From: Illminster, Somerset, UK.
Age: 170 million years old, Jurassic.
Jurassic Harpoceras do not display characteristic ribbing, they do however have very elegant suture lines that form a feathered pattern round the whorls. This a zone fossil, that is a fossil that is used to define a certain geological time span. Ammonites, an extinct order of free swimming predatory cephalopods, make excellent zone fossils as they are geographically wide spread common in the fossil record and evolved quickly and so are only found for limited geological time scales. This fossil defines a subzone within the larger time frame of the Harpoceras falciferum zone of the Lower Jurassic.