Paleontologists have dug the partial skeleton of an enantiomitine (opposite bird) living in what is now Utah about 75 million years ago (Late Cretaceous Epoch). According to an analysis of the fossil, published in the newspaper PeerJLate Cretaceous enantiomithines were the aerodynamic of the ancestors of today's birds, who could fly strong and smoothly.
All birds evolved from springy theropodes – the two-legged dinosaurs like T Rex – which began about 150 million years ago, and developed into many lines in the chalk period between 146 and 65 million years ago.
But after the disaster that threw out most of the dinosaurs there were only a group of birds left: the fathers' fathers we see today.
Why did only one family survive mass extinction? The newly discovered fossil from one of the extinct bird groups, enantiomitins, expands that mystery.
"This special bird, named Mirarce eatoni, is about 75 million years old – about 10 million years before death, "said Dr. Jessie Atterholt, researcher at the Western University of Health Sciences.
"One of the very interesting and mysterious things about enantiomitins is that we find them throughout the Cretaceous, for about 100 million years of existence, and they were very successful."
Mirarce eatoni are among the largest North American birds from Crete; most were the size of chickadees or crows.
The trial was found in Kaiparowit's Formation in the Grand Staircase Escalante National Monument in Garfield County, Utah, 1992, and is the most complete enantiomitine gathered in North America so far.
"Most of the birds from America are from the lower and only known from a single leg, often metatarsus. This fossil was almost complete, just missing its head," said the palaeontologists.
Mirarce eatoniThe sternum or sternum, with the air muscles attached, is deeper than other enantiomines, which means a larger muscle and stronger flight more like modern birds.
The wishbone is more V-format, like modern birds and unlike previous avians and their dinosaur fathers. The wishbone or furcula is flexible and stores energy emitted under the wing.
"The most exciting, however, are large stains on the lower legs. These coarse patches are quill buds, and in modern birds they wrap the wing springs to the skeleton to help strengthen them for active flight," says Dr. Atterholt.
This is the first discovery of quill buds in any enantiornithin bird, telling us that Mirarce eatoni was a very strong flyer.
"We know that birds at the beginning of the war, about 115-130 million years ago, could fly, but probably not as well adapted to modern birds," said Atterholt.
"What this new fossil shows is that enantiomitins, despite being completely different from modern birds, developed some of the same adaptations for advanced advanced flight styles."
If the enantiomers of the late war were as advanced as modern birds, why did they die with the dinosaurs while the ancestors of the eagles did not?
A recently proposed hypothesis claims that they were mostly forestry builders, so that when the forests went up in smoke after the asteroid strike that signaled the end of the chalk period – and the end of non-avian dinosaurs – the enantiomers disappeared.
"I think it's a really interesting hypothesis and the best explanation I've had so far," said Dr. Atterholt.
"But we have to do proper studies of the encyclopedia of the enantiomites, for now that part of the jigsaw is a little wavy."
J. Atterholt et al. 2018. The most complete enantiomitine from North America and a phylogenetic analysis of Avisauridae. PeerJ 6th5910; doi: 10.7717 / peerj.5910