Tuesday, 14 September 2010

Preparing fossils

This, believe it or not, is one of the fossils we found in Burnmouth in July, and it's proving quite enigmatic, even if it actually doesn't look like much at all! (And my apologies that the ruler is upside down!)

The original rock, as extracted from the cliff, was about 8cm square and perhaps 2cm thick, and held a sizeable piece of bone as these things go.  We were quite excited.  Jenny thought it might be the clavicle of a fish from a group called Rhizodonts.  The clavicle in this context is the centre bone from the front of the shoulder girdle (no direct equivalent in human anatomy) but judging from the size of the bone, the fish would have been a couple of metres long, so a decent meal for four!

Now I know you can't see much of the bone, as it's mostly embedded in the rock, but what you can see is a paler area where I've been picking away the rock matrix, leaving a kind of horizontal brownish area just below where the ruler says 15cm upside down.

That, if you magnify it up, is a row of tiny teeth, each only about a millimetre long, and they are getting us excited.

If, and it's a big if, that row of teeth really is part of the rest of the bone in the rock, then it can't possibly be a clavicle.  Shoulder girdles do not have teeth, even in Carboniferous fishes!

But what we're left with, and I know you can't see this, is quite a chunky bone which must be part of the mouth of whatever originally owned it, equipped with minute, but minute teeth.  If an animal has a jaw bone, and it doesn't matter whether that's upper or lower jaw, 10 cm long, how much use is a row of teeth 1mm high?

There's a vertebrate palaeontology conference going on in Cambridge as we speak, and we're showing this beastie to the experts.  So far, the reaction is "Nope.  Never seen anything like that!"

I'll throw another thought your way.  Feel free to discard it.  This animal lived in a 20 million year period known as Romer's Gap.  At the end of the Devonian period, about 360 million years ago, there was a mass extinction, much like the one we're all familiar with at the end of the Cretaceous.  Romer's Gap comes immediately after that, and it's called that because vertebrate fossils are extremely rare from that 20 million year period.  Our beastie comes from about 3/4 of the way through, so about 345 million years ago.

When you get a mass extinction, the few that survive, once things stabilise, have life relatively easy, at least that's my theory. There's less competition and it's generally easier to get by.  If it's easier to get by, even some quite unpromising variations don't stop you reproducing, so a slew of weird animals evolve.  After a bit, and we think less than 20 million years in this case, more and more competing forms evolve, life gets tougher, and only those variants that are actually any good survive.  The rest die out.

We might have one of the survivors here, or one of the doomed weirdos.  We don't know.  Now isn't that fascinating?

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