Ammonite's Feast

Ammonoid Fossil

Ammonites dominate marine fossils from the Late Silurian to the end of the Cretaceous, a period of 300 million years (1). Their abundance over this period of time has made them immensely important index fossils for dating Late Palaeozoic and Mesozoic rocks and for characterising various kinds of marine communities (2). The subgroup Ammonoidea, to which the ammonites (order Ammonitida) belong (along with Anarcestida, Ceratitida, Clymeniida, Goniatitida and Prolecanitida) appeared in the Early Devonian period and are classed as cephalopods, with extant members such as squid, octopuses and the nautiloids (3).

Despite their abundance in the fossil record, little is known about the paleobiology of ammonites due to the lack of a direct living counterpart and a lack of preserved soft tissue (4). However some light has been shed on their feeding habits in an intriguing study of the mouth of the Mesozoic ammonite Baculites using synchrotron x-ray microtomography (5). This method nondestructively generates three dimensional maps of the fossil using x-rays by building up cross sectional images(6). The images generated showed a tiny snail and three tiny crustaceans in one of the ammonite's mouth and jaws and a radula that were adapted for eating prey floating in the water suggesting that these ammonites fed on plankton. The research also suggests a reason for the decline and subsequent extinction of the ammonites around the Cretaceous-Tertiary extinction event as plankton were severely hit at this time.

References:

1. Parker 2007, The Complete Guide to Fossils and Fossil Collecting p. 168
2. Summesberger 1985, Annalen des Naturhistorischen Museums in Wien 87 pp. 145-166
3. Kennedy 1977, Patterns of Evolution as Illustrated by the Fossil Record (ed. Hallam) pp. 251-304
4. Tanabe 2011, Science 331 pp. 37-38
5. Kruta et al. 2011, Science 331 pp. 70-72
6. Flannery 1987, Science 237 pp. 1439-1444