That blend matters. It suggests, in Wang's words, that "the morphological gap between birds and dinosaurs is much smaller than we thought."

Was it really flightless?

The evidence points that way, though it's an inference from anatomy rather than direct observation. Comparisons with 377 living bird species showed the limb proportions of Y. yangyuanensis most closely matched those of terrestrial birds. Wang suggested it neither climbed trees nor waded through swamps. It ran.

"I think there are no directly comparable modern birds," he said.

Despite lacking flight, the researchers place Y. yangyuanensis closer on the evolutionary tree to modern birds than to Archaeopteryx, the famous proto-bird that lived about 150 million years ago and likely could fly.

Why does one odd fossil matter?

Here's the bigger picture. The find hints the earliest birds filled a far wider range of ecological roles than paleontologists previously recognized. Y. yangyuanensis may have occupied a niche that early birds rarely used, Wang said, and its existence argues against any straight-line, step-by-step march toward flight optimization.

In an accompanying article in Nature, Fernando Novas of the Felix de Azara Natural History Foundation in Buenos Aires — who was not involved in the study — wrote that the discovery supports the idea "the ability to fly arose not once, but several times among disparate groups of carnivorous dinosaurs that are closely related to birds."

That list includes early birds, scansoriopterygid dinosaurs with bat-like membranous wing folds, and the four-winged microraptors. Y. yangyuanensis apparently took a different road entirely, abandoning the skies its relatives took to.

"It highlights the extraordinary breadth of ecological and morphological diversification that took place close to the origins of avialans," Wang said — avialans being the branch of the dinosaur family tree that contains birds.

What are the caveats?

This is a single specimen, described only days ago. Jurassic birds emerged between 150 million and 148 million years ago and their fossils are scarce, so the picture of how early birds diversified and gained flight remains incomplete. Interpretations of lifestyle — running predator, flightless habits — rest on limb proportions and claw anatomy, not behavior preserved in stone.

Still, one fossil can shift a timeline. The simplest takeaway: the first birds were a more varied bunch than we thought, and some of them never intended to leave the ground.