Post of the Month: December 1997
by Chris Nedin
In article <email@example.com>, firstname.lastname@example.orgNOSPAM (Mark Isaak) says...
>In article <email@example.com>, >Chris Nedin <firstname.lastname@example.org> wrote: >>In Ben's case, as in mine, no metazoan fossils are found as *organic* >>remains (which was the point of the query). In almost all cases, metazoan >>fossils of Ediacaran age are of the impressions of organisms (usually in >>sandstone). There is no organic component/residue. > >Actually, I really wanted to learn just how good the pre-Ediacaran >fossil record is, as compared with how good we would expect based on the >preservation potential of the deposits we've found.
Oh, right. The PRE-Ediacaran record consists basically of single celled organisms or aggregates thereof, i.e. algae-like and bacteria-like. We do find very similar rock lithologies below Ediacaran fossil-bearing rocks, indicative of similar environments, but, with one or two possible exceptions, there are no obviously metazoan fossils. In South Australia, the first Ediacaran fossils occur in eroded channels, suggesting a period of erosion and lower sea level prior to the deposition of fossil-bearing sediments. However, it is becoming clear that the Ediacara fauna can be divided into separate assemblages, with the most diverse and disparate (variation in bodyplan) occurring at the highest (youngest) levels. So while the Ediacaran fauna tends to emerge into the taphonomic spotlight with very little fanfare, the earliest appearances have relatively low disparity.
>I have heard >suggested several times that the Cambrian explosion might be an artifact >of fossil preservation, and I was wondering how much data we have to test >that hypothesis.
Well one explosion is. There are two aspects to Early Cambrian evolution which although separate are often confused as one and the same. One is the rapid diversification of life during the earliest Cambrian, and the other is the rapid appearance of organisms in the fossil record. Neither event was instantaneous.
The rapid diversification of life in the earliest Cambrian was almost certainly derived from a pre-existing stem stock of organisms, representatives of which appear in the Ediacaran fauna. The Ediacaran fauna appears to contain organisms with a cnidarian-grade of organisation, as well as the more derived, bilaterally symmetric, triploblasic annelid-grade, arthropod-grade and probably mollusc-grade. Trace fossils associated with, but not necessarily formed by, Ediacaran organisms clearly indicate organisms with a coelomic-grade of organisation (triploblasts). These are much smaller than the more commonly found fossils and occur in higher (younger) strata.
All this indicates that Ediacaran organisms appear to have taken two strategic pathways during the late Neoproterozoic. One group (composed of representatives of several -grades) opted for passive acquisition of oxygen via the 'skin' and simple diffusion through the tissues. This allowed them to grow to very large sizes (c. 1 metre) provided they remained very thin (oxygen will not diffuse very far through tissues). The other group, comprised of the triploblasts, opted for oxygen delivery via oxygenated fluids (from oxygen acquired through the 'skin'). This strategy resulted in the ability to have a body plan with a circular cross section (the deeper tissues being supplied via fluids and not simple diffusion) and thus a coelom. With a coelom, centimetric organisms can be mobile and produce simple traces, seen as trace fossils in the rocks. Ediacaran trace fossils are always horizontal - i.e. they do not burrow, probably because 1) there was plenty of organic matter at or near the surface; 2) burrowing covered up the 'skin' cutting down the supply of oxygen.
It seems likely that new biochemical pathways were also being explored, resulting in the production of the metazoan stalwart - collagen. A real tough customer, providing support and strength with flexibility, and difficult to break down.
Since all the Ediacaran organisms so far discovered appear to lack any hard parts it is likely that there were no mobile megascopic predators around (no hard parts = no teeth, its very difficult to gum something to death!). Thus, the presence of possible novel biocompounds, the lack of mobile predators and the lack of burrowers, all combined to provide a taphonomic 'window' which allowed the preservation of soft-bodied organisms. By the start of the Cambrian, this system was breaking down.
It is likely that while the big guns were simply getting bigger, the little guns were getting more complex. The triploblasts were beginning to toy with differentiation (something the arthropod body plan is particularly good at), specifically concentration the job of oxygen scavenging to specific body parts (= gills).
Now, by the very end of the Late Neoproterozoic, two things happened. Probably a combination of increasing dissolved oxygen levels in the oceans, but also probably due to the recognition of gills as a smart move. Whatever the reason(s), organisms began to burrow.
Burrowing is right up there in the list of brilliant tactical manoeuvers. It confers protection, both from dirtyfilthystinkingrottencarbonateprecititators (see below) and from storm activity and the odd vicious undertow. As an added bonus, it allows access to buried food sources which are denied the third-dimensionally challenged suckers confined the the top of the sediment. The possession of gills is a big advantage when burrowing because you can either hang them out in the current while keeping the rest of the body safely in the burrow, or by waving them back and forth, you can create a current which brings oxygenated water into the burrow.
The other thing that happened, was that organisms were toying with the precipitation of calcium carbonate (calcite). This first appears in the Latest Neoproterozoic as the calcified inner lining of worm tubes (a trace fossil called Cloudina). Granted it may well have only been a worm with a flare for interior design, but it was an important first step. From there it is only a short peristaltic motion to the mineralization of the tips of such body parts as legs and jaws. After all it makes burrowing much easier, plus it has the added bonus of allowing access to all that concentrated protein wrapped up in collagen! Predation probably took off faster than you can rub two mineralised jaw elements together.
Thus, with predation mopping soft-bodied organisms off the surface, and burrowing destroying buried remains, the Ediacaran taphonomic window was shut. The 'big is beautiful' organisms inevitably lost out. After all, you may be the biggest redwood in the forest, but you are always going to lose out to the wimp with a chainsaw!
Once the constraints of oxygen scavenging had been confined to a specific part of the body, the body plan in general becomes a good deal more plastic (loose a limb here, fuse some segments there). Thus the seeds of the great Cambrian Diversification were sowed. Fertilized with a rising sea level to open up new living space, organisms rapidly, but not instantly, diversified.
The appearance of organisms in the fossil record is a related issue, resulting from the mineralisation, and hence vastly increased preservation potential, of organisms. However, this too was not an instantaneous event.
At the start of the Cambrian, we find the first evidence of mineralised tissues in the form of what appear to be annelid jaw elements and "small shelly fossils" which are the separate elements of interlocking body armour worn by annelids, molluscs, halkiirids and probably arthropods. This meshwork armour was a first attempt and was composed of separate elements rather than a continuous sheet as it would be later. All this probably came about because of the segregation of oxygen scavenging to a particular part of the body, freeing up the rest of the body and enabling THE fashion accessory of the Early Cambrian to be worn - the calcium carbonate overcoat - without feat of suffocation. The overcoat provided extra support for muscles, allowing better movement, and some protection from predators. It also greatly enhanced the preservation potential of the organism or more importantly wrt arthropods, moults of the organism (thus one trilobite can leave behind several images of itself as it grows, with the added bonus of documenting the growth pattern at the same time - trilobites are cool, they are the best thing on 24 legs!).
Thus the rapid diversification of life during the Early Cambrian and the appearance of organisms in the fossil record are related, but separate, phenomenon. Neither are "sudden" or "instantaneous", but show a sequential, progressive increase.
Article originally posted December 10, 1997
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