Ammonites are common fossils with distinctive chambered spiral shells – so common that they're probably some of the first fossils most of us encounter, as decorative objects sitting prettily in someone's home or workplace. They're cephalopods, extinct relatives of the octopus, squid, and cuttlefish, but unlike these, have an external shell. The still-extant Nautilus is a cephalopod with an external shell, and is often confused with the ammonites, but these two actually represent two different lineages within the cephalopods.
Like most molluscs, ammonites had a (presumably) chitinous feeding apparatus comprising jaws and a radula–a rasping row of teeth-like structures unique to molluscs. These have been recovered occasionally in the past, but using X-rays, these have been studied in-situ in the ammonite Baculites from the Upper Cretaceous of South Dakota, allowing them to be reconstructed in breathtaking, 3-dimensional detail (paper in Science).
What's more, the food of these creatures can also be detected and identified. Within the buccal mass of these ammonites were fragments of small isopods, also called pill-bugs, which are crustaceans that, as their name suggests, look like rounded pills.
Unlike Nautilus and other nautiloids, the jaw structure and the kind of food they ate suggest that Baculites fed on small prey in the water column (i.e. zooplankton), instead of larger prey on the sea floor (benthic). They hypothesize that this preference for planktonic food may be responsible for the extinction of ammonites at the end of the Cretaceous, when the composition of planktonic communities changed considerably, but this theory will certainly have to be tested with more fossil data, as the commentary article points out.