TOP LEFT: Skull of P. mirus with jaws open and closed. Large, flat dermal ossicles associated with the skull are shown floating and with a distributed arrangement in the jaws open posture, rather than confined to the upper or lower jaw.
BOTTOM LEFT: Skull of P. mirus, jaws open and closed, demonstrating positions and relationships of lateral jaw adductors (mAME complex). MAMEP is in yellow and inserts onto the posterior coronoid region; mAMEM is in orange and inserts onto the upper (terminal) coronoid region; and mAMES is in red and inserts onto the lateral main body of the coronoid region, but shown here extending onto the lateral dentary surface along the lateral dentary ridge. No additional muscles would likely contribute to extending muscular tissue onto the lateral surface of the skull any further anteriorly than mAMES; internally, both mPT arms (dorsalis and ventralis) would insert onto the ventral and posterior region of the mandible, near the jaw joint, and mAMP originates on the quadrate and inserts on the medial mandible; origins for mPST are in the region of the anterior braincase and insert onto the medial mandible in the coronoid region, but if they inserted further anteriorly would be obscured and medial to mAMES. Additional muscles of the jaw that would originate from the maxilla require stronger inference than currently presented; novel muscles may be involved, but are not supported currently. Large, flat dermal ossicles are omitted to clearly note muscles’ relationship to mandible.
RIGHT: Head of P. mirus with jaws open and closed, and the position of the mAMES highlighted for contrast. Soft tissue surrounding the head is shown in brown, without definition of its sub-types, and in the open jaw portion describes the possible extent of the rictus, here extending to nearly the margin of the cornified sheets comprising the rostral “beak.” The dorsal surface of the cranium and portions of the mandible are also covered in elaborated modified cranial bones and fused dermal ossicles (e.g., around the external bony nostril), and the outline is elaborated as a consequence. Large, flat bony ossicles associated with the skull in the “cheek” region infer the presence of a continuous tissue between the upper and lower jaw. However, barring direct evidence of a muscular tissue also bridging this region, these ossicles are merely inferring a skin presence, and not any other tissue type beneath them. Unlike the typical reptilian rictus, the skin would be highly cornified to support the embedded ossicles, and this influences the capability of the jaw to gape, yet how far is unclear.
Nabavizadeh, A. 2018 (in press) New reconstruction of cranial musculature in ceratopsian dinosaurs: Implications for jaw mechanics and “cheek” anatomy. The Foundation for American Science in Experimental Biology Journal https://www.fasebj.org/doi/abs/10.1096/fasebj.30.1_supplement.lb27