Fossilised Teeth Suggest Some Ancient Crocodiles Were Vegetarians

Fossilised Teeth Suggest Some Ancient Crocodiles Were Vegetarians

Alligators and crocodiles are some of the scariest carnivores on Earth today, but new research suggests their ancient relatives developed a preference for plants.

A common misconception about crocodilians is that they’ve remained largely unchanged over the course of their 200-million-year history.

New research published in Current Biology provides further evidence that ancient crocodyliforms — a group from which modern reptiles such as crocodiles, alligators, caimans and gharials are descended — were more varied over the course of their evolutionary history than is typically appreciated.

This research from the Natural History Museum of Utah suggests some species of ancient crocodyliforms adapted both omnivorous and herbivorous diets. The authors of the new paper, graduate student Keegan Melstrom and museum chief curator Randall Irmis, reached this conclusion following a careful analysis of fossilised crocodyliform teeth.

Today’s crocodilians have fairly boring teeth, both in terms of their complexity and inter-species variability. These living reptiles all feature sharp, cone-shaped teeth — exactly the configuration you’d expect from aquatic ambush predators. These teeth are well suited for grabbing hold of wriggling prey and ripping out chunks of meat.

False-colour 3D images showing the range in shape of crocodyliform teeth. (Image: Keegan Melstrom)

But as Melstrom and Irmis discovered, ancient crocodyliforms had teeth with a surprising degree of variability.

In all, the team studied 146 fossil teeth from 16 different extinct species. Using high-definition 3D mapping and casts of teeth collected from multiple institutions and researchers, they documented teeth similar to those seen today, as well as crushing molars and canine teeth reminiscent of modern mammals.

Some teeth, however, left them scratching their heads.

“We see all these tooth shapes in the history of reptile evolution, and some of them are fairly straightforward, but others just have no comparison,” said Irmis in a NHMU press release.

To analyse the teeth further, the scientists applied a method that allowed them to assess the physical complexity of each tooth. The technique considered every nook and cranny, evaluating the tooth according to its degree of complexity.

“The more complicated the teeth are, the more plant material the animal eats,” according to Melstrom. At the same time, the teeth of omnivores “fall somewhere in between” in terms of physical complexity, he said.

Armadillosuchus, an extinct omnivorous crocodyliform. (Image: Keegan Melstrom)

This analysis showed that some ancient crocodyliforms who lived alongside dinosaurs were most certainly vicious carnivores, but some were omnivores, who added plants and insects to their diet. The extinct crocodyliform Armadillosuchus is one such example.

And some had, perhaps unexpectedly, adopted an exclusively vegetarian diet. Examples of these herbivorous crocodyliforms included Chimaerasuchus and Pakasuchus.

Fascinatingly, the data suggested that this dietary transition happened independently on three different evolutionary occasions, and possibly as many as six.

Plant-munching crocodyliforms appeared almost immediately after the end-Triassic mass extinction and they hung around until the end-Cretaceous mass extinction, according to the new research. That’s a stretch of 135 million years, running from 200 million to 65 million years ago.

This new study points to the remarkable diversity of ancient crocodyliforms, and their mixed role within ancient food webs.

“Our work demonstrates that extinct crocodyliforms had an incredibly varied diet,” said Melstrom. “The herbivores lived on different continents at different times, some alongside mammals and mammal relatives, and others did not. This suggests that an herbivorous crocodyliform was successful in a variety of environments!”

Speaking to ABC News, palaeontologist Paul Willis from Flanders University said the “real triumph” of the new paper was in how Melstrom and Irmis were able to associate functions to the various teeth.

These researchers were able to “nail down the function of the teeth as an adaptation to herbivory, insectivory or omnivory,” said Willis, who wasn’t involved with the new work. “That’s the breakthrough.”

Looking ahead, the researchers are hoping to better understand how an environment urges a species toward a plant-based diet, and why crocodiles diversified so much after the end-Triassic extinction but not the end-Cretaceous mass extinction.