Heteromorphs

Heteromorphic ammonites, or simply heteromorphs, hold a great fascination for some. It's easy to see why, what with their sometimes odd shapes that look as though the animal wasn't quite sure what it was doing. Shapes vary from simple coils that straighten out, some ending in hooks, to helically wound coils that then become planar, to straight shells that shift to the opposite direction connected by U-turns; and more and in between.

The term, heteromorph, or heteromorphic, means having more than one form or shape. While many heteromortph ammonites are heteromorphic, not all truly are. Some like the Crioceratidae have what are really openly coiled monomorphic (single form) shells, and others like the Turrilitidae have tall spired helically wound shells that are also monomorphic. Because these are related to truly heteromorphic forms, they are generally included in the heteromorphs. Truly hetermorphic forms include the Ancyloceratidae, Macroscaphitidae, Heteroceratidae, Ptychoceratidae, and even the Baculitidae.

A better. and more scientific approach is to refer to these taxonomically, to the Ancyloceratina for Cretaceous forms, or to the Ancyhloceratina and Turrilitina if the distinction is correst, and to the Choristcerataceae (Choristoceratoidea if you want to go along with the rank ambiguous requirement of the ICZN) for Upper Triassic forms.