“I was very surprised at the time,” Dr. Xing said, recalling that the fossil was “undoubtedly the claw of a bird.”

Image A 3D reconstruction of the bird’s foot and leg. Credit... Lida Xing

A handful of Cretaceous bird fossils have been found in Burmese amber, but this is the first to be identified as a new species. Named Elektorornis (“amber bird”) chenguangi, the specimen is described in a study, led by Dr. Xing, published Thursday in Current Biology.

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E. chenguangi was smaller than a modern sparrow and belonged to a family of birds called Enantiornithes, which was abundant during the Cretaceous period.

Its elongated toe structure has never been observed in other birds, living or extinct. Its foot also sported an unusual layer of bristled feathers, “unlike any adult bird known today,” according to Jingmai O’Connor, a co-author and paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.

Long toes are associated with arboreal animals that need a firm grip on tree branches. The bristles suggest that the bird’s foot also had a sophisticated sensory system. Dr. Xing’s team speculated that E. chenguangi may have used the long, sensitive digit to probe cracks in trees for insects and grubs, just as the aye-aye lemur uses a slim finger to extract food in modern Madagascar.