The Radio 4 program "In our Time" is running a program on "the Ediacaran Biota". I'm blogging this as initial thoughts right after the program. It can be heard here.
Oh dear. The photo that accompanies the show on the web site (shown here on the right) has a Dickinsonia costata, but it is upside down! The large segments are considered the head end. So this Dick is doing a head stand!
And it's bad start as they can't pronounce the name correctly. They are pronouncing it "Edi-aaa-car-raaan with long 'a' sounds. It's not pronounced like that. The term is an indigenous Australian word which is pronounced Edi-ak-ra, with the last 'a' pronounced like the last 'a' in Russia, and the middle 'a' not pronounced at all, or Edi-ak-ran with 'ran' pronounced as in "he ran away". All the 'a' sounds are harsh and short. The name means "reedy waterhole'" (Edi- means waterhole).
Ony one of the three guests has worked on Ediacaran fossils, and there are no Australians - they might have got at least one on the phone!
OK, there is a lot of talk about how the appearance of shelly fossils in the Cambrian is sudden, and that this was a problem for Darwin. This is misleading.
It has to be put in context.
The early mapping of what was recognised as the Cambrian rocks (from the name of the Latin name for Wales, where the section was mapped) and became the "Type Section" (the reference section agains which all other sections of the same age around the world are compared), did show that there was a rapid transition from 'barren' 'pre-Cambrian rocks to fossiliferous Cambrian rocks, replete with trilobites, sea shells and other relatively complex organisms.
This rapid transition is what they are talking about, and was the one familiar to Darwin - he actually traversed these Cambrian rocks with the Reverent Adam Sedgewick (who named the Cambrian Period and who was well aware that this represented the earliest evidence of life in the fossil record).
The important point here is that, yes fairly complex fossils appear quite abruptly in this rock section, BUT, the section is incomplete. Basically the section is missing a good deal of the earliest Cambrian rocks. In other words the basal rocks containing the emergence of the Cambrian biota are missing from this area. It's like starting a book at chapter 3 - the introductory chapters have been ripped out at this place.
Rock sections in other places around the world which contain the earliest Cambrian rocks, show a transition from trace fossils, to complex trace fossils to small shelly fossils which comprise bits of the armour of larger organisms, to full body fossils.
This transition has a lot to do with the acquisition of hard parts by organisms - by the process of biomineralisation, where calcium carbonate and calcium phosphate are incorporated into the outer tissues of organisms to produce hard shells (e.g. crabs do this after they molt).
So yes, Darwin conceded that such a rapid transition to complex fossils was a difficulty for his theory, but we now know that the rapid transition he was referring to is an artifact of an incomplete rock record in that area.
There are several references to the Ediacaran biota appearing right after the last major 'snowball Earth' glaciation in the pre-Cambrian. This is incorrect.
The last 'snowball Earth' glaciation ended about 650 million years ago, the earlest Ediacaran biotas appear some 50 million years after that and, in fact, there is some evidence for intervening glacial episodes - albeit not as extensive as the 'snowball earth' ones.
The answer to the question, "Is the Ediacaran biota a failed experiment?" was answered pretty well. Short answer - no.
Longer answer - they lasted for some 40 million years but the bulk of them were done in by a changing environment which took away the conditions required for their preservation as fossils, and the rise of predation (with the rise of mineralisation of tissues allowing jaw elements to be carbonate tipped). However a few groups survived to pass on their genetic legacy to future groups.
Summary, not a bad show all in all. Recommended if you are interested in the Ediacarans.