Witmer, L. M. 1997. Craniofacial air sinus systems. pp. 151– 159 in The Encyclopedia of Dinosaurs, P. J. Currie and K. Padian (eds.), Academic Press, New York. Craniofacial Air Sinus Systems LAWRENCE M. WITMER Ohio University Athens, Ohio, USA n unusual anatomical system pervaded the congested when we have colds and that are involved A heads of dinosaurs. In:sinuated among such in our "sinus headaches." conventional soft tissues as muscles, nerves, blood Paratympanic air sinuses are less common in ar­ vessels, and sense organs was a complicated system chosaurs. In those non-dinosaurian taxa with para­ of air-filled sinuses. These sinuses formed as thin­ tympanic pneumaticity (such as crocodylomorphs walled, epithelial outgrowths (diverticula) of other and pterosaurs), the bones of the braincase are the air-filled cavities, often invading and resorbing sur­ ones that are usually invaded by air sacs. Among rounding bone and producing foramina and cavities perhaps all archosaurs, tympanic recesses are best withinthesebones. This process is called pneumatiza­ developed in theropod dinosaurs. As with paranasal tion and the resulting state of having air-filled bones sinuses, humans and most other mammals have a set is known as pneumaticity. Many dinosaurs are highly of sinuses associated with the tympanic cavity, and pneumatic animals indeed, with most of the bony particularly bad middle-ear infections may spread to skull literally riddled with foramina, channels, and our paratympanic air sinuses. cavities. More technical treatments of this topic have In addition to paranasal and paratympanic si­ been published byWitmer (1990, 1995, 1997) and Cur­ nuses, there are other, more poorly known, systems rie and Zhao (1993a,b) and the articles cited therein. pneumatizing the head skeleton. The first may simply represent diverticula from the cervical system of pul­ Pneumatic Systems in Dinosaurs monary air sacs that extend beyond the neck verte­ There are two well-known pneumatic systems in di­ brae into the occipital region of the skull. Some of nosaurs, one arising as outgrowths of the nasal cavity the pneumatic cavities of certain theropod dinosaurs and the other as outgrowths of the tympanic (middle may result from these pulmonary diverticula. The ear) cavity. Not just dinosaurs, but all archosaurs­ second is the median pharyngeal system that forms living and extinct-have at least one main paranasal as a midline outgrowth from the roof of the throat air sinus, known as the antorbital sinus, that forms and invades the base of the skull in the region of as an outgrowth of the main nasal cavity (Fig. 1). The the basisphenoid and basioccipital bones in many antorbital sinus produces a large cavity and opening archosaurs. It is not always demonstrably of pneu­ inthe side of the face known respectively as the antor­ matic origin in many archosaurs but is almost cer­ bital cavity and antorbital fenestra. In many archo­ tainly so in theropod dinosaurs. Although the re­ saurs, the antorbital sinus itself has subsidiary out­ sulting "basisphenoid sinus" is often regarded as a growths that may pneumatize surrounding bones, derivative of a "medianEustachian tube," it is clearly producing so-called accessory cavities. In addition to distinct from the definitive auditory (Eustachian) the nearly ubiquitous antorbital sinus, a few kinds of tubes that, along with the tympanic cavities, have dinosaurs have air sacs deriving from a different part their embryological origins from the paired first pha­ of the nasal cavity, namely, the front-most portion ryngeal (or branchial) pouches; whereas in some ar­ known as the nasal vestibule. Such vestibular sinuses chosaurs the median system eventually connects up tend to pneumatize the bones surrounding the bony with the tympanic cavity, it does not always do so. nostril (i.e., premaxilla and nasal). Humans and most Some have suggested that the median pharyngeal other mammals have similar (but not homologous) pneumatic system results from aeration of the embry­ paranasal sinuses; these are the sinuses that become onic hypophysial pouch (of Rathke)-a precursor of 151 152 Craniofacial Air Sinus Systems Allosaurus (ragilis Sinraptor dongi lacrimal sUbsidiary diverticula in recess accessory cavities promaxillary recess Facial Pneumatic Recesses • Maxillary recesses • Lacrimal recess sinus antorbitalis in • Promaxillary recess • Jugal recess antorbital cavity • Maxillary antrum • Nasal recess • Excavatio pneumatica • Palatine recess FIGURE 1 Paranasal pneumaticity in theropod dinosaurs. (Left) skull of Allosaurus fragilis in oblique view, showing the antorbital paranasal air sinus and some of its epithelial diverticula (modified from Witmer, 1997, with permission). (Right) skull of Sinraptor dongi in left lateral view with the major paranasal pneumatic accessory cavities labeled (modified from Currie and Zhao, 1994a, with permission). The acces­ sory cavities result from pneumatization by the subsidiary diverticula of the antorbi­ tal sinus. part ofthe pituitarygland-and this is anidea worthy Paranasal pneumaticity, on the other hand, is pres­ of further investigation. ent in probably all ornithischians in that, like all other archosaurs, they possessed an antorbital sinus. This Ornithischian Dinosaurs antorbital air sinus was lodged in a cavity, the antor­ Paratympanic air sinuses are very uncommon in or­ bital cavity, located in front of the orbit and bounded nithischians. The middle ear sac was certainly present by primarily the maxilla and lacrimal, and sometimes (as evidenced by, among other things, the discovery also the jugal, palatine, and nasal. In fact, this descrip­ ofcolumellae-the slender earbones), but apparently tion holds for most dinosaurs (indeed, most archo­ it did not typically send out diverticula that invaded saurs). Pneumatic bony accessory cavities, produced surrounding bones. A few ornithischians, however, by subsidiary diverticula of the antorbital sinus, are such as the basal thyreophoran Scelidosaurus and the relatively rare in ornithischians, although they are ornithopod Hypsilophodon, do seem to have some ex­ present in a few taxa, such as the intramaxillary si­ cavations of portions of the braincase that are best nuses of Protoceratops and its relatives and the maxil­ interpreted as being of pneumatic origin. In these lary recesses of some basal thyreophorans. Higher forms, there is a fairly extensive cavity associated thyreophorans, in particular ankylosaurid ankylo­ with the canal for the major artery supplying the saurians, deserve special mention here in that within brain, the cerebral (or internal) carotid artery. This their highly armored skulls is a maze of pneumatic cavity is directly behind and medially undercuts a sinuses. The precise pattern and arrangement of an­ curving ridge of bone-nearly ubiquitous in archo­ kylosaurid paranasal sinuses remain poorly known, saurs-knownas the otosphenoidal crest, which runs and it is not entirely clear if some are accessory cavi­ from the basipterygoid process to the paroccipital ties of the main antorbital sinus, novel paranasal si­ process and segregates the orbital contents in front nuses, oreven diverticula ofthe nasal vestibule. Pneu­ from the middle ear contents behind. Although such matic evaginations of the nasal vestibule, however, a rostral tympanic recess is, as we will see, fairly are clearly expressed in lambeosaurine hadrosaurids, common in theropod dinosaurs (as well as in a variety such as Corythosaurus. In lambeosaurines, the narial of other archosaurs), it is rather rare in ornithischians. region is greatly enlarged, and the bones enclosing Craniofacial Air Sinus Systems 153 cavity is relatively tiny and completely closed later­ ~ A ~ ally). A similar phylogenetic trend can be identified ~1 ~~ in thyreophorans. These trends almost certainly relate !!~~-~ to (especially in ornithopods) the expansion and elab­ oration of the feeding apparatus (in particular, the ~ '~ dentition and its bony supports). Thus, as the feeding Heterodontosaurus apparatus expanded phylogenetically, the antorbital Lesothosaurus Iguanodonlia sinus and its bony cavity contracted. Euornithopoda Before concluding the discussion of ornithischian _---"_------' Ornithopoda craniofacial pneumaticity, the supracranial cavities of ceratopsids such as Triceratops need to be considered. These cavities, formed by the folding of the frontal bones, are often referred to as "frontal sinuses" and FIGURE 2 The evolving antorbital cavity of ornithischian are commonly thought to be of pneumatic origin. dinosaurs, especially Ornithopoda. Most clades of ornith­ They are often compared to the frontal air sinuses of ischians, such as Ornithopoda, show marked trends for modem bovid mammals (cattle, sheep, etc.) because reduction and enclosure of the antorbital cavity by laminae inboth ceratopsids and bovids the sinuses form strut­ of the maxilla and lacrimal. Modified from Witmer (1997) and references cited therein with permission. ted chambers that extend up into the base of the hom cores. It is possible that the supracranial cavities of ceratopsids are indeed pneumatic, but the source of the air-filled diverticulum remains obscure. It is not the nasal vestibule (the premaxilla and nasal) are yet clear how an outgrowth from either the nasal folded into a complicated collection of passages and cavity or the tympanic cavity could reach the skull chambers, all of which are perched atop the remain­ roof. Its mode of development is also unusual for a der of the skull.