More than a decade ago, fossil diggers unearthed the 50-million-year-old remains of a chicken-sized bird in a former lakebed in Wyoming. Now, researchers reveal that the remarkably preserved fossils belong to a never-before-seen species. They named it Calciavis grandei, and it’s related to modern-day ostriches, kiwis and emus. It was described in the Bulletin of the American Museum of Natural History last week.

Bird fossils that old are exceedingly rare. They’re hollow, more fragile than most mammal bones, and very likely to be crushed during the preservation process. "These are spectacularly preserved fossils,” Virginia Tech’s Sterling Nesbitt said in a statement. One of them is a nearly complete, articulated skeleton with soft tissue (including feathers), and the other specimen is a well-preserved but disarticulated skeleton.

Nesbitt and Julia Clarke from the University of Texas at Austin analyzed both individuals, which were discovered in Warfield Springs at the Fossil Butte Member of the Green River Formation, near Kemmerer, Wyoming. This fossil lakebed – which dates back to the late Early Eocene 51.66 million years ago – is a well-known site for ancient fish, crocs, turtles, mammals, and other birds.

The new species was about the size of a chicken, and like chickens, it was mostly ground-dwelling and flew in short bursts to escape predators as needed. It was a close relative of today’s ostriches of Africa, kiwis of New Zealand, and tinamous, which are found in Central and South America.

"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." It likely went extinct when tropical forests disappeared across the continent.

The new genus name combines “calx,” Latin for stone, with “avis,” Latin for bird, and the species name honors paleontologist Lance Grande.

^{Images in text: Calciavis grandei fossils by Sterling Nesbitt (middle) and Rick Edwards/American Museum of Natural History (bottom)}