Is it a bird? Is it a plane? It’s a dinosaur with the body of a modern bird, but a head like T. rex

We know birds are descendants of dinosaurs like T. rex, but when and how did the transformation from dinosaur to modern bird happen?

This transition is one of the most dramatic in terms of morphology, functionality and ecology, but palaeontologists are still struggling to get to grips with exactly how modern birds came about.

A new fossil discovery by a team at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences, of a bird ancestor dinosaur in China, might place another piece to this feathery puzzle.

Named Cratonavis zhui, the 120-million-year-old fossil has a dinosaur-like skull and modern bird-like body. It also shows surprisingly long scapula and first metatarsal bones, setting it apart from all other birds including ancient birds.

The skull was analysed using high-resolution CT scans before digitally removing the fossil from the surrounding rock to see their original shape.

Life reconstruction of the 120-million-year-old bird Cratonavis zhui. Credit: ZHAO Chuang.

Cratonavis’s skull is not bird like. Morphologically it’s almost identical to that of dinosaurs such as Tyrannosaurus rex.

Read more: Powered flight in feathered dinosaurs confirmed in Chinese fossils showing muscle imprints

“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 a lead author Dr. Li Zhiheng from the IVPP.

Digital reconstruction of the skull of Cratonavis zhui. Credit: WANG Min.

The palaeontologists believe that the elongated scapula and metatarsal were mechanical compensators for an overall underdeveloped flight apparatus in the early bird.

Cratonavis sits on the avian evolutionary tree somewhere between Archaeopteryx and other ancient birds which evolved many of the traits of modern birds. It suggests that early bird skeletons were subject to much variability and plasticity.

When they were discovered in Germany in the 1860s, the 150-million-year-old Archaeopteryx fossils were the first indication that dinosaurs and birds might be related. The fossils were the first to show dinosaurs with feather imprints.

Photograph of the 120-million-year-old bird Cratonavis zhui. Credit: WANG Min.

The authors suggest that changes in the skeletons of theropods which began the transition to flight, and modern bird body plans, show that there was probably interplay between natural selection, skeletal development and ecological opportunities.

Read more: First evidence of a dinosaur eating a mammal found in Chinese Microraptor fossil

The discovery is described in a paper published in Nature Ecology & Evolution.

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