Helicoceras sp.
Heteromorph
Late Cretaceous
Ripley Formation
Coon Creek, Tennessee, USA

40mm heteromorph.

Helicoceras is a heteromorph ammonite genus known from the Late Cretaceous of both Americas.

Cephalopod.

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 any form of efficient swimming.  It’s more likely these ammonites 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.

This specimen comes from a locality lesser-seen on the market.