Avian cranial kinesis is the result of increased encephalization during the origin of birds

Abstract

The origin of birds represents a pivotal transition in vertebrate evolution, marked by significant changes in both brain size and feeding biomechanics. The evolution of the avian skull involved dramatic modifications, such as a segmented palate and the development of powered cranial kinesis in neognath birds. Powered kinesis, the ability to move parts of the skull independently, is considered a key innovation behind the dietary diversity and evolutionary success of birds. However, the processes driving the emergence of avian kinesis have remained unclear until recently. By analyzing data from Mesozoic birds, including reinterpretations of palate homology, 3D jaw muscle biomechanics, and linkage analysis, researchers have quantified changes in muscle forces and their effects on palate mechanics during the transition from theropods to birds. As the neurocranium expanded in non-avian theropods, temporal muscles shifted to more rostrocaudal positions in birds, aiding in the segmentation of the pterygoid. This musculoskeletal transformation increased fore-aft muscle force in neognaths, enabling powered cranial kinesis. A critical change was the separation of the epipterygoid ossification from the braincase, leading to the breakdown of primitive kinematic linkages and the development of a new basicranial joint, which allowed for greater cranial flexibility. These findings shed light on how the neurosensory and feeding systems coevolved during bird origins and offer new methods for identifying cranial kinesis in extinct vertebrates.