A shelled fossil discovered in an amateur’s collection may harbor the first direct evidence of prehistoric sharks eating ammonites some 150 million years ago.

The palm-sized ammonite, an extinct marine animal and distant relative of the modern nautilus, was fossilized with three shark teeth stuck in its shell, plus holes from the bite. Shark bite marks have been found in other fossils, such as crocodile poop, but with tough-shelled ammonites, paleontologists couldn’t pinpoint sharks and rule out other fishes or marine reptiles.

It's not often one knows with extreme certainty what 150-million-year-old predators actually ate.

“For the first time we have a direct link between the predator and prey. We can even give a name to the predator, which is a hybodont shark called Planohybodus,” said paleontologist Romain Vullo of France's Université de Rennes and author of the study published March 31 in Naturwissenschaften.

Hybodont sharks, also named hump-toothed sharks, grew to nearly 7 feet long and roamed ancient oceans for about 200 million years before vanishing with the dinosaurs 65 million years ago. Ammonites floated in the oceans at the same time, growing anywhere from a couple of inches to 10 feet wide. Soft tissue inside their shells was an attractive food source to many creatures.

Some species of hybodont sharks had flat teeth able to crush ammonites and other shelled creatures, but most species were thought to dine exclusively on fish.

“Before this discovery, we thought Planohybodus ate only fish because of its sharp teeth. They seemed better-suited for that kind of predation,” Vullo said. “This specimen shows it probably had a much larger range of prey, including ammonites.”

After reading reports of shark-like bite marks in ammonite fossils, Vullo remembered seeing the fossil in an acquaintance’s collection and asked to study it. He was able to match the teeth – one still embedded, two removed by the collector – to Planohybodus.

Vullo thinks such sharp teeth maimed ammonites by poking holes in their shells' air chambers, which the creatures used for stabilization and steering. Once an ammonite lost control, a shark could conveniently crush it.

"It's impossible to tell what happened with this ammonite, but I think it was bit and somehow escaped before dying," he said. "A scavenging shark wouldn't need to bite. It could eat the soft part without biting the shell."

To learn any more about the relationship between sharks and ammonites, however, Vullo said he needs to get his hands on similarly exceptional fossils.

“A complete skeleton with gut contents, or more tooth-embedded fossils, would be wonderful for making better conclusions about the ecological relationship here,” he said. “Unfortunately, such specimens are exceedingly rare.”

However, paleontologist Adiël Klompmaker of Kent State University in Ohio thinks such fossils may be less unusual than Vullo thinks. As with the collectors' specimen, they might simply have been overlooked.

“I think a lot more could be done,” Klompmaker said. “We should all take another good look at existing ammonite shells for evidence that they were prey.”

*Images: 1) Fossilized ammonite shell and shark tooth found in an amateur collection (Romain Vullo/Naturwissenschaften). 2) Close-up of *Planohybodus tooth embedded in the ammonite shell (Romain Vullo/Naturwissenschaften). 3) Illustration showing how hybodont sharks may have looked (Nobu Tamura/Wikipedia).

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Citation: "Direct evidence of hybodont shark predation on Late Jurassic ammonites." Romain Vullo. Naturwissenchaften, published online March 31, 2011. DOI: 10.1007/s00114-011-0789-9