Description

Allocrioceras hazzardi
Heteromorph
Upper Cretaceous
Boquillas Formation
Study Butte, Brewster County, Texas, USA

32mm heteromorph on 103mm slab. First we have seen.

Cephalopod. Ammonite.

Allocrioceras is an ammonite genus that lived during the Late Cretaceous (Turonian-Santonian).  It’s been found in North America, Europe, and South Africa which hints at an even wider distribution.

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

Here’s an unusual loosely-coiled heteromorph for the collector looking for an example from North America.