Puzzling fossil has the body of a bird, skull of a dinosaur

It is accepted that birds evolved from reptiles, and particularly from small, carnivorous theropod dinosaurs, and that this process did not take place in one burst of evolution. Rather, the huge morphological changes happened in steps, over tens of millions of years, with the typical bird features evolving at different rates, for distinct purposes, and ending up giving the characteristic modern bird body plan. However, paleontologists still wonder how it was that all these dramatic transformations took place. 

A new bird fossil, found in the Jiufotang Formation of Liaoning Province in China, has now been analyzed by paleontologists from the Chinese Academy of Sciences in Beijing, and has shown to be a bizarre mixture of dinosaur and bird features. It is dated at around 120 million years and combines a typical bird body with a reptilian skull that is similar in morphology to the skull of Tyranosaurus rex. This finding supports the idea that evolutionary changes in morphology did not take place at the same rate in all body structures during early bird diversification. 

“The Cretaceous period is a critical time interval that encompasses explosive diversifications of terrestrial vertebrates, particularly the period when the earliest-branching birds, after divergence from their theropod ancestors, evolved the characteristic avian bauplan that led eventually to their global radiation,” said Dr. WANG Min, a researcher with the Institute of Vertebrate Paleontology and Paleoanthropology and the Center for Excellence in Life and Paleoenvironment at the Chinese Academy of Sciences.

Named Cratonavis zhui, this new species is positioned on the avian evolutionary tree, somewhere between the more reptile-like, long-tailed Archaeopteryx and the Ornithothoraces (which include modern birds and their closest ancestors). Its features include wings, feathers, claws on the wings, conical teeth, and the absence of a long, bony tail. In addition, the fossil bird has unusually elongated scapulae and first metatarsal bones, features that are, so far, unique to this species alone.

The researchers used high-resolution computed tomography (CT)-scanning to analyze the fossilized bones, and then they “removed” these, digitally, from their rocky tomb, repositioned them and reconstructed the original shape of the post-cranial skeleton and skull. They found that, while the skeleton was typically bird-like, the skull retained small, conical teeth, along with the feature known as akinesis, which is characteristic of dinosaurs and other reptiles, but not of modern birds. 

“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.

As for the bizarre scapula and first metatarsal bone in Cratonavis, Dr. WANG Min, a lead and corresponding author of this study, said: “The scapula is functionally vital to avian flight, and it conveys stability and flexibility. We trace changes of the scapula across the theropod-bird transition, and posit that 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 behaviour, early in bird diversification.”

In their report, published in Nature Ecology & Evolution, the paleontologists conclude that the elongated first metatarsals in the fossil bird’s feet were subjected to selection pressure during the dinosaur-bird transition, and ended up becoming shorter. They then lost their evolutionary lability (changeability) once they reached their optimal size, less than a quarter of the length of the second metatarsal. 

“However, increased evolutionary lability was present among Mesozoic birds and their dinosaur kins, which may have resulted from conflicting demands associated with its direct employment of the hallux [innermost digit of the hind foot] in locomotion and feeding,” said co-author Dr. Thomas Stidham. For Cratonavis, such an elongated hallux may have been advantageous while hunting for prey.

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By Alison Bosman, Earth.com Staff Writer

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