8 Oct 2013

Jurassic bivalve CSI

Fossil bivalves (molluscs with two shells enclosing the soft bodyparts) are, as I said in the previous post, rare finds on Runswick beach. Here are three, left to right, Pleuromya, the subject of this post, the subject of the last post and, for comparison, the kind of fat little bivalve Dacromya that I find here only occasionally but are everywhere in the lower levels of Whitby's east cliff.

Something traumatic happened to the left hand Pleuromya: one shell has a clean hole punched into it...

On the other side the damage is even more extensive, the hole ragged:

This profile shot shows the damage to one of the shells; it was effectively caved in by whatever befell it:

In 35 years of searching these beaches I have never before seen anything like this. Bivalves worn smooth by being tumbled in shingle for years, yes. Bivalves heavily flattened by milennia of geological pressure, yes. Never, in the hundreds of specimens I've seen have I observed anything like this.

I'm inclined to rule out post-mortem geological damage. I'm not a Jurassic taphonomist, and if such an expert exists I'd be delighted to be contradicted here. Here's what I think happened to Bert (as I have come to call him. Or her.); minding its own business in the mud of a Jurassic seabed some 190 million years ago, Bert the Bivalve became a collateral snack for an icthyosaur (or maybe a plesiosaur). We know from the fossil record that icthyosaurs ate Jurassic shellfish, but I suspect it's unlikely that a small (Bert is 4 cm across) creature like this would be high on a large predator's prey list. Prey has to be worth a predator's time and effort to catch, eat and digest.

It may be that there is the fossil of a toothy marine predator that specialized in raking up molluscs from the Jurassic seabed just waiting to be found in these cliffs; but I think Bert was the accidental victim of a big, toothy predator snapping up something bigger and more worthwhile off the seabed.

In the attack, the sediment is stirred up, Bert is scooped into the icthy- or plesiosaur's jaws, and a second later he's in bivalvhalla as the jaws snap shut and a tooth punches through the tough aragonite of his previously trusty shell. The main meal is gulped down, a shake of the head (maybe Bert spent some time impaled on the fatal tooth, bugging the hell out of the predator) and Bert's remains tumble to the seabed. The benthic fauna clean out the organic remains from the punctured shell, the mud pours in and over the next 190 million years he's buried, fossilised, the rocks are transported by continental drift to 54 degrees North. One day his remains are washed out of a cliff to be found, mislaid round the house and found again.

Bitten Bert is not alone; our aimiable competitors over at Jurassic Trading have seen ammonites with clear bite marks out of them. The above is supposition, it might be complete rubbish. There could be a far more plausible explanation from finer minds than mine, but for now I'm happy to have in my collection what I think is the body in a Jurassic CSI.


Kate said...

How fascinating. I can't tell you any more about it but it's amazing!

Anonymous said...

Thanks for pointing me too your blog today. Reading whilst in Whitby Museum. A wonderful place. hope to return soon. Jim