Researchers in China found the tiny-winged animal with an unusually large digit on each foot, and believe it was used to hook grubs out of tree trunks.
They said it’s the first time they have seen a structure of toe and foot like it in birds, either extinct or living.
Lida Xing, of China University of Geosciences in Beijing, said: “I was very surprised when I saw the amber.
“It shows that ancient birds were way more diverse than we thought. They had evolved many different features to adapt to their environments.”
The study found the bird’s third toe, measuring 9.8mm, was 41% longer than its second toe, and 20% longer than its tarsometatarsus, a bone in the lower legs of birds.
Based on the fossil, the team thinks the bird, which they named Elektorornis, was smaller than a sparrow and would have spent most of its time in trees.
Elektor is from the Greek meaning amber, while ornis is the same language, meaning bird.
The co-author of the report, published in Current Biology, Jingmai O’Connor at the Chinese Academy of Sciences, said: “Elongated toes are something you commonly see in arboreal animals because they need to be able to grip these branches and wrap their toes around them.”
The amber the foot was found it was discovered at the Hukawng Valley in Myanmar, formerly known as Burma, in 2014.
The area around it was full of trees which produced a gooey resin from the bark.
Small animals like geckos and spiders, as well as plants, get trapped in the resin and become fossilised after millions of years.
The team got the piece of amber from a local tradesman, who had no idea what animal the foot belonged to. Some traders through it was a lizard’s foot because of the length of the toe, according to the scientists.
However, the foot in the amber has four toes, like most birds, while lizards have five.
The only known animal with disproportionately long digits is the aye-aye, a lemur, which has long middle fingers it uses to fish larvae and insects out of tree trunks to eat.