Homology of Facial Structures in Extant Archosaurs (Birds and Crocodilians), With Special Reference to Paranasal Pneumaticity and Nasal Conchae

Homology of virtually all major components of facial anatomy is assessed in Archosauria in order to address the function of the antorbital cavity, an enigmatic structure that is diagnostic for the group. Proposed functions center on its being a housing for a gland, a muscle, or a paranasal air sinus. Homology is approached in the context of the Extant Phylogenetic Bracket method of reconstructing unpreserved aspects of extinct organisms. Facial anatomy and its ontogeny was studied in extant archosaurs (birds and crocodilians) to determine the osteological correlates of each soft-tissue component; resemblances between birds and crocodilians comprised the similarity test of homology. The congruence test of homology involved surveying phylogenetically relevant fossil archosaurs for these bony signatures. The facial anatomy of extant birds and crocodilians is examined in detail to provide background and to discover those apomorphic aspects that contribute to the divergent specialization of these two groups and thus obscure homologies. Birds apomorphically show enlarged eyeballs, expanded nasal vestibules, and reduced maxillae, whereas crocodilian faces are dorsoventrally flattened (due to nasal rotation) and elongated. Most facial attributes of archosaurs are demonstrably homologous and in fact characterize much more inclusive groups. Special emphasis has been placed on the nasal conchae and paranasal air sinuses. Within Amniota, the following conchal structures are homologous, and all others are neomorphs: avian caudal concha, crocodilian concha + preconcha, Sphenodon caudal concha, squamate concha, and probably the mammalian crista semicircularis. The avian antorbital paranasal air sinus is homologous with the crocodilian caviconchal sinus; the maxillary sinus of placental mammals is not homologous with the archosaurian paranasal sinus. With regard to the function of the antorbital cavity, archosaurs possess homologous nasal glands, dorsal pterygoideus muscles, and paranasal air sinuses, but the osteological correlates of only the paranasal sinus involve the antorbital fenestrae and fossae. Thus, the antorbital cavity is best interpreted as principally a pneumatic structure. o 1995 Wiley-Liss, Inc. The age-old image of a tiny plover calmly gleaning the leeches from within the gaping mouth of a crocodile hardly suggests the notion of any kinship between these two very different vertebrates. However, among modern vertebrates, birds and crocodilians are indeed sister taxa, representing the only surviving clades of Archosauria. Although extant archosaurs, with their 10,000 species, remain the most diverse group of terrestrial vertebrates living today, during the Mesozoic Era extinct archosaurs (i.e., nonavian dinosaurs, pterosaurs, and a variety of early forms) radiated into virtually all habitats and by all measures were the dominant terrestrial vertebrates. As a result of this radiation, Address reprint requests to Lawrence M. Witmer, Department of Biological Sciences and College of Osteopathic Medicine, Ohio University, Athens, OH 45701. email: [email protected] o 1995 WILEY-LISS, INC