Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion

The Cambrian explosion or Cambrian radiation is the term used for a very rapid appearance in the fossil record of the early representatives of all the later groups of complex animals, plus some that dropped out early in evolution. According to the fossils record, it seems to have began about 530 million years ago. For a long time it seemed like a "Big Bang" of evolution. The major groups of animals suddenly appeared in the rocks after about 530 million years ago. Then the Ediacaran Fauna was discovered in the Ediacara Hills, part of the Flinders Ranges of South Australia. The Ediacaran fauna was from an earlier stage in evolution than those that previously seemed to appear suddenly in the rock strata of the Cambrian, such as the 530 million year old Burgess Shale. The Ediacaran biota show a less specialized level of structure than the Burgess Shale organisms, but displayed a level of organisation that must have had an evolutionary history going back some way. At this stage they lacked hard parts, so required very special conditions for them to be fossilised, hence the rarity of fossils of this stage of evolution.

According to Fedonkin et al. (2007), about 542 Ma many different organisms acquired hard parts (Knoll & Carroll, 1999), mostly of calcium phosphate (Cook & Shergold, 1984). Over a time span of between about 10 and 20 million years, beginning at about 542 Ma, biodiversity of metazoans increased rapidly, at the same time as the there was widespread development of hard parts in the fauna. There is a very large amount of literature covering this large increase of biodiversity (Zhuravlev & Riding, 2001; Schopf & Klein, 1992; Budd & Jensen, 2000; Shu et al., 1999; Chen et al., 1997).

There is much dispute concerning the reality of this apparent explosion of biodiversity, some considering the possibility the sudden increase in diversity may have resulted from a higher proportion of organisms being fossilised once the possession of hard parts had become widespread. The only point all would agree on is that the soft bodied condition of the Ediacaran Fauna was in the past, from now on hard parts were present in most forms up to the present.

Causes of the explosion

This apparent sudden large increase in diversity of metazoans has spawned a proliferation of theories to explain it.

  1. Increased oxygen levels. According to this theory organic carbon was sequestered as a result of increasing amounts of oxygen in the oceans (Kirschvink & Raub, 2003). This theory has many supporters.
  2. Lowering of salinity in the oceans (Kanuth, 2004; Vickers-Rich, 2006).
  3. The rising to the ocean surface of bottom water that was unoxygenated and rich in phosphates as a result of turnover of ocean water (Cook & Shergold, 1984).
  4. The uplift of the Transgonwanan Mountain Range. Vast quantities of continental sediments containing high levels of clay minerals, calcium and nutrients. The very large area of weathered sediments were exposed to the forces of erosion by the rising of the land, depositing the eroded material on the large expanse of the continental shelves that resulted from marine transgressions in the Cambrian (Squire et al., 2006; Kennedy et al., 2006).
  5. The vast areas of these continental shelves that formed as a result of the marine transgressions greatly increased the area providing new habitats to be exploited by biota diversifying to fill the newly available niches (Brasier & Lindsay, 2001)?
  6. Plate tectonic activity. Movement of the tectonic plates possibly brought to the surface sediments rich in carbon and methane that had previously been buried. These materials were released as the moving plates carried them to the tropics where a number of periods of greenhouse conditions were triggered by their release (Kirschvink & Raub, 2003).
  7. Increasing cephalisation and development of sensors, such as eyes. At this time cephalisation took place in many organisms in which the senses, such as photoreceptors and chemoreceptors where being concentrated into sensor organs such as eyes at one end of the body, the head. This allowed better detection of predators as well as prey (Fedonkin et al., 2007)
  8. Development of hard parts. Being able to produce hard parts allowed the animals to be better protected from predators while allowing for more efficient movement as the hard parts also provided attachment sites for more powerful muscles, which in turn allowed the exploitation of new environments as in burrowing. It would also have started an increase in mobility and muscular control, leading to more efficient predators as well as increasing the defences against such predators (Fedonkin et al., 207).

It is believed the Ediacaran Fauna was an earlier stage of organisation that evolved into the Burgess Shale Fauna of the Cambrian., occurring in the 3 known sites, Chengjiang, Southern China, from the Early Cambrian, Emu Bay, Kangaroo Island, South Australia, slightly younger but also from the Early Cambrian, and the Burgess Shale, the youngest, from the Middle Cambrian (Chen et al., 2004).

A number of taxa from the Ediacaran Fauna are believed to be related to taxa present in the Earliest Cambrian (Jensen, Gehling & Droser, 1998). An example is Parvancorina from the Ediacaran Fauna that is a possible early arthropod (Lin et al., 2006). Most Ediacaran forms are believed to have died out, leaving their niches available for the Burgess Shale Fauna to diversify into, the new forms occupying the vacated niches, these forms eventually giving rise to the modern fauna. The demise of the Ediacaran Fauna has been compared to the extinction of the dinosaurs at the close of the Cretaceous, setting the stage for the explosion of diversity that followed. According to this suggestion, most Ediacaran forms went extinct and the survivors rapidly diversified to fill the vacated niches, as did the mammals and birds following the end of the dinosaurs (Knoll & Carroll. 1999).

Conservative interpretation of the molecular evidence indicates a long period of development of body plans leading to the stage when the organisms actually appear in the fossil record. This evidence indicates that diversification of bilateria was underway by 1000-900 Ma (Erwin, 1999; Droser, Gehling, 2002; Hedges et al., 2004; Blair & Hedges, 2005). The Precambrian development of some modern body plans prior to their appearance in the fossil record is supported by some discoveries such as Kimberella (Fedonkin & Wagoner, 1997) and Vendoconularia (Ivantsov  Fedonkin, 2002), that are possibly related to taxa from the Phanerozoic. It is believed diploblastic animals (2 embryonic germ layers) were present by 610-600 Ma and triploblastic animals (3 embryonic germ layers) possibly by 570 Ma. Definite deuterostomes were present by the earliest Cambrian (Erwin, 1999). Finds in Russia have cast doubt on this, though much of this evidence would be doubted by others (Budd & Jensen, 2000), who consider the Cambrian Explosive diversification to have occurred over a relatively short period to time. 

A group of fossils are known from the earliest part of the Cambrian, collectively called Small Shelly Fossils (SSF's), a collection of forms that were spicular, tubular and conical. Some of these small forms are thought to possibly be related to later forms such as some groups of molluscs. Among the SSF's are some forms that have sclerites that display strong similarity to the pyrite armour plating in the "foot" of some gastropods that live around oceanic vents at the present (Waren et al., 2003). Some members of the SSF display a degree of similarity to forms from the Palaeozoic and even more modern forms, though other members of the SSF don't appear to be related to any later forms.

The most rapid diversification of body plans is believed to have occurred between about 540 and 530 million years ago (Erwin, 1999). A fossil site where this can be clearly seen is the Chengjiang Fauna of southern China from 530 Ma. In this assemblage Haikouella (Chen, Huang & Li, 1999) is thought to possibly be a vertebrate. A question of this time is whether there was a surge in new basic metazoan architecture, or was the surge restricted to the acquisition of hard parts by many different groups.

By 555 Ma there were tracks and trails, microfossils and large body fossils in the fossil deposits (Droser, Jensen & Gehling, 2002). The earliest among these were simple, unbranched traces less than a few mm across. They were oriented horizontally close to the interface between the sediment and the water. Among these traces were some that have been associated with animals that are known to be bilateral, being found as late as the Early Cambrian. Deeper burrows and bioturbated sediments, some distance below the water-sediment interface, first appear in deposits from the Middle Cambrian. It is not known why animals began burrowing at this time, possibly being a means of escaping predators or higher oxygen levels in the sediment. Other suggestions are competition for resources on the sediment surface or the water column, or the demise of widespread microbial mats, and many others (Fedonkin et al., 2007).

According to George Williams, a pennatulacean specialist (sea pens), "the problematic nature of interpreting the nature of the Ediacaran fossils is in part due to the specialised state of science in which workers in different fields concerned with similar issues are unaware of each others' work and findings. It would be more productive if neontologists (i.e., systematists) and Precambrian palaeontologists worked more closely together to decipher the affinities of the organisms in the Ediacaran and Burgess Shale faunas" (Williams, 1995). Fedonkin et al. have added "or for that matter the nature of Neoproterozoic and Early Cambrian trace fossil markers". 

The Ediacaran-Cambrian Metazoan Diversification - Ecological Drivers

Cambrian Explosion

The Late Neoproterozoic and Early Cambrian was the time period when the most dramatic changes in environmental conditions occurred during the past 2 Gy. Levels of atmospheric oxygen were much lower in the Late Neoproterozoic than the present and the deep oceans were oxygen-poor and iron-rich, according to the author1 possibly similar to the deepest parts of the Black Sea of the present. Evidence of the increasing oxidation of the oceans during the Ediacaran is provided by a number of geological proxies, such as isotopes of carbon and sulphur, as well as other geochemical proxies. The exact extent of this oxygenation and whether or not it reflects processes that are purely geological, or possibly facilitated by biological innovations, is still being debated by geologists. Tectonic reorganisation resulting from the breakup of Rodinia about, 750 Ma, overlap with these geochemical changes. Several glaciations were very extensive and puzzling geological features accompanied them, a particular feature is the formation of glaciers at sea level in the tropics, so puzzling that they still challenge the ingenuity of geologists to explain them. An environmental revolution had been generated by those changes by the Early Cambrian, the oceans becoming oxygenated to depths of more than several hundred metres, at least. Ferruginous (iron-rich) waters had become only regional phenomena as they had retreated, and the carbon cycle began to settle into a pattern that was more similar to that of the Phanerozoic, having previously been characterised by large fluctuations.

Sources & Further reading

  1. Mary E. White, The Nature of Hidden Worlds, Reed, 1993
  2. Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
  3. Mikhail A. Fedonkin, James G. Gehling, Kathleen Grey, Guy M. Narbonne, Patricia Vickers-Rich, The Rise of Animals, Evolution and Diversification of the Kingdom Animalia, Johns Hopkins University Press, Baltimore, 2007.

Links

  1. Weird Wonders Lived Past the Cambrian
  2. Fossils reveal rapid evolution in ancient eyes
  3. Modern optics in exceptionally preserved eyes of Early Cambrian arthropods from Australia

 

 

    
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 09/07/2013

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading