Humble flint sea urchins and the stories they tell

Flint is a very hard-wearing rock from the chalk of the Upper Cretaceous. Whole beaches made of flint pebbles can be found many miles away from the chalk strata that the nodules originated in, owing to the rock’s ability to withstand the processes that destroy other rocks quickly. Flint sea urchins are especially hard-wearing, as their rounded shapes require a lot of force to damage, while less-rounded flints tend to break up over time if subjected to high-energy environments, such as beaches and fast-flowing rivers. Because of this robustness, it is possible to find flint urchins, which have undergone some very interesting journeys before being collected, adding to their interest for fossil hunters.

All flints start off within chalk strata. Where these strata are exposed at the coast or in quarries and cuttings, it is possible to collect flint sea urchins, which, at first, look very much as if they are preserved like every other urchin found in chalk. They have a white calcite-replaced test and all that can be seen of the flint within is a slight blueish tint or maybe a glimpse of the nodule through the anal or oral apertures. Of course, flints can also be found that partially or fully envelop an urchin and, in these cases, highly aesthetic display pieces can sometimes occur.

Fig. 1
Fig. 1. The hardness of flint and the rounded shape of flint urchins make them extremely robust fossils.

Once worn out of the chalk, the external calcite test will quickly become damaged and abraded in beach environments, owing to its comparative softness. In these cases, to maximise the detail and aesthetics of the fossil once collected, I have sometimes stripped off the battered test using acid, to reveal a perfect flint internal mould, which is often covered in a fine tracery of silicified fragments of the test. Beach-found urchins with their white test still remaining, but damaged, may have been out of the chalk for any amount of time between a few hours and a few years, depending on how exposed they have been to the rolling action of the sea. More time rolling around on a beach will strip off all calcite from a specimen, as well as any silicified traces; and a smoothed, rather undetailed, urchin will remain, with only a neat five-fold pattern to show where the spines were. So, in a highly erosive environment such as a beach, a fossil urchin specimen has a context and a story to tell, even if it is only short.

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Joe Shimmin(UK)
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