Oviraptorid Jaw Muscles Described, Part 3

In the previous two posts (part 1 here, part 2 here), I discussed the shapes and sizes of the muscles and their origins and insertions of the oviraptorid skull. I deigned to provide the basis of the muscles mapped to the skull, supported by reference to updated muscle map locations in extant sauropsidans by Marc Jones for Sphenodon punctatus (tuatara) and turtles, by Eric Snively and Casey Holliday for birds, lizards, and crocodilians in general, and in some specific cases for various theropod dinosaurs. As a restriction, I’ve chosen to put higher weight on avian and theropod dinosaur muscles mapped by these researchers, with attention to sphenodontids and lizards. These maps are harder to determine without personal examination, so in the case of “Big Beak,” generalization is used rather broadly. Even being able to acquire a cast of an oviraptorid skull with excellent preservation would benefit this process.

Skull of “Big Beak” (PIN uncatalogued), a “conchoraptorine” oviraptorid. Skull in various views with origins and insertions colored. The light grey circle in the surangular in medial and lateral views represents the common appearance of a “hole” that may constrain the positions of the muscles mAMP and mPTV. Click to embiggen.

Now, an extension of this research is to look at individual oviraptorids. And in this, the various maps are respective of the differing anatomy of the cranium (braincase, upper temporal arch, supratemporal fossa) and mandible. In some, the position and size of the coronoid process, the degree of relief of the surangular “ridge” (weakly developed in oviraptorids and fairly short), and the fore-aft position of the lower jaw relative to the cranium relate to the size and shape of the muscles, and thus their relative actions and strength, influence the functional considerations of the jaw. There are several ways to model this, including typical beam theory, and these will have to be done for each skull (I hope, anyways). This includes a prospective reconstruction of Oviraptor philoceratops, which has an inordinately elongated rostrum.

To reconstruct the possible motions, we need to know the limits of the jaw, at least as far as we can, and it’s been hypothesized that the biconvex jaw articulation is extremely permissive in movement (Sternberg, 1940; Cracraft, 1971; Barsbold, 1983; Wellnhofer, 1991; Smith, 1993). What is less permissive may be the beak, but as may be seen below, the tips of the jaws are not occluded, unlike in toothed theropods, or even in ornithomimosaurs and potentially birds:

That gap at the front, one can assume, is filled with beak. But otherwise, the jaw is not confined rostrocaudally, so the jaw position is not constrained save by the bony coronoid process.

Shape and Structure of the Beak

Correct articulation of the jaw also relates to the maximums for muscle extent at various positions, and the potential reconstruction for the depths of rhamphothecae (assuming it had them!) and their extent. And on the latter aspect, I noted before that rhamphothecae seem to be related to extent of foramina, bounded as in ornithischians by the presence of evidence for other tissues, such as cornified pads or extranarial tissues:

Skull of MPC-D 100/978, holotype of Citipati osmolskae, from Clark et al., 2002. A indicates the unmodified skull, with views from the paper. B indicates an overlay showing the porosity and regularity of the bone surfaces. Red is the lead regular, with broken or highly porous surfaces; orange moderate with heavy clustering of foramina, and yellow with light clustering of foramina. Anterior is to the left, lateral to the right (skull above, mandible below).

Graded against the number of foramina in the bone, the presence of rhamphothecae is indicated more with grater foramina, a feature found in birds or turtles, with the caveat that in regions of more foramina but irregular presence of bone, with “spongy” bone, a cornified pad or extensive soft-tissue would be present. Indeed, in the rostral tips of several bird and turtle beaks, especially parrots, the irregular preservation is associated with an extensive “hook” on the beak. If we mapped oviraptorid rhamphothecae to the density of foramina, the most densely packed foramina would be the strongest indication for a cornified plate, grading outward towards the rhamphothecal “limits” noted below.

Now, above, there are 5 variations on a possible reconstruction, one in which there are two types of tissue indicated: cornified plates forming the rhamphothecae, divided into premaxillary, maxillary and mandibular plates; and cornified pads, which includes avian “cere” which would expand over the edges of the rhamphothecae and in oviraptorids might also cover the snouts and around the pre-orbital fenestrae. In type 1, a full rhamphothecae with pad model is shown. In type 2, the basic rhamphothecae are shown, at their “maximum” limits, bounded on the cranium by the circumnarial fossa and antorbital fossa dorsally and the jugal caudally, and on the mandible by the external mandibular fenestra. This corresponds to the maximum region of foramina on the “beak” (up to yellow, above). The mandible bears a spline of plate extending towards the coronoid, corresponding to the occlusal margin with the maxilla. In type 3, the rhamphothecae are limited to the moderately dense and densist foramina (orange, above), and I might consider that the mandibular plate would not extend as far. In type 4, the distal splines of the maxillary plate and mandibular plates are either absent, or the maxillary plate is missing entire; the plates are confined to only the densest region of foramina. In type 5, a variation of type 2, rather than distal extent, the dorsal extent is made more brief, and the plates are confined to the margins to rostral ends, and the “cere” would extend all the way to these margins and beyond. Depending on the closure of the jaws, the rhamphothecae could extend further from the jaws at the tip to “close” the jaws, although perhaps (as in the open-billed stork, Anastomus sp., or some parrots, only the tips contacted).

Skull of “Big Beak” (PIN uncatalogued), a “conchoraptorine” oviraptorid. A, described the rostral-most occlusion between the jaws, with arrow position 1 indicating the rostralmost occlusion, and 2, the caudalmost occlusion. Arrow 3 indicates the approximate shape of the space between occlusal tomial margins of the upper and lower jaws, a gap perhaps necessitating a rhamphothecal covering to “contain.” B, describes the shapes of the rhamphothecae required to fill the gap in A. C, ventral view of cranium with rostral (1) and caudal (2) mandibular occlusal positions indicated, showing shape of upper rhamphothecae, which in D, are splayed somewhat to contact mandibular rhamphothecae. Abbreviations: adch, adductor chamber; ch, choanae; dcp, dentary cornified plate; mcp, maxillary cornified plate; pmcp, premaxillary cornified plates; sofe, suborbital fenestra.

The shape of the beak may be constrained by the gap between jaws when the jaws are closed, but determining how the jaws closed is based somewhat on assumptions: the mandible in its rostral third is wider than the skull in its rostral third, and this suggests that, when the occlusal margins are articulated (as might be expected in a beaked animal), the jaw is set substiantially back from the front of the upper jaw, creating a significant overbite. This position is supported by the presence of a ventral “trough” formed by a concave ventral margin of the jugal at the maxillary contact, and by a continuation of this “trough” rostrally onto the maxilla as the medial process of the maxilla descends ventral and medial to the tomial margin. Due to the slender height of the maxillary tomium, it seems unlikely to support a substantial rhamphothecal plate, and so the majority of this plate probably comprised the dentary plate (“dcp” above). Finally, the position of the coronoid process is at maximum rostral extent just rostral to the ascending (postorbital) process of the jugal, and is rostrally constrained by the medial curvature of the jugals, which prevents the mandible from protracting too far. The ascending process of the jugal’s position and orientation roughly parallels the m. pseudotemporalis superficialis, a relationship that bears investigation (as in, is this relationship constrained? how widespread is it among theropods, or indeed among other dinosaurs, or archosaurs, etc.?).

So, the shape of the beak somewhat constrains the position of the jaw, at least its protracted position when the jaw is fully closed. It’s quite another thing as the jaw becomes progressively opened: not only does the “beak” influence how far the jaw can protract as it opens, it also limits (to some degree) translational (side-to-side) movements, which the mandibular articulation may or may not also restrict. This jaw position also shows the resting positions for jaw muscles, to permit a base for a model for jaw muscle mobility as the jaw opens, and that is itself very useful. The shape of the beak as produced here is also rather unique: “Big Beak,” rather uniquely among oviraptorids, has a seemingly “toothed” premaxillary tomium, so that the lateral margins near the maxillae form ventral projections, while the rest form a singular, smooth and continuous curve with its nadir at the rostral tip, producing three “teeth,” and were this specimen ever described would likely represent one of its autapomorphies (as is one of the reasons I refer to this specimen by its beak). The rhamphothecae might very well follow this shape, although that is debatable even though it seems in birds and turtles the rhamphothecae tends to conform to the general shape of the bony tomia, depending on the latter’s morphology (foramina count, surface morphology, microstructure, thickness of the cornified plates that form the rhamphothecae, etc.). If the plates did follow the bone, the beak would possess a pair of lateral “pseudoteeth” which might very well help it grip a bolus in its jaws, or even to help render it, an advantage (as in some falcons, the osprey, and the extinct pelagornithids — or pseudodontorns — which posses similar projections of the beak, all of which are also found on the bony beak) for handling slippery prey. Thus, it might very well seem that Big Beak represents a carnivorous — and possibly piscivorous — oviraptorid. That is not to say the beak morphology might not also be useful in handling less … animate … foodstuffs, and indeed Lindsay Zanno and Peter Makovicky’s hypothesis of herbivory for oviraptorosaurs in general (predicated on the relatively small heads, long necks, and so forth) may well be true.

The structure of the beak hypothesized above does not necessarily conform to my hypothesis of durophagy in these animals. I have, however, deliberate chosen to restrict palatal expansion of the rhamphothecae to the premaxilla, and rostral to the ventral maxillary “prongs.” It is these processes of the maxillae (formed from ventral and medial expansions of the medial vomerine processes of the maxillae and converging ventrally and in some specimens meeting and squeezing the vomer from the ventral aspect of the palate) that seem relatively out of place for either a dedicated herbivore, or a toothless carnivore, piscivore or not; certainly, no living bird has such structures, and the only comparison among turtles is comprised of especially spongy cornified pads, as in Caretta caretta (personal observation).

Skull of MPC-D 100/978, holotype skull of Citipati osmolskae Clark et al., 2001. Skull in A in transverse section (B) and sagittal section (C), describing coronal sections (D) through the premaxilla and maxilla, demonstrating the development of the ventromedial maxillary “prongs.”. From DigiMorph (used with permission).

Instead, these processes arise from ridges without extensive foraminal invasion of their surfaces, implicating smooth, possibly endothelially-covered, bone. It has long been claimed that these “prongs” were sharpened, triangular processes (as, indeed, Smith in 1993 and Wellnohfer in 1991 did, a conclusion not wholly their own as Barsbold Rinchen had developed the description on the basis of general shape, clarified thoughout the 1980s but initially in his Mongolian theropod dinosaur monograph in 1983) that their form would have been “tooth-like.” This has led to some authors and artists, including Greg Paul (in 1998) to reconstruct the palate as fleshy, but which the “prongs” projected through, an imitation perhaps of the hypapophyses of some snakes (such as Elachistodon westermanni or Dasypeltis sp.) which project through the oesophageal tissues into the canal, and which aids in the rendering of swallowed eggshell. (It is no wonder how the idea of egg-eating in these animals caught on early on, and has taken some effort to detract from and towards a more process-driven study of diet.) One interesting point that has come up is how untriangular in actuality these processes actually tend to be in many taxa, in which it may seem rounded, or trapezoidal in shape, but in truth only Citipati osmolskae appears to present a truly triangular process, and the apex of this triangle appears to largely be in line with the medial corpus of the vomer, implying its “projecting” shape is virtual, not actual. What benefit, then, would the processes have?

These processes constrain jaw movement somewhat, though probably not to the degree the mandibular articulation does, by providing a “ramp” against which the mandibular beak slides during retraction while the jaw is closed, or against which the mandible may guide a bolus onto. By the time the mandible retracted to the “prongs” — if indeed they could get that far — the mandible would effectively be “open,” the point lost. This further allows a limitation on the angulation muscles may have as the jaw moves through its propalinal range of protraction-stasis-retraction, and thus allow some quantification of the muscular effects on the jaw while closed, the very position one might infer when the jaw is processing foodstuffs, including vegetation.

Barsbold R. 1983. Жыщнйе динозаврй мела Монголий (Carnivorous dinosaurs from the Cretaceous of Mongolia). Трудй – Совместная Совестко-Монгольской Палеотологыческая Зкспедитсия — Joint Soviet-Mongolian Paleontological Expedition, Transactions 19: 1-120. (in Russian, w/ English summary)
Cracraft, J. 1971. Caenagnathiformes: Cretaceous birds convergent in jaw mechanism to dicynodont reptiles. Journal of Paleontology 45:805-809.
Holliday, C. M. 2009. New insights into dinosaur jaw muscle anatomy. The Anatomical Record 292:1246-1265.
Jones, M. E. H., Curtis, N., O’Higgins, P., Fagan, M. & Evans, S. E. 2009. The head and neck muscles associated with feeding in Sphenodon (Reptilia: Lepidosauria: Rhynchocephalia). Palaeontologica Electronica 12(2):7A, 1-56.
Jones, M. E. H., Werneburg, I., Curtis, N., Penrose, R., O’Higgins, P., Fagan, M. J. & Evans, S. E. 2012. The head and neck anatomy of sea turtles (Crytopdira: Chelonidae) and skull shape in Testudines. PLoS One 7(11):e47852.
Paul, G. S. 1988. Predatory Dinosaurs of the World: A Complete Illustrated Guide. Simon & Schuster (New York City).
Smith, D. K. 1993. The type specimen of Oviraptor philoceratops, a theropod dinosaur from the Upper Cretaceous of Mongolia. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 186:3365–388.
Snively, E. 2006. Ph.D. Thesis. Neck musculoskeletal function in the Tyrannosauridae (Theropoda, Coelurosauria): Implications for feeding dynamics. University of Calgary.
Sternberg, R. M. 1940. A toothless bird from the Cretaceous of Alberta. Journal of Paleontology 14(1): 81-85.
Wellnhofer, P. 1991. The illustrated Encyclopedia of Dinosaurs. Cresent Books (New York City).
Zanno, L. E. & Makovicky, P. J. 2010. Herbivorous ecomorphology and specialization patterns in theropod dinosaur evolution. Proceedings of the National Academy of Sciences of the United States, Philadelphia 108:232-237.