Craniocervical morphological integration in birds

This study investigates the morphological diversity of the neck and its integration with skull anatomy in crown-group birds (Neornithes). The craniocervical articulation, where the skull connects with the neck, can adopt either caudal (rearward) or ventral (downward) orientations. Morphological coherence is crucial for maintaining head stability and precise movement, suggesting that the evolution of these structures must be not only functionally, but also developmentally coordinated (i.e., morphologically integrated). To explore this hypothesis, the relationship between conventional morphometric data of the neck and geometric morphometric data of the skull were assessed using multivariate statistics (Regressions and Two-block Partial Least Squares) across a broad phylogenetic range of Neornithes. Results indicate a significant level of integration between neck and skull morphologies, where variation in neck length, vertebral counts and relative lengths correspond predictably with specific craniocervical articulations. The most pronounced skull variation occurs around the occipital region, which we interpret as possibly relating to the mesodermic origin and shared genetic signalling in the morphogenesis of all craniocervical bones. Additionally, craniofacial changes align with craniocervical modifications, implying that the skull and neck evolve as a unified yet modular system. Analysis of selected fossils (Tsaagan, Archaeopteryx, and Pengornis) suggests that the ancestral craniocervical configuration was caudal, with neck changes associated with the development of a beak and craniocervical ventralization emerging in the lineage leading to modern birds.