Ammonite that’s a common find on beaches

Ammonite fossil
Ammonite fossil

Written by Jeannie Swales

This week’s exhibit is remarkable, beautiful – and similar examples can be found on this coast by anyone with a keen eye.

It’s a fossil ammonite Dactylioceras tenuicostatum. Both it and the 17 other species of Dactylioceras are commonly found on the beaches around here – in fact this specimen, now in the Scarborough Collections, was found by a participant on one of Scarborough Museums Trust’s popular summer fossil-hunting trips.

Three beauties can be seen on the coat of arms for Whitby, where legend has it that St Hilda gathered up all the serpents from the Abbey and hurled them into the sea, where they became ‘snake-stones’.

Ammonites, along with Goniatites and Ceratites, form the fossil group Ammonoids, the largest cephalopod group. Squid-like creatures related to the modern squid, octopus and nautilus, they usually had a flat, coiled external shell, although some were straight, some curved or combined a coil with a straight or hooked section, and, very rarely, some were coiled like a snail.

It’s believed that Ammonoids evolved from Nautiloids during the early Devonian period, about 400 million years ago. They were widely distributed and evolved from the Carboniferous period (around 300 million years ago) until the Cretaceous period, dying out in a mass extinction around 65 million years ago. Our pictured specimen is believed to date from around 190 million years ago.

Ammonoids were predators and scavengers, but were also prey themselves, being eaten by larger sea-living reptiles, such as the dolphin-like ichthyosaur, plesiosaurs and pliosaurs.

What you see here, of course, is the fossilised remains of the shell – the actual creature lived in its last formed chamber, growing new chambers as it increased in size.

The three different types of Ammonoids form an evolutionary sequence, starting with the Goniatites from the Carboniferous and Permian periods, then the Ceratites in the Late Permian and Triassic, and finally the Ammonites in the Jurassic and Cretaceous periods.

There are many fine examples of Ammonites in the Rotunda, the William Smith Museum of Geology, but if you want to find your own, why not join one of the regular fossil-hunting trips run by the Trust? They’re co-ordinated by the Trust’s Head of Public Programmes, geologist Will Watts, who stresses that all specimens found will be in loose material on the beaches, rather than taken from the cliffs – it’s environmentally sounder, and much safer, especially for children.

The trips start on 5 August, and run through the month, taking place at Reighton Sands, Filey Bay, Boggle Hole, Sandsend and Runswick Bay.

For more details, visit the Museums Trust website at, call 01723 384511 or email