Most birds today can lift the upper beak independently of the brain case, enabled by a series of mobile joints and bending zones in the skull. The computed tomography of the skull of Yuanchuavis kompsosoura, a toothed bird that lived approximately 120 million years ago in what is now northeastern China, shows for the first time that the joints were still absent, but also hints at how they may have evolved later.

An illustration showing what Yuanchuavis kompsosoura might have looked like in life. Image credit: Haozhen Zhang.

“Most living birds have what is termed a kinetic skull,” said Professor Min Wang, a paleontologist with the Institute of Vertebrate Paleontology and Paleoanthropology and the Center for Excellence in Life and Paleoenvironment at the Chinese Academy of Sciences, and colleagues.

“This means that upper beak movement is independent of the braincase. This mobility is accomplished by two chains of skull bones aligned from the back of the skull to the front, with one chain along the cheek and the other along the palate (roof of the mouth).”

“These chains of interconnected bones help to transfer forces from the back of the skull to the beak, allowing for its movement.”

“We still don’t know which chain of bones was completed and freed first in bird evolution, or even if it was all completed together.”

“What we see in dinosaurs and the earliest birds like the Early Cretaceous enantiornithine Yuanchuavis kompsosoura is that these chains are missing connections or are locked in place because they connected to more bones that would stop most movements.”

In their study, Professor Wang and co-authors performed a detailed three-dimensional reconstruction of the skull of Yuanchuavis kompsosoura.

This ancient species shows a mosaic of dinosaurian and bird traits such as a feathered bird body with wings, a toothed mouth, and an immovable dinosaurian palate and snout.

“This fossil actually helps to narrow the time when, and where on the bird family tree, components of that moveable beak or kinesis evolved,” said Professor Thomas Stidham, also from the Institute of Vertebrate Paleontology and Paleoanthropology and the Center for Excellence in Life and Paleoenvironment at the Chinese Academy of Sciences.

“We can show that it definitely was not present any earlier in bird evolution.”

Comparison of skull morphology of dinosaurs, Yuanchuavis kompsosoura, and modern birds, demonstrating that key components of the two chains of skull bones critical for cranial kinesis are lacking in Yuanchuavis kompsosoura and dinosaurs. Image credit: Wang et al., doi: 10.7554/eLife.81337.

Among the primitive dinosaurian features of Yuanchuavis kompsosoura is the presence of bar-forming contacts among the bones of the temporal region of the skull behind the eye that are found in dinosaurs, crocodiles, lizards, and snakes (known as the diapsid condition).

These interconnections essentially lock up one of the chains of bones in Yuanchuavis kompsosoura that is otherwise free in living birds, a requirement for kinesis.

The detailed study of the shape of the pterygoid, a palate bone, shows that it had no direct contact with another bone called the quadrate, which is also needed to complete the palatal chain of bones in kinesis.

That absence of contact is seen in most dinosaurs, including Triceratops and Tyrannosaurus, but the bones connect with one another in living birds.

In addition, the authors were able to confirm that the pterygoid of enantiornithine birds retained a unique shape.

It had a two-pronged projection behind the eye like Velociraptor and other close dinosaurian relatives of birds.

Although these features ruled out any kinesis in the skull of early birds, the team was able to uncover a secret hidden in the fossil skull regarding the origin of kinesis.

Detailed analysis and comparison of the palatine, another bone in the palate of Yuanchuavis kompsosoura, shows that the palatine lacks a key contact with the jugal bone, a part of the cheekbone.

Dinosaurs and the oldest bird Archaeopteryx have that contact, which helps stabilize the palate bones and restricts movement.

In contrast, the palatine of living birds does not have that contact, like in Yuanchuavis kompsosoura, thus facilitating back and forth sliding during bird skull kinesis.

“The change in palatine bone shape from having contact with four to just three other skull bones in enantiornithine birds may be where skull kinesis began,” Professor Wang said.

“New features evolve from old ones, and kinesis must have evolved in birds from an ancestor lacking it.”

The findings were published online in the journal eLife.

Source: sci.news