Modern birds are the living relatives of dinosaurs. Take a look at the features of flightless birds like chickens and ostriches that walk upright on two hind legs, or predators like eagles and hawks with their sharp talons and keen eyesight, and the similarities to small theropod dinosaurs like the velociraptors of Jurassic Park fame are striking.
Yet birds differ from their reptile ancestors in many important ways. A turning point in their evolution was the development of larger brains, which in turn led to changes in the size and shape of their skulls.
New research from the University of Chicago and University of Missouri shows how these physical changes affected the mechanics of the way birds move and use their beaks to eat and explore their habitats — adaptations that helped them evolve into the extraordinarily diverse winged creatures we see today.
The benefits of 'wiggly' skulls
Modern birds, as well as other animals like snakes and fishes, have skulls with jaws and palates that aren't rigid and fixed in place like those in mammals, turtles, or non-avian dinosaurs. Alec Wilken, a graduate student in integrative biology at UChicago and lead author of the new study, calls this kind of flexible skull "wiggly." He says this characteristic makes it that much harder to figure out how the pieces work together.
"Just because you have a joint there, that doesn't mean that you know how it moves," Wilken said. "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."
Wilken joined the project in 2015 when he was an undergraduate at the University of Missouri. Casey Holliday, PhD, Associate Professor of Pathology and Anatomical Sciences at University of Missouri, received a grant from the National Science Foundation (NSF) to study how the skulls, jaw muscles, and feeding mechanics changed along the transition from dinosaurs to birds, and Wilken joined his lab to help.
The team began by taking CT scans of a variety of fossils and skeletons from modern-day birds and related reptiles like alligators. Using the data from these images, they then built 3D models to calculate the mechanics of the skulls and jaws in action — muscle sizes and placements, their movements, and the physics involved in how they all fit together.
One of the key differences between modern birds and other animals is that they have what's called "cranial kinesis": the ability to move different parts of the skull independently. This gives birds an evolutionary advantage by literally expanding their palates to eat different kinds of foods or use their beaks as a multifunctional tool.
"Having a wiggly head like this really gives them a lot of evolutionary benefits," Wilken said. Parrots, for example, can use their beaks to help climb; the extra torque helps other birds crack nuts and seeds. "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."
A cascade of changes from dinosaurs to birds
When the team analyzed data from the 3D models, they saw that as brain and skull sizes increased in non-avian theropod dinosaurs, muscles shifted into different positions that allowed the palate to separate and become mobile. These changes in turn increased muscle force, which powers cranial kinesis in most modern-day birds.
"We see this cascade of changes that happened along the dinosaur to bird transition," Holliday said. "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."
As paleontologists discover more details about dinosaurs, the dividing line between them and modern birds becomes murky (yes, birds are technically dinosaurs, but we're speaking in broad terms here). Scientists used to think feathers were the key, but now we know that many bona fide dinosaurs had feathers too. Flight also evolved more than once, and of course many well-known, classic dinos could fly as well.
However, flexible skulls and palates appeared later than transitional dinosaur/bird creatures like Archaeopteryx, and Holliday thinks that may become a key distinction. "Cranial kinesis might be one of the clear dividing lines between modern birds and their more dinosaur-like bird ancestors."
The study, "Avian cranial kinesis is the result of increased encephalization during the origin of birds," was published in the Proceedings of the National Academy of Sciences. Additional authors include Kaleb C. Sellers from UChicago, Ian N. Cost from Albright College, Jul Davis from the University of Southern Indiana, Kevin M. Middleton from the University of Missouri, and Lawrence M. Witmer from Ohio University.
