Arnioceras semicostatum Ammonite, from Dorset, Jurassic Coast, Monmouth Beach, Lyme Regis, UK (No.33)

Ammonites are a form of ammonoid distinguished by their complex suture lines. They were abundant and diverse in the seas of the Mesozoic Era, and they evolved very rapidly to produce a number of species and genera. After a decline in diversity during the late Cretaceous period, ammonites become extinct at the same time as other marine groups, such as Belemnites, and terrestrial groups, such as dinosaurs.

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.

  

Arnioceras is an extinct genus of large, evolute, discoidal ammonite from the Lower Jurassic.

The shell is normal coiled so that all whorles are exposed. Sides bear strong sharp ribs that are straight until reaching the ventrolateral edge where they swing forward and fade. The rim (venter is keeled and free of grooves.

Arnioceras, named by Alpheus Hyatt, (April 5, 1838 – January 15, 1902) an American zoologist and palaeontologist.

Arnioceras distribution is fairly worldwide, and have been found in Europe, South Asia, and North and South America.