However, paleontologists still struggle to understand how this fantastic evolutionary event occurred.

Now, a new, complete 120-million-year-old fossil bird from China further complicates this issue by exhibiting a dinosaur-like skull articulated with a bird-like body. In addition, the fossil specimen, named Cratonavis zhui, preserves a surprisingly elongate scapula and first metatarsal, making it stand out from all other birds including fossil ones.

The study, published in Nature Ecology & Evolution on Jan. 2, was conducted by paleontologists from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences.

Cratonavis is positioned between the more reptile-like long-tailed Archaeopteryx and the Ornithothoraces (which had already evolved many traits of modern birds) in the avian evolutionary tree.

To study the fossil skull, the scientists first used high-resolution computed tomography (CT)-scanning. They then digitally removed the bones from their rocky tomb and reconstructed the original shape and function of the skull.

The result demonstrates that the Cratonavis skull is morphologically nearly identical to that of dinosaurs such as Tyrannosaurus rex, rather than being bird-like. "The primitive cranial features speak to the fact that most Cretaceous birds such as Cratonavis could not move their upper bill independently with respect to the braincase and lower jaw, a functional innovation widely distributed among living birds that contributes to their enormous ecological diversity," said Dr. Li Zhiheng, a lead author of the study.