Apologies for the recent silence… we’ve been trying to finish off all sorts of things before the end of the year, and are flying back to the UK tomorrow for a “holiday”. This actually means going to a conference and a couple of museum visits as well, plus I’ve just been told I’ve got a couple of reports to write up and an hour-long presentation to do as soon as I get back, but we’re still looking forward to a decent break. You never know, we might even manage a blog post or two…
This one I wanted to write as a belated little something about the Burgess Shale. I spent three weeks working in Toronto, so perhaps you’re curious why..? Or perhaps not – but I’m telling you anyway. The reason is, the Burgess Shale contains some of the most astounding fossils to be found anywhere in the world. From the point of view of sponges, they probably are the best anywhere. The fossils come from a series of spectacular locations high in the Rockies in British Columbia. I’ve not been there yet, but I’ve now been invited to join in their fieldwork when I get a chance, which is a deeply exciting prospect.
These rocks are famous because they preserve soft-bodied animals and plants as well as the ones with skeletons – an entire menagerie of weird and wonderful creatures, from very early in the history of animals. They’re middle Cambrian in age (about 505 million years ago), and when discovered over a century ago, they gave us a first glimpse of what the squidgy things were doing at this point in history. The site was made famous by Stephen J. Gould’s book ‘Wonderful Life’, and has now entered mainstream culture through more books, magazines and television. For some reason, metre-long predators with pineapple-ring mouths and huge grasping appendages (Anomalocaris) and strange walking worms with enormous spines on their back (Hallucigenia) have managed to grab hold of popular attention. Strangely, though, the sponges are normally used as a background – literally. In reconstructions, all the interesting creatures are invariably seen scuttling in, on or around the sponges. It’s just not fair, and this is a little attempt to set things straight.
The sponges of the Burgess Shale have been studied in exhaustive detail three times – by Charles Doolittle Walcott (the site’s discoverer) in 1920, by J. Keith Rigby in 1986, and by Rigby and Des Collins in 2004. At each stage the diversity has increased, and it now stands at something near 50 species. Some of these are known from other sites around the world, in Utah or China, but many are endemic and have never been seen elsewhere. Several represent the only example of entire families of sponges in the fossil record, and most of the rest are the best examples. Take this picture here. It’s a sponge called Vauxia bellula, described originally by Walcott, so it’s very well known. This fossil is just astonishing. There are actually three levels of preservation here, and normally you don’t get any of them. The easiest to preserve is probably the organic meshwork that forms the orange lines; I’ve seen this type of preservation once from Scotland, and once from Herefordshire, and it’s also found in the Cambrian faunas of China and a few other famous fossil faunas. This is the type of material that your bath sponge is made of – in fact, it’s probably close to being a direct ancestor of that group. Bath sponges do not generally fossilise at all – they rot away and vanish before they can be preserved. In this case, it seems to have been preserved through being replaced by iron pyrite – fools’ gold – and then weathered to rust. And that’s the easiest bit, remember.
Under a microscope, the fibres are cored by truly tiny spicules. These were originally silica (opal-A), which dissolves very easily in warm water, but is more stable in the deep oceans. Despite the fact that they don’t rot away, these spicules are so tiny that they rapidly dissolve – you hardly ever see sponges with type of skeleton in the fossil record. Even in the famous fossil deposits of China you don’t see them, although I have reason to believe they were probably there. It also takes a different type of preservation to keep the spicules than it does to preserve the fibres they’re embedded in. So, to preserve the spicules as well as the fibres is really astounding.
But what about the third part? Well, you see that black stain covering the entire sponge? That, it appears, is the soft tissue of the sponge, the gooey jelly that has no chance of being fossilised at all. And best of all, that really does appear to be carbon – it’s a remnant of the original organic tissues, preserved without being turned into a mineral. This is the truly staggering bit about the Burgess Shale, and the real mystery behind the preservation of its fossils. To preserve really soft organic materials like this, you need to stop bacteria eating it – and there’s your problem. Take away all oxygen? No problem – the methanogens and sulphate reducers get to work instead. Highly acidic conditions? Not an issue for some of them. Boiling water and toxic metals? Sounds like a holiday for some groups of Archaea. For whatever conditions you can imagine in the sea where the Burgess Shale formed, there should have been absolutely no problem at all with bacteria living in the sediment. So how on earth did these fossils survive?
There are a long list of papers that have tried to address this, some with considerable success. Most of them, however, have focussed on other aspects of the preservation – the clay mineral films that coated the fossils, the calcium phosphate that formed in their digestive tracts, or the bits of pyrite that replaced certain structures. As far as I know, and despite some very intriguing suggestions, the organic carbon is still a mystery…
But what do I care? From my point of view, the result is that I get access to the most exquisite fossil sponges anywhere in the world, and can see both the skeletons and the soft tissues. And when you can all this, there are surprises – things that have been overlooked, despite those three monographic papers. There are vauxiids – sponges related to this one here – with a long, soft peduncle, and the soft tissue separated from the cage of the skeleton. There are others with utterly bizarre, unique spicules making up their skeleton, but with the strange bits composed of organic matter rather than opal. There are some… but no, I can’t tell you about that yet. That’s just too weird, and I don’t even know how to describe it. Let’s just say, for now, that there is more to these sponges than meets the eye. If we didn’t know better, we might think that they were much more complicated organisms way back in the Cambrian than they are now. Of course, we do know better… right?
For more information on this wonderful fauna and a whole stack of fabulous photos, please visit the recently launched website at: www.burgess-shale.rom.on.ca
Saturday, 10 December 2011
Burgess Shale sponges