UPDATED: World's Smallest-known Dinosaur Has Been Discovered Inside 99 Million-Year-Old Amber

UPDATE: The study this article is based on has been retracted by the authors.

The world's smallest known species of dinosaur has been discovered trapped inside a 99 million-year-old piece of amber. The fossilized tree resin was found to contain the tiny skull of a previously unknown dinosaur, which scientists have named Oculudentavis khaungraae.

The team, from the U.S., Canada, and China, came across the amber in 2016. "You can see the skull easily by holding up the amber to the light so as soon as I was shown a picture I knew it was something special," Jingmai O'Connor, from the Chinese Academy of Sciences and the Natural History Museum of Los Angeles County, told Newsweek.

O'Connor and colleagues have now described the new species in a study published in Nature. In it, they show how this non-avian dinosaur had a skull measuring just 0.2 inches in length. They estimate it would have been around the size of a bee-hummingbird, the world's smallest species of bird. It is also believed to be the smallest species of dinosaur from the Mesozoic era, between 250 and 65 million years ago.

The amber was found in Myanmar, which, 99 million years ago, would have been a mangrove swamp or part of an island arc. This environment, the team believes, is why O. khaungraae became so small. Species can become miniaturized in isolated environments, as being smaller has a number of benefits, such as being better for thermoregulation and requiring fewer resources.

O. khaungraae, the researchers discovered, had a number of unusual traits. It had a bird-like beak that was full of teeth, with 29 or 30 teeth identified. It was also found to have a large eye socket similar to a lizard, with a narrow socket that only let a small amount of light in, indicating it would likely have been active during the day. They believe it was a predator, feeding on small arthropods or invertebrates.

O'Connor said its strange features suggest it is a bird, and that it may have flown. "But there are no features of the skull that define birds or dinosaurs, so it's possible it's a non-avian dinosaur and not a bird, or even something else," she added. "It's so hard to say with something so weird and so incomplete.

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"There is no bird alive or fossil that would have behaved like this one, which of course makes it really hard to understand what it's lifestyle was really like. It has eyes indicative of acute visual abilities, numerous teeth and a reinforced skull. Based on these morphologies, it appears it was a tiny, predatory bird, most likely an aerial insectivore feeding on very tiny insects."

The team says this discovery opens up a new world of previously unknown creatures, with new technologies allowing scientists to identify species trapped in amber millions of years earlier. "We are only just at the very beginning of what we can discover about small animals from this amber locality," O'Connor said.

Study author Luis Chiappe, from the Natural History Museums of Los Angeles County, added, "Amber deposits tend to trap very small organisms and our new discovery is an example of that. There's no reason why you could not imagine very small vertebrate animals living in this ancient tropical forest.

In a related News & Views article, Roger Benson, from the U.K.'s University of Oxford, and was not involved in the study, said the "bizarre features" of O. khaungraae means its place in the evolutionary tree is unclear. The past decade has generated much data on the dinosaur–bird transition, greatly advancing our understanding of this major evolutionary event," he wrote. "In the past few years, Burmese amber has yielded surprising insights, including previously unseen feather and skeletal structures in other extinct birds.

"The study of small vertebrates preserved in amber, their ecosystems and their evolutionary relationships with one another is in a nascent phase. But Oculudentavis suggests that the potential for continued discovery remains large—especially for animals of diminutive sizes."