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Tools of time

There couldn’t be a better example than this to illustrate the usefulness of ammonites in dating the relative age of the rocks in which they are found. Dactylioceras commune is a very common ammonite in the thick, black shales near Whitby on the Yorkshire coast. Yet it also appears in a completely different rock type from the Dorset coast. This is because ammonites evolved quickly through time, and each species only lived for a short time. Therefore the black shales at Whitby and the Beacon Limestone Formation near Seatown are exactly the same age. Why is that important? It helps geologists understand past environments, placing the relative age of the animals in context so that we can understand how life evolved.



Find out more about ammonites on the molluscs pages.

Common name


Scientific name

Dactylioceras commune


Molluscs > Cephalopods


Upper Lias, Junction Bed

Time period

Lower Jurassic



Where found

Thorncombe Beacon

Found by

Paul C.Ensom


Dorset County Museum, Dorchester

Accession number