A nautiloid with ventral bitemarks from the Charmouth Mudstone Formation (Lower Jurassic) near Lyme Regis, Dorset, UK

Abstract

Locally within the Charmouth Mudstone Formation (Sinemurian, Lower Jurassic) of the Dorset coast fatally bitten ammonites with ventral bite marks are common. Ventral bite marks occur at the rear of the body-chamber. The predator probably severed attachment muscles, thus removing its prey more easily. We report the first ventrally bitten fossil nautiloid, which is unusual in that the large bite mark is entirely within the phragmocone. The nautiloid is juvenile with a very short body-chamber extending 46° back from the apparently undamaged aperture. The whorl cross-section is nearly circular, giving the predator little to grip when biting. Nautiloid shells are also thicker than ammonite shells. Despite these difficulties, the predator bit repeatedly and damaged both the shell and septa. Five rounded bites occur on the left side, separated by sharp points. This suggests the predator was probably another cephalopod with suckered arms and powerful jaws. The centre of the bite is 160.5° back from the aperture, close to the mean value for ammonites (184.5°), suggesting the predator located the position of the first bite opposite the victim's aperture. Thus, we think the predator attacked an incomplete nautiloid shell which explains why the damage affects only the phragmocone.