This is an excellent polished goniatite fossil from the Devonian of North Africa. Unlike most of these that are mainly blank rock with a small fossil center, this is a COMPLETE and nice size example with a full display of the fossil to the outer edge. These ammonoids were formed by calcite filling the shell, turning it to a sparkling and translucent gray with a black limestone interior. The result of the fossilization process beautifully highlights the classic anatomy of these ancient cephalopods of the prehistoric sea.
Goniatites survived the Late Devonian extinction but eventually died out at the end of the Permian Period, the Earth's most deadly mass extinction event in history. All goniatites possessed an external shell, which is divided internally into chambers filled with gas giving it buoyancy during the life of the animal. An open chamber at the front of the shell provided living space for the goniatitid animal, with access to open water through a ventral siphuncle. The general morphology and habit of goniatites was probably similar to that of their later relatives the ammonites, being free swimming and possessing a head with two well developed eyes and arms (or tentacles).
Goniatite shells are small to medium in size, almost always less than 15 centimeters (5.9 inches) in diameter and often smaller than 5 centimeters (2.0 inches) in diameter.