Bite marks in Jurassic and Cretaceous ammonites: what did it?

Ammonites are among some of the best fossils to collect. They are relatively easy to find, for example, in the Lower Jurassic Posidonia Shale in Germany and the Jurassic sediments of Dorset in the south of England. The larger, well-preserved ones will always be sought after and, if of sufficiently high quality, may even be displayed in museum exhibitions. However, while these nicely-preserved, complete ammonites are ideal for identifying species, they often do not say much about the life history and, more specifically, the death of the ammonite itself. On the other hand, studying the sub-lethal or lethal damage to the fossil shell certainly does.

This article is about a relatively newly discovered type of bite mark. It is found on Jurassic and Cretaceous ammonites, might have occurred worldwide, is easy to recognise and is also fairly common.

Ventral damage

When I was browsing through the ammonite collections of several Dutch museums (including, Naturalis and Oertijdmuseum De Groene Poort) and the Geologisch-Paläontologisch Institut der Universität Münster in Germany, it became apparent to me that there were many specimens with damage to the outer whorl, on the back side of an ammonite (the ventral side) in its living position. This damage was not only visible on the ventral side, but could also be seen on both lateral sides, if preservation permitted.

When viewed from the lateral side, the damage has different shapes, including triangular, sub-circular, rectangular and irregular. These appear to be related to the degree of ornamentation of the ammonite shell – the stronger the ribs, the more rectangular the damage becomes laterally.

Ventral damage to ammonites cannot be found in every collection, as based on my study of about 35,000 ammonites spread over many such collections. However, Cretaceous and Jurassic collections do display this sort of damage (Figs. 2 and 3). I found it in Maastrichtian ammonites from the Netherlands and Belgium, notably in the species, Hoploscaphites constrictus. I also discovered that the Lower Cretaceous of southeast Spain (Miravetes Formation), in a collection I examined, yielded many ammonites with ventral damage, for example, in the genera, Barremites and Pseudothurmannia. In total, 17.1% of the ammonites in that collection were ventrally damaged. In addition, I found similar damage in Lower Cretaceous ammonites from Colombia, South America.

Cretaceous ammonites from southeast France also showed ventral damage – 17.9% of the collection I looked at. I also found similar ventral damage in Upper Jurassic ammonites from southwest Germany (Plettenberg and Geisingen) and found the same damage in Middle Jurassic, Indonesian, Irianites moermanni, and Middle Jurassic ammonites from Pamproux, France. Lower Jurassic collections from Dotternhausen, which have two-dimensionally preserved ammonites, are no different. As mentioned by Taverne (2000), the genera, Dactylioceras, Hildoceras and Harpoceras, show abundant ventral damage. Later on, our team discovered that Lytoceras specimens also exhibit this sort of damage. The percentage of ventral damage in ammonites in some layers of the Posidonia Shale in Dotternhausen is quite high, with up to 50% of specimens showing evidence of such injuries. Lower Jurassic ammonites also exhibit ventral damage, notably in specimens of Dactylioceras from Dorset studied mainly by Natascha Waljaard, as well as some Pliensbachian ammonites from Germany (Fig. 4).

The position of the ventral damage is remarkably uniform across collections. The position of this damage was measured by two methods, which are explained in Fig. 1. The greater majority of the damage can be found at about 180° from the aperture. Additional analysis of the large collections of the Lower Cretaceous of Spain and Lower Jurassic of southwest Germany clearly shows that most damage is located at the end of the living chamber, close to the last septum. Apparently, this was a favourite position to break.

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