Finite element modeling suggests functional divergence in the skulls of palaeognathous and neognathous birds

The basal bifurcation in the phylogeny of modern birds is between ostrich-like birds (ratites and tinamous; Palaeognathae) and all other birds (Neognathae). Most differences between the Palaeognathae and Neognathae lie in the reduction or loss of the ability to fly, but the crania of palaeognaths are also more robust and more fenestrated than those of neognathous birds. The specific morphology of the palaeognath cranium has been attributed to neoteny, but recent studies suggest that it must have functional significance. Within the avialan lineage the cranium becomes increasingly fenestrated leading to a reduced number of lateral elements. This trend is more pronounced in palaeognaths than in neognaths. To test how fenestration affects cranial function, we made a finite element model of a neognath (chicken) skull and in two analyses applied experimentally validated forces and displacements to it. We then removed the lateral bars, which are lacking in palaeognaths, and applied the same forces and movements. When the lateral bars were present, we found lower stress concentrations in the maxilla during beak opening, suggesting that lateral bars serve to reinforce the beak. The presence of the lateral bars during beak closing also appears to reinforce cranium as well as increase in beak reaction (bite) force. These results suggest that differences in the degree of fenestration in the skulls of palaeognathous and neognathous birds have functional consequences that may have played a role in the divergence of the two lineages.