A widely distributed group of soft-bodied marine organisms that are preserved as fossils in rocks of latest Proterozoic age (600–543 million years ago; Ma). The biota characterizes a geological period, known as the Ediacarian or the Vendian, which precedes the widespread appearance of animals with mineralized skeletons. The name Ediacara refers to an abandoned mining area about 380 mi (600 km) north of Adelaide, South Australia.
Although Ediacaran fossils were described in the 1930s from southwest Africa (Namibia), it was the discovery in 1946 of abundant fossil “jellyfish” at Ediacara that sparked international interest in this biota. Subsequently, similar or identical fossils were found in England; Newfoundland; Russia, the Ukraine, Siberia; North Carolina; Canada; Nevada; and elsewhere. About 30 localities have been described, with the most diverse biotas found at Ediacara, Namibia, and on the coast of the White Sea in northern Russia. As some of these sites could not have been less than about 6000 mi (10,000 km) apart, no matter how the continents were arranged at the time, there is no doubt that the Ediacaran biota was a globally distributed marine biota.
Most known occurrences of Ediacaran organisms precede the earliest great radiation of skeletal fossils (archaeocyaths, trilobites, mollusks, brachiopods), or else they can be placed as latest Proterozoic on other evidence. Radiometric dates have confirmed that there was no significant time gap between the disappearance of the Ediacaran organisms and the Cambrian radiations. In fact, a few Ediacaran fossils have been found in Cambrian strata; the biota did not entirely die out before the Cambrian. See also Cambrian.
At almost all sites, the fossils are preserved as impressions in some kind of detrital sedimentary rock. Commonly, they are found on the bases of sandstone beds (South Australia), within sandstone beds (Namibia), or below volcanic ashes swept into deep water by wind and turbidity currents (Newfoundland). The organisms appear to have lived in continental shelf to slope environments and are normally preserved in sediments that were deposited under fairly quiet conditions below normal wave base.
At a conservative estimate there are probably 40–50 distinct genera or probable genera of Ediacaran organisms worldwide. Assigning these genera to higher taxa, however, has been controversial. A real problem is that few of these fossils can unequivocally be referred to living or extinct animal taxa. Because many of the fossils are simply circular structures with or without radial or concentric markings, they impart little information and are difficult to interpret. The recognition of many critical features is hampered by the nature of the preservation. In addition, many Ediacaran fossils, notably most forms from Namibia, are so unusual in shape that they cannot be placed firmly in any modern group. Many workers have placed the more unusual organisms in an extinct higher taxon of phylum grade, commonly named the Petalonamae. Others have used differences in symmetry and body organization to identify and characterize several major taxonomic groups regarded, in general, as extinct higher taxa of phylum or class grade. Some have proposed that all of the Ediacaran organisms were constructed on a single basic plan that was radically different from any other animal. According to this hypothesis, the Ediacarans were “quilted” organisms lacking heads, muscles, or digestive systems, and consisted of parallel sheetlike walls held together by regularly spaced internal partitions, and the whole organism was inflated by body fluids. Proponents of this hypothesis classify the Ediacaran organisms in an extinct kingdom of multicellular life, the Vendobionta. Still others have proposed that the Ediacaran organisms belonged to extant kingdoms other than the animals; they were lichens, algae, or single-celled or colonial protists. Controversy still reigns, but in all likelihood no hypothesis is entirely incorrect.
The marks left in soft sediments by otherwise unknown animals provide another source of knowledge of late Precambrian animal life. The figure-8-shaped trail in the illustration indicates that animals capable of directed, muscular, gliding motion (like that of a garden snail) coexisted with more typical members of the Ediacaran biota. In a similar fashion, strings of fecal pellets demonstrate the existence of animals with one-way guts, and closely meandering marks imply an ability for systematic grazing. There is little evidence for vertical burrowing in rocks of this age.
Ediacaran trace fossil Gordia found on the sole of a sandstone bed in South Australia gives clear evidence for the existence of mobile animals in the late Precambrian.
Although many Ediacaran fossils are enigmatic, there is sound evidence that sponges, cnidarians, bilaterian worms, and possibly arthropods and other phyla were present. This implies that the Animalia originated even farther back in time. The largest Ediacaran fossils, reaching up to 3 ft (1 m) in length, are flattened “fronds” that had large surfaces compared with their volumes. Proponents of the Vendobionta hypothesis have claimed that such large organisms, lacking guts or muscles, must have been photosynthetic or chemosynthetic and probably contained symbiotic, photosynthetic microorganisms. However, such a lifestyle is also found in modern reef corals and various other marine animals. Alternatively, the high ratio of surface to volume may represent adaptations to an atmosphere and hydrosphere relatively low in oxygen. See also Paleontology.
