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Chilesaurus diegosuarezi has a puzzling mix of anatomical features that make it hard to classify. First thought to be a theropod, a new study suggests it’s an ornithischian. 

Sebastian Silva/EFE/Newscom

Scientists struggle to fit this strange vegetarian dinosaur into the family tree

When scientists said in 2015 they had found a plant-eating theropod dinosaur, it was as if a vegetarian had crashed a meat lovers’ barbecue. Almost all theropods—the group of fierce predators that included Tyrannosaurus rex—were hypercarnivorous, eating a diet of more than 70% meat. Now, researchers are reporting that the dinosaur in question—the parrot-beaked Chilesaurus diegosuarezi—wasn’t a theropod after all, and instead belonged to a group of primarily plant eaters called ornithischians. They say it might even be a missing link between the two groups. But other scientists aren’t so sure the simplest answer is the right one.

“Since its discovery, Chilesaurus has been an enigmatic dinosaur,” says David Evans, a vertebrate paleontologist at the University of Toronto in Canada who was not involved in the discovery of the dinosaur or the new paper. The dinosaur is a strange jigsaw of features from different groups, he notes, “that make placing it on the family tree of dinosaurs difficult.”

Indeed, vertebrate paleontologist Fernando Novas of the Bernardino Rivadavia Argentine Natural Science Museum in Buenos Aires, who reported the dinosaur’s discovery, says he and his co-authors weren’t initially sure how to classify the 150-million-year-old Chilesaurus. It clearly ate plants with its flat teeth, and had a slender neck and horny beak rather than the thick neck and sharp teeth of a carnivore. But it also had numerous theropodlike characteristics, from the air spaces in its vertebrae to its stocky arms (although they ended in thick, short fingers rather than sharp claws). And it shared some features with other dinosaurs, such as primitive plant-eating sauropodomorphs and a particularly bizarre group of long-necked, vegetarian theropods known as therizinosaurs.

After numerous statistical analyses, Novas and his team concluded that the animal was probably a theropod. But, he says, he’s been expecting to hear alternate hypotheses from his colleagues.

First up: Matthew Baron, a Ph.D. student at the University of Cambridge in the United Kingdom, and paleontologist Paul Barrett, also at the University of Cambridge. In March, Baron and Barrett found themselves in a swirl of controversy—still ongoing—after they and others suggested a “shakeup” of the dinosaur family tree based on their analyses of a massive data set of fossil dinosaur characteristics. Their data set had many more ornithischians—the group that includes Stegosaurus and Triceratops—than other such analyses; one of the conclusions of that study was that theropods and ornithischians were more closely related than once thought.

Now, using the same data set, they decided to reanalyze Chilesaurus. “It was striking, it had a very ornithischianlike pelvis,” Baron says. The hip structures of ornithischians (“bird-hipped”) and saurischians (“lizard-hipped”) were one of the earliest features used to distinguish the groups. Theropods have traditionally been classed as saurischians.

Baron and Barrett came to a different conclusion. Unlike Novas, they report today in Biology Letters, they conclude that Chilesaurus was probably a primitive ornithischian. The theropodlike features, they suggest, actually lend support to their original hypothesis: that theropods and ornithischians are more closely related than previously thought. And Chilesaurus might be one transitional form that links them.

If their reclassification is correct, there would also be one less theropod that went vegetarian—an unusual transition. Herbivorous diets tend to be lower in nutrients than meat, and they’re tough on the teeth; such diets also require digestive adaptations such as gastroliths (gizzard stones) to break down tough plant matter. Even if Chilesaurus itself isn’t a theropod, scientists still have a handful of other theropods that have “gone green” to puzzle over, such as the therizinosaurs and another oddball called Limusaurus.

Other researchers, however, continue to raise questions about the reliability of the data set used for Baron and Barrett’s analysis. It’s certainly possible that Chilesaurus is something other than a theropod, “but the analysis they use to test this is problematic,” says Martin Ezcurra, a vertebrate paleontologist at the Argentine Natural Science Museum who was an author on the original Chilesaurus paper. Although the original paper classified the animals as a type of theropod known as a tetanuran, the new analyses included no tetanurans. “How can you test this hypothesis if you are not including any tetanuran in your analysis?” he says.

Novas, for his part, isn’t convinced that Baron and Barrett have the right answer, stating that their interpretation overlooks the many features Chilesaurus shares with theropods. Still, he says, “I welcome the novel interpretation by Baron and Barrett, because it promotes a necessary debate on poorly known aspects of dinosaur evolution as a whole,” he says. “I guess the discussion on Chilesaurus has just begun.”