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This Enigmatic Dinosaur May Be the Missing Link in an Evolution Mystery
A 7-year-old boy discovered Chilesaurus diegosuarezi in southern Chile in 2010 during a geology expedition with his parents.
Credit: Gabriel Lío

A bizarre-looking dinosaur discovered by a young boy in Chile may be the missing link showing how members of one major dinosaur lineage evolved into a completely new dinosaur group, a new study finds.

Researchers in the United Kingdom say the species, dubbed Chilesaurus diegosuarezi, explains how some theropods, mostly meat-eating, bipedal dinosaurs, evolved into the herbivorous, long-necked ornithischians.

Previously, it was unclear how the "ornithischian group just suddenly appeared and became this well-adapted herbivorous group," said the study's co-lead researcher, Matthew Baron, a doctoral student of paleontology at the University of Cambridge in England. "There was no intermediate step. This is the first one we've found." [See Photos of the Missing Link, Chilesaurus diegosuarezi]

If future research confirms this finding, this would make Chilesaurus the earliest member of Ornithischia, a group that includes the armored dinosaurs, such as Stegosaurus and Ankylosaurus, as well as horned dinosaurs, such as Triceratops.

But not everyone is on board with this interpretation. Rather, more "grunt work" is needed to determine Chilesaurus' true identity, said Thomas Carr, an associate professor of biology at Carthage College in Wisconsin and a vertebrate paleontologist. Carr was not involved in the study.

This isn't the first time Chilesaurus has turned heads. In 2010, 7-year-old Diego Suárez, the son of two geologists, found the 145-million-year-old beast in southern Chile's Toqui Formation.

After Diego found the first specimen, excavations in Chile yielded more than a dozen Chilesaurus individuals, including four complete skeletons that ranged from turkey-size young dinosaurs to nearly 10-foot-long (3 meters) adult dinosaurs. But despite the abundance of fossils, Chilesaurus' anatomy was a real head-scratcher.

The creature looked like a mixture of lineages. It had the long neck, small skull and clunky feet of a sauropodomorph (a group of long-necked, herbivorous dinosaurs with lizard-like hips); the beak, teeth and pubic bone of an herbivorous, bird-hipped ornithischian; and the bipedal stance, robust forelimbs and ilium (the upper part of the pelvic bone) of a meat-eating theropod.

An artist's representation of the enigmatic, herbivorous species found in Chile.
An artist's representation of the enigmatic, herbivorous species found in Chile.
Credit: Gabriel Lío

To determine where Chilesaurus fit in the dinosaur family tree, the South American researchers looked at four data sets to compare the dinosaur's features with those of theropods, mainly from the Jurassic and Cretaceous periods, as well as with sauropodomorphs. In the end, they deemed Chilesaurus an enigmatic plant-eating theropod, a relative of Tyrannosaurus rex and the fearsome Velociraptor, according to a 2015 study published in the journal Nature.

<i>Chilesaurus diegosuarezi</i> walked on its hind legs as other theropods did. It also had robust forelimbs that looked like those of other Jurassic theropods, such as <i>Allosaurus</i>.
Chilesaurus diegosuarezi walked on its hind legs as other theropods did. It also had robust forelimbs that looked like those of other Jurassic theropods, such as Allosaurus.
Credit: Gabriel Lío

However, not everyone was satisfied with Chilesaurus' classification as a theropod. Earlier this year, Baron and his colleagues stunned dinosaur researchers when they published a study revising the dinosaur family tree. According to their analyses, theropods and ornithischians were more closely related than previously thought.

Baron wanted to see where Chilesaurus fit on the new family tree. So he reached out to Fernando Novas, a paleontologist at the Bernardino Rivadavia Natural Sciences Museum in Buenos Aires, Argentina, who was the lead researcher on the 2015 Nature study.

Novas provided data on Chilesaurus to Baron and study co-lead researcher Paul Barrett, a paleobiologist at the Natural History Museum in London. Once Baron and Barrett had Chilesaurus' information, they plugged it into their enormous data set, which has data on the earliest dinosaurs on record. [Gory Guts: Photos of a T. Rex Autopsy]

"[Chilesaurus] came out as basically the first diverging member of one of the major groups, which is a position that had never been suggested before," Baron told Live Science. "It was a bit of a surprise."

However, evolution is a long, complicated process. There are ornithischians that are older than the Late Jurassic Chilesaurus, but they likely evolved from earlier theropods, Baron said. The fossils of these older, transitional creatures have yet to be found, he said.

"More and more evidence is now appearing that the ornithischian group might just be entirely Jurassic and Cretaceous, that they weren't present in the first period of dinosaur history [the Triassic]," he said.

It's difficult to say which interpretation is correct — that is, whether Chilesaurus is a theropod or an early member of Ornithischia, Carr said.

"Having read these works side by side, I can understand why the [2015 researchers] thought what they thought: The evidence is convincing that it's a theropod," Carr told Live Science. "[But] this new paper is just as convincing that it's an ornithischian."

There's only one way out of this conundrum, Carr said: "All of these data sets have to be combined" so researchers can determine, once and for all, where Chilesaurus fits — a task that can't be completed unless all of the relevant and available data from the Mesozoic is used.

Chilesaurus diegosuarezi has characteristics of three different dinosaur groups. Its pubic bone points backward like that of an ornithischian dinosaur, perhaps because it provided the gut more surface area with which to digest plant matter.
Chilesaurus diegosuarezi has characteristics of three different dinosaur groups. Its pubic bone points backward like that of an ornithischian dinosaur, perhaps because it provided the gut more surface area with which to digest plant matter.
Credit: Gabriel Lío

Even though the family trees are different, it's possible to combine the data sets of the early dinosaurs that Baron used and the Sauropodomorpha and later theropod data sets that Novas and his colleagues used, Carr noted.

"It does take a lot of work, but in the end, what you get is a data set that includes all of the relevant characters from all of the relevant analyses," he said. Only then can researchers "get the single truth, the historical truth of the universe as it happened," Carr said.

Baron accepted the critique in stride. "That's genius because that's exactly what we need to do," Baron said. "And that's exactly what I am doing at the [moment]."

Combining data sets is arduous work and could take four or five years to complete, Baron said. But the end result would shine a light on dinosaur evolution, which is a valuable step forward because "we actually are finding we know less and less about dinosaur evolution," Baron said. [Photos: Newfound Tyrannosaur Had Nearly 3-Inch-Long Teeth]

If that's the case, then the discussion on Chilesaurus' relationships has just begun, Novas told Live Science. "However, 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," Novas said.

The new study was published online today (Aug. 16) in the journal Biology Letters.

Original article on Live Science.