Analysis of fossil remains dating 50 million years represent a new species that is a previously unknown relative of the modern-day ostrich, researchers say.
The bird fossils were found more than a decade ago, completely intact with bones, feathers, and soft tissues in a former lake bed in Wyoming in the US.
"This is among one of the earliest well-represented bird species after the age of large dinosaurs," said study co-author Sterling Nesbitt, Assistant Professor at Virginia Polytechnic Institute and State University (Virginia Tech).
"You can definitely appreciate how complete these fossil are," Nesbitt added.
The new species is named Calciavis grandei -- with "calci" meaning "hard/stone," and "avis" from the Latin for bird, and "grandei" in honour of famed paleontologist Lance Grande.
The bird is believed to be roughly the size of a chicken, and similar to chickens, were mostly ground-dwelling, only flying in short bursts to escape predators.
Two fossils of Calciavis dating from the Eocene epoch -- roughly between 56 million and 30 million years ago -- were found by fossil diggers within the Green River Formation in Wyoming, a hot bed for extinct fish.
"These are spectacularly preserved fossils, one is a nearly complete skeleton covered with feather remains, the others are nearly are nearly as complete and some show soft tissue remains," Nesbitt said.
The findings were published in the Bulletin of the American Museum of Natural History.
"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," Nesbitt said.
"Back when Calciavis 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," Nesbitt pointed out.