There are some odd cranial-facial oddities among amniote vertebrates out there, doing all sorts of odd things with their skulls, such as cyamodontoid placodonts, which lost virtually all of their margin teeth but expanded their palatal teeth while forming thin struts with the tips of their jaws, or the strangely “upside down” skulls of flamings, or the twisted and Hoover-vacuum-like skull of Nigersaurus (and apparently other rebbachisaurid sauropods). So it would then appear odd to speak of another group as potentially more odd: that of members of the strange group called Oviraptoridae.
It should be noted that, despite being theropods, which are generalized for the most part, oviraptorids are in some strange company when it comes to skull variation. See, oviraptorosaurs generally exhibit a lot of craniofacial shortening; that is, the region of the skull rostral to the eye has become increasingly shorter and shorter. This is only emphasized in oviraptorids. Here, I illustrated the variety of oviraptorid “snouts” out there (crests, prongs, serrated margins, etc.). I also illustrated the skull of an oviraptorid in a myriad of views here. I’ll let you absorb all of those links.
Here we have the holotype skull of Citipati osmolskae (GIN 100/978, which includes a nearly complete skeleton lying atop a nest, but undescribed), shown in whole lateral (A) and ventral (B) views, and in sagittal section (C) through the premaxilla. Those lines in B and C represent the sections through which D1-5 are taken, illustrating a feature often glossed over in oviraptorid discussion — but I’ll get to that in a bit.
Because oviraptorid skulls are short, certain parts of them get moved around to make room for the innards of the head (nasal passage, eyeballs, muscles, etc.). This has resulted in a drastic reconfiguration of the skull. It’s seen in sauropods to a degree, and hadrosaurids as well, among dinosaurs, and among a variety of mammals and reptiles. What generally happens is that a section of the skull is simply shortened, while to make room, segments will become rotated to preserve muscle attachment and functionality of parts without loosing pieces of the whole (the posterior palate in sauropods — along with the jugal and postorbital — are essentially rotated around 90 degrees, while the posterior portion of the skull is less rotation, and the snout is not rotated at all … relatively.
Now, ordinarily, in most dinosaurs (indeed, in most archosaurs), several bones extend below the jaw margin; this is especially notable in crocodilians, where the pterygoid and ectopterygoid are expanded and make up virtually half of the skull’s lateral profile!
But in most typical dinosaurs, this is limited to merely these two bones. In most oviraptorosaurs, these bones are unknown, with the exception of oviraptorids, but in many of them, such as a specimen referred to Chirostenotes pergracilis, ROM 43250, the maxilla has clear indications that the palate was probably not expanded below the jaw margin, especially due to a large tomial blade of the maxilla. This tomial blade is absent in oviraptorids (but I’ll get to that in a later post). In oviraptorids, the entire palate is everted (or “pushed outward”) below the jaw margin.
Part of this “eversion” complex of the ventral portion of the skull is the appearance of portions of the secondary bony palate, a structure formed from the medial plates of the premaxilla and maxilla, which meet on their midlines. Because the vomer, which usually lies between the premaxillae and divides the maxillae from one another, is ventrally extended, the palatal surface is as well, and we can see the ventral surfaces of these bones … in lateral view.
Schematic of an oviraptorid palate in top/lateral and bottom/ventral views. Hatching indicates region of overlap with the maxilla. EcPt, ectopterygoid; Ept, epipterygoid; Pal, palatine; Pt, pterygoid; V, vomer. Hatching indicates overlap with other bones, with maxillae overlying all hatched portions. Dotted lines show maxillae, jugal, quadratojugal, and quadrates in articulation — bottom only.
The eversion of the palate is so extensive, that as a complex, it provides a substantial value to the degree to which oviraptorids (Oviraptoridae) differs from literally all other theropod dinosaurs, or really all of the rest of Archosauria. It is true that there are a number of birds that have everted palatal bones (parrots have nearly their entire palatine below the jaw margin) none others have their entire palate everted to this degree.
Note anteriorly in the palate, where even the ventral surface of the maxilla becomes a portion of the palatal complex, while the premaxillae are also partially exposed laterally. These are seen in the first figure (D) with gradual fluting arranged mostly longitudinally to the skull axis. The functional significance of these has hardly been touched on, yet they are apparent in all oviraptorid skulls. Something more interesting has been noted by various researchers, and that’s how a process of the medial maxilla, a portion of the palatal plate, is expanded into a small triangular spur. This plate is sometimes rounded or oblate in some oviraptorids, but mostly it is triangular, and has consistently reminded various observers of a “tooth.” Some workers have even so guilty of this characterization that they illustrate is as a lone hard element sticking out from a sea of soft-tissue “gum.” But this characterization is very wrong, and very unstudied. There are two of them, and as in the figure (D5), there are two of them and they are paired close together on the midline of the palate, effectively forming a single small “process.”
The functional significance of these processes, unlike the ridges, has been speculated on (it was argued to be a device for breaking eggs by various workers including Norman and Paul, and of other shells by Barsbold), but never analyzed in depth. It should be interesting to see what further research may provide.
Barsbold R. 1986. Raubdinosaurier Oviraptoren (The robbing dinosaurs — oviraptors). pg.210-223 in Vorob’yeva (ed.) Herpetologische Untersuchungen in der Mongolischen Volksrepublik. Академия Наук СССП Институт Еволюсыонной Морфологий и Экологий Зывотнйк им. A.M. Syevyerstova, Moscow. (in Russian w/ German and English summary)
Norman, D. M. 1986. The Illustrated Encyclopedia of Dinosaurs. Gramercy Publishing (New York City).
Paul, G. S. 1988. Predatory Dinosaurs of the World: A complete Illustrated Guide. Simon & Schuster (New York City).