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Hammerhead marine reptile with teeth like needles was actually vegetarian

Hammerhead marine reptile Atopodentatus unicus ate algae

This artist’s rendition depicts Atopodentatus unicus, a hammerhead marine reptile that lived during the Middle Triassic.

(Y. Chen, Copyright IVPP)

Appearances can be deceiving. An ancient marine reptile that swam the seas long before the time of the dinosaurs sported a distinctive hammerhead jaw, along with two intimidating groups of teeth – some like chisels, others like needles.

But in spite of this fearsome-looking face, Atopodentatus unicus used its jaw for less-than-terrifying purposes: to scrape and eat algae off rocks. The fossils described in the journal Science Advances shed light on the diversity and recovery of animal life in the wake of a devastating extinction event that occurred roughly 252 million years ago.    

Scientists discovered fossil remains of this strange animal in 2014 in current-day southern China. It appears to have roamed the seas some 242 million years ago during the Middle Triassic. Measuring around nine feet long from head to tail, Atopodentatus unicus has a somewhat elongated neck, strong fore-limbs and a surprisingly small head for its size. But as implied by its name, which means “unique strangely toothed,” the standout features of A. unicus appeared to be its jaw and its many weirdly shaped teeth.

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At first, researchers believed that the marine reptile had a downturned snout that it used to stir up and eat tiny invertebrate animals in the mud of the sea floor, by sucking in water and then filtering it out through needle-like teeth, leaving the edible critters behind. The process, if true, would have been “a bit like a flamingo,” said Nicholas Fraser, one of the study authors and a vertebrate paleontologist at National Museums Scotland in Edinburgh, in an email.

But it turns out that the fossil on which this hypothesis was based, while fairly complete, was kind of squashed. And now, with two new and mostly intact skulls, an international team of researchers were able to more accurately describe its face, shifting the downturned snout into a hammerhead jaw.

“We have given it a face-lift – literally,” Fraser said.

The researchers believe that A. unicus used chisel-like teeth at the front of its mouth to scrape algae off of rocks in shallow coastal waters and then would filter it through a dense mesh of needle-like teeth. The researchers used colorful children’s modeling clay and toothpicks to figure out exactly how the mouth might work.

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“This was such a unique dentition that we need to reconstruct how the two jaws would interlock and work ... just to be sure we had a workable interpretation!” Fraser said.

Even so, the scientists didn’t expect to come to these bizarre conclusions, he added.

Not only is this a very strange shape for a face, but it also means that this animal was an herbivore – quite rare for marine reptiles at the time, probably in part because of relatively low plant diversity as well as what Fraser called the “restricted nature of plant life at sea.”

“This fossil took us very much by surprise. However, this was a whole different world,” he said, pointing to animals like Dinocephalosaurus, a roughly 15-foot-long, suction-feeding, seafaring reptile whose neck was at least half the length of its body. “So now we are beginning to accept this strange and wonderful environment that gave rise to very unfamiliar body forms.”

A. unicus also would have lived roughly 10 million years after the end-Permian extinction, a brutal mass extinction event that wiped out around 90% of life in the seas and 70% of life on land. How the oceans recovered and diversified after that devastation, filling up all the vacated nodes in the food web, is of great interest to researchers – and this fossil shows that, at least in some quarters, life was springing back.

“After the end-Permian mass extinction it is widely accepted that it took several million years for the trophic levels to fully recover,” Fraser said. “However, with our work, we are finding that by 243 million years or so ago (i.e. 9 million years after the extinction), there was a very complex and diverse assemblage of marine reptiles. The discovery of this completely new niche (herbivory in the sea) just amplifies our hypothesis that the recovery happened much more rapidly than previously thought.”

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