Monday, 24 January 2011

Charnia - About Time!

One of the most important fossils ever discovered, especially when you are interested in the Ediacaran period as I am, is Charnia masoni, yet I have never blogged about it. I barely ever mention this iconic find. On top of its importance it can be found in Britain and I have looked at casts of it, yet still no mention. Until now...

The original Charnia masoni fossil. 
The discovery of Charnia is a very well known one within palaeontology. In the past year it has been discussed on Bang Goes the Theory by the lovely Liz Bonnin, and more recently by Sir David Attenborough in his First Life documentary. The absence of fossils before the Cambrian had baffled Darwin, causing him much embarrassment which creationists sadly still think is a problem. In the 1950s fossils were found in Australia, but due to circular reasoning were not recognised internationally as they should have been. As Precambrian rocks were devoid of fossils, so the logic went, the fossiliferous rocks must have been Cambrian. It wasn't until a budding young geologist in Leicestershire's Charnwood Forest discovered a frondose fossil in unequivocally Precambrian rocks.

Modern sea pens.
Like all Ediacaran fossils good old Charnia is difficult to classify and its life habits have remained mysterious. It was first classified as algae (in the journal of the Yorkshire Geological Society believe it or not) before being reinterpreted as a sea pen (pennatulacean cnidarians), an interpretation which stuck for quite some time and resulted in many Ediacaran forms being shoe-horned into modern phyla.

This interpretation was first challenged when Dolf Seilacher presented his Vendozoa classification, suggesting that the Ediacaran forms were an evolutionary experiment in multicellularity which left no descendants. The nature of what are now referred to as Vendobionts has changed a lot since Seilacher's ingenious proposal. Originally they were a separate, metacellular, kingdom, before later becoming an extinct phylum which diverged before true animals evolved. Eventually Seilacher settled on the idea that Vendobionts were large, quilted protists.

Seilacher's view of a separate phylum is widely accepted, though his classification of the Ediacaran Vendobionts as protists is less well accepted. Charnia is classified as a Rangeomorph - a taxon containing the frondose fossils of the Ediacaran, believed to potentially be a monophyletic clade (meaning they are not from separate groups but have similar morphologies because they are closely related) morphospace data appears to bear this out.

Charnia's connection to the sea pens was not completely thrown out, particularly because the classification of Charnia is hugely important. If it can be connected to modern forms then the Garden of Ediacara was not so bizarre after all, perhaps evolution is actually quite predictable. If, however, it was a failed experiment, then the Ediacaran remains ridiculously enigmatic, positively alien to us. The connection to sea pens was made through a Cambrian frond-like fossil called Thaumaptilon, which I elucidated here. At the time I did not want to comment on a potential relationship with Charnia, but now I know that they were unlikely to have been related. One simple reason I will give is that Charnia has no stalk running down the centre, yet Thaumaptilon does, though a much better case can be made. Charnia's potential connection to the sea pens took its heaviest blow when it was shown that it grew in a very different way to sea pens. Ontogeny is important in connecting disparate fossil forms as it can be inferred from fossil data and is often quite evolutionarily rigid. Sea pens grow by adding extra polyps to the bottom, whereas Charnia did the opposite and added to the tip.

In working out the evolutionary relationships of many Ediacaran forms, particularly rangeomorphs, it seems that working out their life cycle might be the key. Many have overlapping morphologies which may be different growth stages. With heterochronic evolution changes in developmental timing produces different adult-stage morphologies, allowing a juvenile form to reach sexual maturity for example. These overlapping forms may also be different variations of the same organism, but from different environmental conditions (ecophenotypic variation) much like the variation seen in people from different continents. Bradgatia is one such form, as it is like a bush of Charnia. Many fossils are spindle forms (e.g. Fractofusus) which resemble two Charnia stuck end to end, branching outwards. All have fractal branching frondlets.
Bradgatia from Charnwood Forest. Photograph by Tina Negus. 
How it lived is perhaps the biggest mystery. It is believed to have lived quite deep, so photosynthesis seems to have been out of the question (whether it was an autotroph or used symbionts). Filter feeding is a possibility, but it seems to have evolved ways of folding which were inefficient. We may never know how it lived and fed.

There are many other rangeomorph fossils which resemble Charnia. Unsurprisingly Charniodiscus is one such fossil, also found in Charnwood Forest, a cast of which is pictured to the right, upside down, with myself using it for a rather lewd pose. This specimen is small compared to the 2m long Charnia found in Newfoundland.

The mysterious ivesheadiomorphs, or "pizza-discs", have recently been interpreted as the decayed remains (taphomorphs) of Charnia and related taxa.

An ivesheadiomorph.
Now all I need to do is see Charnia up close. I intend to visit Charnwood Forest some time over the summer if possible, though the Charnia found in Newfoundland, Canada, do greatly appeal.

Antcliffe, J.B.; Brasier, M.D. (2007a). Charnia and sea pens are poles apart. Journal of Geological Society 164 (1): 49.

Antcliffe, J.B.; Brasier, M.D (2007b). Towards a morphospace for the Ediacara biota. 377–386. In VICKERS-RICH, P. and KOMAROWER, P. (eds). The rise and fall of the Ediacaran biota. Geological Society of London Special Publication 286, London, 456 pp

Brasier, M.D. & Antcliffe, J.B. 2004. Decoding the Ediacaran enigma. Science, 305, 1115–1117.

Ford, T.E. 1958. Precambrian fossils from Charnwood Forest. Proceedings of the Yorkshire Geological Society, 31, 211–217.

Liu, A. G.; McIlroy, D.; Antcliffe, J. B.; Brasier, M. D. (2011). Effaced preservation in the Ediacara biota and its implications for the early macrofossil record. Palaeontology: In press.

Seilacher, A. (2007) The nature of vendobionts. 387-397. In VICKERS-RICH, P. and KOMAROWER, P. (eds). The rise and fall of the Ediacaran biota. Geological Society of London Special Publication 286, London, 456 pp

Shu, D.-G., Conway Morris, S. & Han, J. et al. 2006. Lower Cambrian vendobionts from China and early diploblast evolution. Science, 312, 731–734


tina negus said...

I see you have used my photo of Bradgatia from my flickr stream, in your blog. Did you request permission to use this? You have not credited me for it as far as I can see, neither have you mentioned my part, belatedly recognised in Charnia's discovery in Charnwood Forest.

The Palaeobabbler said...

Would you like me to remove the image?

The blog was not about the discovery, which I largely skipped over. The point of mentioning the discovery was to show the importance of the fossil in changing thought, not to shed light on how it was discovered. Nobody was mentioned by name in that part of the blog; it would have been a superfluous detail.

tina negus said...

I would have given you permission - if you had asked for it.

So I wuill allow you to use it, but would like credit given, please, if that is ok with you. The other remark was due to being peeved that you had used the pic.....

I DID like the article btw.....

The Palaeobabbler said...

My apologies, since Google changed the way they show the pictures I don't always see where the picture is from (I actually thought Flickr prevented this). I'll edit the blog now.

After you commented I checked out your Flickr stream, there are some beautiful images on there.

Thank you for the comment, your name popped up a fair bit when I was researching Charnia. Your own pictures of it are excellent.

tina negus said...

Thanks for the credit. You probably havve seen that I have pics of "holdfasts" and Cyclomedusa on my stream, and will be adding some others.

thankyou for your appreciation of my photos!