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Up until 1947, it was believed that the Cambrian Explosion marked the first true abundance of multicellular life. However, this was discovered to be untrue after Sir Douglas Mawson and R. C. Spring mistakenly came across numerous "fossil jellyfish" in the Ediacaran Hills while observing what was originally believed to be sandstones belonging to the lowest strata of the Cambrian. At first, these finding were dismissed as "fortuitous inorganic markings. " (AAS Biographical Memoirs. ) Several years later however, other discoveries of segmented worms, worm tracks, and impressions of two other assemblages that bear no resemblance to any known organism, living or extinct, prompted the South Australian Museum and the University of Adelaide to undertake a joint investigation of the region. Further studies by M.
F. Glaessner, a paleontologist at Adelaide showed that the fossils were found well below the oldest Cambrian strata and that the strata actually dated from the Precambrian era. Several thousand specimens have since been collected in the Ediacaran Hills. All the fossils collected were soft-bodied animals and their tissues were strengthened by spicules-needles of calcium carbonate that functioned as their support. The Ediacaran organisms were marine animals, some crawled, some were attached to the sea floor and others would swim or just freely float. Their impressions were molded in the moving sands that washed over the mud flats and were preserved as casts in the sandstone.
It is difficult to conceive how fossils of delicate soft-bodied animals could be preserved given the evidence of strong currents in the strata. However, extensive research has provided an explanation. Most of the animals settled on mud patches out of the water during calm currents. Some of these patches dried between tides and developed deep cracks. The next shifting current would then cover these cracks with a layer of sand and the lower surfaces preserved the mud in the form of perfect casts. (Glaessner 67) The nature of these soft-bodied fossils justifies the characterization of the Precambrian as the "age of the jellyfish, " however the term jellyfish only refers to a number of diverse forms, which belong to the Phylum Cnideria. (Glaessner 64) Six principle forms of animals have been discovered. The first are the rounded, discoid al impressions, resembling the modern day jellyfish.
The second form is the stalk-like fronds with grooved branches that also belong to the Cnideria Phylum. Next come the elongated worm-like impressions with a horseshoe shaped head followed by 40 identical segments and rounded flattened, worm-like impressions with a central groove and strong segmentation. These worm-like impressions belonged to the Annelida Phylum. The last two forms, which were oval shaped impressions with T shaped groves and circular impressions with three "bent arms, " resemble no known organisms and are believed to represent the Phylum Arthropoda. The most abundantly distributed fossil at the South Australian site, the Dickinsonia, was represented by over one hundred specimens.
These impressions may be related to some modern day flatworms and were covered with transverse ridges. The number of ridges varies with size-while smaller forms had 20 ridges, larger ones could have as many as 550. Another notable find was the Spriggina founders, which was named after R. C. Spring and Ben Flounder; the two geologists credited to their discovery. These small annelid worms ranged between one and three inches in length and had small horseshoe shaped heads.
These worms resemble the modern Tomopteridae, which have heads similar to those of the Precambrian fauna, only wider. It now appears as though these marine worms are direct descendants from the ancient forms. The Ediacaran organisms are a clear foreshadowing of one of the most dramatic events in all of life's history-the rapid expansion of shell-bearing organisms. The earth's early atmosphere was made up largely of carbon dioxide, water vapor and nitrogen.
While the present atmosphere contains the preceding three elements, it also contains large quantities of oxygen. Geologists now believe that this expansion of life was a direct result of the fact that the atmosphere first accumulated abundant oxygen at this time. To contrast this, many modern descendants of the organisms present during the Precambrian such as blue-green algae do not require oxygen to survive. Oxygen is released as a waste product of photosynthesis by the blue-green algae, but would have combined with oxygen-deficient elements in the water, rather escaping into the atmosphere. However, eventually, most of these elements would become oxidized and the oxygen would begin to accumulate in the ocean before being released into the atmosphere.
This paved the way for the rapid expansion of oxygen dependant life that brought an end to the Precambrian era. (McAlester 27) Until the 1980 's the fossil record of early multicellular organisms was interpreted to be one of rapid and simple diversification. Adolf Seliacher, along with other paleontologists argued that this was incorrect and that the earliest forms of multicellular organisms include few or no species that are directly related to later faunas. As evidence, they pointed to the earliest fauna from the Ediacaran period-organisms living at the end of the Precambrian era, 600 - 700 million years ago. Seliacher argued that similarities between the modern living organisms and the oldest known fauna were too superficial. He supported his argument by noting that the branches in the frond fossils lack passages through which water currents might pass, while the modern sea pen have these openings.
He also noted that modern jellyfish have radial structures in their center and centric structures around their periphery, while the discoid al forms of the Ediacaran are opposite to that of today. As for the similarities between the Dickinsonia and Spriggina and modern worms, Seliacher points out the lack of evidence of organs, such as a mouth, stomach or anus, in the fossils that are essential to the living worms of today. Because of these dissimilarities, he suggests that the Ediacaran animals should not be placed in existing phyla, rather their own taxonomic category which he gave the name Vendor. (Levin 267) Seliacher believes that these creatures were "failed experiments, " in the evolution of multicellular organisms leaving behind no ancestors, unlike the Cambrian animals. Another interpretation of these organisms suggests that the Ediacaran fossils weren't even animals. Gregory Retallack, a paleontologist from the University of Oregon, suggests that these fossils were lichens. Retallack's argument rests on the fact that the impressions were buried five kilometers under ground.
He believes the fossils should have been crushed by the weight of the overlying sediment. But Retallack observed that the fossils "were as compaction resistant as some kinds of fossil tree trunks!" Because of this, Retallack concludes that these forms weren't animals, rather lichens made up of sturdy molecules such as chitin. Further evidence of this interpretation can be found in the Ediacaran organism's growth patterns and microscopic structure. (Woodmorappe 1) Despite these controversial implications many paleontologists still believe the Ediacaran animals are in fact ancestors of the animals in the existing phyla. This controversy might not be resolved until the discovery of fossils providing more information... (Levin 267) Charles Darwin found himself in this same situation some 150 years ago.
Just as he was befuddled by the absence of the ancestors of the Cambrian animals, we are puzzled by the absence of direct ancestors of these Ediacaran fossils.
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