Country diary: fossils with the treasure factor draw a crowd

The crashing waves throw up a misty-grey haze as they break against the bottom of crumbling ashen cliffs that slouch on to the top of the beach. The backdrop is the imposing hulk of Golden Cap, its sandstone-topped, ragged, seaward face cascading at 45 degrees for 200 metres to meet the churning waters.

The foreground is less majestic but perhaps more surprising, with 400 or so intrepid fossil hunters filtering along the cliff base as the tide retreats. They might have considered, as I did, that a February trip to the British coast would be likely to involve some discomfort, but in the event the sun shines, coats are left in cars and the ice-cream van does a brisk trade.

The ammonite is the epitomic Jurassic Coast fossil, and with good reason – any prolonged attention to the fine shingle will turn up the corrugated whorls of Polo-mint-sized Androgynoceras lataecosta, their treasure factor amplified by a burnish of the false gold (iron pyrites).

For 350m years ammonites swam through the planet’s seas in their coiled shells. Tentacled relatives of squid, their shells’ complex internal compartmentation, evolved to control buoyancy, is diagnostic. Today’s eroding Charmouth cliffs spill ammonites back into the sea that they last gambolled in 183m years ago.

Soon we are spotting ammonites in lumps of clay and impressed on large slabs, and are discerning patterns in form that suggest different species. Amaltheus stokesi, for example, has broad swirls but is discus- not doughnut-shaped, with the acute rim delicately crimped. These streamlined, coaster-sized molluscs are thought to have been the fast movers of the genre, perhaps hunting swifter prey than more classically ram’s-horn-shaped relatives.

Alongside fossils, the beach is also littered with amorphous, bobbly nuggets of dustily sparkling pyrites. Closer inspection of one big nodule reveals an embedded row of 1cm conical teeth. For a brief period of excitement we think we have found an ichthyosaur jawbone fragment, but it turns out to be part of a defensively armed, hand-sized ammonite, Eteoderoceras obesum.

Unusual, intriguing and beautiful, these fossil treasures will always broaden our horizons.