Emerging in the Upper Palaeozoic and rapidly diversifying through the Mesozoic, thereby surviving over a period of 340 Ma, ammonites proved to be a stratigrapher’s dream organism as well as being the most widely collected fossils. As well as their rapid evolution of form, they were able to spread widely though the oceans in larval form, through the jet propulsion they shared with other cephalopods and because they floated when dead and drifted with currents. Much of ammonite taxonomy has centred for almost two centuries on their external for: ribs, keels, knobbles, intricacy of the sutures separating each body chamber and the previous one and whether or not their growing shell coils hid earlier parts or developed into an open spiral. These characteristics enables such a wealth of easily recognised genera and species that as zone fossils ammonites have been used to finely divide Mesozoic sediments; Jurassic ammonites locally divide the Jurassic (199 – 145 Ma) into time slices each of which represent a few hundred thousand years.
What is least familiar to non-specialists is the feeding apparatus of ammonites and what they actually ate. Thanks to the use of high energy X-ray images it turns out that, unlike squid, octopuses and the similar looking modern Nautilus, some Cretaceous ammonites would not have been able to rip apart large prey (Kruta, I et al. 2011. The role of ammonites in the Mesozoic marine food web revealed by jaw preservation. Science, v. 331, p. 70-72). Instead of a large beak-like process the ammonites studied sported a rasp-like radula, similar to that used on lettuce by the slug. The radula is armed with tiny but quite fearsome looking barbs, suitable for grating but not gnawing. The analysed ammonites may probably have eaten plankton. Indeed, one specimen turned out to have fragments of its last meal lodged in its radula; an isopod and a small gastropod. That diet tallies with the likely habitat of some ammonites; they were probably able to change their buoyancy by manipulating the gas and water content of their abandoned earlier body chambers to move up and down in the upper ocean. However, such was the stratigraphic duration, global spread and diversification of the ammonites, further studies of this kind would be needed to verify general plankton feeding. However, such a diet may well explain the conundrum of the total extinction of ammonites at the end of the Cretaceous while the superficially similar nautiloids survived and live today. The Cretaceous-Palaeocene (K-P formerly K-T) mass extinction devastated plankton, while larger marine organisms lived on to serve as nautiloids prey.
- Ammonites’ last meal: New light on past marine food chains (eurekalert.org)