediacaran forms bear towards sea anenomes and jellyfish led some scientists to conclude that they are true animals, precursors to the animals that exist today. Various Ediacarans have also been mistakenly classified in the past as algae, lichens, or giant protozoans.
However, some scientists believe that the Ediacarans were not animals as we know them, and they did not evolve into such animals. These scientists focus upon characteristics of the Ediacarans that are not found in the body plans that evolved during the Cambrian. Based on information gleaned under close examination of the fossil traces, they concluded that the Ediacarans underwent a set of embryonic/morphological development processes that differs radically from the normal pattern of development experienced by true animals.
see also Cambrian Explosion; Cambrian Period; Geological Time Scale.
Judy P. Sheen
Erwin, Douglas, Jim Valentine, and David Jablonski. "The Origin of Animal Body Plans." American Scientist 85 (1997):126-137.
Knoll, Andrew H., and Sean B. Carroll. "Early Animal Evolution: Emerging Views from Comparative Biology and Geology." Science 284 (1997):2129-2137.
Gould, Stephen Jay. Wonderful Life: The Burgess Shale and the Nature of History. New York: W. W. Norton and Co., 1989.