What fossil did you find? Answer four questions about its shape, pattern, size and where you found it, and the key narrows it down to the likely groups, from ammonites and belemnites to shark teeth, with their ages, what to look for and the stones most often mistaken for fossils.
Likely groups
One group fits your answers:
Best matchAmmonite Fossil
Age
About 415 to 66 million years ago: ammonoids appeared in the Early Devonian and died out at the end of the Cretaceous.
Look for
A coiled shell divided into chambers. Where the chamber walls met the shell they left suture lines, which in ammonoids are complex and frilly.
Where
The foreshore and cliffs at Lyme Regis and Whitby are famous collecting places; Runswick Bay in Yorkshire too. Lyme Regis is a famous ammonite beach; its shiny pyrite ammonites are usually found inside hard limestone nodules.
Don't confuse with
A nautilus has simple, gently curved suture lines; a snail (gastropod) coils into a raised spire instead of a flat spiral.
Start with the overall shape, because that sorts most finds at once. The second question only appears for shapes that need it: a flat spiral could be an ammonite or a nautilus, two shells could be a brachiopod or a bivalve, and a round lump could be a nodule, a flint or a sea urchin. Size and place re-rank the answers and add local notes, such as Gryphaea in the shingle at Lyme Regis.
Clean it gently. A soft brush and water show ribs, sutures and growth lines better than scrubbing.
Look at broken ends. A broken belemnite shows calcite fibres set at right angles to its surface.
Check the lookalikes. Septarian nodules, flints and “beef” fool many collectors.
The key covers the common groups only. For anything unusual, ask a museum or a heritage coast centre, and note exactly where you found it.
Printable fossil ID sheet
Ammonite
Age: About 415 to 66 million years ago: ammonoids appeared in the Early Devonian and died out at the end of the Cretaceous.
Look for: A coiled shell divided into chambers. Where the chamber walls met the shell they left suture lines, which in ammonoids are complex and frilly.
Belemnite
Age: Jurassic and Cretaceous; belemnites died out at the end of the Cretaceous, 66 million years ago.
Look for: Called “fossil bullets” for their pointed, conical shape. Broken across, they show fibrous calcite crystals set at right angles to the surface, and often a cone-shaped hole at the wide end.
Crinoid (sea lily) stem
Age: From the Cambrian or Ordovician to today: the BGS says the first appeared in the mid Cambrian, the Digital Atlas of Ancient Life dates the group from about 485 million years ago; some survive in today's seas.
Look for: The stem is a stack of pieces called ossicles or columnals, which can be round, oval, square, five-sided or star-shaped. Single pieces were once called “St Cuthbert's beads”, “star stones” or “fairy money”.
Brachiopod (lamp shell)
Age: From the earliest Cambrian, over 540 million years ago, to today, though they are rare in modern seas; most common in rocks 541 to 66 million years old.
Look for: Two shells of different sizes. The line of symmetry cuts across both shells, so each shell is symmetrical down its own middle.
Bivalve (clam, scallop or oyster)
Age: From the middle Cambrian, over 500 million years ago, to today; they flourished in the Mesozoic and Cenozoic.
Look for: Two shells (valves) that are mirror images of each other: the line of symmetry runs between them, along the hinge. Often single valves are all that is left.
Gastropod (sea snail)
Age: From the early Cambrian, about 541 million years ago, to today; they have been most diverse in the last 65 million years.
Look for: Most coil into a spire (a helix), unlike the flat coil of an ammonite.
Gryphaea (devil's toenail)
Age: Jurassic: the classic devil's toenail, Gryphaea arcuata, is a Jurassic oyster.
Look for: An extinct oyster with a strongly curved-over shell, grey with growth lines. In limestone, cross-sections look like white commas or apostrophes.
Echinoid (sea urchin)
Age: From the Late Ordovician, about 450 million years ago, to today.
Look for: A test (shell) with five-part symmetry, covered in small knobs called tubercles where the spines were. Regular urchins are round; heart urchins and sand dollars are irregular.
Fossil shark tooth
Age: Cartilaginous fishes go back almost 450 million years. Megalodon lived about 23 to 3.6 million years ago.
Look for: A shiny enameloid crown, often triangular and sometimes serrated, on a duller root. Shark skeletons are cartilage, so teeth (and sometimes vertebrae) are usually all that is left.
Coral
Age: The oldest corals lived in the Cambrian, over 500 million years ago. Rugose (horn) and tabulate (honeycomb) corals lived from the Ordovician to the end of the Permian; modern-type scleractinian corals since the Middle Triassic.
Look for: Solitary rugose corals are often shaped like a bull's horn, with partitions (septa) in groups of four; scleractinian septa come in sixes. Tabulate corals were all colonial, some like a honeycomb (Favosites).
Petrified wood
Age: Trees and the first forests appeared by the end of the Devonian, so petrified wood can be any age from then on.
Look for: Wood turned to stone by minerals (often silica) filling and replacing it, so the grain and growth rings show but no original wood remains.
Septarian nodule (not a fossil)
Age: A rock, not a fossil: it formed as hardening sediment shrank during burial.
Look for: A grey nodule with a network of white calcite-filled cracks, sometimes called a dragon stone. It can resemble the plates of a turtle shell or a cracked egg.
Flint nodule (usually not a fossil)
Age: Silica that collected in the chalk, much of it from the dissolved skeletons of sponges and tiny organisms.
Look for: Irregular nodular or elongated lumps of silica. Many formed by replacing old burrow systems in the chalk sea floor, which gives them their odd shapes. Some hold real fossils, such as sea urchins.
Nodule with a fossil inside
Age: The nodule is rock, but it often protects a real fossil: at Lyme Regis, shiny pyrite ammonites are usually found inside hard limestone nodules.
Look for: A rounded limestone ball, sometimes with part of a shell or ammonite showing. Ask at a museum or fossil centre before trying to split one.
“Beef” (fibrous calcite, not a fossil)
Age: A rock layer, not a fossil.
Look for: Forms when calcite crystals grow like fibres, so a slab shows parallel needles that can look like wood grain or bone.
Ages and descriptions from the British Geological Survey, the Natural History Museum, Lyme Regis Museum, the Charmouth Heritage Coast Centre, the Digital Atlas of Ancient Life, the UC Museum of Paleontology, the Florida Museum and the Maryland Geological Survey; each card above links its sources. Drawings by Fossil Wharf.
Questions
How can I tell an ammonite from a snail?
An ammonite coils flat, in one plane, and shows frilly suture lines where its chamber walls met the shell. Most snails coil into a raised spire.
Is it a brachiopod or a clam?
In a brachiopod each shell is symmetrical down its own middle and the two shells differ. In a bivalve, such as a clam or scallop, the two shells are mirror images of each other.
Can I keep the fossils I find?
Often, but not everywhere. Loose finds on the West Dorset beaches are yours to keep, while national parks in the US allow no collecting at all. See the fossil collecting rules before you go.