TOP LEFT: Skull of E. regalis with jaws open and closed.
BOTTOM LEFT: Skull of E. regalis, 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.
RIGHT: Head of E. regalis 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. Without further inferences for peculiar jaw tissues, the lateral extraoral tissues might be “lip”-like in the manner of lizards and snakes, as there is no evidence there was further muscular or cornified dermal modification of the facial tissues. Thus, when the jaw is closed, the upper and lower tissues surounding the jaw would close. When the jaw is opened, there is likely passive tissue retaining a lizard-like oral margin, and the teeth were likely not fully visible.
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