Fatally Bitten Ammonites – Predatory Behavior

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

Predatory behaviour

Klompmaker et al. (2009) argued that the lack of shell chips associated with the damaged ammonites together with the absence of damage to the shell apertures argue against scavenging and in favour of predation as the cause of the damage. Furthermore, they considered the lack of associated shell chips to indicate that the attacks took place in the water column. We agree with both points.

Fig 19

Most of the Liassic ammonites that were bitten have body chambers that are between 194 and 290 degrees in extent (mean 237 degrees). This means that the midpoint of the damage is generally opposite the aperture (mean 185 degrees back from the aperture, n = 45). Such a position is close to the points where the muscles attached the body to the shell. Several pyritized ammonites preserve evidence of the muscle scars (Fig. 19). Thus, breaking the shell in this position would make it easier to sever the attachment muscles and remove the body from the shell. The most important point here is that the positioning of the damage is very precise, which implies complex and precise predatory behaviour.

Roll (1935) argued that the damage was caused by a pincer-like action. However, the hinge axis of the jaws of vertebrates and cephalopods is horizontal, whereas to bite an ammonite in life position requires a vertical hinge axis. Broadly speaking, the same argument applies to lobster claws as well. It seems unlikely that a sudden grabbing attack would have achieved such a precise position for the damage. Surely sometimes the predator would cause damage to other parts of the ammonite shell? Thus, we think the predator first grabbed the ammonite and then manipulated it into position. We also think the latter action involved reorientating the ammonite into a horizontal position relative to the jaws of the attacker and then rotating it until the predator was able to bite the shell precisely opposite the aperture. If our interpretation is correct, then the predator had to be able to hold and manipulate even smooth ammonite shells. This implies both considerable prehensile ability and considerable intelligence. Thus, the manipulation that we suggest was necessary could not have been easily achieved by predators with nothing but jaws, such as vertebrates.