Ammonite Eats

A few weeks ago I wrote about how mosasaurs deftly cracked into the coiled shells of ammonites. The immense, seagoing lizards had a well-honed technique for breaking open the outer defenses of cephalopods – more squishy ammonite meat, less hard shell pieces.

But what did ammonites eat? Even though their fossils are extremely common, much of what we know about ammonites comes from shells, or pieces of shell. We don’t find ammonite gut contents in the same way that we can pick apart the fossilized food scraps in the stomachs of their predators, such as mosasaurs. The best clues about ammonite diets come from occasionally-preserved jaws – similar to the hard beaks of modern octopus and squid. When paleontologists looked in detail at the feeding apparatus of the straight-shelled ammonite Baculites, they were lucky enough to find the cephalopod’s prey preserved, too.

Isabelle Kruta and colleagues outlined the diet of Baculites in a Science paper last year. The key was the cephalopod’s buccal mass – the upper jaw, lower jaw, and a rasping feeding structure called a radula. These delicate hard parts are sometimes preserved inside ammonite shells, but, most often, are only seen when the creature’s shell breaks in just the right place or the sediment around the structures erode just enough to reveal their presence. Thanks to high-resolution x-ray imaging techniques, however, Kruta and co-authors were able to scan the contents of three roughly 66 million year old Baculites specimens from South Dakota’s Pierre Shale to reconstruct what the mouths of these animals looked like.

The Cretaceous cephalopod had asymmetrical jaws. In each specimen, the lower jaw was more than twice as long as the upper, and it seemed that the upper jaw might have been slightly more flexible than its counterpart. And the ammonite’s radula was a nasty, net-like arrangement of hard teeth which could have folded and unfolded in life. As scary as the reconstructed radula looked, though, Kruta and collaborators pointed out that the mouthparts didn’t represent a rapacious lifestyle of tearing chunks out of fish and other prey. The buccal mass of the prehistoric cephalopod most closely resembled its counterpart in small, open-ocean octopuses that feed on krill, copepods, and other small fare.

A bit of comparative anatomy narrowed down the list of possible prey. But some fortuitous fossilization provided the paleontologists with a rare view of what the ammonite may have actually be eating. One Baculites specimen – AMNH 66253 – contained the remains of several small invertebrates inside the buccal mass. Portions of several small isopods and a larval snail were preserved in the mouth area.

As Kruta and colleagues pointed out, though, there’s more than one explanation for the association. The small invertebrates could have been eaten by the ammonite, might have been feeding on the buccal mass of the dead ammonite, or might have been washed into the body of the ammonite during preservation. Just because fossil remains of multiple creatures are found together doesn’t necessarily mean that they were interacting with each other in life – every association demands an explanation.

In this case, predation is the most likely possibility. If the tiny invertebrates were washed in, then additional small fossils should have been found in the rest of the ammonite’s body chamber. Instead, the diminutive animals were only found in the buccal mass. And the fact that the isopods were broken up threw support to the idea that the ammonite had actually eaten them. If the isopods and the snail were scavenging a cephalopod carcass, then we would expect the preservation process to keep their bodies intact. The isopods were already cleft into multiple pieces, hinting that the Baculites died right in the middle of a meal. The long-shelled Baculites was most likely a predator of plankton.

Of course, Baculites is just one of many, many, many ammonite forms. Ammonites undoubtedly differed in diets just as they differed in size, shape, and shell arrangements. Forms closely related to Baculites – called aptychophorans – may have also fed on plankton, but we don’t know for sure. Additional scans of ammonite shells will give paleontologists a better look at the mouth parts of these prolific cephalopods and, if we’re lucky, what some of those ammonites had for their last meals.

References:

Kruta, I., Landman, N., Rouget, I., Cecca, F., & Tafforeau, P. (2011). The Role of Ammonites in the Mesozoic Marine Food Web Revealed by Jaw Preservation Science, 331 (6013), 70-72 DOI: 10.1126/science.1198793

Tanabe, K. (2011). The Feeding Habits of Ammonites Science, 331 (6013), 37-38 DOI: 10.1126/science.1201002