About the size of a turkey vulture, a fossil found in the Grand Staircase-Escalante National Monument in Utah is not only one of the largest dinosaur-era birds ever found in North America, but it is also the continent’s most complete skeleton for a long-extinct group of birds called the enantiornithines.
The enantiornithines, or “opposite birds,” were a primitive lineage related to modern birds that thrived globally during the Cretaceous period but were killed off the by the same mass extinction event that wiped out the nonavian dinosaurs 66 million years ago.
While numerous beautiful specimens of enantiornithines have been found in early Cretaceous rocks in China and pieces of amber from Myanmar, most are no bigger than small- to medium-sized songbirds, such as chickadees or finches. And while other enantiornithines have been uncovered in North America from the late Cretaceous—including one that may be slightly larger than the new species—most of those fossils only consist of a single bone.
The new specimen, described today in the journal PeerJ, is dated to 75 million years ago. It includes parts of the bird’s legs, wings, shoulders, wishbone, tail, and spine, adding up to about 30 percent of the overall skeleton, and it reveals new details about the flight capabilities of this early lineage of birds.
“It is by far the most complete skeleton of an enantiornithine bird from North America,” says lead author Jessie Atterholt of the Western University of Health Sciences in Pomona, California. “We have bits representing the entire skeleton except the skull. It shows it had advanced adaptations for flight that we haven’t seen in any other enantiornithine.”
Built for flight
Atterholt’s team named the species Mirarce eatoni. Mirarce (pronounced meer-ark-ee) is derived from the Latin mirus, for wonderful, and the Greek Arce, after the mythological winged messenger of the Titans. The species’ epithet, eatoni, honors paleontologist Jeffrey Eaton, who worked for many years on the Kaiparowits Formation where the fossil was originally found.
Study co-author J. Howard Hutchinson had unearthed the fossil in 1992. But in the intervening decades, the bird’s skeleton had been languishing on a shelf at the University of California Museum of Paleontology, until Atterholt resurrected it from the archives for her research there as a graduate student.
“[It’s] a great reminder of the incredible value of museum collections, which hold untold specimens waiting for their secrets to be revealed by enthusiastic future researchers,” comments Daniel Field a paleontologist and expert on fossil birds at the University of Cambridge in the U.K.
“This really is a great piece of work that … sheds amazing new light on the evolution of bird-like forms during the age of dinosaurs,” he says. The research reveals the surprising extent to which these ancient birds paralleled the evolution of modern birds by developing similar physical features more than 70 million years ago, Field says.
While enantiornithines would have looked similar to modern birds, most of the crow-sized or smaller species discovered so far would not have been as skilled at flight. But Mirarce bucks this trend, showing flight features that are similar to those of modern birds, except that they must have evolved independently—characteristics such as a narrow wishbone, suited to powerful flapping flight, and a sternum with a deep keel for the attachment of large, flight muscles.
Most exciting are pronounced “quill knobs” on the forearm bones, where the flight feathers of living birds are anchored with ligaments.
“In modern birds, that is an adaptation to making flight feathers stronger and reinforcing them,” Atterholt says. “[Mirarce] would have been a very strong flier, completely adapted to active flapping flight, possibly even for long distances.”
Dig site under threat
All of the other late Cretaceous enantiornithines “are named after fragments of bone, or a single bone, so finding a specimen this complete in North America is quite a discovery,” says Alida Bailleul, an expert on fossil birds at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, who was not involved with the research.
The relative completeness makes it “a very important specimen that fills a gap in our knowledge of bird evolution in America, right before the dinosaurs went extinct,” she says. It is also among the initial fossil finds that led to the protection of Grand Staircase-Escalante National Monument back in 1996.
Sadly, Atterholt notes, fossil finds such as this are under threat as President Trump’s administration last year slashed the extent of the national monument from 1.9 million acres down to one million.
“The park is full of beautiful, important geological features, many of which contain priceless fossils,” she says. “If the land is not protected, these fossils will be much more vulnerable to fossil poachers, destruction from development, and damage from recreational off-road traffic.”
Field concurs: “The work of Atterholt and colleagues illustrates the immense importance of this locality for reconstructing how the modern world has come to be,” he says. “Scaling back protections on such an important scientific site for short-term economic and political gain is very troubling.”