More salad fancier than fearsome predator, relatives of today's crocodiles evolved to eat a plant-based diet millions of years ago during the age of the dinosaurs, according to new research.



Key points: Ancient crocs ranged from giant carnivores to small plant eaters

Ancient crocs ranged from giant carnivores to small plant eaters Researchers can work out what the ancient crocs ate from analysing fossil teeth

Researchers can work out what the ancient crocs ate from analysing fossil teeth Eating plants was a successful strategy for ancient crocs in a range of environments

Prehistoric reptiles developed a predilection for plant eating at least three times between 65 – 200 million years ago, according to research published today in Current Biology.

So, how did scientists pin down the diets of long-dead crocs? The clues were in their weird and wonderful teeth, said lead author, Keegan Melstrom.

"Extinct crocs had weirder teeth than I ever could have possibly imagined," said Mr Melstrom of the University of Utah.





Being a diet detective

Many ancient croc teeth were different to anything found in living reptiles, which made investigating their function challenging.

But the structural pattern of the modern teeth can provide clues about what the ancient teeth were used for.

A 3D tooth print of the extinct crocodyliform Chimaerasuchus. The tooth's structural complexity suggests that Chimaerasuchus was a plant eater. ( Supplied: Keegan Melstrom )

Mr Melstrom and his colleagues compared 146 fossil teeth from 16 species of extinct crocodyliforms with those of modern reptiles.

Crocodyliforms were a big, diverse group of ancient reptiles, a subgroup of which evolved to become modern day alligators and crocodiles, as well as their rarer cousins the gharial and caiman.



The researchers used computer modelling to determine the complexity of the biting surface of each tooth.

The more complex a tooth, the more likely it is to be used for eating plants over meat, Mr Melstrom said.

The analysis showed some crocodyliforms stuck to a traditional meat diet; others were omnivorous, eating meat as well as plants. Some ate insects, while still others were herbivorous, relying solely on plants for their fuel.



Different species of ancient crocodyliforms had a wide variety of tooth shapes. ( Supplied: Keegan Melstrom )

The "real triumph" of this paper is that it pinpoints the function of these weird forms of teeth, said Paul Willis, an ancient crocodile expert at Flinders University, who was not involved in the study.

"They can nail down the function of the teeth as an adaptation to herbivory, insectivory or omnivory. That's the breakthrough," he said.

Some croc ancestors lived at bottom of food chain

It was previously thought that crocodyliforms were always near the top of the food chain in this era.

But, surprisingly, this paper suggests ancient croc species could appear anywhere on the food chain, Dr Willis said.

"There are [ancient] crocodiles that would have taken down Tyrannosaurus without a problem. What you've got here is crocodyliforms that are actually at the bottom of the food chain."

Keegan Melstrom and Randall Irmis, with fossil and recent crocodilian skulls. ( Supplied: Keegan Melstrom )

Before embarking on this research, Mr Melstrom thought that crocs would only evolve to be herbivores when mammals, which may out-compete them, were absent.

But, he said, it turned out that plant-eating crocs could live in a range of environments, including, alongside mammals.

And the fact that ancient crocs evolved a taste for leafy greens on at least three separate occasions suggested this diet worked for them in a number of very different environments.



What did these old crocs look like?

Reconstructions of extinct crocodyliforms. Differences in tooth shape are related to differences in diets. ( Supplied: Jorge Gonzalez )

Ancient crocs were far more diverse than their surviving cousins. Some were fully aquatic, some lived entirely on land, while others were very similar to the fearsome semi-aquatic creatures of today.

One plant-eating ancient croc, Simosocus clarki, which lived in the Late Cretaceous up to 66 million years ago, looked more like a marine iguana from the Galapagos than an Australian salty.

"A normal person wouldn't look at that in a dark alley and be like 'Oh my gosh, this is a croc relative'," Mr Melstrom said.

Whereas another prehistoric croc, Armadillosuchus, which lived up to about 83 million years ago, looked more like a crocodile crossed with an armadillo. Its teeth suggested it was an omnivore, Mr Melstrom said.

Could today's crocs evolve to be plant eaters?

The evolutionary branch which gave rise to our modern crocodiles seems to be particularly carnivorous.

However, if you flush a modern croc's stomach, as much as 10 per cent of what's in their gut may be plant material, Mr Melstrom said.

"Probably most of this is picked up when they're eating other animals."

This tooth is from Armadillosuchus, an armadillo-like croc, which was likely omnivorous. ( Supplied: Keegan Melstrom )

Today's crocs have very simple peg-like teeth, because their meat-rich diet does not need much processing in the mouth.

Prehistoric crocs that ate plants as well as meat needed more complex teeth to process the plants, Mr Melstrom said. If all a croc was eating was plants, it would need even more complex teeth.

Even though alligators don't have the tools for the job, they have been observed picking fruit off trees in captivity, he said.

"[Captive alligators] want to eat fruit. It's an active thing."

"Animals have a more flexible diet than people give them credit for because we like to place them into wonderful discrete categories."

