When archaeologists discovered the fossilized remains of Chilesaurus diegosuarezi in southern Chile, they were perplexed.
The dinosaur had a unique mix of features, some of which belonged to the theropod group of dinosaurs—mostly carnivorous beasts that included Tyrannosaurus rex —but also other characteristics that placed it outside that classification, such as flat, leaf-shaped teeth that indicated a vegetarian diet.
Researchers decided in 2015 that Chilesaurus should be classified as an unusual form of theropod. But now, a new study suggests that the dinosaur was actually an early member of a completely different group, known as ornithischia. And, the researchers say, this species could be the missing link between herbivorous dinosaurs and their meat-eating counterparts.
Dinosaurs first appeared around 230 million years ago and have long been divided into two basic categories based on their hip structure. Ornithischians —a Greek term meaning “bird-hipped”—have a four-pronged pelvic structure with a pubic bone that points downwards and are the ancestors of modern birds. Saurischians, or “lizard-hipped,” have a three-pronged pelvis with the pubic bone pointing upwards and are the forebears of modern reptiles like crocodiles. Theropods like Tyrannosaurus and Velociraptor are a subdivision of saurischia.
In the new study, published in Biology Letters , paleontologists from the University of Cambridge, in the U.K., and the Natural History Museum in London analyzed more than 450 anatomical features of early dinosaurs in order to classify the Chilesaurus, which lived during the Late Jurassic period around 150 million years ago. They determined that the dinosaur likely belonged to a “transitional” group that marked the early evolution of ornithischian dinosaurs.
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“Chilesaurus almost looks like it was stitched together from different animals, which is why it baffled everybody,” said Matthew Baron, a postdoctoral student at the University of Cambridge and the paper’s joint first author.
A Chilean family discovered the first fossilized remains of Chilesaurus in 2004; the species was named after the seven-year-old boy, Diego Suarez, who stumbled upon a fossil at the Black Hill site near General Carrera Lake in southern Chile, the Guardian reported.
The dinosaur, which grew to the size of a small horse, had a strange collection of anatomical features. Some traits were consistent with ornithischia, which include popular dinosaurs like Stegosaurus and Triceratops. For example, Chilesaurus had the bird-like hip structure and had flat teeth that were adapted for grinding down plant matter. But it also lacked features that were considered fundamental to ornithischian dinosaurs, such as a predentary bone, a stump on the lower jaw that, together with a complementary formation on the upper jaw, formed a beak-like structure used for clipping leaves or other plant material.
Chilesaurus also had stocky forelimbs like the carnivorous allosaurus. But instead of sharp claws, it had two stumpy fingers at the ends of its forelimbs.
The researchers concluded that Chilesaurus was probably an early ornithischia that may mark the point at which the two main divisions of the dinosaur family tree split. “It seems it became more advantageous for some of the meat-eating dinosaurs to start eating plants, possibly even out of necessity,” said Paul Barrett, a professor at the Natural History Museum and a study co-author.
The findings on Chilesaurus appear to fit a hypothesis made by the same two researchers, Baron and Barrett, in a seminal study published in Nature in March proposing a new evolutionary history for dinosaurs. Rather than dividing them into two main divisions based on the hip bone—saurischia and ornithischia—Baron, Barrett and co-author David Norman, a paleontologist at the University of Cambridge, proposed a new division between saurischia and ornithoscelida; ornithischian dinosaurs like Triceratops and theropod creatures like Tyrannosaurus would then be classed under the same broader group.
The March study received some positive feedback but was also greeted with skepticism by some in the scientific community. “This is just one analysis, and lots of recent studies recovered the more traditional grouping. Since this new result contradicts such a vast legacy of research,” Steve Brusatte, a paleontologist at the University of Edinburgh, told The Atlantic of the proposed new division. “I think the bar [to accepting it] should be high,”