BioEd Online

A roadrunner-like bird lived in China 110 million years ago, say researchers who have analysed fossil tracks found in a quarry. The tracks are 50 million years older than any other evidence of similar birds.

As well as being the size of a roadrunner, which weighs about half a kilogram and measures about half a metre from beak to tail, the bird shared the same unusual foot structure.

Most birds have three toes facing forwards and one — the equivalent of our big toe — facing back. Roadrunners have two facing forwards and two facing backwards. Feet with this shape are called zygodactyl.

"No bird like that has ever been found from that time period," says Jerald Harris, a palaeontologist at Dixie State College in St George, Utah, and an author on the study.

Based on the spacing of the tracks and the estimated size of the bird, the researchers think that it was running at about 8 kilometres per hour when it left the footprints. The analysis is reported in Naturwissenschaften¹.

Fossil birds from the early Cretaceous period, when this bird lived, include small wading birds, tree-perching birds and possibly a hawk-like raptor or two. But we know of nothing as large, as fast, or with the same foot structure as the one that made these tracks.

Runs in the family

The tracks were found in a quarry in Shandong Province; they are named Shandongornipes muxiai. Not expecting to find a zygodactyl bird in rocks that old, researchers originally thought the footprints belonged to a shorebird.

"There are no ground-dwelling zygodactyl birds at all in the fossil record, unless you count a roadrunner from the last million years or so," says the study's leader, Martin Lockley of the University of Colorado, Denver. But further analysis revealed the tracks as belonging to zygodactyl bird.

The prehistoric bird was not a direct ancestor to the roadrunner — zygodactyly evolved independently in both birds.

Most running birds have three toes, not four. There's no obvious advantage to being zygodactyl, and no obvious reason for two running birds to evolve this sort of foot, says ornithologist Gerald Mayr of the Senckenberg Research Institute in Frankfurt am Main in Germany. "It's a big enigma," he says.

The modern roadrunner is the only ground-dwelling, running member of the cuckoo family, all of which are zygodactyl. Rather than developing zygodactyly to aid running, Harris suggests that both the roadrunner and its predecessor began as tree dwellers and moved to the ground later.

"Nobody's really worked out how modern roadrunners got these feet," says Harris. "But they probably didn't evolve for running."