It was like nothing Ryan McKellar had ever seen.
Two tiny bird wings were encased in amber. They were 99 million years old, but they looked as though they could take flight any minute: delicate bones were arched in the middle of the wings and branched into fingers at the wing tips; a network of veins was woven through the preserved flesh; every barb of every feather was visible in the rich, brown plumage. In his whole career working with creatures caught for millennia in amber, no bird specimen ever stood out like this.
"It gives us all the details we could hope for," said McKellar, a who is curator of invertebrate paleontology at the Royal Saskatchewan Museum in Canada. "It's the next best thing to having the animal in your hand."
An analysis of the two fossils conducted by McKellar and his colleagues was published in the journal Nature Communications on Tuesday. They are the best remains of Cretaceous wings scientists have ever found, and they offer some surprising insights into the lives of these ancient animals, as well as the evolution of the birds we know today.
That's important, because traditional fossils from ancient birds are few and far between. Because bird bones are hollow, they don't mineralize very well. And feathers almost never survive the fossilization process. Most of what we know about prehistoric feathers comes from other amber fossils, but usually those contain single feathers, which don't mean much out of the context of the creature they belonged to.
The two new specimens, which were discovered in amber extracted from mines in Burma, illuminate details that scientists don't often get to see. They include the network of veins that supply the fleshy part of the wing, the follicles where feathers attach to the flesh, the pigmentation of the feathers, the complicated microstructures called barbs that "zip" flight feathers together so they stay firm and perfectly arranged.
The owners of these wings were members of an extinct group of hummingbird-size creatures called Enantiornithes. Coming along about a third of the way through bird history (the earliest avialan, which includes birds and bird-like dinosaurs, lived about 160 million years ago) these fossils show that the Enantiornithes were already far more similar to their modern bird relatives than to their dinosaur ancestors. The last vestiges of their reptilian origins are teeth in their beaks and claws at their wing tips.
The wings reveal another key difference between Enantiorithes and modern birds — something that paleontologists had a hunch about but haven't been able to prove until now: Enantiornithes were born basically fully developed.
"They were coming out of the egg with feathers that looked like flight feathers, claws at the end of their wing," McKellar said. "It basically implies they were able to function without their parents very early on."
By comparison, "modern birds are lucky if they're born with their eyes open" he joked. Most birds are born naked and utterly helpless. Even "precocial" creatures — ones that are able to move around almost immediately after hatching — come out of the egg with fluffy down feathers that eventually molt and are replaced by the feathers needed for flight.
"But we're not seeing any signs of molting in these specimens," McKellar said, "which suggests that may have been a secondary development in modern birds; it happened later in evolutionary time."
The owners of the amber-encased wings probably died when their feathers caught in the resin, or not long after. Their species would live another 33 million years, until the asteroid impact that wiped out the dinosaurs killed them off as well.
But their wing tips would survive, along with the amber that contained them. Their discovery is entirely thanks to the substance's incredible preservative powers. It starts as sticky resin, which injured trees secrete to stop insects from boring into them or bacteria from invading. It's intensely acidic, so it dries things out quickly, and potently anti-microbial. And it polymerizes as it ages, changing from a sap-like substance into what McKellar calls "nature's plastic."
"It basically seals things in" he said. "Like a time capsule."