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Most complete enantiornithine bird fossil from North America

November 13, 2018

A 75-million-year-old bird skeleton from a threatened national monument in Utah represents the most complete skeleton ever found in North America for a long-extinct group of birds called enantiornithines. This large, fossilized bird provides important new insight into the evolution of flight. It has several advanced adaptations for flying, which show that the Enantiornithes evolved these features separately from living birds.

Enantiornithine birds ("opposite birds") were a group that existed during the Cretaceous. They were super successful -- so far scientists have found enantiornithine fossils from every continent except Antarctica, and they are known from rocks as old as 130 million years right up to the end of the Cretaceous 65 million years ago. In many ways, they looked quite similar to modern birds and are in fact closely related. They were fully-feathered and flew by flapping their wings like modern birds.

In North America, the enantiornithine fossil record is restricted to the Late Cretaceous - about 100-65 million years ago. Nearly all of these fossils are single bones from the feet, so we don't know what most of these birds looked like. In our new paper published in peer-reviewed journal PeerJ - the Journal of Life & Environmental Sciences, we describe a very special enantiornithine fossil from the Kaiparowits Formation in Utah (~75 million years old), which we named Mirarce eatoni (Meer-ark-ee).

Mirarce pays homage to the incredible, detailed, three-dimensional preservation of the fossil ("mirus" = "wonderful" in Latin), and anatomical evidence that it was an advanced flyer (Arce was the winged messenger of the Titans in Greek mythology). The species name honors Dr. Jeffrey Eaton for his decades of work as a paleontologist on fossils from the Kaiparowits Formation.

Mirarce is unique and important for several reasons. First, it is by far the most complete enantiornithine bird fossil ever discovered in North America. We have bones from almost all regions of the skeleton, except the skull. Additionally, it is also one of the largest birds known from North America from the entire Age of Dinosaurs.

Most Cretaceous birds were the size of chickadees or crows, but Mirarce was significantly bigger, about the size of a turkey vulture or great-horned owl. This fossil also is critical to helping us understand trends in enantiornithine evolution, and bird evolution in general. Though they superficially look like modern birds, enantiornithines have many anatomical differences. Most enantiornithines had a sternum (breast-bone) with a low keel and a wide wishbone. Though they could fly, they weren't as well-adapted for flight as modern birds.

Mirarce, however, shows us that by the Late Cretaceous opposite birds had separately evolved adaptations for advanced flight, similar to what we see in modern birds. The wishbone is narrower, and the sternum has a deeper keel for bigger flight muscles. What is most exciting, however, are large patches on the forearm bones. These rough patches are quill knobs, and in modern birds they anchor the wing feathers to the skeleton to help strengthen them for active flight. This is the first discovery of quill knobs in any enantiornithine bird. Thus, we know that Mirarce was a very strong flier.

This important fossil was found in Grand Staircase Escalante National Monument in Utah, a monument currently under threat of severe size reduction by the national government. Originally discovered in the early 1990s, this find (among many others) contributed to the original establishment of the monument in 1996. Mirarce is one of thousands of fossils discovered from Grand Staircase, and the rocks of the Kaiparowits Formation contain one of the best Cretaceous fossil records in the entire world, underscoring the critical importance of protecting and preserving these parts of our natural heritage. Reducing the size of the protected area puts some of our nation's most valuable natural and scientific resources at risk.

PeerJ is an Open Access publisher of two peer-reviewed journals and a preprint server. PeerJ's mission is to help the world efficiently publish its knowledge. All works published by PeerJ are Open Access and published using a Creative Commons license (CC-BY 4.0). PeerJ is based in San Diego, CA and the UK and can be accessed at

PeerJ is the peer-reviewed journal for Biology, Medicine and Environmental Sciences. PeerJ has recently added 15 areas in environmental science subject areas, including Natural Resource Management, Climate Change Biology, and Environmental Impacts.

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