This Dino-Era Bird Was as Advanced as Modern Species. So Why Did It Disappear?

An enantiornithine fossil found in the western U.S. sat in storage for 25 years before it was officially described

Artist's rendition of Mirarce eatoni. Brian Engh

During the age of the dinosaurs, there were lots of species of birds that came from many different lineages. But about 65 million years ago, when the fifth great extinction took place, all of those birds—except for a few flightless species—went extinct along with the dinosaurs and half the living creatures on Earth. For decades, paleontologists have wondered why so many groups of early birds went extinct. Now, reports John Pickerell at National Geographic, a new fossil from the American West deepens the mystery.

The new fossil is actually an old find. Discovered 25 years ago in the Kaiparowits Formation in the recently downsized Grand Staircase-Escalante National Monument, the bird remains sat on a shelf at the University of California Museum of Paleontology until Jessie Atterholt, then a graduate student and now an assistant professor at the Western University of Health Sciences in Pomona, California, took a look at it. Atterholt found that the fossil came from an enantiornithine, an early lineage of birds that flitted around during the dinosaur age.

While researchers have lots of bits and pieces from the fossil animals, this one was the most complete fossil found so far with about 30 percent of the skeleton intact, including almost every part of the body except the skull. The specimen is also larger than most of its relatives, about the size of a turkey vulture while others are closer to crow size or smaller. This particular species, dubbed Mirarce eatoni, is about 75 million years old, meaning it dates from the end of the Cretaceous period. It is described in a new paper published in the journal PeerJ.

The completeness of the fossil allowed researchers to figure out how the birds flew and shows that by the end of the Cretaceous some enantiornithines had developed many features that made them good flyers with traits similar to those that modern birds developed after the mass extinction.

“We know that birds in the early Cretaceous, about 115 to 130 million years ago, were capable of flight but probably not as well adapted for it as modern birds,” Atterholt says in a press release. “What this new fossil shows is that enantiornithines, though totally separate from modern birds, evolved some of the same adaptations for highly refined, advanced flight styles.”

In particular, Mirarce eatoni has a deeply keeled breastbone, unlike earlier enantiornithines, meaning it had strong flight muscles similar to modern birds. Also, the flexible furcula, or wishbone, is V-shaped versus the U-shape found in earlier species, which would have given the bird extra power when flapping its wings. The fossil also shows evidence of “quill knobs” an adaptation on the forearm where ligaments would have attached to flight feathers, making the birds able to fly longer distances. It’s believed the birds also had long tails used in mating displays. “It is quite likely that, if you saw one in real life and just glanced at it, you wouldn’t be able to distinguish it from a modern bird,” Atterholt says in the statement.

Enantiornithines fossils have been found on every continent and are more common than the group of birds that led to our modern feathered friends. The big question is, if enantiornithines were so advanced and so widespread, why didn’t they survive the extinction event? And why did the ancestors of modern birds, which were flightless, make it?

Paleontologist Steve Brusatte of the University of Edinburgh, not involved in the study, tells Ryan F. Mandelbaum at Gizmodo that we can only speculate:

“Maybe they had beaks and could eat seeds—a nutritious food source that can survive in the soil for decades or centuries, a food bank for when the world went to hell when the asteroid hit. Or maybe these birds nested on the ground, so they weren’t wiped out with the tree-living birds when forests collapsed after the asteroid hit. Or maybe they could fly longer, or grow faster, or hide easier. We don’t really know. But this new discovery tells us that the birds that lived with the last dinosaurs were even more diverse than we used to think, so it’s more of a mystery why so few of them survived the asteroid.”

Researchers do have a working theory as to why enantiornithines didn’t make it. The group has strongly recurved claws which are used for perching in trees and climbing, meaning their primary habitat may have been forests. It’s possible that an asteroid, comet, volcanic eruptions or runaway greenhouse effect wiped out the world’s forests, taking the enantiornithines with them, a possibility that Atterholt says researchers need to look into.

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