Intraskeletal Pathway of the Trigeminal Nerve in Modern Bird Beak: 3D Comparative Anatomy Using the diceCT Method

The morphology of the trigeminal nerve extending through the snout region in Archosauria has recently been the focus of extensive research, particularly in paleontology. However, the pathways of this nerve within cranial bones in extant birds have not been described in detail. Here, we describe trigeminal nerve pathways in the skull of nine avian species using a non-destructive soft-tissue imaging technique (diceCT) with a particular emphasis on osteological correlates, including intraskeletal neurovascular canals as well as foramina and grooves on the bony surfaces. In many species, a branch of the trigeminal nerve, the ophthalmic nerve, extends through the anterior premaxillary canal. Similarly, another branch of the trigeminal nerve, the maxillary nerve, passes through a bony canal in the maxillopalatine process at the posterior end of the snout in many species. However, apparently clade-specific unique pathways are also observed for the ophthalmic nerve around the external naris in Falconiformes and Psittaciformes and the maxillary nerve on the palatal surface in Anseriformes, the latter of which is likely associated with the specialized tactile sensation. Based on comparison with the condition in Crocodylia, the "reptilian-like" course of the maxillary nerve within the premaxilla is considered to have apomorphically been lost, whereas that of the ophthalmic nerve within the premaxilla has become extremely prominent in Aves. These changes in the intraskeletal nerve pathways in Aves have likely been associated with the enlargement of the premaxilla and the loss of teeth in the avialan lineage. The present observation of the trigeminal nerve pathway in Aves potentially contributes to the discussion of the homology of other associated soft-tissue structures and the presence of special sensory functions in fossil archosaurs.