Softbodied Ediacarans

As with so many apparent first discoveries in Earth Time, detailed investigation reveals some previous unsung and forgotten record that predates the generally accepted 'first' find. So it is with the strange soft-bodied and extinct Ediacaran organisms. Back in 1877, two English geologists, E. Hill and T. G. Bonney, found and described some 'curious arrangements of concentric rings which have been supposed to be organisms' on some ancient seabed surfaces in the late Precambrian sandstones of Charnwood Forest near Leicester, England. These subsequently turned out to be genuine Ediacaran fossils, but at the time Bonney and Hill dismissed them as inorganic.

The real recognition of these novel fossils had to wait until the late 1940s, when an Australian mining geologist, Reg Sprigg, was reassessing the economic value of some abandoned lead-silver mines in the Ediacaran Hills of the Flinders Range north of Adelaide, South Australia. Among the hillside outcrops of sandstone strata of the Rawnsley Quartzite, Sprigg found remarkably well-preserved impressions of some very organic-looking structures. There was no sign of any hard parts and the preservation was curious: the fossils seemed to form positive casts filled with sand. Although his palaeontological knowledge

Rev. Thomas George Bonney, 1833-1923, theologian (Canon of Manchester), petrologist and professor of geology at University College, London (from 1871), president of the Geological Society of London (1884).

was not great, he was a widely experienced field geologist and knew that they were quite unlike any other fossils he had ever seen.

Over the next few years Sprigg published reports of his finds and suggested that they might be the remains of some kinds of jellyfish. Although he thought they were probably of early Cambrian age, he did also claim that they might be among the oldest fossil evidence for animal life. However, his reports were published in an Australian scientific journal that was not widely read outside of that country and it took some time before the palaeontological world began to sit up and take notice of what was a remarkable and very important find.

On the other side of the world, back in Charnwood Forest in April 1957, a schoolboy enthusiast by the name of Roger Mason rediscovered similar fossils to those dismissed as inorganic by Bonney and Hill. The low sandstone crags in this part of Leicestershire are ideal for clambering and climbing over. They were a favourite after-school haunt of Roger and his friends and on one of these excursions Roger's companion, knowing of his palaeontological interests, alerted him to some fossils on the rock surface.

Fortunately, Roger Mason told academic palaeontologist Trevor Ford of Leicester University of the discovery and showed him a paper rubbing he had made of the fossil. Ford was initially sceptical, but the clearly organic form reproduced by the rubbing persuaded him to take a look for himself. He was quickly convinced and in the following year published a description and illustrations of the fossils. One of the most striking of the new fossils was a strange little plume or feather-shaped frond that Ford called Charnia masoni after the location and Roger Mason. Most importantly, Ford could argue convincingly that the Charnwood fossils were late Precambrian in age rather than early Cambrian as the Australians thought. The Ediacaran 'bandwagon' slowly began to roll. Now, over 50 years later, there is an amazing amount of information about these strange fossils, but there is still a very real uncertainty about what kind of organisms they were.

One of the favourite early interpretations saw them as some kind of jellyfish, and indeed a number of them did have circular dish and blob shapes. But when palaeontologists tried to replicate the preservation of soft-bodied recent jellyfish in sand, they were singularly unsuccessful. It seemed that the Ediacarans must have had tougher body tissues than living jellyfish. Another approach was predicated by the evolutionary Darwinian expectation that the ancestors of the major groups of invertebrate sea creatures from jellyfish through molluscs and arthropods to echinoderms must be present in these latest Precambrian strata. Otherwise, where would all the Cambrian shelly fossils have come from? As a result, the various Ediacaran forms were at first shoe-horned into these groups and claimed to represent the soft-bodied ancestors of coelenterates (jellyfish, sea anemones), segmented worms, trilobite arthropods, sea urchins (echinoderms) and so on. The radical alternative explanation is that they were some kind of failed evolutionary experiment.

The last few decades have seen Ediacaran fossils turning up all over the world in latest Proterozoic strata dated at between 575 and 541 million years ago. Some forms are widespread, while others are endemic to a few nearby localities. Overall, the diversity has increased enormously with some 100 species now known. Apart from vastly increasing our knowledge about their diversity, this global distribution has revealed Ediacarans with less common modes of preservation, especially in fine-grained muddy sediments, such as seen in Newfoundland, Canada and in the White Sea region of Arctic Russia. Such occurrences have given new insights into their mode of life and biology.

The form of the Ediacarans varies from simple blobs, discs and sac shapes to ribbons and fronds up to 2 m in length. They are the first large creatures to have lived on Earth. Some of them lived within the seabed sediment, others could move slowly over the sediment surface like some sort of flatworm, and yet others were anchored in the sediment with leaf and ribbon-shaped, stiff but flexible fronds projecting up into the water in a manner reminiscent of living seapens (pennatulaceans). Many had strangely quilted bodies, some of which were filled with sand during life, while some were very like primitive sponges. But as yet there is no consensus on whether they form a mixture of known and unknown animal groups or whether they are all extinct 'aliens'. In that case, where did the Cambrian invertebrates come from?

Curiously, although Californian strata of late Proterozoic age are thought to extend through this time interval and the sediments are quite similar to those in which many Ediacarans have been found, none has yet been uncovered in the Californian strata. However, a few have been found not too far away over the border to the south in Mexico and somewhat further away in North Carolina. By far the most interesting Ediacarans of Laurentia are to be found in Newfoundland, a rocky island off the northeast coast that is a paradise for geologists.

Ediacarans, including Charnia masoni, were first found in southeastern Newfoundland in 1969. They occur within a 6-km-thick succession of late Proterozoic strata that record a depositional transition from a deep marine basin through shallow coastal to land-based alluvial deposits. The most abundant fossils were found at Mistaken Point in strata containing a volcanic ash that was radiometrically dated at around 565 million years old. Ediacarans continue to occur sporadically throughout another kilometre and half of strata below this.

The lowest and therefore oldest known Ediacarans still include Charnia masoni and another similar frond called Charnia zvardi. This latter species is the biggest Ediacaran, growing from an anchorage in the sediment to nearly 2 m long. Numerous fronds of both species have been found lying parallel to one another, bent over and flattened onto the seabed by a submarine current before being covered by a volcanic ash deposit. Over a kilometre below the lowest-known Ediacarans some glacial diamictite deposits, around 595 million years old, are found interbedded with the predominantly marine deposits. If this date is correct, it indicates that these earliest Newfoundland Ediacarans are perhaps 580-570 million years old. As such, they are not only the biggest Ediacarans known but also the oldest by some 5 or 10 million years.

The Ediacarans seem to appear suddenly in late Proterozoic time, already significantly larger and more complex than any previous life form. In addition, they seem to appear 'fully fledged' within some 15 million years of a glaciation that is supposed to have shut down primary production in the oceans of the world. But is this the reality of the situation? Is it just that we have not yet found older Ediacarans nor the organisms from which they evolved? If this is true, then they will be even closer in age to the glaciation. If so, does this weaken the case for a catastrophic impact on life by the last 'snowball' event? Perhaps the Ediacarans evolved before the last glaciation? We just do not know yet, just as we do not really know what kind of organisms the Ediacarans represent. Despite all the uncertainties, one thing is sure: the glaciations were real and the evolution of the Ediacarans mark a critical stage in the story of life on Earth, which may be linked in some way to these major environmental changes.

This exploration of early Earth Time has been detailed and has taken us well beyond the confines of the Grand Canyon. If it is necessary to excuse such attention, I think it can easily be justified when we remember that the Precambrian does represent by far the greatest proportion of Earth Time. The rock strata laid down over the 4 billion years and more of early (Precambrian) Earth Time is one of the last great unexplored regions of the Earth's surface, apart perhaps from the deep oceans. The last few decades have seen remarkable advances in our understanding of the remote past and I guarantee that the next few decades will bring yet more surprises from Precambrian strata.

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