ARTICLES Holes in the Head: Evolutionary Interpretations of the Paranasal Sinuses in Catarrhines TODD C. RAE AND THOMAS KOPPE Paranasal sinuses are hollow spaces in the facial skeleton that are partially separated by bone from the nasal cavity proper (Fig. 1). They arise from the cartilaginous nasal capsule during growth of the face; each sinus starts as a outpocketing of the nasal cavity lining, or epithe- lium, that invades the surrounding bone by resorption. 3 Although named for the principal bone in which they are found in humans, the four mammalian sinuses, maxillary, sphenoidal, ethmoidal, and frontal, are anatomically delineated by the position of their opening, or ostium, within the nasal cavity. 2,4,5 Historically, there is disagreement over who first alerted the scientific world to the presence of paranasal pneumatization. It was once widely held that Galen, a second-century philosopher and doctor to the Ro- man Emperor Marcus Aurelius, dis- covered these unique hollows in the bones of the cranium. 6 Indeed, even Hippocrates is cited as expressing his opinion on craniofacial pneuma- tization. 7 However, the earliest di- rect evidence of scientific interest in paranasal sinuses is found in the notebooks of Leonardo da Vinci, 8 one of which contains a drawing of a sagitally and coronally sectioned hu- man skull with the frontal and max- illary sinuses exposed (see cover). The study of the paranasal sinuses is hampered by the fact that they are hollow structures; in essence, they are a lack of bone. In addition, their location inside the cranium raises a host of difficulties. As da Vinci found, often the best way to examine sinuses is to section the cranium. Indeed, until recently, most of the major work on cranial pneumatiza- tion was based either on casting, in which the sinus is filled with a resis- tant substance and the surrounding bone is destroyed, or on dissection. Although a few studies of primate cranial pneumatization relied on flat-plane x-ray examination, 9,10 it was not until computed tomography (CT) imaging techniques achieved widespread availability that com- mensurate attention began to be paid to this important aspect of cra- nial anatomy. 11 CT imaging and the three-dimensional virtual recon- structions that can be generated from CT data (Fig. 2) revolutionized the study of internal cranial struc- tures in primates. 11–16 This nonde- structive, noninvasive technique al- lows detailed data on cranial pneumatization to be obtained eas- ily, even from fossils in which the internal cavities are filled with ma- trix. 17 As a result, new discoveries about the evolution, growth, devel- opment, and possible functions of the paranasal sinuses in primates have increased noticeably in the last decade. EVOLUTION Based on the work of a few pio- neers, in particular Seydel, 18 Paulli, 4 and Cave and colleagues, 5,19 by the middle of the twentieth century a basic outline of paranasal sinus dis- tribution in catarrhine primates had been enunciated (Fig. 3): Maxillary and sphenoidal sinuses are present in all hominoids, ethmoidal and frontal sinuses are seen only in Afri- can apes (here and throughout, in- cluding humans), and paranasal pneumatization is completely lack- ing in most cercopithecoid mon- keys, 4,20,21 with the exception of ma- Todd C. Rae is a member of the Evolu- tionary Anthropology Research Group and a Lecturer in the Department of An- thropology at the University of Durham, UK. He has investigated craniofacial mor- phology, paleoprimatology, and the the- ory and method of phylogenetic system- atics. E-mail: t.c.rae@durham.ac.uk Thomas Koppe is an Oberassistent of the Institute of Anatomy at the Ernst Moritz Arndt University, Greifswald, Germany. His main interests are the growth and development of cranial pneumatization in mammals and the application of com- puted tomography to morphological problems. E-mail: thokoppe@uni-greif- swald.de © 2004 Wiley-Liss, Inc. DOI 10.1002/evan.20036 Published online in Wiley InterScience (www.interscience.wiley.com). Everyone who has ever experienced a head cold is familiar with the paranasal sinuses, the bony hollows above and beside the nasal cavity that contribute, sometimes painfully, to upper respiratory tract disorders. These internal cranial structures have a wide distribution among eutherian mammals and archosaurs. 1,2 Sinuses have languished somewhat in the shadow of their better known and more accessible morphological cousins (dentition, postcrania), but new imaging tech- niques, growth studies, and explicit phylogenetic evaluation 3 are beginning to fill in the gaps in our knowledge of the evolution of these enigmatic spaces in primates and promise to yield insights into the evolution of the facial skeleton. Evolutionary Anthropology 13:211–223 (2004)