Goniatites are an extinct group of ammonite, which are related to the nautiloids. Goniatites evolved from nautiloids in the early Devoian period, about 400 million years ago. They were abundant in world seas for the next 370 million years, after which they vanished suddenly at the end of the Cretaceous period.
The goniatites all possessed an external shell, which is divided internally into chambers. The animal lived in the largest of the external chambers, and the internal chambers would have been filled with gas, making the animal buoyant in the water.
The general structure of the goniatites would have been similar to that of their relatives the ammonites, being a free swimming animal possessing a head with two well developed eyes and arms (or tentacles).