COLLECTIONS & RESEARCH

Fatally Bitten Ammonites

An abridged version of the paper Fatally bitten ammonites from the Lower Lias of Lyme Regis by Museum  Education Officer – Chris Andrew, Geologist – Paddy Howe, Trustee – Chris Paul and Steve Donovan

Pablo Garcia-Gil has recently contacted us saying he has discovered cretaceous ammonites with similar bite marks. Examples are shown below.

We would be interested to hear from anyone who has made similar finds. Contact candrew@hotmail.co.uk

Table of Contents

  1. Summary & Introduction
  2. Fatally Bitten Ammonites – Results and Interpretation
  3. Fatally Bitten Ammonites – Predatory Behavior
  4. Fatally Bitten Ammonites – Identity of the Predator
  5. Fatally Bitten Ammonites – Discussion and References

Cretaceous bitten ammonite 1 Cretaceous bitten ammonite 2

Fatally bitten ammonites from the Lower Lias of Lyme Regis

By Chris Andrew, Paddy Howe, Chris Paul & Steve Donovan

Full text in Proceedings of the Yorkshire Geological Society, 2010, vol. 58, pp. 81-94. doi: 10.1144/pygs.58.1.276
Available at http://pygs.lyellcollection.org/content/current

Summary

Location of the sites mentioned in the text

In the Lower Liassic Charmouth Mudstone Formation, up to 20 % of small ammonites (Promicroceras, Xipheroceras, Cymbites, Arnioceras, Asteroceras, Caenisites, Eoderoceras and Paltechioceras) show ventral damage at the rear of the body chamber that is far too consistent in morphology and position to result from accidents. Single pieces of shell, which extend almost symmetrically on either side and usually reach the umbilical seam on one side, are missing. Midpoint of the damage lies at 80 % of the body chamber length with very little variation. Damage at this position would allow a predator to sever attachment muscles and remove the body from the shell more easily. Lack of shell chips preserved adjacent to damaged ammonites precludes scavenging or post-burial crushing as causes of the damage. Ammonites were caught in the water column, manipulated into a horizontal position and then bitten at a precise point. The most likely predator was an active swimmer, with the capability to hold and manipulate a smooth, possibly slippery, ammonite shell. We favour squids over belemnites and think that the slightly asymmetrical damage is consistent with bites made by robust, parrot-like, squid jaws. Lobster-like crustaceans are less likely to have been able to catch, hold and manipulate shells. Lack of tooth marks precludes most vertebrate predators.

Introduction

Although it is generally accepted that ammonites were predators in Mesozoic seas, much less is apparently known about ammonites as the victims of predation. For example, only two previous papers described British ammonites lethally damaged by predators; Martill (1990) recorded a single specimen of Kosmoceras preyed on by a semionotid fish, whereas Ward & Hollingworth (1990) documented a second specimen of Kosmoceras, which they interpreted to have been bitten by an unknown marine reptile. Globally the literature on lethally damaged ammonites is similarly sparse. However, Roll (1935) described injuries on Upper Jurassic ammonites in which many shells were consistently damaged by a single irregular hole at the rear of the body chamber. Klompmaker et al. (2009) described similar damage from several Mesozoic sources and pointed out that such damage occurred in as much as 20 % of the populations they examined. They distinguished this type of damage as ‘ventral bite marks’ in contrast to the lateral teeth marks left by vertebrate predators.

Photograph of a typical slab from the Topstones below Black Ven Right side of a pyritised Promicroceras

Here we describe damage to Lower Liassic ammonites, discuss its possible causes, and document its occurrence. We demonstrate that some single Liassic concretions contain more evidence of predation on ammonites than the entire hitherto published literature from Britain! We find it improbable that the Liassic of Dorset is the only British source of such evidence and hope this paper will stimulate others to look for similar damage elsewhere, both geographically and stratigraphically.

Material and Methods

The damaged ammonites come from the foreshore below Black Ven, east of Lyme Regis, or below Stonebarrow Hill (Fig. 1). Damaged ammonites either occur within eroded concretions (Fig. 2) or as pyritic internal moulds (Figs 3, 4). Line drawings have been prepared using a Wild camera lucida to measure the position of several key landmarks on the ammonite shell, in order to describe the damage quantitatively (Fig. 5). Our methods follow those used by Klompmaker et al. (2009), thus enabling our results to be compared with theirs. Four principal landmarks were used as follows: lines from the origin to the aperture (A in Fig. 5), the last suture (LS), the start of the damage (D1) and the end of the damage (D2). These lines allow measurement of the length of the body chamber (BC) and the position of the mid-point of the damage (M), which can then be expressed as a percentage of the length of the body chamber. Such measurements allow comparisons between specimens independent of their size. Measurements of the commonest ammonite genus, Promicroceras, from nodules and pyritic moulds yield very similar results. Illustrated specimens have been deposited in the Lyme Regis Philpot Museum (LYMPH registration numbers 2009/52-60 and 2010/1-20).

Left side of a pyritized Promicroceras Camera lucida drawing to illustrate the measurements taken on damaged ammonites

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