The Burgess Shale Fauna

THE BURGESS SHALE FAUNA The Burgess Shale Formation in British Columbia, Canada, contains the remains of strange, soft-bodied animals that first appeared in the lower Cambrian about 540 million years ago soon after the appearance of complex organisms. This collection displays more diversity of basic anatomical designs than all the oceans in the world today. These include about two dozen types of plants and animals that do not have modern counterparts. Many of these strange animals were probably brought from the upper Precambrian but never made it beyond the middle Paleozoic. They are so peculiar that they defy attempts to classify them into existing taxonomic groups. The animals are very complex with special adaptations for living in various environments. Several species appear to still be living Ediacaran fauna, most of which became extinct towards the end of the Precambrian. Indeed, the Cambrian explosion may have been triggered in part by the availability of habitat vacated by the leaving Ediacaran species. The fauna of the Burgess Shale comprises more than 20 different body plans. The leechlike animal called Pikaia (Figure 54) is the first known member of the phylum chordate and one of the rarest fossils of the Burgess Shale Formation, which contains the bestpreserved Cambrian fauna. It had a rigid dorsal trunk called a notochord along the back made of cartilage that supported organs and muscles, the precursor of the spine in vertebrates. The fauna of the Burgess Shale originates from the shallow waters of a giant coral reef that dwarfs Australia's Great Barrier Reef, the single most massive structure built by living things today. Ancient coral reefs surround Laurentia, the ancestor of the North American continent, and are covered with easily trapped mud and fossilized organisms. Most occurrences originate in North America's western Cordillera, an ancient mountain range overlooking the open ocean in the central Cambrian. Similar fauna existed in other cratons, including the North and South China blocs, Australia, and Eastern European platforms. Their widespread distribution around other continents suggests many members have a swimming way of life. One strange animal named Hallucigenia (Figure 55), in honor of its strange characteristics, is a worm-like creature that propels itself across the ocean floor on what appear to be seven pairs of pointed stages. However, a more recent interpretation states that two rows of spines on his back were used for protection. A strange Burgess Shale animal named Wiwaxia was another spiny creature about an inch long, possibly related to modern scaleworms known as sea rats. It resembled an underwater hedgehog, with large scales and two rows of spikes running on its back apparently to thwart predators. It is fed by scraping pieces of food from the seabed with hoarse organs similar to horn-toothed tongues. Unusual worms have large eyes and protruding fins. A chunky burrowing carnivorous worm called Ottoia was found with the animal clams still in its intestines. When reaching out from the bottom mud, it reaches out a muscular, toothed mouth and swallows its prey whole. The strange creature called Opabinia (Figure 56) has five eyes arranged on its head, a vertical tail fin to help direct it along the ocean floor, and a forward-projecting grasping organ that might be used to catch prey. Its long, hose-like front appendages gave it the name "swimming vacuum cleaner." About half the fauna of the Burgess Shale is made up of arthropods, with fossil remains of about 20 different possible arthropods that did not survive. One giant arthropod reaches 3 feet in length. The most interesting of all arthropods of the Burgess Shale due to its unusual shape is the Aysheaia, an animal with a fat body and fat limbs. An extraordinary arthropod called Anomalocaris (Fig.57), whose name means "strange shrimp," is probably the largest Cambrian predator as well as the oldest known large predator in the fossil record. These earliest monsters reached a length of 6 feet and had a mouth surrounded by spiny plates on the lower part of the body. Up to eight rings of teeth up to 10 inches are arranged concentrically, pointing toward the mouth. Anomalocaris propelled itself by raising and lowering a set of side flaps, swimming in a manner similar to modern manta rays. It also has a wide tail with a pair of long trailing spines used for steering and stability. The body was flanked by a pair of jointed appendages apparently designed to hold and crush invertebrate armor plates. The animal was apparently well equipped to devour crustaceans and was nicknamed the "trilobite terror." Many fossil trilobites were found with round pieces bitten from their sides, indicating they somehow escaped the monstrous clutches of Anomalocaris. Some species of trilobites evolved long spines that probably served as protection against anomalocalis attack. Most Burgess Shale species suddenly became extinct by the end of the Cambrian. Although many mass extinctions and proliferations of marine organisms have occurred since then, no new body styles have emerged over the past 500 million years. After the late Cambrian extinction, only a few ancient forms survived into the middle Devonia. Had they triumphed and produced offspring, Earth would now be blessed by an entirely different set of life forms.