Ediacaran organisms have been found in Australia, Newfoundland and England. However they were discredited as being Ediacaran in age until 1957 with the discovery of Charnia in Charnwood forest.
The atmosphere leading up to the Ediacaran underwent some vital changes. Oxygen built up in two stages corresponding to periods of glaciation. The oxygen was produced as a by-product of activity by cynobacteria. The glaciation periods prevented complex life from developing before the Ediacaran. Bacteria which inhabited the waters during these snow ball events sheltered in areas of open water where ice did not cover.
The biosphere changed from simple life before the Ediacaran to complex organisms. These have been found in Australia, Newfoundland and England. Charnia and Aspidella are two of these organisms which have been preserved as a result of microbial mats, which held the shape of these creatures. The Ediacaran life forms were wiped out at the beginning of the Cambrian as a result of competition from mobile organisms.
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The evolution of the biosphere and atmosphere in the Ediacaran allowed the first complex organisms to inhabit our oceans. The rise in oxygen combined with melting of ice allowed conditions to be favourable for life to flourish.
It is well understood that there was life on Earth in the Cambrian 542-588 million years ago (Gradstein,Ogg et al 2004). However life before this time was much disputed until 1957. A school boy called Roger Mason found a fossil of Charnia in Pre Cambrian rocks in Charnwood Forest Leicestershire (Ford 1958). Before this time Precambrian fauna had been discovered in areas of Newfoundland and Australia, but they had been discredited and said to be Cambrian in age. The find in Charnwood forest was a turning point in geological history and now Ediacaran fossils have been found across the world. In this report I will look at the how atmospheric changes allowed the first complex organisms to appear in the Ediacaran. I will also consider the mode of life of these organisms and how they were preserved.
Figure 1 and 2, Show the Ediacaran period 630-542Ma. The Ediacaran period was in the Proterozoic Eon of the PreCambrian.
Source: Taken from Gradstein, Ogg et al 2004
Main Findings on the Ediacaran Atmosphere
Glaciation in the Neoproterzoic
Organisms on earth originated at a critical point of Earths history. The break up of the supercontinent Rodinia approximately 750Ma caused widespread glaciation (Hoffman & Scharag 2002). It is proposed by Paul Hoffman that the ice, 1km thick, covered the entire Earth's Oceans. However ice this thick could not support photosynthesis and therefore marine areas would quickly become anoxic, making respiration impossible. Joseph Kirschvink on the other hand favoured a 'soft snowball' where Earth maintained areas of open ocean, even at glacial maxima (Kirschvink 1992). This is supported by the survival of diverse eukaryotic lineages and unequivocal fossils of crow groups of marine red, green, heterodont and yellow-brown algae prior to 750Ma implying that marine photosynthesis continued though glaciations (Narbonne 2004). These organisms would have found refuge in open oceans of the 'soft snowball' world.
History of Oxygen
It is not known precisely when oxygen reached its present levels in the atmosphere, though it is widely agreed that a moderate level was reached around 2 billion years ago in the Neoproterzoic. Work by Paul Hoffman and Don Canfield has shown that oxygen built up in two stages. The first was 2200Ma and is broadly associated with a period of glaciation called the Makganyene, which occurred between the Archean and Proterozoic. The second rise occurred around 635Ma. This corresponds with a period of glaciation called Marinoan between the Proterozoic and the Phanerozoic (Cranfield 2007). (See Figure 3)
Figure 3 shows periods of glaciation compared to the oxygen levels in the Precambrian. In the diagram one can see that glaciation event corresponded greatly with oxygen build up.
Source: Taken from Paul Hoffman's website www.snowballearth.org
Oxygen is constantly removed from the atmosphere by the process of respiration and is returned by photosynthesis and by the break down of H2O, in the atmosphere by the suns rays (Stanley 1993). Therefore levels of oxygen over the past 500Ma have remained fairly constant. Life in the Ediacaran period was restricted to oceans. The oceans became oxygenated as a result of cynobacteria; they produced oxygen as a bi-product of activity (Nabonne and Gehling, 2003). Evidence to support an anoxic world before this time is from the building blocks of life 'amino acids' these occur in anoxic environments and are inhibited by oxygen. Today these bacteria are restricted to swamps and lagoons. The fact that the simplest bacteria on earth are anaerobic suggests that there was virtually no oxygen in the atmosphere when they came into being (Stanley 1992).
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Photosynthesis caused oxygen to build up. However before this could occur oxygen sinks had to be filled. Iron, sulphur and over elements readily combine with oxygen. It took 3.5 billion years before the oxygen had filled the sinks and reached a high level. This is due to the low abundance of photosynthesising organisms in the Archean (Stanley 1992).
Main finding on the Ediacaran Biosphere
The Ediacaran biota consists of frond-shaped sessile organisms which lived 588-542Ma. These organisms are pioneers and were the first multi-cellular, complex organism to inhabit Earth (Antcliffe and Brasier 2008). Before this time only eukaryotic and bacterial life had existed. (See Figure 4) The appearance of the first complex organisms occurred as the Earth thawed from the Gaskiers glaciation period 580Ma, which had previously prevented the introduction of complex life. The Ediacaran organisms were non-skeletal and soft boded. They included sponges and cnidarians. They had a range of morphologies including spines, fans and frond like organisms. These were mostly attached to the sea floor by holdfasts, which are normally preserved in isolation to the body.
Figure 4 shows the plankton explosion between the Marinoan glaciation and the Gaskiers glaciation which lead to a rise in oxygen levels in the oceans. After the Glaskiers glaciation one can see the introduction on complex life in the form of Calcisponges in the Ediacaran.
Source: Taken from Paul Hoffman's website www.snowballearth.org
The earliest fossil communities disappeared from record at the beginning of the Cambrian leaving only fragments of the once thriving ecosystem. There are many hypotheses as to why the Ediacaran organisms became extinction. These include change in environment, the introduction of predators, and competition from Cambrian life forms. Cambrian organisms were mobile and had skeletons and therefore proved too greater competition for the simple Ediacaran biota (Canfield, Narbonne et al 2008).
Ediacaran Avalon Assemblage
The oldest Ediacaran assemblage is found in the Mistaken Point Formation in the Avalon zone of Newfoundland. It is dated radiametrically using U-Pb dating which yields dates of 565Ma for the volcanic ash which coves the most fossiliferous surface, near the top of the formation (Benus, 1988). The assemblage contains Charnia and Aspidella. In central Britain these fossils are known only from Charnwood Forest in the Avalon zone.
A) Charnia- classification and mode of life
Charnia was discovered in Charnwood forest in 1957. This was a major turning point in the history of Precambrian fossil research. It was the first accepted macrofossil from Precambrian rocks, before its discovery life in the Precambrian had been much disputed. Charnia is very useful as it has a wide geological range. It is found in the Drook formation in Newfoundland, the Pond Quartzite in South Australia and Charnwood forest in England. Charnia appeared 575Ma and persisted for 20Mya, this has been shown by dating of U-Pb of the Charnwood forest fossils (Narbonne and Gehling 2003). Charnia it has the widest time range of any Ediacaran fauna.
Charnia is a frond-like life form with segmented ridges branching alternately to the right and left from a zigzag medial structure (Ford 1958). The species can be split into three groups.
Charnia Masoni which first discovered in Charnwood Forest by Rodger Mason in 1957.
Charnia Wardi, discovered in 1978 in southeast Newfoundland. This species was bigger and narrower than Charnia Masoni and measured up to 2 meters.
Charnia Antecedens, discovered in Newfoundland. This species branches at high angles
Charnia was at first classed as an alga however it was recast as a sea pen in 1966. This was further discredited in 1984 when Charnia was classed as an extinct group which was confined to the Ediacaran called Vandobionta. It is unknown where this group should be placed on the tree of life.
Charnia was discredited from the group sea pen due to the mannor in which it grows. Sea pens grow by basal insertion whereas Charnia grows by apical insertion of new buds (Antcliffe and Brasier 2008).
Figure 5, diagram of a sea pen and figure C shows how it grows. You can see from the diagram that the youngest part of the sea pen is at the bottom and the oldest part is at the top. In contrast the Charnia In figure B and D is youngest at the bottom and oldest at the top. This if the reason Charnia has been reclassified as a Vandobionta rather than a sea pen.
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Source: Antcliffe and Brasier 2008
Little is known about the mode of like of Charnia. It was benthonic and anchored to the sea floor using a holdfast. It is thought to have lived in deep water below the wave base and therefore was unable to photosynthesis. There is no obvious feeding apparatus and it is therefore thought that Charnia directly absorbed food.
B) Aspidella classification and mode of life
Figure 6 shows Aspidella found in black shale in Newfoundland.
Source: Taken from www.palaeos.com
Aspidella was discovered in Newfoundland by Elkanah Billings in 1872. They were found in a Precambrian outcrop of black shale. However the find was disputed as life in the Precambrian was thought to not exist. Some argued that the shapes were gas bubbles or formed inorganically. Aspiella was not excepted as a Ediacaran fossil until later discoveries in British Columbia which dated back to the same age.
Aspidella was 1-180mm in size and is ellipses shaped made up of concentric rings. It was originally thought to be a scyphozoan jellyfish however this has been disputed and there are now two theories of what Aspidella could be (Gehling J.G, Narbonne G.M, et al 2000).
A holdfast of an organism. This is supported by some specimens having a small stalk attached.
Microbial colony which is backed up be similar fossils
Preservation of Ediacaran organisms
Ediacaran organisms were almost exclusively soft bodied. They are most commonly preserved as impressions on the base of sandstone and volcanic ash beds, which were deposited on the sea floor. Ediacaran fossils lack carbonisation or mineralisation and are commonly found in high relief. This differs greatly form the Phanerozoic LagerstÈtten where organisms were preserved as two-dimensional carbonised or mineralised films (Briggs 2003). Some scientists believed that the organisms were firm bodied composed of wood material instead of flesh. However Ediacaran trace fossils are flexed and have evidence of wrinkles, this implies that the organisms were mainly soft bodied. Scientists like Jim Gehling argue that preservation of organisms in the Ediacaran was made possible by moulding of the nonmineralised organisms and colonies by microbial filaments (Gehling 1999).
Microbial mats are colonies of microbes which live in sediment. They secrete a sticky fluid which binds the sediment particals together. The individual microbes are unable to move, they instead grow and reproduce. However if a layer of sediment deposits onto them before they are able to grow, they die. This leaves a layer of characteristically wrinkled and tubercular texture. If an organism dies onto of the microbial mat the microbes will die maintaining the shape of the organism. This is the reason so many Ediacaran organisms have been preserved.
Further studies by Guy Narbonne have shown evidence of "a microbial role in preservation in the form of Ediacaran-type fossils preserved on carbonaceous sheets and sheet like intraclasts" (Narbonne 1998).
Though once disputed it is now evident that there was complex life in the Ediacaran. The discovery of Charnia in Leicester was a major turning point for this period of Earths history. It has allowed scientists to move further back in time to the very beginning of life on Earth. It has provided links to other countries including Australia and Newfoundland where the same organisms have been discovered. This shows that these multi-cellular beings were widespread across the globe and dating has shown how they were a thriving community for 20Ma.
Glaciation and an oxygen deficit are the reason that life was unable to evolve before the Ediacaran. However once the ice receded and oxygen levels began to rise due to photosynthesis, ideal conditions for life on earth were met. The first complex, multi-cellular organisms live in deep marine areas. They had a benthonic and sessile mode of life in which they were attached to the sea floor by a holdfast. These organisms feed through absorbsion of nutrience.
Preservation of soft bodied organisms is rare through Earths history and it is therefore surprising that such an abundance of Ediacaran life has been preserved. Microbial mats have allowed the shapes and details of these organisms to be conserved.
The organisms of the Ediacaran became extinct in the Cambrian due to either predication, a change in environment or competition by new biota.