Jurassic (Callovian)
St. Laon, France
Very fine 21mm heteromorph ammonite on 36mm matrix. Full lappets seem to be present.
Ammonite, Cephalopod.

Oecoptychius is a genus of ammonite known from the Middle-Late Jurassic of Europe and Madagascar.  Collectors know it for its very small size and its nickname, the “elbow ammonite.”

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 seriously impeded efficient swimming.  It’s more likely these animals either drifted with 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.