Oculudentavis khaungraae is had a skull that was less than 2 centimetres long.Credit: Lida Xing

A miniature creature exquisitely preserved in amber for almost 100 million years is probably the smallest dinosaur ever discovered.

The animal’s bird-like skull, described in Nature on 11 March1, is less than 2 centimetres long — suggesting that the creature was the size of the bee hummingbird (Mellisuga helenae), the smallest living bird. Its discovery could help scientists understand how such creatures evolved to be so small.

“It reveals to us a whole new lineage of birds,” says Jingmai O’Connor, a palaeontologist at the Chinese Academy of Sciences Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, who co-led the study. O’Connor and her team assigned the animal a new genus and species, Oculudentavis khaungraae; the genus name means ‘eye-teeth bird’. The dinosaur weighed perhaps two grams and lived during the Mesozoic era, which lasted from about 250 million to 65 million years ago.

The fossil — which was found in Kachin state in northern Myanmar and dated to 99 million years ago — is exceptionally well preserved for a specimen of its size, O’Connor says. Its tiny beak is crammed with dozens of sharp teeth, suggesting that in life, it preyed on insects and other small invertebrates. Its eyes protrude from either side of its skull, meaning that unlike most modern predators, this dinosaur did not have binocular vision. And the size and age of the creature means that miniaturization in birds occurred much earlier than scientists previously thought.

“It’s a truly amazing specimen,” says Amy Balanoff, an evolutionary biologist at Johns Hopkins University in Baltimore, Maryland. The fossil record is biased towards large creatures, which can be preserved more easily in sedimentary rocks. But if the authors’ interpretation is correct, she says, it’s evidence that the ecological and morphological diversity that we see in modern birds goes way back. “And wherever it fits, it’s still an important fossil,” she adds.

Further research on the fossil — for example, looking for biomolecules in its preserved soft tissue — will require advances in research techniques that don’t damage the specimen, O’Connor says.

While amber specimens can give scientists detailed windows into the past, the fossil’s discovery — in war-torn northern Myanmar — highlights a dilemma faced by many scientists who work with the material, says Victoria McCoy, a palaeontologist at the University of Wisconsin in Milwaukee. The conditions in the amber mines there, as well as the political situation in Myanmar, have left some scientists questioning whether they should be working on these fossils at all. And although McCoy is continuing to work on such specimens, “it’s something that everyone should think about as they do this kind of research”, she says.