Description

Lower Cretaceous (Aptian)
Samurskaya layer
Kurgips river, Apsheron, Krasnodar region, Russia
15mm heteromorph
Ammonite, Heteromorph. Cephalopod.

Ptychoceras is a genus of ammonite 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 but what is clear is that their uncoiled shells would have made them very poor swimmers.  Open shells, especially those with spines and ribs, create a lot of drag. More importantly, the orientation of the shell with the body hanging below the buoyant part of the shell, would have seriously impeded any form of 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.

Here’s an affordable specimen of a very unusual ammonite group.