Ediacaran fossils

Ediacaran fossils provide evidence of the oldest remains of animal life with moderately complex body structures. These organisms, known as metazoans, lacked internal skeletal structures and lived in ancient oceans, as bottom dwellers or as primitive swimmers and floaters. Ediacaran fossils are now known from widely scattered locations worldwide and represent essential data relative to the interpretation of ancient plate tectonic activity.

Discovery of Ediacaran Fossils

The Ediacaran fossil assemblage is a group of animal remains representing the oldest metazoan life forms of significant size and diversity known from the fossil record. The Ediacaran assemblage comprises several different fossils in a part of the Earth's stratigraphic record that dates from about 565 million to 543 million years ago. Fossils, now considered part of the Ediacaran assemblage, were initially described in 1933 from a location in southwest Africa (Namibia) and South Australia in 1949. However, the age and significance of these fossils were only realized in the mid-to-late 1960s because, until that time, the particular rock units that include the Ediacaran assemblage were not of interest to most geologists. Since then, geologists in many parts of the world have been researching the Ediacaran fossils and the rocks that contain them. These ancient metazoans, the Ediacaran assemblage, are now recognized from stratigraphic units in several geographically isolated areas, including North Carolina, Newfoundland, British Columbia, England, China, Africa, Australia, Ukraine, and the White Sea coast of Russia. However, significant numbers and varieties of individual fossils have been found only in southwest Africa and South Australia.

Rocks of the appropriate age and type that could contain Ediacaran fossils belong to what geologists define as the Late Precambrian eon on the geologic time scale or calendar. Because the Earth's crust constantly changes, rocks of the Late Precambrian age are not commonly preserved at the planet's surface. Where preserved, they are generally unfossiliferous or have undergone such intense alteration or metamorphism that any fossil evidence has been destroyed. Also, locations of preserved rocks of this age occur in such widely separated areas that geologists working in one location frequently do not have the opportunity to become familiar with the geology in other areas. Thus, discovering this unique and easily identified fossil assemblage from widely scattered locations provides geologists with a correlation tool to help unravel the Earth's history during the Late Precambrian.

The Ediacaran fossils described to date represent the remains of several distinct soft-bodied animals. These organisms were very different in appearance from modern creatures, and consequently, geologists struggled to establish evolutionary connections between ancient and modern organisms. In 1983, A. Seilacher of the University of Tübingen proposed that the Ediacaran organisms represented distinct taxa unrelated to the modern groups that appeared in the Cambrian, thus constituting a “failed experiment.” Some Ediacaran organisms were bottom-dwelling, or benthic, creatures with bilateral symmetries. Some fossils have symmetries and shapes analogous to modern swimming and bottom-dwelling medusoids (jellyfish-like organisms). Other Ediacaran fossils display a body geometry similar to living flatworms and were swimmers or crawlers on the ocean floor. Many of the Ediacaran fossils look like an elm leaf and were originally mistakenly classified to the biological class Petalonia because of this appearance. Fossil sizes for organisms represented by the Ediacaran assemblage range from several millimeters in length or diameter to several centimeters and, in some cases, several tens of centimeters.

One of the unusual qualities of this assemblage is that its components appear to have left little or no direct evidence of evolutionary descendants in the fossil record. This situation, which could result from the incomplete nature of the geologic record, makes it difficult to compare the Ediacaran organisms with either modern creatures or those preserved in the early Paleozoic stratigraphic record.

Disk-like and Frond-like Fossils

There are two main body geometries of the Ediacaran fossils. One of the most common shapes is discoidal, similar to many living jellyfish. One sees Ediacaran fossil remains preserved in the rock as plate-like or disk-like impressions like those left in beach sand by a washed-up jellyfish. This impression is commonly marked by several concentric rings, narrow ridges, or grooves. These features can be located near the rim of the disk, or they can be more or less evenly spaced across the disk's surface or concentrated at intervals in different places across the disk surface. Some of the discoidal fossil impressions suggest tentacles on the margin of the disk. In addition, many Ediacaran discoidal fossils have radial ornamentation and various bumps and depressions near the disk's center. These latter have been interpreted as either feeding or reproductive organs.

The fossil record is not sufficiently detailed to establish an evolutionary line from these metazoans to fossils with similar features preserved in younger rock units or to modern medusoids.

The other common shape of fossils in the Ediacaran assemblage is the generally flattened, bilaterally symmetrical shape that is overall very similar to that of a leaf from an elm tree. This frond-like or leaf-like shape is preserved in many fossil localities worldwide and was common to several benthic Ediacaran organisms. These bottom-dwellers were somewhat inflated, sac-like creatures whose exterior appears to have had a supple, leathery texture. The creatures lived attached to the bottom in a manner very similar to that of a modern sea pen. The attachment mechanism was a bulbous expansion of the lower end of a basal stalk. This expanded portion of the stalk was anchored within the bottom sediment and is best preserved in the form Charnodiscus, described from England. Some variations, such as Pteridinium, exhibited a triradiate symmetry from a central stalk. The specimens from Newfoundland's Mistaken Point localities are termed rangeomorphs. Exquisite preservation by ash falls shows details of a fractally pleated surface that greatly increased the surface area of these organisms. The frond-like organisms apparently fed by filtering food materials from the surrounding water.

The several varieties of frond-like and disk-like Ediacaran organisms are preserved as impressions in shaly sandstone layers, as if they were footprints in mud. (There are no actual remains or even altered remains of the Ediacaran organisms known.) These impressions indicate that the organisms lacked an internal skeleton and that the external body covering was probably leather-like. This leathery exterior and inflated habit have created difficulties in classifying many remains. When the organisms died and fell into the soft mud of the ocean bottom, sediments would fill the open sac in a haphazard and incomplete manner.

As the sediment compacted and squeezed the organic remains during the fossil-making process, various preservation styles from the same type of organism resulted. Consequently, a particular frond-like organism may be represented by multiple preservation styles. For example, body geometries from opposite sides of the creature can be superimposed on each other, giving the impression of a very different creature. Also, there are situations in which internal and external body geometries are superimposed on each other, and situations even exist in which internal geometries from opposite body walls appear as a single fossil. Thus, several different names have been assigned to the same organism.

There is evidence that some bilaterally symmetrical Ediacaran fossils are remains of swimming organisms or perhaps swimming or crawling flatworms. Similarities to living annelids are sufficiently convincing that some Ediacaran fossils—such as the fossil Spriggina—have been tentatively classified as annelids.

The Ediacaran assemblage of fossils is preserved in rocks whose compositions, textures, sedimentary structures, and positions in sequence indicate that the original sediments were deposited in an ancient ocean, near the shoreline. Preserved sedimentary features in the rocks suggest that the depositional environment was generally marine, probably in nearshore shallow water and intertidal situations on relatively low-energy sandy bottoms. Thus, when Ediacaran fossils are recognized in previously undescribed rocks, it is possible to interpret the original depositional environments.

Study of Fossils

Fossils are one essential piece of geologic data used to determine time equivalence (correlation) of stratigraphic units separated by great distances, such as between fossil-bearing rocks in North Carolina and South Australia. Fossils are also important to the evolutionary biologist or paleontologist interested in the history and development of life forms on the Earth throughout geologic time. Fossil evidence is routinely employed by geologists who interpret ancient depositional environments. The general assumptions are that fossils indicate the depositional environment preserved in the rock in which the fossils are found and that identical fossils found in different areas of a state or on other continents are essentially the same age.

The fundamental techniques in utilizing fossils are critical observation, accurate data collection, synthesis of data, knowledge of the biology of living organisms, and knowledge of modern sedimentary processes. The latter two items can be gained from reading, while the former three require practice. The geologist or paleontologist makes detailed observations of fossils and the enclosing rock characteristics before the fossils are collected for study, noting such things as the position of the fossil relative to the probable living positions and searching for any indications that the fossil was transported after the animal died. The geologist also looks for relationships to other fossils, rock types, and relative abundances of particular fossils. After the fossils are collected, they are carefully prepared for laboratory study, where detailed observations and accurate descriptions of fossil morphology or shape are made. These analyses form the basis for biological comparisons with other fossils and living organisms.

The occurrence of Ediacaran fossils in both South Australia and North Carolina, as well as other parts of the world, has been established by careful fieldwork and descriptive work. These data tell the geologist that the enclosing rock units are the same age even though they are not now connected. In those unfossiliferous parts of the stratigraphic column, such interregional or intercontinental correlations are possible only by using expensive techniques that employ radioactive elements or materials. These materials are in such low concentrations they are detectable only with sensitive instruments. Thus, the discovery of metazoan fossils in stratigraphic units previously thought unfossiliferous has been of tremendous value to geologists working in different parts of the world.

Some geologists are very interested in the biological aspects of the fossil record. The working hypothesis is that creatures with similar anatomical features or creatures living in similar environments can be used as indicators for the physical surroundings and to determine biological affinities. Based on this hypothesis and the hypothesis of uniformitarianism (the notion that the present is the key to the past), geologists seek to find similarities between preserved or implied anatomical structures of fossils and similar features in living organisms. Also, the geologist examines the rock record and compares it with modern depositional environments, attempting to reconstruct the ancient conditions. In the case of the Ediacaran fossils, geologists have established ancient depositional environments based on enclosing rock characteristics and from these environments to interpret how the organisms lived. By comparing the preserved anatomical features of the fossils with apparent living analogs, geologists then make interpretations about the organisms themselves. The geologist thus compiles many lines of evidence to support a hypothesis.

In the twenty-first century, new Ediacaran fossils continue to be discovered across the globe, and scientists categorize these fossils to better understand evolution and Earth's history. Some of these fossils have exhibited a putative tunicate relationship. Research of Ediacaran fossils has helped explain some of the origins of biomineralization that facilitated life on Earth and the Earth's carbon cycle.

Principal Terms

correlation: the establishment of the fact that an event in one place occurred at the same time as a similar event in another place

Early Paleozoic: that part of geologic history that is somewhat younger than about 550 million years before the present

Ediacaran fossils: fossils of marine animals with moderately complex body structures (metazoans) that lived some 570 to 670 million years ago; the term “Ediacaran” was derived from the well-known Ediacara Hills fossil locality in south Australia

Late Precambrian: that part of geologic time from about 550 million years to one billion years before the present

stratigraphic unit: any rock layer that can be easily recognized because of specific characteristics, such as color, composition, or grain size

Bibliography

Fedonkin, Mikhail A., et al. The Rise of Animals: Evolution and Diversification of the Kingdom Animalia. Johns Hopkins University Press, 2007.

Glaessner, M. F. The Dawn of Animal Life: A Biohistorical Study. re-issue. Cambridge University Press, 2010.

Martyshyn, A., Uchman, A. New Ediacaran Fossils from the Ukraine, some with a Putative Tunicate Relationship. PalZ, vol. 95, 2021, pp. 623–639. https://doi.org/10.1007/s12542-021-00596-1.

Prothero, Donald R. Bringing Fossils to Life. 3rd ed. McGraw-Hill, 2013.

Schopf, J. William, ed. Major Events in the History of Life. Boston: Jones and Bartlett, 1992.

Vickers-Rich, Patricia, and Patricia Komarower, eds. The Rise and Fall of the Ediacaran Biota. Special Publications, 2007.

Wan, Bin, et al. "A Tale of Three Taphonomic Modes: The Ediacaran Fossil Flabellophyton Preserved in Limestone, Black Shale, and Sandstone." Gondwana Research, vol. 84, 2020, pp. 296-314. https://doi.org/10.1016/j.gr.2020.04.003.

Wicander, Reed, and James S. Monroe. Historical Geology. 8th ed., Brooks/Cole, Cengage Learning, 2016.

Yang, Ben, et al. "Taxonomic Revision of Ediacaran Tubular Fossils: Cloudina, Sinotubulites and Conotubus." Journal of Paleontology, vol. 96, no. 2, 2022, pp. 256-273. https://doi.org/10.1017/jpa.2021.95.