Ammonites’ last meal: new insight into past marine food chains

Ammonites are among the world’s most well known fossils but until now, there has been no experimental evidence of their place in the food chain. Using synchrotron X-rays, a Franco-American team of scientists led by Isabelle Kruta has discovered exceptionally preserved mouth organs of ammonites, along with the remains of a meal that show that these ammonites dined on plankton. Plankton was largely destroyed in the wake of the same asteroid impact that led to the demise of the dinosaurs and other species. After losing their source of food, ammonites and many other marine groups could not survive this cataclysmic event. These findings are published this week in Science.

Ammonites are extinct relatives of the squid and octopus. The Nautilus, a present-day marine invertebrate, is similar in appearance to many ammonites but is a more distant relative. Ammonites appeared about 400 million years ago (the Early Devonian) and experienced a population explosion in the Early Jurassic. In fact, ammonites became such an abundant and diverse part of the marine fauna that they are used by paleontologists as classic “index” fossils to determine the relative ages of marine Mesozoic rocks in which they are found.

Artist's impression of three Baculites sp. fossils. Although the spiral-shaped Ammonites are best known, many other forms existed, notably in the Cretacean. Baculites sp. had an elongated conical shape, in which the animal's soft body occupied the largest segment of the shell, at the bottom. The smaller elder segments had been emptied, and the mollusc could maintain its buoyancy by filling these with gas. This explains why they floated upside down.   Credit A. Lethiers, UPMC

The team of researchers, led by Isabelle Kruta (MNHN, CNRS, UPMC), used the ESRF to perform X-ray scans of exceptional quality of Baculites fossils found on AMNH expeditions to the Great Plains in the United States. Results suggests that the large group of ammonites to which Baculites belongs, had jaws and radula (a kind of tongue covered with teeth) adapted for eating small prey floating in the water.

The study used synchrotron X-ray microtomography to check the presence, and then digitally reconstruct the mouths of three fossils found in South Dakota. The three-dimensional reconstructions are of such high quality that the jaws and teeth are revealed in their complete form. In addition, one specimen has a tiny snail and three tiny crustaceans in its mouth, one of them having been cut into two parts. Because these planktonic fossils are not found anywhere else on the specimen, the team thinks that the specimen died while eating its last meal rather than being scavenged by these organisms after death.

	Reconstruction in 3D of the radula of Baculites sp. (AMNH 6653). Credit I. Kruta, MNHN

“I was astonished when I saw the teeth for the first time, and when I found the tiny plankton in the mouth,” says Isabelle Kruta (MNHN). “For the first time we were able to observe the delicacy of these exceptionally well preserved structures and use high quality details to obtain information on the ecology of these enigmatic animals.”

“When you take into consideration the large lower jaws of ammonites in combination with this new information about their teeth, you realize that these animals must have been feeding in a different way from modern carrion-eating Nautilus,” says Neil Landman (AMNH). “Ammonites have a surprisingly large lower jaw with slender teeth, but the effect is opposite to that of the wolf threatening to eat Little Red Riding Hood. Here, the bigger mouth facilitates feeding on smaller prey.“

“X-ray synchrotron microtomography is currently the most sensitive technique for non-destructive investigations of internal structures of fossils. It started ten years ago with primate teeth, but is now widely applied in palaeontology,” says Paul Tafforeau (ESRF). “ We made a first test on one of the Ammonite specimens after a test with a conventional scanner failed, and the results were so impressive that we scanned all the other available samples, discovering nearly each time a radula and for one of them and many other structures.”

Fragment of the body chamber of a Baculites sp. (AMNH 55901) ready for a scan at the ID19  microtomography beamline at the ESRF. The laser shows the path of the synchrotron X-ray beam. Credit I. Montero, ESRF

Ammonite jaws lie just inside the body chamber. The research team’s new scans of Baculites, a straight ammonite found world-wide, confirms older research that ammonites had multiple cusps on their radula teeth. The radula can now be seen in exquisite detail: the tallest cusp is 2 mm high, tooth shape varies from sabre to comb-like, and teeth are very slender. The jaw is typical of this group of ammonites (the aptychophorans) in that the lower jaw is larger than the upper jaw and consists of two halves separated along a midline.

Until recently, the role of ammonites in the marine food web was unknown, although some clues were provided by Landman and colleagues on the shape of the jaw, as well as a 1992 paper by Russian scientists that reconstructed some of the internal structures by slicing fossils.

"The plankton in the Baculites jaws is the first direct evidence of how these uncoiled ammonites fed. This helps in understanding their evolutionary success in the Cretaceous." says Fabrizio Cecca (UPMC).

“Our research suggests several things. First, the radiation of aptychophoran ammonites might be associated with the radiation of plankton during the Early Jurassic,” say Landman. “In addition, plankton were severely hit at the Cretaceous-Tertiary boundary, and the loss of their food source probably contributed to the extinction of ammonites. This research has implications for understanding carbon cycling during this time".

Isabelle Rouget (UPMC) agrees, adding that “we now realize that ammonites occupied a different niche in the food chain than we previously thought.”

Reference: I. Kruta et al., The Role of Ammonites in the Mesozoic Marine Food Web Revealed by Jaw Preservation, Science 7 January 2011.

This research was supported by the Centre National de la Recherche Scientifique (CNRS, France), the Museum National d´Histoire Naturelle (MNHN, Paris, France) , the Université Pierre et Marie Curie (UPMC, Paris, France), the American Museum of Natural History (AMNH, New York, USA) and the European Synchrotron Radiation Facility (ESRF, Grenoble, France).

	Photo of a fragment of the body chamber of a Baculites sp. investigated at beamline ID19 at the ESRF.   Credit I. Montero, ESRF

Top image: 3-D reconstruction of the radula of a Baculites fossil. Teeth in yellow and fragments of fossil's last meal in blue (crustacean) and pink (snail)