Oviraptorid Jaw Muscles Described, Part 1

Here follows a small, basic and minor depiction of the jaw muscles of an oviraptorid, largely specifically based on landmarks provided by the extant phylogenetic bracket (EPB) but also informed by examining for cranial landmarks in which these muscles typically occur and mapping them to likely muscle position.  Muscle arrangements also tend to be conserved due to intervening nerves that innervate structures such as the eye or associated regions, but must pass from the lateral wall of the braincase past these muscles, but tend to appear between particular muscles (it is thus assumed that where the nerve may pass, the muscle boundaries are limited).

Adductor (jaw-closing) muscles of an oviraptorid, based on "Big Beak" (PIN collection.), a "conchoraptorine" with an unusually large orbit. Muscles depicted follow common terminology. Also shown is the m. depressor mandibulae, which is here depicted as a single muscle but may be divided in some nonarchosaurs.

Adductor (jaw-closing) muscles of an oviraptorid, based on “Big Beak” (PIN collection.), a “conchoraptorine” with an unusually large orbit. Muscles depicted follow common terminology. Also shown is the m. depressor mandibulae, which is here depicted as a single muscle but may be divided in some nonarchosaurs.

There is a lot we won’t know about the cranial anatomy of oviraptorids, or indeed most dinosaurs and extinct animals, but what we do know is a substantial amount nonetheless. Certainly, knowing the likely origin and insertion points for various cranial muscles is a result that comes with some speculation, much of it fueled by various degrees of inference: If a muscle is always attached in a certain way to two different structures, then it is very likely that that muscle attaches that way to those structures when the structures are preserved in a fossil animal but the muscle is not; but if a muscle is only attached one way to those structures in some animals, but another way to a second group, and our animal seemingly related to both, there is equivocation. We can’t be certain, but we can make a reasonable prediction. Using the EPB in this manner, we may then make a model which can then be tested by direct means, by looking at surface anatomy of the bones in question to examine for relationship of the connective fibers (enthesial surfaces), as well as examine the surety of relationships of hard structures or foramina to the arrangement and distribution of bones, muscles, etc., and the variance within the framework of the bracket used. The EPB is not a crystal ball, but it is a great tool for the anatomist to use in inferring extinct animal appearances.

The models above depict the cranial muscles of a single oviraptorid, segregated into several “groups”: the mAME group is the major jaw-adductor group, which originate in the upper adductor chamber and surround the margins of the surpatemporal fenestra and insert either on the dorsal surface of the surangular, or slightly onto the lateral surface; the palatal group, which originate lower in the adductor region or on the palatal bones, and act primarily as assistant jaw-adductors or protractors (pulling the jaw forward or stabilizing against displacement), and insert either on the medial or ventromedial surfaces of the mandible, or in the case of the mPTV, the ventral and lateral surfaces, wrapping around (this isn’t always the case in animals); and the mPst group, which are smaller muscles that largely defined the anterior margins of the jaw muscles and act as jaw retractors (serving a similar, stabilizing role). It is not intended to be a general model for all oviraptorids, and indeed it is likely that some animals vary in some muscle relationships more than others.

Inferred position of the m. depressor mandibulae.

Inferred position of the m. depressor mandibulae.

In this animal, the paroccipital processes of the skull are elongated and pendant, composed from elongated processes of the squamosal and opisthotic bones: A common error in dinosaur anatomy depictions is to call the bone on either side of the foramen magnum and occipital condyle which articulates with the squamosal the “paroccipital,” which corresponds to two different, sometimes distinct bones, the medial exoccipital which forms portions of the condyle and foramen margin, and the opisthotic which forms virtually the entire “bone.” But the paroccipital process is a small section of the lateral end of the latter bone, the part that curves downward and connects with the squamosal, forming a process somewhat distinct from the rest of the skull. It has also been long depicted as the side of origin for the m. depressor mandibulae (mDM), but this is wrong: the mDM always originates much further up, on the squamosal, and may even cross onto the parietal or postparietal (when present), and on the caudal surface. In birds, the origin site for this muscle has shifted laterally, and is thus broadly exposed in that direction, but that is due to the lateral expansion of the braincase and the internal expansion of the cerebrum, forcing the endocranium to become more spherical, distorting the surrounding bones. This isn’t an issue in all non-avian dinosaurs, so it should never be depicted on the lateral surface of anything without direct reference. It is shown above passing medial to the paroccipital process, and inserting onto the distal end of the dorsomedial section of the retroarticular process of the mandible, comprised generally of the articular bone in this case (in this specimen, that portion of the illustrated side is not preserved, but in other oviraptorids is is, and depicts a broad, spatulate and shallow element: the mDM in oviraptorids has a broad insertion region).

Inferred positions of the m. psuedotemporalis superior (mPstS) and m. pseudotemporalis profundus.

Inferred positions of the m. psuedotemporalis superior (mPstS) and m. pseudotemporalis profundus.

The m. pseudotemporalis (mPst) is a series of muscles that extend from the braincase to the mandible, and help define the anterior edge of the muscle bodies that help close the jaw; they are also fairly slender, and small, jaw adductors. In most archosaurs, there are two main bellies of the mPst (the superficialis, or mPstS, and the profundus, or mPstP); additional muscles in this region, such as the m. levator pterygoideus, do not reach the mandible. MPstS originates on the laterosphenoid around or beneath the lateral process of the frontal which forms the articulation to the postorbital, and is generally easily recognizable by this site. A major branch of the mPst is the mPstP, and this muscle originates on the upper end of the epipterygoid, when present, or along its length, though Holliday (2009) and Holliday & Witmer (2007) note there is much ambiguity on locating the muscle’s presence definitively. The medial surface of the mandible at its dorsal margin produces a coronoid emminence or process (processus coronoideus), and is it roughly to this structure which the m. psuedotemporalis (mPst) inserts. The medial surface of this region in oviraptorids has been noted to possess a distinct coronoid bone (os coronoideum), but in many dinosaurs, this bone may be lost or merge into the middle, or second, coronoid: The bony coronoid in dinosaurs is the remnant of the third tetrapod coronoid, which run as a series from the mandibular symphysis to the muscle-bearing bones, and it has been suggested that the supradentary bone, a “unique” ossification found in tyrannosaurids and dromaeosaurids (Osborn, 1912; Brown & Schlaijker, 1940; Currie & Hurum, 2000) — among many other saurtischians — is part of the coronoid, and which may fuse to the “coronoid” bone, or where segementation occurs in taxa in which these bones may not be continuous. MPstS inserts ventral to mPstP on the medial jaw, and roughly correspond to the ventral and caudal extents of the coronoid region, which often exhibit a clear roughened region and enthesial surface for their attachment; mPstS may extend caudally beyond the coronoid proper but not so far as the medial adductor fossa, which in oviraptorids occupied virtually the entire medial face of the postdentary bone anterior to the articular structure.MPstP represents a part of the palatal-originating group of muscles, though is collected under the pseudotemporalis group given its function and insertion, and origin as a division of the m. pseudotemporalis. However, two other major palatal muscles, the main palatal group, receive a lot more attention as their origins, insertions, and size are far more noticeable and significant in estimating jaw function and bite strength. Indeed, while the adductor group are often modeled as a single element given their generally parallel lines of action, the palatal group differs remarkably enough to deserve modelling on its own.

Inferred position of the m. pterygoideus dorsalis (light) and ventralis (dark).

Inferred position of the m. pterygoideus dorsalis (light) and ventralis (dark).

M. pterygoideus (mPT) originates on the pterygoid (os pterygoideum), and in typical archosaurs this bone is largely a dorsoventrally flattened plate, with the dorsal surface originating the dorsalis branch (mPTD), and the ventral surface originating the ventralis branch (mPTV). mPTD may extend caudally towards the lateral surface of the quadrate and the pteryoid process of that bone, but is generally restricted to the pterygoid caudally, but extends onto the palatine rostrally; mPTV extends rostrally and may expand onto the lateral ramus of the bone, which supports the ectopterygoid is present. In the absence of the ectopteryoid, or its far rostral placement as in oviraptorids, which may (or does, in this case) result in a non-triradiate pterygoid (the rostral portion is divided, but the two rostral rami run parallel and do not broadly fork into a T-shaped structure), mPTV extends rostral towards the lateral ramus and its divergence towards the jugal/maxilla. In typical archosaurs, the pterygoid twists at its midsection and does a fair imitation of farfalle pasta in which one end becomes twisted around 90° along its long axis; but in oviraptorids, the broad horizontal plate is restrict to a small portion at the mid section, and the lateral, ectopterygoid process becomes a deep, narrow plate while the vomeral process remains a slender rod-like structure. Thus, the pterygoid is a narrow, and deep, bone, and inverts the typical shape. This causes the origin sites for the pterygoid muscles to twist around from the ventral surface towards the lateral; a longitudinal ridge extends along most of the lateral surface of the pterygoid, from the quadrate to the ectopterygoid, and a broad fossa extends dorsal to this along the ectopterygoid. It is my assumption that this ridge divides the origin sites for the m. pterygoideus branches, and that the muscles’ origins are thus both exposed in lateral view, unlike virtually all other archosaurs. It may also be that only mPTD attaches on the now lateral surface of the pterygoid/palatine, which may extend onto a fossa that defines the division towards the lateral process of the pterygoid, and that mPTV is extensive along the ventral half of the pterygoid. Both muscles insert on the ventral margin of the mandible, and are generally confined to the angular (mPTD), or the angular and surangular (mPTV). In the case of the former, mPTD inserts on the medial and ventral surface of the retroarticular process and somewhat ventral to the articular bone itself, and occupies a small facies on the medial surface of the angular and may even extend onto the prearticular. mPTV, however, typically wraps around the ventral surface of the mandible, and while it may attach partially to the ventral surface of the prearticular, is more notably present on its lateral surface, where a ridge may be present. In oviraptorids, this ridge is distinctive and extends not merely along the broad, flattened retroarticular process (which is diagnostic to Oviraptoridae) but ventral to the articular and the lateral contylar process of the surangular, and then onto the lateral surface of the surangular. In the lateral bone a distinct feature — the surangular ridge — is present in theropod dinosaurs and defines the dorsal extent of mPTV (as well as the lateral and ventral insertions for the adductor group muscles, but that’s a discussion best left for Part 2), although clear signs of its shape and size are limited to broader inferences. For example, as suggested by Holliday (2009), the ventral process of the jugal in some dinosaurs may house a spot for the insertion of a portion of mPTV, which would thus extend around the mandible, inserting onto it, and then joining with the jugal, and effectively insert lateral to its origination, “cradling” the mandible.

However, it seems the broad, and distinctive, mandible and the absence of such a feature on the jugal (so far) indicates limiting ourselves to the “mandible only” insertion. The lateral surface of the surangular in oviraptorids exhibits almost universally a large fossa lacking any definite margins, and this fossa can extend caudally beneath the lateral cotylus of the articular but also rostrally nearly to the edge of the external mandibular fenestra. Curiously, though, several oviraptorid specimens (including AMNH PR 6517, holotype of Oviraptor philoceratops) possess curious holes in the surface of the surangular, a feature that may indicate regions of resorption which would also be regions of limited stress, and thus unlikely to support muscle-based tissues (which place stresses on bones and ligaments).

Skull of AMNH FR6514, holotype of Oviraptor philoceratops Osborn (1924)

Skull of AMNH FR6514, holotype of Oviraptor philoceratops Osborn (1924).

Here, the foramen is a broad structure, is present on each mandible, and preserved as smoothly-margined and generally equally-sized fenestrae. If the foramen is not a preservational artefact, then it implies that at least in some taxa, but perhaps not all, mPTV would not extends across the entire lateral surface of the surangular towards the surangular ridge, and indeed in oviraptorids such a ridge is very slight or even absent, and delimited only it seems as a dorsal lateral thickening of the surangular forming a dorsolaterally-inclined “facet” onto which the mAME muscles would have inserted. MPTV itself would have been restricted to the ventral margin of the mandible. In the reconstruction above, the [slightly] smaller, non-Oviraptor clade of oviraptorids lack this feature as a common attribute of preservation, and indeed may present a more rigid surface with a far thicker and distinct “surangular ridge” for insertion of mPTV, but it remains to be seen whether the muscle attached so broadly to the lateral surface of the jaw.

This represents Part 1 of a post on the muscles. Next week I will present Part 2, which will focus on the adductor-group muscles.

Brown, B. & Schlaijker, E. M. 1940. A new element in the ceratopsian jaw with additional notes on the mandible. American Museum Novitates 1092:1-13.
Holliday, C. M. 2009. New insights into dinosaur jaw muscle anatomy. The Anatomical Record 292:1246-1265.
Holliday, C. M. & Witmer, L. M. 2007. Archosaur adductor chamber evolution: integration of musculoskeletal and topological criteria in jaw muscle homology. Journal of Morphology 268:457–484.
Hurum, J. H. & Currie, P. J. 2000. The crushing bite of tyrannosaurids. Journal of Vertebrate Paleontology 20:619-621.
Osborn, H. F. 1912. Crania of Tyrannosaurus and Allosaurus. Memoirs of the American Museum of Natural History 1:33-54.

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5 Responses to Oviraptorid Jaw Muscles Described, Part 1

  1. Pingback: Oviraptorid Jaw Muscles Described, Part 2 | The Bite Stuff

  2. Pingback: Oviraptorid Jaw Muscles Described, Part 3 | The Bite Stuff

  3. Pingback: Oviraptorid Jaw Muscles Described, Part 4 | The Bite Stuff

  4. Pingback: A Look Back at the Bite Stuff, 2013 Edition | The Bite Stuff

  5. Pingback: The Emaciated Tyrannosaur – a Reply to Ford, 1997 | The Bite Stuff

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