Description

Lower Cretaceous (Aptian)
Samurskaya layer
Kurgips river, Apsheron, Krasnodar region, Russia
19mm heteromorph on 34mm slab
Ammonite, Heteromorph. Cephalopod.

Ptychoceras is an ammonite genus that lived almost all the way across the Cretaceous.  It is known from sites around the world (the Americas, western and eastern Europe, Madagascar, and even Antarctica).

The biology of the heteromorph ammonites is unclear (why such bizarre shell shapes?) but one thing is clear: their uncoiled shells would have made them 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 impediment to efficient swimming.  It’s more likely these ammonites either drifted in the plankton, capturing small animals on long tentacles like modern jellyfish, or they crawled along the sea floor feeding on sessile or slow-moving animals such as clams.