A 120 million-year-old fossil depicts a modern bird skull that evolved from a mix of Dinosaur and bird features

A 120 million-year-old fossil depicts a modern bird skull that evolved from a mix of Dinosaur and bi ...

Yuanchuavis kompsosoura's skull is digitally recreated. Credit: Wang Min

Researchers from the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology and the Field Museum of Natural History have discovered that birds retained key dinosaurian characteristics for millions of years after the split between dinosaurs and birds.

Yuanchuavis kompsosoura, a 120-million-year-old bird fossil, was analysed and reconstructed. It also revealed that this ancient bird retained many of its dinosaurian ancestors.

The research was just published in the electronic publication eLife.

The upper beak's movement is independent of the braincase, according to most living birds. Two chains of interconnected bones connect the skull's back to the beak's body, allowing it to move.

Comparison of dinosaur skull morphology with Yuanchuavis and modern birds, demonstrating that key components of the two skull bones critical for cranial kinesis are absent in Yuanchuavis and dinosaurs.

Professor Wang Min from IVPP, the lead and corresponding author of the study, said: "These chains are missing connections or are locked in place because they connect to more bones that would prevent most movements."

"This fossil helps to decipher the time when and where components of that moveable beak or kinesis evolved in birds," according to Professor Thomas Stidham of IVPP, the coauthor of the study.

Yuanchuavis is a member of a group of extinct birds called enantiornithines or "opposite birds" owing to significant differences in their skeleton from living animals at the end of the Cretaceous age.

Yuanchuavis kompsosoura's skull is reconstructed digitally. Credit: Wang Min

Yuanchuavis shows a mosaic of dinosaurian and bird traits, including a feathered bird body with wings and a toothed mouth.

The presence of bar-forming contacts among the skull bones of dinosaurs, crocodiles, lizards, and snakes is one of the first dinosaurian features of Yuanchuavis (referred to as the diapsid condition). These interactions basically "lock up" one of the bones in Yuanchuavis that is otherwise free in living animals, and this requires kinesis.

A detailed investigation of the pterygoid, a palate bone, shows that it had no direct contact with another bone, the quadrate, which is also required to complete the palatal chain of bones in kinesis. Most dinosaurs, including Triceratops and Tyrannosaurus, have this absence of contact, but the bones connect with one another in living birds.

The research team was able to verify that the pterygoid of enantiornithines retained a unique shape. It had a two-pronged projection behind the eye, similar to Velociraptors and other close dinosaurian relatives of birds.

The team of researchers also found that the pterygoid of enantiornithines retained a distinctive shape, comparable to Velociraptors and other dinosaurs closely related to birds.

Although these features ruled out any kinesis in early birds, the paleontological team discovered a clue hidden in the fossil skull that explains why kinesis was formed. The palatine, another bone in the palate of Yuanchuavis, has that important contact, which helps stabilize the palate bones and restricts movement. In contrast, the palatine of living birds, like Yuanchuavis, does not have that contact, thus facilitating back-and-forth sliding during bird skull kines

According to Professor Wang, skull kinesis might have begun when the palatine bone shape changed from having contact with four to only three other skull bones in enantiornithines.

Professor Wang believes that new features emerge from old ones, and kinesis must have evolved in birds from an ancestor who did not have it.

Professor Stidham noted that most people expect early birds to have bird skulls that matched their feathered wings and bodies. "However, these early birds hadn't completely gone beyond their dinosaurian roots," he said, noting that bird evolution has its "detours and dead-ends."

Min Wang, Thomas A Stidham, Jingmai K O'Connor, and Zhonghe Zhou, Reference: eLife, 5 December 2022, DOI: 10.7554/eLife.81337

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