Think of a hammerhead, and you probably imagine a carnivorous shark with a mallet-shaped head patrolling the seas for prey. But a 243-million-year-old fossil found in the Yunnan Province of southwestern China may just turn that image on its head.

This ancient hammerhead was no fish; it was a reptile. And instead of feasting on schools of hapless prey, its jagged teeth helped it forage for mouthfuls of algae—making it the first marine reptile that was a vegetarian.

“We were blown away by this,” says Nick Fraser, a paleontologist at the National Museum of Scotland. “There were some strange animals around [back then], and this reminds me of one of those Dr. Seuss creations in Oh, the Thinks You Can Think!”

The first sample of this species was unearthed in 2014. At the time, the fossil seemed largely like any other Triassic marine reptile, with a long body measuring about three meters, a short neck, and flippers equipped for ocean life. However, the researchers were captivated by its extraordinary mouth, which housed hundreds of tightly packed needlelike teeth.

Based on the skull, which was slightly disfigured, the team hypothesized that the mouth took the shape of a down-turned snout, similar to the beak of a flamingo. Named Atopodentatus unicus (Latin for “unique peculiar teeth”), this marine creature was believed to use its snout to dig into the ocean floor for crustaceans and other small prey then filter them out using its thin teeth, like a baleen whale collecting krill.

“The animal described in 2014 was based on one specimen, which was pretty complete to be honest,” says Fraser. “But the skull was a bit mashed up, particularly on the front end, and it was difficult to see how it all went together. The original authors obviously did the best they could.”

Life in Bloom

While the new Atopodentatus fossil largely matched up with the original, there was one distinctive feature that immediately caught the team’s eye: a hammerhead.

A closer analysis of the new skull, which is a mere 4.5 inches (11.4 centimeters) wide, showed that the front of the wide mouth was lined with an array of large chisel-like teeth, followed by the collection of thin teeth toward the back of the jaw.

“It was very difficult just looking at the fossil itself to see how the thing could have worked,” says Fraser. “So we bought some clay and toothpicks and arranged them in this jaw and saw what was going on.”

Their results, published today in Science Advances, show that Atopodentatus was most likely an early herbivore.

View Images An artist's impression of what the Triassic hammerhead would look like while feeding Illustration by Y. Chen

While the vast majority of post-Permian species were carnivorous, Atopodentatus was using its large front teeth and wide mouth to loosen algae from the ocean floor, the researchers argue. The reptile then vacuumed up plant-laden water and filtered it through its densely packed teeth, trapping the vegetation in the process.

"This is just such a bizarre [animal], eating something that we really didn't think about [as a food source] before this fossil," says Sterling Nesbitt a paleontologist at Virginia Polytechnic Institute and State University who was not involved with the study. “It’s basically an underwater lawn mower. There is absolutely nothing like it alive today.”

This prehistoric John Deere lived during a unique period in geologic history—about six million years after the Permian extinction. The largest mass die-off in Earth’s history, the mysterious event killed up to 95 percent of all marine life.

“The general feeling is that it took some time to recover after such an extinction,” says Fraser. But the deposits in China paint a picture of life bouncing back with a huge array of new forms to fill the variety of ecological niches.

“The theme of the Triassic is ‘expect the unexpected’ in terms of what you are going to find, because it’s the time of experimentation with these reptiles,” says Nesbitt. “They are evolving very quickly after this mass extinction, and you just get so many weird forms.”

Such an explosion of life might explain the evolution of never before seen features for grazing on newly available food sources.

“Vertebrates were not the only ones affected by this Permian extinction—all organisms were,” says Nesbitt. “Potential food sources were going through their own evolution too. So maybe the food source Atopodentatus ate was all over the place because of this extinction.”

Hammerhead History

The find raises questions about how and why other animals adapted to a vegetarian lifestyle, as well as why hammerheads have appeared at various times in Earth’s history.

According to the researchers, a turtlelike reptile called Henodus, which lived later in the Triassic period, used a similar feeding technique as Atopodentatus, but it did not have a hammerhead.

Meanwhile, other animals have developed hammer-shaped heads but for vastly different functions. An amphibian known as Diplocaulus, for example, lived around 270 million years ago and had a boomerang-shaped head that helped provide lift while underwater.

And the contemporary hammerhead shark is believed to use its wide head for more efficient hunting—eyes positioned at each end of the head allow for 360-degree vision, while the placement of the shark’s electroreceptors across a wider area enables better tracking of prey.

Fraser and his team are continuing to excavate in the Yunnan region and are optimistic about the potential for new discoveries to better understand how Atopodentatus and its weird head fit in the ancient ecosystem.

“We are finding new species all the time, and once we have got a few more under our belt, we might be able to start putting together a little bit more about the ecosystem and how the whole thing interacts,” says Fraser. “There is huge potential for so much more to be done, and this is possibly just the tip of the iceberg.”