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Modern birds are descended from dinosaurs, making them the only dinosaur lineage that survived the planet-shaking asteroid impact that wiped out the rest of their kind around 66 million years ago. But exactly how birds evolved from the rest of the theropods — a bipedal group with hollow bones and three toes or claws on each foot, which includes avian dinosaurs as well non-avian dinosaurs, such as raptors like Velociraptor — is still unclear.

Researchers unearthed the new species, which they named Cratonavis zhui, at a fossil site in China. The fossil's age suggets C. zhui likely appeared somewhere between the earliest known bird, Archaeopteryx, which lived about 150 million years ago during the Jurassic period, and the Ornithothoraces, a dinosaur-era group which had already evolved many traits of modern birds.

In a new study, published Jan. 2 in the journal Nature Ecology and Evolution, researchers analyzed the new fossil to see what traits it shared with both groups. After studying the fossils with a high-resolution computed tomography (CT) scan, which allowed them to virtually reassemble the bones in 3D, the team found that, despite a majority of the skeleton being very similar to Ornithothoraces, certain bones shared a surprisingly strong likeness to non-avian dinosaurs. The most striking similarity was in the skull, which has a shape that is "nearly identical to that of dinosaurs such as T. rex," researchers wrote in a statement.

The specimen's raptor-like skull is notable because it would have prevented C. zhui from moving its upper bill in relation to its lower jaw. Modern birds are capable of moving both parts independently, which is believed to have greatly contributed to their enormous ecological diversity today, study lead author Zhiheng Li, a paleontogolost at the Chinese Academy of Science's Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), said in the statement. It is therefore surprising to know that this trait developed so late in birds' evolutionary history, he added.

C. zhui also has an unusually elongated scapula, a shoulder bone used during flight, and first metatarsal, a bone found in the foot, compared with modern birds.

The scapula plays an important role in flight because it helps rotate birds' shoulders and beat their wings. The elongated scapula in C. zhui likely "compensated for the overall underdeveloped flight apparatus in this early bird," study co-author Min Wang, a paleoornathologist at IVPP, said in the statement. 

However, the extended metatarsals are likely leftover from land-dwelling raptors who required longer versions of the bone to help them run. Over time, these bones evolved to be much shorter in birds to allow them to use their hallux, or big clawed toe, to land on branches and grab prey from the air instead of running, study co-author Thomas Stidham, a paleoornathologist at IVPP, said in the statement.

The unexpected lengths of both the scapula and first metatarsal "highlight the breadth of skeletal plasticity in early birds," study co-author Zhonghe Zhou, a paleoornathologist at IVPP, said in the statement. This plasticity suggests that certain skeletal traits could have evolved independently from one another across the birds' evolutionary tree, a phenomenon known as convergent evolution, but more fossils are needed to tell for sure.