Researchers have discovered a bird foot from 99 million years ago preserved in amber that had a hyper-elongated third toe.
Enantiornithes were wiped out along with the non-avian dinosaurs, while Neornithes went on to become the diverse group of birds - from ostriches to penguins, eagles to hummingbirds - that now inhabits our planet.
The soft tissue, found on the bird's foot, is also without parallel. Wiped out in the Cretaceous-Paleogene extinction event around 66 million years ago, they were the most common birds of the Mesozoic era.
The foot was so distinctive that O'Connor and her team declared the bird, which was probably about the size of a sparrow, a new species, naming it Elektorornis chenguangi. "But this extreme difference in toe lengths, as far as we know, has never been seen before".
During the area was full of trees that produced a gooey resin that oozed out of the tree bark.
Small animals like geckos and spiders, and plants, often get trapped in the resin and become fossilised in the amber after millions of years. Scientists have discovered many extinct animals, including the oldest known bee and a feathered dinosaur tail, in amber from this valley.
It was first presented to Chen Guang, a curator at China's Hupoge Amber Museum, and initially suspected to be an extinct lizard. "Like most birds, this foot has four toes, while lizards have five".
The elongated toe has been compared to those observed in the case of tree-climbing lizards and lemurs, and it infers that some of the early bird types may have had an exciting lifestyle.
"I was very surprised at the time", Dr Xing told the Times, recalling that the fossil was "undoubtedly the claw of a bird".
It remains unknown why the amber bird evolved such an unusual feature. The aye-aye is a lemur that uses its long middle fingers to fish larvae and insects out of tree trunks for food. Therefore, the researchers suggest Elektorornis might have used its toe for the same goal. Without these fossils, "we would never have direct evidence of the fanciful deep evolutionary history of birds", he said. "There is no bird with a similar morphology that could be considered a modern analog for this fossil bird". A lot of ancient birds were probably doing completely different things than living birds.
The Chinese team now hope to study proteins and pigments in feathers found on the surface of the amber in the hope of better understanding how the bird was adapted to its environment, for example by using its feathers for camouflage.
This work is supported by the National Natural Science Foundation of China, the National Geographic Society, USA, the Fundamental Research Funds for the Central Universities, and Natural Sciences and Engineering Research Council of Canada.