‘Wiggly’ skulls helped birds evolve from dinosaurs

Modern birds are closely related to dinosaurs, as seen by comparing flightless species like chickens and ostriches, which walk on two hind legs, or predatory hawks and eagles with their keen senses and sharp talons, to small theropod dinosaurs. 

Still, birds have diverged in crucial ways from their reptilian predecessors. One vital turning point was their development of larger brains, which spurred changes in skull size and shape.

Recent work by researchers at the University of Chicago and the University of Missouri demonstrates how these physical transformations affected how birds move and utilize their beaks to eat and interact with the world – adaptations that helped them become one of the most diverse lineages on Earth.

Complex nature of a “wiggly” skull

Unlike many other animals, modern birds, along with certain snakes and fishes, have skulls whose jaws and palates are not firmly fixed in place. 

Alec Wilken, a graduate student in integrative biology at the University of Chicago and lead author of this new study, calls this kind of flexible anatomy “wiggly.” He notes that such a trait complicates attempts to understand its function. 

“Just because you have a joint there, that doesn’t mean that you know how it moves. So, you also have to think about how muscles are going to be pulling on the joint, what kind of torque they have, and how other joints in the head limit the mobility,” he explained.

Investigating how bird skulls evolved

Wilken first became involved in this project in 2015 while studying at the University of Missouri.

There, Casey Holliday, an associate professor of pathology and anatomical sciences, had received support from the National Science Foundation (NSF) to investigate how bird skulls, jaw muscles, and feeding mechanics evolved from their dinosaur lineage to modern birds.

Wilken contributed to the research by analyzing data from computer tomography (CT) scans of fossils and existing birds, as well as the skulls of other reptiles such as alligators.

The gathered data then supported the creation of 3D models that track muscle positions, movements, and the physics of how they operate together.

Advantages of modern bird skulls

One distinctive feature of modern birds is referred to as “cranial kinesis,” which is the capacity to move different portions of the skull independently. This affords birds an evolutionary edge, letting them open or expand their palates and use their beaks in various ways.

“Having a wiggly head like this really gives them a lot of evolutionary benefits,” Wilken said. Parrots, for instance, employ their beaks to climb, while other birds break open tough seeds and nuts. 

“In some ways, the beak functions like a surrogate hand, but being able to move the palate around while eating is also mission critical to helping them acquire food and survive.”

The path from dinosaurs to birds

From analyzing these 3D models, the scientists observed that as non-avian theropod dinosaurs developed increasingly larger brains and skulls, muscles shifted into arrangements that allowed for more mobile palates and jaws. 

Consequently, these transformations increased muscle force and supported cranial kinesis in the majority of today’s birds.

“We see this cascade of changes that happened along the dinosaur to bird transition,” said Holliday. “A large part of it hinges upon when birds evolved a relatively large brain. Just like in humans, bigger brains drive a lot of changes in the skull.”

Over time, paleontological findings have shown that the line between dinosaurs and birds is considerably blurred, especially since many dinosaurs sported feathers and some were capable of flight. 

However, the scientists noted that truly flexible skulls and palates appeared later than species such as Archaeopteryx, leading them to speculate that cranial kinesis may turn out to be a key factor distinguishing modern birds from more primitive dinosaur-like birds.

A blueprint for future understanding

While it is well established that prehistoric reptiles gave rise to the birds we now see flying overhead, the genetic and mechanical steps along that journey remain a topic of intense investigation. 

This latest research strengthens the idea that the interplay between a bigger brain and skull modifications allowed birds to free up their jaws and palates, leading to vast ecological success.

Ultimately, the improved skull flexibility and diversified feeding methods among birds mirror the complicated interplay of evolutionary pressures that have continued since the time of dinosaurs. 

The new study represents only one step in unravelling how birds learned to harness their beaks’ capabilities, and how earlier dinosaur species formed the foundation for today’s avian biodiversity.

The study is published in the journal Proceedings of the National Academy of Sciences.

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