Although ammonites were the most species-rich group of cephalopods and are among the most abundant fossil animals to be found, in many ways they remain an enigma. For example, what did they feed upon and why did they all die out at the end of the Cretaceous, about 65 million years ago?
The major problem in trying to answer the first question is that very few non-‘shell’ remains of ammonites exist, and although it is tempting to make comparisons with the still-extant (just) nautiloids, this is dangerous, as we do not know how similar these two groups really are. Jaws have been found in 43 ammonite genera, but radulae in only 9 genera. One view, which we put forward in the book, is that ammonites were largely bottom-feeders, pushing their shovel-like lower jaws along the sea bed and thus sweeping up detritus and small creatures.
Recently Isabelle Kruta and colleagues in France and USA (Kruta et al. 2011) have examined the upper and lower jaw and radula (together termed the buccal mass) from the ammonite Baculites, using, for the first time, synchrotron x-ray microtomography. This has enabled them to produce a three-dimensional reconstruction of the jaws and radula apparatus. They conclude that the buccal mass of these ammonites, including small delicate teeth on the radula, is most consistent with these creatures feeding by capturing and eating small organisms in the water column – plankton. This view is strengthened by the finding of possible planktonic prey in the buccal mass of this specimen.
Can this new information be used to answer the second question above? Kruta and colleagues think so. They postulate that plankton feeding may have contributed to the extinction of the ammonites, as several groups of plankton also suffered a severe decline at the end of the Cretaceous, and they compare this with the nautiloids which fed on other food sources and did not become extinct. This may be taking the results from the study of one fossil a bit far: ammonites, at least in shell shape and size, were a very diverse group, and it is certainly possible that they had a range of feeding habits, and yet they all went extinct. However, in a useful commentary to this report, Kazushige Tanabe (Tanable 2011) has developed this idea, suggesting that ammonites laid a large number of small eggs, producing small larvae that would be plankton feeders, and thus the late Cretaceous decline in plankton abundance would have affected both the adult plankton feeders, and the possibly greater numbers of larval plankton feeders. We now need an examination of further ammonite fossils to add support, or otherwise, to this argument.
Kruta, I, Landman, N, Rouget, I, Cecca, F, Tafforeau, P (2011) The role of ammonites in the Mesozoic marine food web revealed by jaw preservation. Science 331, 70-72
Tanabe, K (2011) The feeding habits of ammonites. Science 331, 37-38
Links to these papers will be available on Blackboard for students registered on the module.