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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Ammonite wars

Neale Monks (UK) Ammonites have been studied intensively for the last 200 years but, between experts, there is very little agreement on what ammonites looked like or how they worked as living organisms. Could they float? Did they swim? How did they catch their food? How long did they live? Why did they disappear at the end of the Cretaceous? All these questions remain essentially unresolved. In fact, ammonites are a quite poorly understood group of fossils in many ways. By far the majority of scientific papers written about ammonites concentrate purely and simply on what is known as primary taxonomy — is this ammonite species distinct from all the others so far discovered and, if it is, how can it be recognised reliably and where else can it be found? The reason most scientists concentrate on these questions above all others comes down to the usefulness of ammonites for biostratigraphy. Many ammonite species evolved and died out within fairly short periods of time, perhaps a few hundred thousand years, but their fossils are often abundant and, most crucially of all, often very widely distributed. So, if a particular ammonite species can be found in sediments at two different localities, it’s a good indication that those two sediments were laid down within the same, rather narrow period of time. Just taking British palaeontologists as an example, virtually all the major scientists working on ammonites did so to further their studies of biostratigraphy: WJ Arkell, R Casey, MR House, MK Howarth, … Read More

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