The ancient bird, named Elektorornis chenguangi, lived 99 million years ago (Cretaceous period) and had a hyper-elongated third toe.
Elektorornis chenguangi belongs to a group of extinct birds called Enantiornithes, the most abundant type of bird known from the Mesozoic era.
The bird was smaller than a sparrow, and it was arboreal, meaning it spent most of its time in trees as opposed to on the ground or in water.
“I was very surprised when I saw the amber,” said Dr. Lida Xing, a paleontologist at the China University of Geosciences.
“It shows that ancient birds were way more diverse than we thought. They had evolved many different features to adapt to their environments.”
The fossil that Dr. Xing and colleagues examined is an incomplete right hindlimb with feathers preserved encased in amber, measuring 3.48 x 3.44 x 0.82 cm and weighing 5.51 g. It was found in 2014 at the Angbamo locality in the Hukawng Valley of Myanmar.
The researchers scanned the specimen with micro-CT and created a 3D reconstruction of the foot.
They found that the bird’s third toe, measuring 9.8 mm, is 41% longer than its second toe and 20% longer than its tarsometatarsus (a bone in the lower legs of birds).
They also compared the ratios with those of 20 other extinct birds from the same era and 62 living birds. No bird has a foot that resembles this one.
“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,” said Dr. Jingmai O’Connor, a scientist in the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences.
“But this extreme difference in toe lengths, as far as we know, has never been seen before.”
It remains unknown why Elektorornis chenguangi evolved such an unusual feature.
The only known animal with disproportionally long digits is the aye-aye (Daubentonia madagascariensis), a lemur that uses its long middle fingers to fish larvae and insects out of tree trunks for food.
Elektorornis chenguangi might have used its toe for the same purpose.
“This is the best guess we have. There is no bird with a similar morphology that could be considered a modern analog for this fossil bird,” Dr. O’Connor said.
“A lot of ancient birds were probably doing completely different things than living birds. This fossil exposes a different ecological niche that these early birds were experimenting as they evolved.”
The discovery is reported in a paper in the journal Current Biology.
Lida Xing et al. A New Enantiornithine Bird with Unusual Pedal Proportions Found in Amber. Current Biology, published online July 11, 2019; doi: 10.1016/j.cub.2019.05.077