Dorset Ammonites and Belemnites: A Field Guide for First-Time Fossil Hunters
Nearly every first-time fossil hunter on this beach comes home with one of two things: a coiled ammonite or a bullet-shaped belemnite. Knowing what you are actually holding turns a lucky find into something you understand.
Published July 2026Of the thousands of fossils picked up along the Lyme Regis and Charmouth foreshore every year, the overwhelming majority fall into one of two groups: ammonites and belemnites. Both are the remains of extinct marine molluscs related to modern squid and octopus, both are common enough that finding one requires patience rather than luck, and both are legal to collect from the open foreshore in reasonable quantities for personal use under the code of conduct that governs collecting on this stretch of the Jurassic Coast.
Ammonites: the coiled shells
An ammonite is the fossilised, coiled external shell of an extinct cephalopod, and what you are looking at is essentially a stone cast or mineral replacement of that original shell, often preserved in iron pyrite or calcite that gives it a metallic or crystalline sheen when the light catches it right. Sizes on this beach range from ones smaller than a fingernail up to specimens the size of a car wheel, though anything beyond dinner-plate size is a genuinely rare find rather than a routine one. The most commonly found genera around Lyme Regis and Charmouth include Promicroceras, usually small and tightly coiled, and various Arietites and Asteroceras types, larger and more ribbed. A whole, well-preserved ammonite still showing its original shell ribbing is worth keeping regardless of size; a squashed or badly weathered fragment usually is not, and part of learning to fossil hunt well here is learning which finds are actually worth the pocket space.
Belemnites: the bullet-shaped guards
A belemnite looks nothing like an ammonite and is easy to misidentify at first: it is a smooth, solid, bullet- or cigar-shaped piece of dense calcite, usually a few centimetres long, sometimes with a faint pointed tip at one end. What you are holding is not the whole animal but its internal guard, a solid structure that sat at the rear of a squid-like creature’s soft body and is almost the only part of it dense enough to survive fossilisation intact. Because belemnite guards are solid rather than hollow shells, they tend to survive the tumbling and grinding of the beach environment better than ammonites do, which is part of why loose belemnite guards are so much more common as isolated finds than complete ammonites of equivalent age.
Where the two turn up, and why
Both fossil types weather directly out of the Blue Lias and Charmouth Mudstone cliffs that make up most of this coastline, laid down as fine marine sediment roughly 195 to 200 million years ago when this part of England lay under a shallow tropical sea. Storms and winter erosion are what actually deliver fresh material: a calm high summer beach with no recent cliff falls will usually be picked clean of easy surface finds by the volume of visitors working it, while a beach walked the morning after a big autumn or winter storm can be scattered with fresh material within reach of anyone willing to look properly. This is one reason the general fossil hunting guide for this coast puts so much emphasis on timing a visit around tides and recent weather rather than just turning up whenever is convenient.
What to leave, and what to check first
Not everything worth finding should be picked up on your own. Anything embedded in the solid cliff face itself, rather than loose on the foreshore, should be left alone and reported rather than dug out, both because cliff collecting here is genuinely dangerous and because it is restricted under the local code of conduct. Larger or unusual finds — anything that does not look like a routine ammonite or belemnite, particularly bone material or anything with visible vertebrae or teeth — are worth showing to staff at the Lyme Regis Museum before you do anything else with them, since occasional finds by ordinary visitors have turned out to be scientifically significant. The Natural History Museum publishes accessible background material on Jurassic marine fossils and their formation for anyone who wants to go beyond simple identification into the wider science behind what is washing out of these cliffs.