These creatures, once the most abundant marine animals on Earth, likely went extinct because of a food shortage.
The last meal of a Dinosaur-Era ammonite was found still lodged in the marine animal's mouth.
High-powered X-rays reveal that ammonites had jaws and a toothed tongue-like structure.
Ammonites likely went extinct because of a shortage of certain key prey at the end of the Cretaceous Period.
The last supper of an ammonite, a shelled cephalopod relative of squid and octopus, was stuck in the individual's mouth when it died, described in a new study published in the journal Science. Extremely powerful X-rays revealed that the Baculites ammonite's final meal was a small snail and three tiny crustaceans.
Ammonites, one of the Dinosaur Era's most abundant marine animals, first emerged around 400 million years ago before dying out, along with dinosaurs and numerous other animals, 65.5 million years ago.
"Unfortunately we do not know what killed the Baculites," lead author Isabelle Kruta of Paris' National Museum of Natural History told Discovery News.
Kruta and her team, however, were able to unveil amazingly vivid details about this specimen, and other ammonites that are all extinct.
In a scientific first, a process called synchrotron X-ray microtomography, which creates cross sections of 3D objects without harming them, was used on several ammonites. Some of the fossils were discovered at a site in Belle Fourche, S.D.
The researchers determined ammonites possessed jaws and a radula, which is like a tongue covered with teeth. In this case, the slender teeth varied from saber- to comb-like.
"Ammonites used their radula to trap the food in the mouth and convey it through the esophagus," Kruta explained.
This feeding system, along with the last supper remains, suggests that ammonites were adapted for eating small prey, such as small crustaceans and plankton, floating in the water. Although ammonite fossils are readily available, and are even sometimes sold as jewelry, no one previously knew much about their diet and lifestyle.
Now scientists suspect that the meteor impact thought to have wiped out dinosaurs also killed many of the small creatures hunted by ammonites.
In a separate paper also published in Science, paleontologist Kazushige Tanabe of The University of Tokyo proposes that newly hatched ammonites were particularly dependent upon such small prey.
"The abrupt decline of phytoplankton at the end of the Cretaceous led to the collapse of marine food webs and would have greatly affected the survival of newly hatched ammonites," Tanabe told Discovery News.
"Ammonites are some of the most famous invertebrate animals in Earth's history," he added, "yet as biological entities they are poorly understood, largely owing to the absence of a direct living counterpart."
The new high-resolution 3D images help to solve those mysteries, he said. They also help to explain why one of the world's most successful animals, in terms of abundance and species longevity, suddenly bit the dust with dinosaurs. If just a single link in the food chain disappears, it can have devastating consequences on many other animals.
"Known predators of ammonites were, for example, mosasaurs and plesiosaurs," Kruta said. Mosasaurs were large, toothy marine lizards. Plesiosaurs were carnivorous aquatic reptiles. Both groups of animals are now extinct, but it remains unclear how much the ammonite die-off affected their demise.
Kruta and her team hope additional research can help to further unravel what happened to ammonites and other prehistoric animals.
Those answers may come sooner rather than later, as Tanabe believes the new uninvasive X-ray technique "can be widely applicable to other fossilized delicate organismic structures preserved in sedimentary rocks, such as specimens preserved in museum collections."