Scientists have identified a previously unknown relative of the modern-day ostrich from well-preserved fossils dating back 50 million years.
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, US.
"This is among one of the earliest well-represented bird species after the age of large dinosaurs," said Sterling Nesbitt of Virginia Tech's College of Science in the US.
"You can definitely appreciate how complete these fossils are," said Nesbitt.
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 who has studied the fossil fish from the same ancient North American lake for decades.
A skeleton of the Calciavis Grandei is nearly complete.
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 hotbed for extinct fish.
"These are spectacularly preserved fossils, one is a nearly complete skeleton covered with feather remains, the others are nearly as complete and some show soft tissue remains," said Nesbitt.
"Fossil birds are rare," said Nesbitt, adding that as bird bones are hollow, they are far more fragile than most mammal bones, and more likely to be crushed during fossilisation.
One of the fossilised birds in this rare case apparently was covered in mud soon after death.
The former lake in which the fossil was found 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 roughly 50 million years old.
Included in the extinct group of early Palaeognathae birds, the Lithornithidae, researchers call the bird a close relative of living ostriches, kiwis and tinamous now living in the southern continents.
After tropical forests disappeared in North America, Calciavis and other more tropical birds went extinct, researchers said.
"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," he said.
The Calciavis skeleton will be important to interpreting new bird fossils and other fossils from the Eocene epoch that were collected decades ago.
