A new analysis of the earliest known ancestor of modern-day birds suggests it used all four of its featured limbs to fly.

Doctoral student Nick Longrich at the University of Calgary examined fossils of Archaeopteryx lithographica, a Jurassic-era dinosaur first discovered in Germany in 1861.

Eight more specimens of the species have been found since then. The fossils of Archaeopteryx, with its feathers and bird-like wishbone, are part of the evidence that birds evolved from dinosaurs.

Scientists have known that Archaeopteryx had feathers on its back legs since it was first discovered, but they were thought to be insulating feathers that weren't involved in flight.

"I believe one reason for this is that people tend to see what they want or expect to see. Everybody knows that birds don't have four wings, so we overlooked them even when they were right under our noses," said Longrich in a statement.

Longrich calculated that the feathers on the rear legs of Archaeopteryx have an aerodynamic shape and could make up 12 per cent of the creature's wing area.

The finding also supports the theory that flying birds evolved from creatures that glided from the tops of trees, "similar to modern flying squirrels," Longrich said.

This "trees up" hypothesis is opposed by the "ground up" hypothesis, which states that bird ancestors began flying by taking off from the ground while running.