14.6" Jurassic Ammonite, Belemnite & Gastropod Cluster - England

At the base of the Belemnite Marls is Apoderoceras or Armatus Limestone (Bed 105). This is a .35 meter (~14 inches) limestone which crops out a reef visible at low tide at Westhay Water. Above that come the main sequence of marls with a cyclical variation in carbonate content. The top of the unit is taken as the Belemnite Stone (Bed 121), a laterally persistent limestone full of belemnites, ammonites, bivalves, and some rhynchonellids.

About Ammonites

Ammonites were ancient marine cephalopods, similar to today's squids and octopuses, but with a defining feature: their distinctive, tightly coiled spiral shells. These shells, resembling those of modern nautiluses, served as both a protective home and a buoyancy aid, allowing ammonites to navigate the prehistoric seas with ease. First emerging around 240 million years ago in the Triassic Period, ammonites thrived for over 175 million years, adapting through numerous forms and sizes. As predatory creatures, they likely fed on smaller marine organisms, using their tentacles to capture prey. However, their long reign came to an end 65 million years ago at the close of the Cretaceous, coinciding with the mass extinction event that also eliminated the dinosaurs.

Belemnites are probably the most well known extinct cephalopod after the ammonites. They lived during the Jurassic and Cretaceous periods and are fairly common fossils found throughout the world. They had a hard, internal, cone-shaped structure that is often preserved as a fossil, though it is not technically a shell. They had ten arms but, unlike modern squid, these arms had small hooks instead of suckers.