Description

Late Cretaceous (Campanian)
Belo Tsiribihina, near Tulear, Madagascar
90mm heteromorph
Ammonite, Heteromorph. Cephalopod.

Nostoceras is a genus of heteromorph ammonite known from Late Cretaceous sites all over the world – sites in Alaska, British Columbia, California, Texas, Columbia, Angola, Madagascar, New Zealand, and Japan.   However, it appears on the market only occasionally.

The biology of the heteromorph ammonites is not clear but it’s apparent that 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 prevented them from swimming efficiently.  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.