Part 3 of a 5 part series: for part 2 click here!

Banner image: Assorted ammonite fossil IDs, The Fossils of the Oxford Clay by The Palaeontological Association (1)

Let's journey to the Millennium Country Park's very own marine Jurassic Park...!

By Duncan Willox ǀ Local geology enthusiast and poet

I hope that you’re enjoying our trip back in time! Please remember to take all your belongings with you as we leave - if you lose an electronic device in the seafloor sediment, we’re all in big trouble, but you will be the one to have to try to explain to the Ranger team…

I know the questions you’re all desperate to ask me: how do I know that what I’ve shown you is what the sea was really like in the distant, blink-of-an-eye moment in geological time, rather than some fantastical hypnotic dream?

Well, the answer is under our feet. The Oxford Clay that was quarried here, and which leaves us the legacy of these lakes, contains the fossilised remains of all the creatures I’ve shown you this morning. Where that clay hasn’t been dug out, the fossils remain there, silently entombed. Indeed, some of them may be new species waiting to be discovered, like some unremembered Pharoah in his dark stone walled chamber.

So what fossils can we expect to find in our Millennium Country Park Oxford Clay?

Well, generally not creatures’ soft parts - they would have decayed long before they had a chance to be fossilised. The fossils are therefore generally of creatures’ shells, bones, teeth, scales and other hard parts. Some of the very individuals that we saw alive in our previous expeditions may be preserved as fossils under the park today.

The Clays and grey limestone nodules are full of the shells of the creatures we saw on the seabed. There are also other fossils including many species of ammonite. In the clay itself the fossils of these creatures are generally crushed flat. However, the bullet shaped internal guards of the belemnites are hard enough to be preserved uncrushed. In some layers they are so common that they were a problem for the brick making process, as their presence would crack the bricks during firing in the kilns. Therefore belemnites, bone and other larger fossils had to be removed by hand, and for many years workers were rewarded for each bucketful of fossils that they had removed from the clay. Over the centuries belemnites were such common and puzzling objects that they acquired the folklore name of ‘Thunderbolts’.

You remember the seemingly endless beds of Gryphaea, the oysters that I pointed out in the last installment? There are several species in the Oxford Clay and others in earlier and later Jurassic rocks. The changes in the humble Gryphaea over the millennia have, since the 1920’s, been studied in testing Darwin’s Theory of Evolution. You may find this hard to believe, but there are probably even more Gryphaea fossils in the world than Kim Kardashian has Twitter followers! Gryphaea also acquired a gruesome folklore name, “Devil’s Toenail”.

I owe a personal debt to Gryphaea because it was the first fossil I ever found as a young over-excited boy. I treasured it then as much as if I’d just discovered a whole Tyrannosaurus Rex dinosaur with every terrifying tooth in place. Over half a century later I still have it (and still don’t have a Tyrannosaurus Rex).

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(1) (2) Kelly, S. (1992). D. M. Martill & J. D. Hudson 1991. Fossils of the Oxford Clay. Palaeontological Association Field Guide to Fossils, number 4. 286 pp. London: The Palaeontological Association. Price £15 (paperback). ISBN 0 901702 46 3. Geological Magazine, 129(3), 374-375. doi:10.1017/S001675680001935X