We can confidently say that birds are dinosaur descendants, though paleontologists are still puzzled as to how this incredible evolutionary event occurred.
Now a complete fossilized skeleton of a bird that lived in what is today China around 120 million years ago might help clarify key steps in the transformation process, presenting with a more archaic, dinosaur-like head atop a body that has more in common with modern birds.
The transition from dinosaur to bird includes some of the most dramatic changes in shape, function, and environment, which ultimately led to the body plan that is typical of today's birds.
Some of those shifting features can still be seen in the way modern birds develop. But the order in which these changes occurred, and the nature of the evolutionary pressures that gave rise to strictly avian characteristics, is still open for debate.
The fascinating, newly found fossil, named Cratonavis zhui, may provide important insights into the evolution of modern birds.
Researchers discovered the body print of Cratonavis, the bird with a dinosaur head, during excavations conducted in northern China.
Body prints of feathered dinosaurs and early birds, including Confuciusornis sanctus, have been discovered in this region, in sedimentary rocks formed about 120 million years ago, during the Cretaceous period.
Led by paleontologist Zhou Zhonghe from the Chinese Academy of Sciences (CAS), the scientists began their investigation of the fossil skull with high-resolution computed tomography (CT) scanning.
Using the digital versions of the mineralized bones, the team reconstructed the shape and function of the skull as it was during the bird's life.
The result shows that the shape of the Cratonavis skull is almost the same as that of dinosaurs like Tyrannosaurus rex, and not like a bird's.
"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," says CAS paleontologist Zhiheng Li.
The unusual combination of a dinosaur's akinetic skull with a bird's skeleton adds to previous studies on the importance of evolutionary mosaicism in the early diversification of birds.
Among the avian branches of the dinosaur's family tree, Cratonavis is between the long-tailed Archaeopteryx, which was more like a reptile, and the Ornithothoraces, which had already developed many of the traits of modern birds.
Also of interest is the fact the Cratonavis fossil has a surprisingly long scapula and first metatarsal (foot bone) – features which are rarely seen in the fossils of other dino-ancestors to birds, and altogether absent in modern birds.
Evolutionary trends show reduced length in the first metatarsal as birds developed.
The study authors propose that during the change from dinosaurs to birds, the first metatarsal went through a process of natural selection that made it shorter. Once it reached its optimal size, which was less than a quarter of the length of the second metatarsal, it lost its earlier functions.
The unique feature of an enlarged metatarsal in Cratonavis is more comparable to the Late Cretaceous Balaur, a member of a group of feathered carnivores known as dromaeosaurids.
The elongated scapula has been observed previously in Cretaceous birds such as Yixianornis and Apsaravis.
The fact that Cratonavis had a very long scapula probably made up for the fact that it didn't have a breastbone adapted to provide the meaty pectoral muscles a larger surface to attach to. This extinct species may have contributed to a biological experiment in flying behavior.
One of the lead authors, paleontologist Min Wang, explains "the elongate scapula could augment the mechanical advantage of muscle for humerus retraction/rotation, which compensates for the overall underdeveloped flight apparatus in this early bird, and these differences represent morphological experimentation in volant behavior early in bird diversification".
The authors mention the abnomal morphologies of the scapula and metatarsals preserved in Cratonavis highlight the breadth of skeletal plasticity in early birds.
Cratonavis zhui's unique mix of anatomy is less a stepping stone between two majestic categories of animals, but a sign of how all living things represent increments of change, and the evolution of birds of all feather occurred simultaneously along a wide variety of divergent paths.
The research has been published in Nature Ecology & Evolution.