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Modern birds are recognized as the descendants of dinosaurs. Observing the characteristics of flightless birds such as chickens and ostriches, which walk on two feet, juxtaposed with the predatory nature of eagles and hawks—featuring sharp talons and acute vision—highlights striking parallels with small theropod dinosaurs, including the infamous velociraptors brought to life in Jurassic Park.
Despite these similarities, birds exhibit significant differences from their reptilian ancestors. A pivotal moment in their evolutionary journey was marked by the emergence of larger brains, leading to alterations in skull shape and size.
Recent research conducted by the University of Chicago and the University of Missouri has shed light on how these morphological adaptations have influenced the mechanics of avian movement and feeding behaviors. These changes have been crucial in the evolution of birds into the incredibly diverse group we observe today.
The advantages of flexible skull structures
Modern birds, alongside other species such as snakes and fish, possess skulls characterized by movable jaws and palates, unlike the rigid structures seen in mammals, turtles, and non-avian dinosaurs. Alec Wilken, a graduate student in integrative biology at the University of Chicago and the primary author of the study, refers to this unique feature as “wiggly.” He notes that this flexibility complicates the understanding of how the skull’s components interact.
“The presence of a joint doesn’t inherently clarify its movement,” Wilken stated. “It’s essential to consider how muscles exert force on the joint, the type of torque produced, and how other joints in the head can restrict movement.”
Wilken’s involvement in this project began in 2015 during his undergraduate studies at the University of Missouri. Under the guidance of Dr. Casey Holliday, an Associate Professor of Pathology and Anatomical Sciences at the same university, Wilken contributed to a study funded by the National Science Foundation (NSF) that aimed to explore the changes in skull structures, jaw musculature, and feeding dynamics throughout the transition from dinosaurs to birds.
The research team utilized CT scans of various fossils along with skeletons from both modern birds and related reptiles such as alligators. They created 3D models from these images to analyze the mechanics of the skulls and jaws in action, considering factors like muscle dimensions, placements, movements, and the underlying physics of their connectivity.
A notable distinction of modern birds compared to other animals is their “cranial kinesis,” the capacity for different skull parts to move independently. This feature bestows evolutionary advantages by enabling birds to expand their palates, allowing them to consume diverse food sources or utilize their beaks as versatile tools.
“The wiggly nature of their skulls affords them numerous evolutionary benefits,” Wilken explained. For instance, parrots employ their beaks for climbing, while other species exploit additional torque to crack open nuts and seeds. “In several ways, the beak acts as a replacement for a hand, and being capable of repositioning the palate while feeding is critical for food acquisition and survival.”
A series of transformative changes from dinosaurs to birds
Upon studying the 3D model data, the researchers discovered that as non-avian theropod dinosaurs experienced increases in brain and skull sizes, the musculature rearranged, enabling the palate to become more mobile. This mechanism ultimately amplified muscle force, which underpins cranial kinesis in many contemporary birds.
“We observe a cascade of modifications that transpired during the transition from dinosaurs to birds,” Holliday remarked. “A significant aspect of this evolution relates to the point at which birds developed larger brains. As seen in humans, an increase in brain size incites numerous changes in skull morphology.”
As paleontological discoveries continue to reveal intricate details about dinosaurs, the line demarcating them from modern birds becomes increasingly indistinct (technically, birds are classified as dinosaurs, although this discussion considers broader categories). While feathers were once perceived as the critical distinction, it’s now understood that many authentic dinosaurs also sported feathers, and the capability of flight has arisen multiple times among various species.
Nevertheless, the flexibility of skulls and palates likely emerged after the evolution of transitional species such as Archaeopteryx, which Holliday proposes might serve as an essential differentiation criterion. “Cranial kinesis may represent one of the definitive traits that separates modern birds from their dinosaur-like ancestors.”
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