Reviewing the morphology of the jaw-closing musculature in squirrels, rats, and guinea pigs with contrast-enhanced microCT.

Rodents are defined by their unique masticatory apparatus and are frequently separated into three nonmonophyletic groups--sciuromorphs, hystricomorphs, and myomorphs--based on the morphology of their masticatory muscles. Despite several comprehensive dissections in previous work, inconsistencies persist as to the exact morphology of the rodent jaw-closing musculature, particularly, the masseter. Here, we review the literature and document for the first time the muscle architecture noninvasively and in 3D by using iodine-enhanced microCT. Observations and measurements were recorded with reference to images of three individuals, each belonging to one of the three muscle morphotypes (squirrel, guinea pig, and rat). Results revealed an enlarged superficial masseter muscle in the guinea pig compared with the rat and squirrel, but a reduced deep masseter (possibly indicating reduced efficiency at the incisors). The deep masseter had expanded forward to take an origin on the rostrum and was also separated into anterior and posterior parts in the rat and squirrel. The zygomaticomandibularis muscle was split into anterior and posterior parts in all the three specimens by the masseteric nerve, and in the rat and guinea pig had an additional rostral expansion through the infraorbital foramen. The temporalis muscle was found to be considerably larger in the rat, and its separation into anterior and posterior parts was only evident in the rat and squirrel. The pterygoid muscles were broadly similar in all three specimens, although the internal pterygoid was somewhat enlarged in the guinea pig implying greater lateral movement of the mandible during chewing in this species.