Polyptychoceras sp.
Heteromorph
Late Cretaceous
Hokkaido, Japan

92mm heteromorphic ammonite on 95mm concretion.

Cephalopod.

Polyptychoceras is an ammonite genus that lived during the Late Cretaceous.  It is known from sites around the world (the Americas, western and eastern Europe, Angola, and Japan).

The biology of the heteromorph ammonites is not clear but it’s certain that 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 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 else they crawled along the sea floor feeding on sessile or slow-moving animals such as clams.

Here is an excellent heteromorph for the collector looking for a displayable matrix piece from Japan.