Early Cretaceous (Albian)
Granpré, Ardennes (08), France
27mm heteromorph on 87mm slab.

Ammonite, Heteromorph. Cephalopod.

Protanisoceras is a genus of heteromorph ammonite known from the Early to Middle Cretaceous of various parts of the world (South America, Europe, Madagascar, and South Africa.

The biology of the heteromorphs is not clear but their uncoiled shells would have made these forms very poor swimmers.  Open shells, particularly ones with spines and ribs, create a lot of drag; but more importantly, the orientation of the shell with the body hanging below the buoyant part of the shell, would have created a serious hindrance to efficient swimming.  It’s more likely these ammonites either drifted in the plankton, capturing small animals on long tentacles like modern jellyfish, or else they crawled along the sea floor feeding on sessile or slow-moving animals such as clams.

This is an example of the more obscure “hairpin” heteromorphs.