The ancestral ammonite
This humble crushed ammonite, Psiloceras is the oldest ammonite, regarded as the common ancestor to the ammonites found in the Jurassic and Cretaceous rocks of the Jurassic Coast. It is also a survivor from an older type of ammonite (‘ammonoid’ to be more precise) known as a Ceratite and these lived during the Permian and Triassic periods of geological time.
Ceratites differ from ammonites by having a less complex suture line, the pattern created where the chamber walls meet the outer shell. The great extinction event that took place between the Triassic and Cretaceous periods wiped out the Ceratites but Psiloceras survived to a new world with no competition. As a result, it evolved into a staggering range of ammonite forms.
Psiloceras is found near the base of the Blue Lias and marks the Planorbis Zone, the first of the ammonite zones used to date the relative age of the rocks.
