Homology of the palpebral and origin of supraorbital ossifications in ornithischian dinosaurs

The palpebral is a small ossification that projects across the orbit in some ornithischian dinosaurs and its presence is considered a synapomorphy of the clade. By contrast, other ornithischians lack the palpebral but possess accessory ossifications, commonly termed supraorbitals, which form the dorsal margin of the orbit. The homology of the ornithischian palpebral to one or more of the supraorbitals is widely accepted in the literature, but this homology has never been explicitly tested and no hypotheses have been proposed regarding the function of the palpebral or why it was incorporated into the orbital margin. As homology is synonymous with synapomorphy, incorrect homology statements can lead to incorrect hypotheses of relationships being obtained during cladistic analysis. The primary and secondary homologies of the ornithischian palpebral and the anterior supraorbital of more derived members of the major ornithischian clades are tested and we demonstrate that these homology hypotheses can be accepted. Osteological correlates indicate that the palpebral supported a layer of connective tissue that roofed the orbit; ossification of this connective tissue resulted in the incorporation of the palpebral into the skull roof and gave rise to additional supraorbital elements, which are neomorphic ossifications. Large-scale structural changes in the ornithischian skull, including dermal ossifications associated with display or defence and the development of complex feeding mechanisms, may have led to the incorporation of the palpebral into the skull roof.