Fossil, with dinosaur-like head, sheds light on evolution of birds
What's the story
A 120-million-year-old fossil uncovered in China has one of the most bizarre anatomical features: a head resembling a dinosaur and a body resembling a bird.
Scientists from the Chinese Academy of Sciences studied the fossilized specimen, named Cratonavis zhui, and discovered that the skull was shaped like that of the Tyrannosaurus rex dinosaur instead of being like birds.
Context
Why does this story matter?
It has been widely accepted that birds evolved from dinosaurs, a transition that bears drastic changes morphologically and functionally. The recently discovered Chinese fossil sheds more light on this evolutionary process.
Talking about the avian evolutionary tree, Cratonavis zhui is positioned between the long-tailed, reptile-like dinosaur Archaeopteryx and the Ornithothoraces. The latter has evolved many of the characteristics that are seen in modern birds.
Study
Scientists reconstructed the shape and function of the fossil's skull
Scientists first studied the specimen by means of high-resolution computed tomography (CT).
Later, they digitally removed the bones and reconstructed the original shape and functions of the skull, which led to the discovery that the fossil's skull resembled that of dinosaurs.
It was found that Cratonavis could not move their upper bill independently with respect to the braincase and lower jaw.
Findings
Cratonavis has features that are absent in birds
Interestingly, the Cratonavis fossil had a "surprisingly long scapula," a bone in the shoulder and first metatarsal (foot bone), which are features that are absent in modern birds, including fossilized ones.
According to Wang Min, co-author of the study, the nature of the scapula allowed these species to better rotate their wings and compensated for their underdeveloped flight capabilities.
Explanation
The evolutionary process favored a shorter metatarsal bone
Talking about the foot bone, the study explains that during the transition from dinosaurs to birds, the first metatarsal of Cratonavis was subjected to the process of natural selection that made it a shorter bone.
Once the bone attained its optimal size, which is less than a quarter of the length of the second metatarsal bone, it lost its evolutionary lability.
Information
The fossil provides insights into the evolution of birds
Cratonavis zhui has a "unique mix" of anatomical features and serves as a sign as to how all living things represent increments of change, and the evolution of birds occurred simultaneously, along a wide variety of divergent paths.