Australia: The Land Where Time Began
Sedimentary deposits from the time of the Ediacaran Era that contain traces of life are not common. One of the reasons that has been suggested for the fossilisation of the Ediacaran Fauna is that at the stage of evolution they had reached, prior to the development of hard parts, efficient scavengers hadn't yet evolved, so that any animals that settled on the sea floor after death were not scavenged by other animals. The fact that the fossils could be discernable at all is surprising considering the soft nature of the animals and the coarseness of the sandstone of which the Ediacaran Hills are composed. The deposit is believed to have formed on the sea bed where silty sand accumulated in shallow water near the shore. In modern environments of this type the only remains that survive for any length of time are shells, and they are usually broken and worn. Before the Ediacaran impressions were determined to actually be the remains of animals it was believed that soft-bodied animals could not be fossilised in rocks such as coarse-grained sandstone.
A number of suggestions have been made to explain how such animals that completely lacked hard parts could be preserved in this deposit. As with many modern marine animals, it is believed the Ediacaran animals were probably covered with mucous. One suggestion is that sand stuck to this mucous layer so that when the organism decayed the surrounding sand was held in place by the mucous in the shape of the animal. Another suggestion has been that on death the body was covered by a mat of bacteria that could have contributed to the holding together of the surrounding silt in the shape of the decaying organism. It is believed that the mucous theory and the bacterial mat theory actually work well together, also providing an explanation for the crumpled surface found on many Ediacaran fossils. In addition, it would also explain the fact that they have a higher iron concentration than that of the surrounding rocks. The bacteria are believed to have been able to fix iron in the form of pyrite, this would add cement to the accumulating bacterial mat in the shape of the organism.
It has been difficult to determine the appearance of the animals in life, as when they died their soft bodies tended to slump into a gelatinous mass on the sea floor, thus obscuring their 3-D shape.
DNA studies have indicated that the major animal groups separated about 1 billion years ago. This would allow 400 million years of evolution to get to the stage seen in the Ediacaran animals.
The organisms here, and elsewhere around the world from the period, all seem to appear "out of nowhere", as did the later Burgess Shale biota. Animals that preceded them have still not been found. An explanation for the lack of an evolutionary line leading to them is probably that before the Ediacaran stage of development life was too soft to be preserved. It is believed that prior to this stage all life forms lacked the fibrous protein collagen, which adds just enough strength to the body of the animal to allow preservation under very favourable conditions. It is known that animals cannot produce collagen at oxygen pressures less than 3 % of the present atmospheric level of oxygen. By 630 million years ago oxygen levels in the atmosphere had been increasing for some millions of years and it is thought that atmospheric oxygen levels crossed this 3 % threshold about the time of the Ediacaran Fauna, allowing the formation of collagen as soon as the ability to produce it had evolved. The Ediacaran Fauna differed from the 530 million-year-old Burgess Shale Fauna by being unspecialised. By the time of the Burgess Shale Fauna specialisation had progressed a long way towards the level of specialisation found today.
In the Ediacaran deposit are found traces of organisms moving on and beneath the microbial mats that covered the sea floor at the time, about 565 million years ago. They seem to have been made by organisms resembling earth worms, in size, shape and the method of movement. The fact that they burrowed indicates that they must have had a level of organisation that had head and tail ends. Maybe they were already bilateral. As the organisms that made the burrows have not been found so far probably means that they had not reached the level where harder body parts, or even collagen, could be produced, but they had the pre-existing structure to lead to the more highly organised organisms once harder body parts could be produced. Once they had reached this level of organisation they would be "ready to roll" once the harder body parts were available, shortening the time necessary to evolve structures to use the harder parts. Once collagen and harder materials had been added to the pre-existing body structures many would have a better chance of fossilisation, so this is probably the origin of the Cambrian explosion, previously soft-bodied, naked organisms suddenly appeared in the rock strata in a wide variety of forms, but now with harder body parts that seemed to "come out of nowhere". The addition of harder body parts probably allowed for additional radiation, on top of the apparent radiation displayed by the now more easily fossilised forms, as new niches would become available to animals with firmer bodies. Hence the "explosion".
It has been suggested that rather than permitting the evolution of complex biological systems it was actually a forcing factor that pushed evolution in the direction of complex systems. The decreasing concentration of heavy metals that resulted from the locking up of catalytically active elements, that were used in many enzymes, in insoluble compounds forced cells to adapt to the changed conditions.
V.A.Gostin et al. found glacial debris in the ejecta from the impact at Lake Acraman. He suggests this is evidence of glacial conditions at the site, indicating that it wasn't responsible for the triggering of a low latitude glacial event, as had been proposed earlier. He suggests the Acraman impact could have triggered the surge of evolution by releasing the biota that survived it from the harsh conditions of the "Snowball Earth".
Recently the oldest known chordate was found among the Ediacaran Fauna from the Ediacara Range of South Australia. At 560 million years old (at least) it is 20 (or 30) million years older than the previous vertebrate record holder which was found in China several years ago. The 5 cm long tadpole-shaped animal had a head and muscles and a fin on its back, which makes it different from the many other non-chordate animals found in the Ediacara Hills.
The Ediacaran Fauna were of a soft-bodied form, that lived in shallow-water, marine environment. The fossils consist of impressions of the organisms that mostly look like jellyfish, seapens, annelids (segmented worms) and primitive arthropods. It is not known if they were actually the earliest forms in the lineages that led to later forms that they resemble. Some believe they were animals from a period where all sorts of forms were evolving that would die out, being replaced by the ancestors of the later lineages. Others believe that at least some of them were the earliest known stages in the evolution of lineages that led to later, more easily recognised, forms.
The Ediacaran animals lived in the Neoproterozoic Era, that covered the time between about 800 and 542 million years ago, at a time of great climatic fluctuation, including a succession of ice ages. Evidence from this time, in the form of dropstones from icebergs in the Adelaidean Sea, and thick diamictites deposited by sea level ice sheets, and this at a time when Australia was positioned on the equator. This evidence contributing to other evidence that it was a time of Snowball Earth, when even Australia, on the equator, was affected by worldwide glaciation. Thick carbonate series of deposits formed at the end of each of the ice ages. One such rock unit, the Marinoan (aka Elatina), has been designated as the basement of the Ediacaran Period, that covered a time span of 635-542 million years ago, reaching a thickness of about 4 km. This series of sedimentary layers records the changing climate throughout the period, as well as the changes in sea level that took place during the Period.
In sediments dating to about 570 million years ago there is evidence of a large meteorite impact, the Acraman impact horizon, that is believed to have ejected debris over a distance of more than 300 km from the impact site. Such a large impact probably had a severe, possibly catastrophic, effect on the life in the Adelaidean Sea.
At about 1200 m below the overlying Cambrian strata, the oldest fossils known from the Ediacaran Biota are found in a thin layer near the top of the Wonoka Formation. About 400 m below the earliest Cambrian strata is the Rawnsley Quartzite, that contains the richest, most diverse Ediacaran fossil fauna. This layer dates to much later than the impact event.
The base of the Cambrian sequence contains the first fossils of a diverse fauna of shelled animals and burrows penetrating the sea floor. In the limestone layers from the Early Cambrian are found the first instance of metazoan reef builders, the archaeocyathids, and the tabulate corals. It is believed the water was warmer than during the Ediacaran Period.
Some of the animals in the Ediacaran Fauna are Dickinsonia, Spriggina, Parvancorina, Rangia
Dickinsonia, was a broad, flat sheet-worm, up to about 50 cm long, oval leaf-shaped. There was a central furrow with lines radiating out from the central furrow to the margins of its body, most of these lines connecting directly to the furrow. It is usually thought to be an early form of marine worm, to which it bears a strong resemblance. There is not universal agreement on this point, some believing it more closely resembles a polyp of Fubgia, the banana coral.
Study of Dickinsonia among fossils of Ediacaran Fauna in a White Sea site in Russia has found what appeared to be a group of 4 individuals, all the same size and shape, 3 of which were impressions of one side but the 4th was an imprint of the other side, prompted the suggestion that 3 of the impressions were made by the animal as it flopped around on the sea floor, the 4th being an impression of the actual organism as it died. Study of the Ediacaran Hills site has turned up the same pattern. The fact that these largish animals, up to about 50 cm long, could apparently move enough to flop around on the sea bed indicates that they had already developed musculature that was obviously capable of being operated in an organised manner requiring some level of nervous system. This leads to the conclusion that they were apparently more complex than was originally believed.
Study of Dickinsonia costata material from the Ediacaran Hills site in South Australia in the Natural History Museum, Oxford, has found that all the characteristics of the Dickinsonia can be explained by the hypothesis that its body was based on a simple hydraulic system, such as that found in coelenterates, where movement is achieved by changes in turgor pressure in the body
At this stage in the evolution of life the life forms lacked the firm supporting structures that allowed later forms to develop structures such as a circulatory systems to move oxygen and nutrient to all parts of the body, however deep they were in the body. The only way such organisms could get larger was to increase their surface area, maintaining a high surface to volume ratio - becoming flat and wide or long, while remaining thin enough for the necessary gas exchange to take place and for nutrients to reach all the component cells of their multicellular bodies.
The classification of many of the Ediacaran fauna is uncertain and disputed, and subject to change in the future. In a number of cases the same specimen has been given 2 or more names. It will be some time before the confusion is sorted out.
About 580 million years ago a 4 km-wide meteorite impacted at the site of Lake Acraman, the impact crater, on the Eyre Peninsula north of the Gawler Ranges and about 300 km west of the Flinders Range. A layer in the sediments of the Flinders Range is composed of the debris of this impact. It had a big effect on the Ediacaran biota, there was a complete change of the acritarch fauna.
Some Australian species of Ediacaran fauna
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|