Fatally bitten ammonites from the lower Lias Group (Lower Jurassic) of Lyme Regis, Dorset

摘要

Little is known about ammonites as prey. Two papers report single British examples of fatally bitten ammonites. Globally, two previous papers describe damage similar to that recorded herein. Nevertheless, in the Charmouth Mudstone Formation of the lower Lias Group at Lyme Regis, 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 reach the umbilical seam on one side, are missing. Midpoint of the damage lies at 80% of the body chamber length back from the aperture, with a coefficient of variation of 10%. Damage at this position would sever attachment muscles and facilitate removal of the body from the aperture. 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 bitten at a precise point. The most likely predator was an active swimmer, able to hold and manipulate a smooth, possibly slippery, ammonite shell. The slightly asymmetrical damage is consistent with bites made by robust, parrot-like, cephalopod jaws (rhyncholites), which are known from the same stratigraphic interval. Ammonites are less likely predators than teuthoids. Belemnites, lacking suckers, are discounted. Lobster-like crustaceans are unlikely to have been able to catch, hold and manipulate shells. Lack of tooth marks precludes most vertebrate predators. These bite marks are referred to Bicrescomanducator rolli Donovan, Paul, Andrew & Howe igen. et isp. nov.