A nearly 50-million-year-old bird fossil unearthed in Wyoming represents a new species that is a close relative of living kiwis, ostriches, and emus, according to a team of paleontologists from the American Museum of Natural History and the Virginia Polytechnic Institute and State University.

The ancient bird, named Calciavis grandei, is believed to be roughly the size of a chicken and was mostly ground-dwelling, only flying in short bursts to escape predators.

“This is among one of the earliest well-represented bird species after the age of large dinosaurs,” said co-author Dr. Sterling Nesbitt, from Virginia Tech.

The exceptionally well-preserved specimen of Calciavis grandei dating from the Eocene epoch — with bones, feathers, and fossilized soft tissues — was found more than a decade ago in the Green River Formation, a former lake bed.

The Eocene lake is best known for producing scores of complete fish skeleton fossils, but other fossils such as other birds, plants, crocodilians, turtles, bats, and mammals from an ecosystem 50 million years old.

According to Dr. Nesbitt and his colleague, Prof. Julia Clarke from the American Museum of Natural History, Calciavis grandei belongs to the extinct group of early Palaeognathae birds, the Lithornithidae.

The bird is a close relative of the modern-day kiwis, ostriches, and tinamous now living in the southern continents.

“The new bird shows us that the bird group that includes the largest flightless birds of today had a much wider distribution and longer evolutionary history in North America,” Dr. Nesbitt said.

“Back when Calciavis grandei was alive, it lived in a tropical environment that was rich with tropical life and this is in stark contrast to the high-desert environment in Wyoming today.”

“After tropical forests disappeared in North America, Calciavis grandei and other more tropical birds went extinct,” the paleontologists said.

“Relationships among species in this lineage of birds have been extremely contentious.”

“We hope the detailed new anatomical data we provide will aid finding a resolution to this ongoing debate.”

Research describing Calciavis grandei is published online in the Bulletin of the American Museum of Natural History.

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Nesbitt, Sterling J. & Clarke, Julia A. 2016. The anatomy and taxonomy of the exquisitely preserved Green River Formation (early Eocene) lithornithids (Aves) and the relationships of Lithornithidae. Bulletin of the American Museum of Natural History, no. 406, 91 pp.