Gastrioceras, genus of extinct cephalopods (animals related to the modern squid, octopus, and nautilus), found in Pennsylvanian marine rocks over a wide area, including North America and Great Britain (the Pennsylvanian Subperiod began 318 million years ago and lasted about 19 million years). The shell of Gastrioceras is round and coiled and consists of a series of chambers. The sutures between successive chambers are represented by a sinuous line of arches and troughs in a simple pattern known as a goniatite suture pattern.