Saturday, 31 July 2010

Sean Carroll and Ediacaran Reductionism

It is no secret that I am a fan of Sean B. Carroll, a prominent geneticist who often gets to voice his opinions on evolutionary theory. I mentioned him in a previous blog which was about the genetic changes involved in form and function, wherein Carroll's prediction was validated by the study. He is one of the main scientists at the forefront of evo-devo, investigating through genetics and embryology the relationship between genotype and phenotype. He often discusses the role of gene switches in the evolution of phenotypic change. His first book, Endless Forms Most Beautiful, looked at how gene expression produces form such as patterns and limbs and is one which I fully recommend to anyone wanting to get insight into this field. His second book, The Making of the Fittest, looks into the genetic evidence for evolution and also comes recommended, being better written than his first and containing some very interesting examples. I have not yet read his third book, so I can't comment on that one.

Recently, in the New York Times, Carroll published an article titled Translating Stories of Life Forms Etched in Stone  about the impact of the Ediacaran fossils. He chronicles the thought surrounding these fossils, starting with Darwin's dilemma of a lack of fossils before the Cambrian, through the discovery of the first Ediacaran fossils to modern views surrounding the fossils. There's some interesting information in the article, I particularly like the mention that Andrew Knoll likens the Ediacarans to a Rorschach test, as everyone interprets them differently. Reading through the article though and it seems Sean Carroll, who is not a palaeontologist (so I wonder why he wrote this article), favours the shoehorning interpretations of Gehling. The first clue can be seen in the image which accompanies the article:

This is a Pteridinium from Namibia, which looks bilateral in this image even though other fossils show that it is not and actually has a three lobed body. For example:
This choice of image puts the idea of bilateral forms in mind, the fossil doesn't actually look too unusual compared to some Ediacarans. Accompanying the article online is a slideshow with some incredible photos. The first is a small and beautiful Dickinsonia from the Ediacara Hills, chosen because it is mentioned in the article and maybe also because it looks superficially like many extant forms. Also from Australia is Spriggina, another which is iconic of the Ediacaran forms and is often thought to be related to modern forms (despite the difference in symmetry). Next is Archaeaspinus from the White Sea in Russia, a form which is often labelled as from the group proarticulata, a proposed phylum which also includes the two previous organisms. These are often linked with the chordates, an interpretation which would likely please Carroll.

Next in line on the slideshow is  Fractofusus from Mistaken Point in Newfoundland, a spindle-like fossil which would be difficult to shoehorn into modern groups. Kimberella comes next and was mentioned in the article, this one is from Russia. This fossil has often been considered to be a possible mollusc and potential radula marks have been found around the fossils. Following Kimberella is Charniodiscus, this one from Newfoundland and curiously the image is upside down. Charniodiscus is an interesting frond fossil which is sometimes linked to the sea pens. A favourite of mine follows, Parvancorina,  from Australia, a shield like fossil which can be tentatively linked to the trilobites. The final image is Yorgia from Russia and is often linked to the proarticulata. Anyway, back to the article.

The first hint of Sean Carroll's chosen interpretation comes subtly in the following sentence:

Dickinsonia, for example, has been interpreted as being a relative of jellyfish, a marine worm, a lichen, or even as a member of a completely extinct kingdom.

The concept of the Vendobionts is not an unpopular one as this phrasing might suggest. Carroll makes it sound like an extreme view. His view becomes more obvious soon enough, as he says, "But, in fact, such simple bodies are exactly what should be expected of primitive forerunners of later animals." He says this in relation to the lack of features which we use to identify extant phyla today and in that I agree with him, however, he must also ignore the features found in some Ediacaran forms which exclude them from known phyla. He also states that a few bilateral forms are known and gives Kimberella as an example, but it is not clear whether or not he takes into account that many display glide symmetry and not typical bilateral symmetry; Kimberella is one of the few which seems to actually be bilateral. 

Carroll then goes on to explain why these multicellular forms took so long to appear and does a good job, but then ends by saying, "Our earliest animal ancestor probably had no head, tail, or sexual organs, and lay immobile on the sea floor like a door mat." 

Overall the article is a good read and quite informative. It is odd that it is written by a geneticist and not a palaeontologist, but Carroll does not get things wrong, he simply puts his own slant on things, a slant which is shared by many palaeontologists. It would be interesting to see what a palaeontologist like Seilacher would think of the article. 


Anonymous said...

The constant drivel of glide symmetry keeps recurring in your threads it is an invention of Fedonkin and repeated adinfinitum by his acolytes -(such is the Russian system) and those wanting to dispose of the Ediacaran biota as an evolutionary sideline and not worthy of study -and hence a diversion of grant monies away from the Cambrian. Notice how the great detractors are Cambrian researchers?
I'll state the obvious using Dickinsonia as an example (back to symmetry again). Ediacaran fossils are found on the undersides of siliciclastic rocks , in negative or hyporelief, dickinsonia having segments that arc across the body, often interupted by a midline which in the fossil is a deeper groove. Most Dickinsonia fossils are bilaterally symmetrical, then there are a few that display "glide symmetry". Why? I would argue -on the basis of viewing hundreds of Dickinsonias that the glide symmetry is a taphonomic artifact, while the Russians and the new kids on the block argue the clearly bilateral fossils (the vast majority of fossils found)are distorted from glide to bilateral. Think about it a while -how is that possible?
An experiment: draw on a piece of paper a little dickinsonia with segments that arc across the body from one side to the other -perfectly bilateral, now fold it along the midline and fold paralell to this fold so that the midline is a deeper v shape just like the fossil.
Result -what do you see when viewed from above - a perfectly bilateral Dickinsonia right? now change your viewing angle because this is what we do with the fossils look from the the side at about 45 degrees from the midline and what do you see? Glide symmetry..Now the problem for the glide symmetry camp is the reverse does not work -try it. They also fail to explain why on all of the smaller Dickinsonias (presumably thinner also)that do not have a midline there is never an example of glide symmetry......

The Palaeobabbler said...

Thank you for your very interesting response, Anonymous. If you manage to see this, could you perhaps drop me an email at As this is a subject I am wanting to explore in depth and I am no longer using this blog (I only logged in pretty much to post about posting). As you posted anonymously I have no idea who you are or how to contact you.

Many thanks.