A giant reptile with a hammerhead-shaped snout and a mouthful of teeth swam the Triassic seas of southern China some 242 million years ago. According to researchers studying new fossils of Atopodentatus unicus, this crocodile-sized animal is the earliest known herbivorous marine reptile. It used its unique snout for grazing plants underwater. The findings are published in Science Advances this week. 

The first fossils of Atopodentatus unicus – or "unique strangely toothed" in Latin – were described in 2014. However, the skull wasn’t well preserved, and at the time, researchers thought the reptile ate by using its downturned snout to stir up invertebrates in the soft sediment, like a flamingo. 

Now, a team led by Li Chun from China’s Institute of Vertebrate Paleontology and Paleoanthropology examined two new fossil specimens of Atopodentatus unearthed from Middle Triassic outcrops of the Guanling Formation in Luoping County, Yunnan Province. According to their analyses, Atopodentatus didn’t have a downturned snout. Rather, the front of the snout was shaped like a hammerhead. The front edges of its jaws were lined with chisel-shaped teeth, and further in, there were densely packed needle-shaped teeth that formed a mesh for filter-feeding. 

"To figure out how the jaw fit together and how the animal actually fed, we bought some children's clay, kind of like Play-Doh, and rebuilt it with toothpicks to represent the teeth," study co-author Olivier Rieppel from the Field Museum of Natural History explained in a statement. Atopodentatus fed like a baleen whale, not a flamingo.

Using its chisel-like teeth, Atopodentatus scraped off pieces of algae from rocks and other underwater surfaces. Once these were loosened and suspended in the water, they would be sucked in and then filtered by the closely packed bunches of long, thin teeth. Water goes back out, but the tooth sieve keeps plant matter trapped.

These findings reveal that Atopodentatus is the earliest marine reptile to have evolved herbivory. This evolved a second time in an unrelated marine reptile called Henodus during the Late Triassic. This animal was an efficient suction feeder, though it didn’t have a hammerhead. Even though a dazzling diversity of feeding styles popped up after the end-Permian mass extinction 252 million years ago, no other case of herbivorous marine reptiles have been documented for the rest of the Mesozoic Era.

^{Life restoration of Atopodentatus. Y. Chen/IVPP}

^{Image in the text: Researchers made models out of children's clay and toothpicks to see how the jaw worked. Olivier Rieppel/The Field Museum}