The smallest dinosaur ever found – a bird-like creature weighing less than a tenth of an ounce – has been discovered inside a drop of amber, where it was preserved for 99 million years.
The fossil skull of the new species, dubbed Oculudentavis, meaning "eye tooth bird," is just over half an inch long, and researchers think the animal was smaller than the bee hummingbird, the smallest known modern bird.
"It's really tiny," said one of the scientists who studied the new dinosaur, Jingmai O'Connor, a paleontologist at the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. "And it's just weird on multiple levels."
That weirdness is of particular interest to dinosaur experts, particularly because there are still disputes about whether the tiny dinosaur is also a primitive bird.
Modern birds evolved from flying dinosaurs, but even with just the surviving skull, the researchers say Oculudentavis is very different from modern birds. "It's a new ecology, never seen before," O'Connor said.
Hummingbirds feed mostly on nectar from flowers, but Oculudentavis had rows of sharp teeth and probably ate insects.
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The skull has large eyes with small pupils, so the dinosaur probably had good eyesight and hunted during the day.
But its eyes don't point forward like those of modern birds. Instead, Oculudentavis had eyes that bulged out to the sides and no binocular vision, the researchers found.
O'Connor said she thinks Oculudentavis evolved its tiny size to live on islands. As such, the dino lived at the same time as some giant dinosaurs about 99 million years ago but perhaps not in the same place.
Such miniaturization, called insular or island dwarfism, can be a response to an island's limited food sources, and it is seen in modern animals. "The bee hummingbird comes from Cuba. The tiniest vertebrate is a little frog from Madagascar," O'Connor said.
The Oculudentavis fossil was found in 2016 in northern Myanmar inside a block of amber – the dried resin of ancient trees. It made its way into a museum in China before it was studied by paleontologists.
The scientists were able to make detailed scans of the fossil with powerful X-rays while leaving the amber intact, said one of the researchers, Luis Chiappe, a paleontologist at the Natural History Museum of Los Angeles County.
Oculudentavis is one of only a few dinosaurs fossilized in amber that have been recovered by researchers. The more common process of fossilization in rocks preserves only much larger animals, Chiappe said.
"Amber has the potential of capturing very small animals," he said. "It opens up a window into a portion of the biodiversity that lived during the age of the dinosaurs that the normal fossil record does not capture."
Some other dinosaur experts are perplexed by the fossil and what it suggests. Although the researchers aren't certain that Oculudentavis was a primitive bird – a flying dinosaur, in other words – that's the conclusion they've reached based on the shape of its skull.
"This is a highly interesting fossil," said paleontologist Gerald Mayr of the Senckenberg Research Institute in Frankfurt, Germany, who was not involved with the new study. "If the authors are correct that it is a bird skull, it would show that our current understanding of avian evolution is still very incomplete."