Paleontologists unearthed the partial skeleton of an enantiornithine (opposite bird) that lived in what is now the state of Utah about 75 million years ago (Upper Cretaceous era). According to an analysis of the fossil, published in the journal PeerJThe enantiornithines of the Upper Cretaceous were the aerodynamic equals of the ancestors of today's birds, capable of flying strong and agile.
All birds evolved from feathered theropods – the two-legged dinosaurs T. rex – beginning about 150 million years ago, and developed in many strains in the Cretaceous, between 146 and 65 million years ago.
But after the cataclysm that swept most dinosaurs, only one group of birds remained: the bird ancestors we see today.
Why did only one family survive mass extinction? The newly discovered fossil of one of these groups of extinct birds, the enantiornithines, deepens this mystery.
"This particular bird, called Mirramce eatoniis about 75 million years old – about 10 million years before extinction, "said Dr. Jessie Atterholt, a researcher at the Western University of Health Sciences.
"One of the really interesting and mysterious things about the enantiornithines is that we find them all through the Cretaceous period, for about 100 million years of existence, and they were very successful."
Mirramce eatoni is among the largest North American birds of the Cretaceous; most were the size of chickadees or crows.
The specimen was found at the Kaiparowits Formation at the Grand Staircase-Escalante National Monument in Garfield County, Utah, in 1992, and is the most complete enantiornitino collected in North America to date.
"Most birds in the Americas are from the Late Cretaceous period and are known only by a single foot bone, usually the metatarsus. This fossil was almost complete, with only the head missing, "paleontologists said.
Mirramce eatoniThe bone of the chest or sternum, where the muscles of the flight attach, is more deeply attached than other enantiornithines, which implies a larger muscle and a stronger flight, more similar to modern birds.
The lucky bone is more V-shaped, like the bone of the modern bird fate and, unlike the U-shaped lucky bone, from previous birds and their dinosaur ancestors. The wishbone or wishbone is flexible and stores the energy released during the course of the wing.
"What is most exciting, however, are large patches on the bones of the forearm. These rough spots are feather buttons, and in modern birds they anchor the wing feathers to the skeleton to help strengthen them for active flight, "said Dr. Atterholt.
This is the first finding of feather buttons on any bird enantiornithine, which tells us that Mirramce eatoni was a very strong leaflet.
"We know that birds early in the Cretaceous, about 115 to 130 million years ago, were able to fly, but probably not as well adapted to it as modern birds," Atterholt said.
"What this new fossil shows is that the enantiornithines, though totally separate from modern birds, have developed some of the same adaptations to highly refined advanced flight styles."
However, if the enantiornithines at the end of the Cretaceous were as advanced as modern birds, why did they die with the dinosaurs while the ancestors of modern birds did not?
A recently proposed hypothesis argues that they were primarily forest dwellers, so that when forests soared in smoke after the asteroid attack that signaled the end of the Cretaceous – and the end of non-avian dinosaurs – the enantiornithines also disappeared.
"I think it's a really interesting hypothesis and the best explanation I've heard so far," Atterholt said.
"But we need to do really rigorous studies on the ecology of enantiornithines, because right now that part of the puzzle is a bit cloying."
J. Atterholt et al. 2018. The most complete enantiornithine in North America and a phylogenetic analysis of Avisauridae. PeerJ 6: e5910; doi: 10.7717 / peerj.5910