Hooks, paperclips and balls of string: Understanding heteromorph ammonites

Neale Monks (UK) Heteromorph ammonites were a group of externally shelled cephalopods that were particularly diverse during the Cretaceous period. Many species were abundant and geographically widespread and, for this reason, they have been used to date and correlate rocks. Unlike regularly coiled ammonites, which underwent a steady decline in diversity through the Cretaceous, the heteromorphs continually produced new and often bizarre species indicating a certain level of success at occupying new ecological niches. Only at the final mass extinction, at the Cretaceous-Tertiary boundary, did the heteromorphs finally fail. Fig. 1. Anisoceras armatus is a typical hamiticone heteromorph. In this reconstruction, it is shown as a benthic animal with the head oriented towards the substrate, though some recent work suggests that they were in fact planktonic animals that inhabited deep water. What makes a heteromorph? Broadly speaking, heteromorphs are ammonites with shells coiled in something other than the normal way. Whereas most ammonites had shells that can be described as flat, closed spirals where each whorl at least partially enclosed the one before it, heteromorphs had shells that coiled in a variety of ways. Some were simply open spirals, while others were helical like snails, or consisted of approximately parallel shafts connected by tight bends, so that the resulting shell looked a bit like a paperclip. At the most extreme, there was Nipponites. This is an ammonite with a shell formed from connected U-bends, each at an angle to the preceding one, resulting in something that looks more like a … Read More

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