The Ediacaran Fauna

THE EDIACARAN FAUNA Earth underwent many profound physical changes towards the end of the Proterozoic, driving rapid species radiation. At this time, a supercontinent called Rodinia (Fig. 34), Russian for "homeland," is located on the equator. It then cracks, generating intense hydrothermal activity that causes fundamental environmental changes. With these major changes on Earth, the increase in marine habitat area gave birth to the largest explosion of new species the world has ever known. As a result, the ocean contains a large population of widespread and diverse organisms. Sea levels rose and flooded most of the land. The extended coastline spurred an explosion of new species. Life forms are developing rapidly, and unique and strange creatures wander in the depths of the sea. The dominant animals are coelenterates. It is a radially symmetrical invertebrate animal, including giant jellyfish-like floaters up to 3 feet wide and colonial fur shapes, possibly coral predecessors. They are attached to the seabed and grow to more than 3 feet in length. The remaining organisms are mostly sea worms, unusual naked arthropod animals, and small naked starfish that looked curious with three rays instead of the usual five. Vase-shaped archaeocyathans (Figure 35) resembled sponges and corals and built the earliest limestone reefs, eventually extinct in the Cambrian. Some of these creatures looked like modern animals. The proliferation of these new organisms, representing nearly every major group of marine species, set the stage for the evolution of the ancestors of all animal life on Earth today. The Ediacara Formation in South Australia's Ediacara Hills contains fossil traces of very curious creatures. The radiation of the Ediacaran fauna was sudden, caused by unprecedented changes in the environment. Some Ediacaran fauna were giants of their time, reaching sizes of 3 feet and ranging from radius wheels and miniature anchors to wavy leaves and lettuce. The Ediacaran giants provided the first evidence of complex life on the planet, previously inhabited by single-celled microscopic organisms. Complex life forms suddenly appeared about 600 million years ago shortly after the last major episode of late Precambrian glaciation, perhaps the most severe in Earth's history. The emergence of the Ediacaran fauna is closely related to major changes in the physical environment of the Earth. These included the breakup of continents and the increase in oxygen that allowed the evolution of large animals. About 30 species of these simple and beautiful creatures have been identified from fossil traces in rocks from 20 sites scattered around the world. Many impressions are left by animals that seem to be associated with modern jellyfish, corals, and segmented worms. Others appear to represent arthropods, annelids, and possibly even echinoderms. The bodies of some forms have a threefold symmetry unlike that seen in modern organisms. Ediacaran fossils represent marine life very different from today's oceans, including hairy leaves, puckered pouches, flattened clumps, and etched discs. Many are characterized by radiating, concentric, or parallel folds, whereas others are inscribed with fine branches. The organism does not seem to have a head or tail; circulatory, nervous, or digestive system; eye; Mouth; bone; Muscle; or internal organs, making classification very difficult. Ediacarans are found around the world in rocks that just preceded the Cambrian explosion, when shellfish fauna first erupted onto the scene. Indeed, the loss of Ediacarans made the next Cambrian explosion possible. Only when the Precambrian sea was cleared of primitive life forms could more advanced species flourish and diversify. The evolutionary heyday that marked the beginning of the Cambrian gave birth to most of the animal phyla that today swim, roam, or fly around the world. Animals appeared for the first time covered with shells, carrying jaws, claws and other biological innovations. The Ediacaran fauna appears to be a preview of these animals and has a body style or morphology that has never been seen before or since in the fossil record. Most, if not all, are unrelated to modern forms. They may have been failed evolutionary experiments in multicellular organisms that were completely separated from all known kingdoms of life and perished in a previously unknown mass extinction. Yet some Ediacaran organisms seem to inhabit the planet much longer than previously thought, surviving into the Cambrian. The Ediacaran fossils display an unusual body architecture (Figure 36) that is completely alien to anything seen on Earth today. Some Ediacaran fauna called vendobionts, after the Vendian period, the late Precambrian stage, developed unique solutions to the problem of large body growth. They simply use a network of tubes, such as blood vessels, to transport nutrients and oxygen to individual cells. Fossils show no gaps for swallowing food and eliminating waste. They may absorb oxygen and nutrients directly from seawater or symbiotic algae that convert sunlight into energy. They do not show a clear internal digestive or circulatory system, which does not appear to fossilize. Nevertheless, some animals leave what look like fecal pellets as evidence of an advanced digestive system. The highly flattened body of the Ediacaran fauna maximizes the ratio of surface area to volume, allowing efficient absorption of nutrients and oxygen and light collection for symbiotic algae. Living algae are embedded within the host animal's tissues, which offer protection from predation in exchange for nutrients and disposal of waste products. This adaptation works well for the marine conditions prevailing in the late Precambrian, when shallow seas lacked nutrients and oxygen. The Ediacaran fauna arose from adaptation to extremely unstable conditions during the late Precambrian. Overspecialization on a narrow range of environmental conditions, however, brought about the great extinction of Ediacaran species about 540 million years ago on the doorstep of the "Cambrian explosion," mass diversification of species. Therefore, the rapid evolution of multicellular animal life ushered in the Cambrian period. Marine animals that survived death were very different from their Ediacaran ancestors and participated in the largest diversification of new species the world has ever known. The descendants of the Ediacaran fauna split into two major lineages of modern life, which include protostomas such as mollusks, annelids, and arthropods, and deuterostomes such as echinoderms and chordates, the phylum to which humans belong.