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Introduction to the Ediacaran Fauna


The Ediacaran fauna (pronounced edi-akran) is a Precambrian(Neoproterozoic) assemblage, which existed from about 600 million years ago to approx 545 million year [Note: the new timescale for the Cambrian has the Period dominated by the Lower Cambrian and the whole timespan has been reduced to approx. 40 million years; 545 - 505 million years ago. Hence the Cambrian-Precambrian boundary is now at approx. 545 mya] The fauna has now been found on all continents except Antarctica. However, the most important sites are; Namibia, Newfoundland & MacKenzie Mountains Canada, the White Sea Coast Russia and the Flinders Ranges, South Australia. One of the best localities and the place where the significance of the fauna was first recognised is here in South Australia. The name Ediacara comes from the site where the fossils were first found here. Fossils were found in Namibia about 25 years ealier, but due to a mistake over the age and the fact that the finds were published in foreign language journals (German) their significance was not realized at the time. Whilst the fauna has a world-wide distribution, it is important to note that there are significant differences in the make-up of the fauna at different localities. This is due, in the main, to environmental conditions. The Australian and Russian forms are similar and the rocks are indicative of a shallow water ecosystem. The Newfoundland forms are decidedly different and the rocks are indicative of a deep water setting. In South Australia, the fossils occur as depressions up into or extentions down from the bottom of thin quartzite beds. The fossils were formed by the covering of the muddy shallow sea floor and the organisms on it by mantling, thin sand bodies. Those organisms which where able to support the sand created depressions up into the overlying sand body (Fig 1).

Fig. 1

Beast _
       |            --------------         --------------
____(^^^^)____  ->  ____(^^^^)____SAND ->  ____(^^^^)____SANDSTONE
      MUD                 MUD                     |
______________      ______________         Fossil_|

Those organisms which were either lying in or were forced into the mud by the sand, allowed sand to fill in the void left as they decayed (to produce downward extentions on the bottom of the sand body) (Fig 2)

Fig 2

Beast _ | -------------- -------------- _____ | ___ -> ____ ____SAND -> ____ ____SANDSTONE MUD (____) MUD (____) (____) _____________ ______________ | Fossil-

The majority of fossils are of rounded forms, reminiscent of jellyfish and in fact these were classified as jellyfish for a long time eg. Cyclomedusa, Mawsonites. (up to 35 cm across) Other forms included occasional 'sea-pen-like organisms (colonial octocorals) which appear very similar to forms extant today (up to half a metre tall). A couple of possible annelids such as the large sheet-like form Dickinsonia which looks like a flat pancake with segmentation, a gut and a definate head end (up to 75 cm long); and Spriggina which looks like a cross between a bristle worm and a trilobite (5 cm). A possible arthropod is also present, Parvancorina a recent specimen of which shows gills and possibly legs (3 cm). A new Phylum appears to be represented by Tribrachidium, which as it's name suggests is based on a tripartate body plan, but may well be some form of lophophore (similar to brachiopods and bryozoa)(3 cm). Until recently it was thought that the fauma was dominated by the motile, free-swiming medusoids, which created a problem of preservation since medusoids do not, as a general rule spend a lot of time on the sea bed in the adult form. However recent work has shown that, whilst medusoid forms are represented, the vast majority of rounded forms are the anchors of sea-pens. Modern sea-pens have a round, bulbous structure near the base of the organism which is highly muscled. The organism uses this 'organ' to burrow into the soft muddy sediment and then as an anchor to hold the organism in place. During burial by the mantling sands, the stem of the 'sea-pen breaks off and the body of the 'sea-pen'. Since the 'sea-pen is held up by hydrostatic pressure, the rip deflates the 'blade', the 'blade' becomes mixed with the sand, thus diminishing its preservation potential. The bulb, on the other hand, is already buried. In life the bulb is filled with water, so when the stem breaks away, the bulb fills with sand (Fig 3).

Fig 3

/^\ // \\ / | | \ / | | \ ( | | ) ( | | ) \ | | / \ | | / \ | | / (--) \| |/ / | | / | | --> -------/ / ------------- | | SAND : : : SAND _____| |_____ _____ : _____ _____ _____ | | |:| * * * * * * : * _____________ * * * : : * * * * : * /|\ /|\ MUD * MUD * | | _____________ _____________ | |

Since the underlying mud is approx. 80% water, as it dries out the thickness of the bed diminished to only a few cms, resulting in a flattened, rounded outline to the fossil. The various classifications on the 'medusoids' was due to surface ornamentation (ribs, concentric circles etc.), these are now thought to be the manifestation of muscle bands due to different degrees of decay before final 'molding'. Thus the fauna has a decidedly benthonic bias, rather than being made up of free swiming forms as previously thought. There are two main theories as to the affinities of the Ediacara fauna. One, put forward by Martin Glaessner is that most of the forms are related to modern forms, if not direct precursers. The other, proposed by Dolf Seilacher is that the Ediacaran fauna represents a unique bodyplan which arose early in metazoan evolution and became extinct before the Cambrian and thus all the forms within the fauna are members of a now extinct, separate phylum - the Vendozoa, with no connection to modern forms- or even Cambrian forms. This later idea holds particular prominance in the US (eg. Stephen J Gould) because (I think) it fits very neatly into the Punctuated Equilibrium model. whereby you have a rapid evolutionary event followed by an extinction event, then another evolutionary event (the Lower Cambrian). However, close examination of the fossils shows that many of the forms do indeed have a striking resemblance to Cambrian if not modern forms. Recent finds of 'sea-pen'-like organisms in the Burgess Shale, which are very similar to Ediacaran forms appears to extend the range of such forms well into the Cambrian. The form Kimberella can be placed with confidence within the Class Cubozoa (box jellyfish) Likewise the form Chondroplon can be placed in the Suborder Chondrophorina. The form Arkarua can be placed in the Class Edioasteroidia. Thus several groups within the Ediacaran fauna exist today and so the whole fauna did not becone extinct. This is not to say that there are not some unique forms, there are, but the idea that they are all unique is oversteping things. My own opinion is that several groups of extant organisma can be traced back to the Ediacaran fauna. However, the origin of the metazoans is another matter. The Ediacaran fauna appears as a fully intergrated ecosystem with some quite advanced forms (eg. the colonial octocoral 'sea-pens'), so the question of origins has to be pushed back even farther, probably IMHO to the late Proterozoic glaciation approx 900 mya. And IMHO body fossil evidence will never be found, since they occur in meiofauna - too small to leave anything but chemical traces. BTW the Newfoundland fauna are found in tubidites, covered by ash fallout, settling through water. Such forms apparently existed in deep water and were almost certainly heterotrophic, which might not mean much to most people, but is another nail in the Vedozoa hypothesis.

The various elements of the Ediacara fauna are united by one common character, none have any hard parts. There is no evidence of mineralisation in any fossil so far found. Thus the preservation of essentially 'soft bodied' organisms presented something of a quandry, especially as they are preserved in what is now quartzite. It was thought that fossilization was due to a unique sedimentological facies, namely the ripple-topped sands mantling muds and that the fossils were constrained by the occurrance of this facies (eg. Mount 1989) (see Figs 1 & 2). However, the Ediacara Member in the Flinders Ranges contains 5 separate facies, ranging from thinly laminated silts to high energy, coarse sandstones, each of these facies if fossiliferous to one degree or another. Thus fossilization is not facies controlled, but occurs due to the interplay of a number of factors (a common occurrance compared with a simple 'cause and effect' answer - a fact which makes science interesting). Amongst the factors which allowed the preservation of the Ediacara fauna are (in no particular order): Collagen bioturbation - the lack of predation - the lack of The ability to produce collagen is important because collagen is relatively inert, strong and flexable. A collagen outer layer helped hold the organism together. It also allowed the organism to retain it's shape when covered by the mantling sand to produce the fossils. Also, since collagen was a relatively new compound (it's synthesis was probably related to the crossing of a threshold level of oxygen in the atmosphere), the micro- community took a while to realize that collagen was a food source - see predation below. The lower level of oxygen in the atmosphere (compared with present levels), plus the absence of suitable bodyplan made vertical burrowing virtually unknown during this period. The lower oxygen levels meant that bodies had to be kept small or thin - so because oxygen was adsorbed through the surface of the organism (no lungs or gills - or more importantly a method of delivering oxygen to the tissues from such organs [i.e. blood]). Therefore, tissues had to be close to the surface in order to obtain oxygen by simple diffusion. This meant thin bodies. There is very little constraint to the size such organisms could reach, provided they stayed thin, hence half metre long 'flat' worms. Flat, thin bodies are very bad at burrowing, so no vertical burrows, which meant that any organism which was buried was not disturbed, disrupted and ultimately distroyed by bioturbation (as it common today). Since there were no hard parts about, predation was well nigh impossible, except possibly by disgorging some sort of dissolving fluid and sucking up the resultant gastronomic soup. But, definately no chewing! Therefore, once the organism shuffled off to join the ranks of the choir immortal, it's mortal remains did just that - remained. They hung around on the surface, undisturbed for a considerable period of time, waiting for the mantling blanket of sand. As was mentioned before, collagen was probably still a relatively novel compound at this time, so it was resistant to decay - i.e. large numbers of collagen munching bacteria had yet to make an appearence. However, these conditions did not last. An interrelated series of events which included, the rise in oxygen levels, the aquisition of mineralization capabilities, the rise of predation and the ability to produce a round cross-sectional bodyplay (oxygen depended) condusive to burrowing, soon demolished what was a pristene preservational environment. The so called extinction of the Ediacara fauna is IMHO largely illusionary for several reasons:

    1. Of 7 cnidarian divisions represented in the Ediacaran fauna, 4 appear to be anscestral to living taxa.
    2. There is no close time control in respect of the supposed episode of extinction.
    3. The disappearence of the fauna is largely due to the closure of a taphonomic or preservational 'window'.
    4. The uppermost facies of this period throughout the world indicate a shallowing upward cycle, resulting in environments likely not condusive to preservation.


Ediacaran Rogues' Gallery


Cyclomedusa
Dickinsonia
Tribrachidium
Mawsonites
Spriggina
Parvancorina



References

Jenkins, R. J. F. (1989) The 'supposed' terminal Percambrian extinction event' in relation to the Cnidaria. Memoir of the Association of Australasian Palaeontologists, 8: 307-317.

Mount, J. F. (1989) Re-evaluation of unconformities separating the "Ediacaran" and Cambrian Systems, South Australia. Palaios, 4: 366-373.

Related References

Jenkins, R. J. F. (1992) Functional and ecological aspects of Ediacaran Assemblages. In: Origin and Early Evolution of the Metazoa. J.H. Lipps and P.W. Signor.131-177. Plenum Press, New York.

McMenamin, M. A. S. & McMenamin, D. L. S. (1990) The Emergence of Animals: The Cambrian Breakthrough. Columbia University Press, New York.

Seilacher, A. (1989) Vendozoa: organismic construction in the Proterozoic biosphere. Lethaia, 22: 229-239.

Seilacher, A. (1992) Vendobionta and Psammocorallia: lost construction of Precambrian evolution. Journal of the Geological Society of London, 149: 607-613.