Fossil preparation of an Arnioceras ammonite from Charmouth, UK

This Arnioceras sp. ammonite was found by a couple of one of The Wobbly Fossiler’s guided walks on Charmouth beach. Whilst not the biggest ammonite of this type ever found, its size (80mm) and quality are very uncommon from the location. The particular rock is usually not worth preparing as the ammonites are often crushed and poorly preserved, and therefore the difficult prep job is not worth the time in doing. Not to be perturbed, James though this particular was one was worth a crack!

You can see the tiniest bit of outer whorl sticking out with only the slightly worn tops of the ribs initially showing on this nodule; this has had less then a minute of cleaning to expose the shell and check the quality. These first moments of exploratory work suggested something perhaps a little more promising than usual inside. You can see the ammonite shell hidden within the rock has been protected from the damaging effects of rolling around on the beach, with the exposed rib tops the only giveaway of the potential contents within.

James carefully started uncovering the ammonite by working down close to it with an air pen (the ZOIC Chicago), and then air abrasive techniques using dolomite powder to reveal the tops of the ribs. With this genus of ammonite it is important to work slowly and carefully as the ribs become very narrow and pronounced as the ammonite gets bigger. This is a decent size at 8cm and so you can only imagine how fragile and delicate those ribs were!!

James made the stylistic decision to ‘bowl’ the rock around the ammonite quite tight to the shell itself. The main reason for bowling is to draw the eye to the ammonite like a fruit sat within a fruit bowl; leaving a relatively tight bowl around the ammonite maintains much of the original stone surface and offers a focused look inside whilst minimising needless extra work.

This bowling took a great deal of skill, as any vibration transmitted from the air pen towards the ribs could have damaged them. Instead, James carefully used his air pen parallel to the ribs at all points when removing the matrix; the smallest amount of shock transmitted in the wrong direction could have shattered the fragile calcite of the thin ribs.

So much of fossil preparation comes down to aesthetic decisions such as this, relying on the natural intuition and experience of the preparator to visualise the finished product.

James uncovered a bit of the ammonite at a time, firstly by working down near the ribs with the air chisel, using air abrasive to remove the rock from above and between the ribs and profiling the bowl with a mixture of both tools. He chose dolomite powder for this particular job due to the fact that it is hard enough to cut through limestone but still retains some delicacy when it comes in contact with the ammonite shell (for the split second that he allows it to!). He worked carefully on each rib at a time, being very careful not to ‘gouge’ by being conscious of the angle at which he directed the abrasive powder, and by not hovering in any one place for even a moment too long. This piece took just over fourteen and a half hours to prepare.

All in all, it is safe to say that this is a stunning specimen!

About the Preparator

James Carroll is a professional fossil preparator and fossil guide with walks based out of Charmouth, Dorset on the Jurassic Coast. His website, under his pseudonym, The Wobbly Fossiler, can be found by clicking this link. He also regularly updates his Instagram with his latest work.

In his 15 years of experience collecting the fossils around Lyme Regis and Charmouth, he has gained a reputation for having sharp eyes both on the beach (looking for insect and lobster fossils) and more recently when cleaning fossils. His incredible eye for detail when preparing fossils gives his work a unique finish.