Psittacosaurus lujiatunensis skull

Psittacosaurus lujiatunensis skull and adductor muscles

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TOP LEFT: Skull of P. lujiatunensis with jaws open and closed.

BOTTOM LEFT: Skull of P. lujiatunensis, 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.

Two inset versions of the “open” model demonstrate plausible, but less likely insertions for mAMES: above, a lateral circular depression on the mandible, and below, a large ventral flange can both be considered adjacent to muscle insertion points. However, neither is consistent with Nabavizadeh’s research indicating relationship of the lateral dentary ridge supporting an extension of mAMES from the coronoid (a small ridge is present), nor does it support the extension of the suptemporal foramen anteriorly as a subjugal foramen. This aperture supporting an extension of muscle would be present, but not utilized, in these two models. They are thus less likely.

RIGHT: Head of P. lujiatunensis 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 short rostrum and overall the short skull reduce additional tissues of the oral margin. Here, mAMES implies an anteriorly extended adductor musculature that would only marginally affect the extension of the rictal tissues anteriorly from one without an extended mAMES. This is made less apparent due to the shortness of the skull overall, and thus the mAMES distal insertion increases the adductor volume *relative to the skull* only by the skull’s extreme brevity. Oral tissues of the facial region imply elaborate and possibly glandular structures of the lateral face, of additional features of the circumnarial region, and a large depressed area of the lateral premaxilla implies that the small depression immediately around the bony nostril may not have confined the soft-tissues of the external fleshy nostril. Likewise, the upper beak would have likely extended partway onto the ventral premaxilla to properly occlude with the lower beak, and so this is reconstructed.

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

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