The skulls of humans as well as other mammals are characterized by air sinuses associated with the nasal cavity and the middle ear. The mammalian sinuses tend to be minor aspects of skull form. However, another group of vertebrate animals, the Archosauria, certain extinct dinosaur groups and their presumed living descendants the birds and crocodiles, exhibit air sinuses in the skull but of different shape and position from those of mammals. In many archosaurs, sinuses dominate skull morphology. Although archosaurs have been the most diverse land vertebrates for over 200 million years, their important anatomical system of sinuses has been neglected in morphological studies. Consequently it has not been possible to determine which sinuses are strictly comparable among archosaurs, that is, which are "homologous" in an evolutionary sense, having evolved from the same developmental and anatomical source in ancestral forms. Faculty sponsor David Weishampel and graduate student Lawrence Witmer are studying the air sinus systems in several representative samples of living birds and crocodiles and in well preserved skull fossils of extinct archosaurs. Their methods of study include sophisticated clearing and double staining procedures with the modern forms, and the use of elaborate casts from fossil forms. Their comparative data will help determine which features of the air sinus system are similar or different among archosaurs and which features provide strong evidence for close genealogical relationship within the group. When correlated with what is known about the life history of the animal, whether fossil or living, the skull sinus data may help in solving the puzzle about what functions these cavities perform in the life of the organism.
