Promicroceras planicosta Ammonite Group, from Monmouth Beach, Lyme Regis, UK (No.156)

Promicroceras is an extinct ammonite genus from the Lower Jurassic period

Promicroceras species are commonly found in South West England, particularly along the Dorset coast, also know as the Jurassic Coast.

  

Ammonites were free swimming creatures distantly related to squid and octopuses. Like these modern relatives they would have been predators, catching prey with their long tentacles. Their shell was divided up into chambers filled with liquid and gas, which kept them buoyant in the water, much in the same way as a submarine. They can be preserved in a number of different ways.

Ammonites first appeared around 400 million years ago and became a very successful group of animals, dying  out around the same time as the dinosaurs about 65 million years ago.