Tuesday, 25 May 2010

Minerals in fossils

Dear Readers,

Some of you may have noticed a news story recently about some rather pretty fossils from Morocco. I've been lucky enough to be part of the group working on these things, and now it's been published, I can tell you about them and how minerals are (of course) vital to them being preserved.

We even made the cover - that's what I call fame!

The story goes something like this... Once apon a 542,000,000-years-ago-sort-of-time, there weren't many animals. There were some strange things we call Ediacarans, and at least some of these were genuine animals. Most of them, though, may well have been simpler organisms of unknown types, long since extinct. At precisely this time, though, and for the next twenty million years or so, things started happening in the sea that led to lots of animals, skeletons, and generally complicated things appearing in the fossil record. For those in the know, this was the Cambrian Explosion.

A trilobite. This one is from Wales, rather than Morocco, but these creatures are some of the most iconic critters that emerged during the Cambrian Explosion.

During this time (the Cambrian Period), a wide range of extraordinary creatures evolved, ranging from spectacularly spiky sponges to large arthropod predators such as anomalocarids. We know an extraordinary amount about these animals due to a fluke in the fossil record. For some reason, at this time we have a number of remarkable windows into these ancient ecosystems. The most famous is the Burgess Shale in Canada, but there are others such as the Chengjiang Biota of China, the Sirius Passett of Greenland, and the Emu Bay Shale of Australia. All these sites preserve not just the hard skeletons, but also the 95% of creatures without a mineral skeleton. For example, trilobites are famous fossils with a calcite skeleton that covers the soft parts, and pieces of their skeleton are common in the right age rocks. The legs and other soft parts have no mineral content, though, and these normally rot away completely, along with all the other jointed-legged critters that didn't have the calcite skeleton of trilobites. The Burgess Shale-type faunas have preseved all these lost animals, and shown us what the communities really looked like...

The wonderfully spiky sponge Pirania auraeum -one of a host of survivors from Cambrian times.

There's a twist to this tale, though - they virtually all vanish from the fossil record at the end of the Middle Cambrian. For decades we have wondered whether there was an extinction event that wiped them all out, or whether they carried on into the Ordovician and later periods but have not been preserved. And... well, now we know.

The scenery of the Draa Valley of Morocco, near the edge of the Sahara. Not the place to get your car stuck on a boulder...

The new sites in the Lower and Upper Fezouata Formations of Morocco preserve a wide range of Burgess Shale-type creatures - not the same species, but certainly very similar animals. And the unique thing about the Fezouata fossils is that they're in the Ordovician, some 30 million years after the last examples from anywhere else. They prove that these creatures survived long after they appear to have vanished, and shared the world with animals that appeared in the Ordovician Period. The fauna includes, for example, the earliest known barnacles and horseshoe crabs.

The Ordovician was also an extraordinary time in the history of life, in which animals diversified to an amazing degree. By the end of the Ordovician, there were reefs, corals and even fish; the world would have looked relatively familiar... sort-of (under the water, anyway - there was still virtually nothing on land).

But what about minerals? Well, the question of course is how these soft creatures were preserved, and we think we know roughly what happened. Of course, it all depends on chemical reactions. The fossils are preserved as bright orange or red shapes on green rocks, and that simple fact tells you most of what you want to know. The colouring is due to iron oxides - basically, rust. These rocks are very deeply weathered, so we know that they would have started out as something else - something iron-based - before they rusted.

Cubic crystals of pyrite, starting to go rusty. Leave it in Morocco for another few hundred million years, and it would be a tad bit more rusty...
They couldn't have been pure iron, though - that simply doesn't form in this sort of way. Look really closely, and you find there are a few crystal shapes preserved. The shapes include tiny cubes and octahedra... the tell-tale signature of pyrite.
Pyrite (iron sulphide) is also relatively easy to form chemically. In fact, we know of several other places where pyrite has preserved soft tissue of fossils. In these cases, the pyrite forms because of a group of microbs called sulphate-reducing bacteria. When these anaerobic beasties start eating a carcass, they get their energy from converting sulphates to sulphides. Very rarely, there can be enough of the right sort of iron dissolved in the water that these sulphides react with it to make tiny crystals of pyrite. The crystals end up coating the animal that's rotting away, sometimes just on the surface but sometimes also the internal organs. And that seems to be what's happened here.

Exactly that appears to have happened at some sites in Wales that I work on, as well. In that case, the rocks aren't yet completely weathered, and the pyrite can be seen in X-ray... like so:
An X-radiograph of pyritised beasties from a site in Wales - hydroids and sponges, in this case.

It's rarely quite that simple, though, and there are some specimens that show a range of different minerals apparently being involved. There are 3D worms, for example, that seem to have calcite (calcium carbonate) filling the centre of where the body was, and some sponges with what appears to be a thin sheet of quartz (silicon dioxide) over the soft tissue. The latter is probably because the skeletons of these sponges was made of opal (a type of semi-crystalline silica). There are even different shapes of pyrite crystals preserving different tissue types. There's bound to be more to make of this story as we uncover more and better preserved fossils, but what we know of the original mineralogy is always going to be a little limited... the rocks are so deeply weathered that we'd have to dig a well to find examples with the original crystals still there!

Hmm... might well be worth it...