In the last post, I discussed the one jaw to cranium muscle that isn’t a depressor that bears on its mechanics (m. depressor mandibular, or mDM) and the palatal and psuedotemporal groups of muscles (m. pterygoideus and m. psuedotemporalis, respectively). I left out the primary, “vertical” adductors because it would be neater to include them all together, as they are for the most part of a unit. I should note before I continue and get to those remaining adductor muscles mention the muscles I’m NOT discussing. There are an arrangement of smaller muscles of the skull that are localized within the cranium, and attach to the palate and braincase, but do not extend to the jaws; some of these muscles are important for jaw movement in some dinosaurs due to the passive presence of cranial kinesis, but for the most part in oviraptorids, I’ve chosen to simply ignore them … for now. This is due to the fact that oviraptorid skulls are remarkably non-kinetic, with several of the bones generally involved in mobility fixed tightly to one another. This is not to say there wasn’t some form of mobility, but it was very slight, and likely allowed minor distortion to alleviate stresses in the skull. Anyways…
M. adductor mandibular posterioris (mAMP) is a short muscle that originates on the anterior surface of the quadrate and, in archosaurs where there is a process that contacts the pterygoid bone (the “pterygoid process,” of all things), mAMP attached to that, and across most of its lateral surface. It can extend towards the pterygoid, but for the most part its localized to the process. One can expect that, with a larger, taller and longer process, the attachment for mAMP gets larger, and so the muscle may increase in size. The pterygoid process of the quadrate extends for about 1/3 of the length of the adductor chamber, and thus imparts a fairly sizable surface. This muscle attaches to the medial surface of the surangular, and there it typically occupies the surface between the ascending process of the prearticular and the medial surangular shelf, which supports the mAME-group muscles. As I mentioned last time, the surangular in several specimens of oviraptorid jaw exhibits a small hole that penetrates the bone, and gives the appearance of a foramen. In the only specimen of Oviraptor philoceratops, this has the smoothed-margins of a natural fenestra, but in others it is irregular. In Khaan mckennai, it is the latter, or the foramen is not present (Balanoff & Norell, 2012). In oviraptorids, the surangular in this region is extremely thin, and it is likely that any hole is a factor of preservation, i.e., it is taphonomic, and that normally this region is closed, albeit by thin plates of bone. Despite this, the question of whether this region could support muscular tension (laterally from mPTV, medially from mAMP) is a matter of structure, and analysis. It seems more than reasonable to reconstruct this muscle extending over the entire potential surface, including over the “hole,” but for safety’s sake this can be confined to the region in which the bone is thicker. The area for attachment of mAMP on the mandible is smaller than that on the quadrate under a restricted model, but on a larger model is is larger — determining if there is a constraint on the two surfaces (even in size, or whether the size of one can predict the size of the other) is a subject of research. The muscle generally acts as a “retractor,” pulling the mandible backwards.
The primary adductors (m. adductor mandibular externus medialis and m. adductor mandibulae externus profundus) extend from the dorsal margin of the adductor chamber and insert on the dorsal posterior 1/3 (or so) of the mandible in all sauropsidans. In turtles, and other taxa with a closed dorsal margin of the adductor chamber (such as some pachycephalosaurids or some ankylosaurids; or turtles, or ichthyosaurs, etc.) these muscles attach to the medial and lateral margins of this chamber. When the supratemporal fenestra is open, mAMEM and mAMEP extend further dorsally, while mAMES is restricted to the supratemporal bar, and remains fixed to its medial margins. In turtles, mAMEP and mAMES extend caudally and attach to the supraoccipital spine (crista supraoccipitale) and the squamosal, both of which can extend outward, and thus produce a sort of “open-ended” caudal adductor chamber. In theropod dinosaurs, the two muscles are demarcated from one another by a small ridge, and in coelurosaurian theropods, this ridge is generally confluent with a fossa that extends onto the dorsal surface of the skull, and expands onto the frontal caudally and the postorbital medially. Beneath this fossa, the frontal produces a lateral process that supports the postorbital, and beneath that is the attachment for mPstS, as mentioned previously. Oviraptorids have a fairly shallow, but broad, supratemporal fossa extending onto the frontal and postorbital, and a broad supratemporal fenestra: the postorbital and squamosal bones form a laterally-bowed bar which defines a very large, oval opening. This opening is generally “filled” with both mAMEM and mAMEP. The fossa in oviraptorids, as mentioned, is small, and thus mAMEM would have a short but broad origin site, and thus it is likely that this muscle was significantly smaller than mAMEP; it is also somewhat supressed by mAMEP, which extended dorsally and probably cranially onto the dorsal surface of the parietal. A clear ridge appears to circumscribe the adductor fossa here in “Big Beak,” but not in other oviraptorids. Such a margin, describing a “dorsolateral” and a “lateral” facet of the parietal, appears to mark the boundary of the adductor fossa in oviraptorids such as Khaan mckennai (MPC-D 100/973 and 100/1127; Clark et al., 2001; Balanoff & Norell, 2012) and Citipati osmolskae (MPC-D 100/978; Clark et al., 2002); more complicated is MPC-D 100/2112, holotype of Nemegtomaia barsboldi; Lü et al., 2004) in which the ridge appears to be present, but is close to the midline, where a thin ridge is present in virtually all oviraptorids. This ridge may describe the “true” origin point for mAMEP, in which case the entire dorsal surface of the parietal forms a muscle site, though I have my doubts about this. In Oviraptor philoceratops (AMNH PR 6517) and Rinchenia mongoliensis (MPC-D 100/32A), the parietal is expanded dorsally into a tall midline crest, though the morphology in both specimens is unclear and both specimens in this region are extremely eroded, making the solution questionable.
Both muscles insert on the dorsal mandible, between the coronoid process as the “pedestal” of the surangular which supports the articular, mAMEP rostral to mAMEM, where it attaches to the coronoid process on its dorsal and lateral surface, and in a line with one another. This region in all oviraptorids forms a convex arch in cross-section across the dorsal surface, and a broad lateral surangular ridge. Along the median of this surface, exclusive to the surangular, a narrow groove may be present, with what appear to be oblique foramina; it is suggested that this groove (when present) divides mAMEM+mAMEP from mAMES, the latter which occupies the lateral surface, and the former two the medial. The lateral half is tilted laterally and forms the dorsal surface of the surangular ridge, and the medial half dorsomedially, and would not be exposed in lateral view. The lateral surface of the coronoid process is a smooth, medially inflected process that rises as a trapezoidal or triangular structure, the largest such in all Theropoda, and is continuous with a portion of the lateral surface of the mandible, but mAMEP would converge on the tip dorsally with mPst, and caudally would meet mAMEM, and bounded laterally by mAMES.
The supratemporal bar formed by the postorbital and squamosal bones forms a somewhat oblate triangle in cross-section, with a broad lateral face smoothly flowing into the dorsal face, and with a somewhat concave medioventral face. It is likely that this face would present the origin for m. adductor mandibulae externus superficialis (mAMES), although in dinosaurs this region tends to be smooth, and it isn’t quite clear whether this surface, or the ventral edge, would support the tendon that attaches mAMES to the bone. The attachment for mAMES may have extended around the rostral periphery of the infratemporal fenestra, but always on the inner or medial margins of the bones so bounding the fossa, save perhaps the squamosal, where the medial process “cups” the quadrate head and is broadly visible in lateral view. This region presents a very shallow fossa, and the ridge that separates this from the lateral face of the squamosal may indeed be the margin that attaches mAMES, but it would likely be limited to to the supratemporal fossa for the most part. Insertion of the muscle on the lateral, laterally-facing surface of the broad surangular is highly likely. MAMES also supports m. levator anguli oris, but this muscle, associated with the rictus, leaves no osteological correlates. This muscle also forms the lateral wall to the adductor chamber, and medially defines portions of the paranasal sinus system which extend caudally in theropods, and would not have the “depressed” feature seen in so much paleo-art. Below is a version of the “bulging” infratemporal caused by this muscle, which still defines the fenestra; typically, this won’t occur.
So, these are the many muscles of the jaw that define oviraptorids. Or they will once described more thoroughly than this. Oviraptorids exhibit a remarkably engineered jaw, and this is peculiar for many reasons:
- Oviraptorid jaws are massively muscled, but
- The mandible is very thin and the conjoined symphysis short and shallow (and unfused), despite
- Being very akinetic, with cranial bones also fused or firmly locked together, though
- The skull is also extremely pneumatic, so much so that only modern birds and pterosaurs are likely this pneumatic.
One might suspect the jaws are very advanced for a typical carnivore, and that is true. I’ve argued that this indicates a form of durophagy, and this need not even arise from carnivory, or egg-eating, which it might be “over-engineered” for, but even processing certain types of vegetation would require such muscles. One interesting idea is that the rostral placement of the coronoid displaces several muscles anteriorly to form a “pseudocheek,” and I hesitate to suggest this because it is my concern to examine for mechanical advantages prior to assuming some secondary masticatory feature such as increasing intraoral processing and the “need” to retain foods. Mammalian cheeks are useful for retaining food while translational jaw movements push food side to side, but the jaw morphology of oviraptorids suggests translation is limited, if not impossible, and especially while the jaw was closed, a presumption based on the preservation of oviraptorid mandibles with the coronoid process firmly and deeply placed within the very elongated adductor chamber, seemingly for this purpose.
Like turtles and birds, dicynodonts and some groups of mammal and sea-going archosauromorphans, oviraptorids have very anteriorized adductor muscles and very shorted rostra. What this means, and whether this influences recent suggestions (Zanno, 2010; Zanno & Makovicky, 2010) that they were herbivorous, remains to be seen.
Balanoff, A. M. & Norell, M. A. 2012. Osteology of Khaan mckennai (Oviraptorosauria: Theropoda). Bulletin of the American Museum of Natural History 372:1-77.
Clark, J.M., Norell, M. A. & Barsbold R. 2001. Two new oviraptorids (Theropoda: Oviraptorosauria), Upper Cretaceous Djadokhta Formation, Ukhaa Tolgod, Mongolia. Journal of Vertebrate Paleontology 21:209-213.
Clark, J.M., Norell, M. A. & Rowe, T. 2002. Cranial anatomy of Citipati osmolskae (Theropoda, Oviraptorosauria), and a reinterpretation of the holotype of Oviraptor philoceratops. American Museum Novitates 3364:1-24.
Lü J.-c., Tomida Y., Azuma Y., Dong Z.-m. & Lee Y.-N. 2004. New oviraptorid dinosaur (Dinosauria: Oviraptorosauria) from the Nemegt Formation of southwestern Mongolia. Bulletin of the National Science Museum (Tokyo), Series C, Geology and Paleontology 30: 95-130.
Zanno, L. E. 2010. A taxonomic and phylogenetic review of Therizinosauria. Journal of Systematic Palaeontology 8(4):503-543.
Zanno, L. E. & Makovickey, P. J. 2010. Herbivorous ecomorphology and specialization patterns in theropod dinosaur evolution. Proceedings of the National Academy of Sciences of the United States of America, Philadelphia 108(1):232-237.