The fossil of a small lizard-like, flying reptile with a complex set of feathers challenges the theory that birds evolved from dinosaurs, a new study says.

Researchers say the fossil is 220 million years old, which proves feathered animals existed long before the appearance of the dinosaurs believed to be the ancestors of modern birds.

The fossil has been named Longisquama insignis and is thought to be an archosaur, a group of early reptiles that resembled small dinosaurs but belonged to a different lineage. The first known bird, Archeopteryx, appeared about 145 million years ago, some 75 million after the date for Longisquama.

Feathered fossil scientists say predates first known bird

Oregon State University paleontologists John Ruben and Terry Jones conducted the study published in the journal Science this week.

But other experts on the evolution of dinosaurs dismiss the study. They say the lone specimen is not enough proof to throw out the theory that birds evolved from dinosaurs, an idea that can be traced back through the work of hundreds of scientists over many years.

The fossil was actually found in Kyrgystan, in central Asia, in 1969 and ended up in a museum in Moscow. It went virtually unnoticed until Ruben and Jones spotted the fossil at a travelling exhibition at a Kansas shopping mall. The two scientists identified appendages on the back of the fossil as feathers and began a long study of the specimen.