Bringing the incisors together or using them to chisel away at a surface requires muscles that forcefully brings the lower jaw forward. In rodents, this is done primarily by the masseter muscle. Primitively, the masseter was divided into three portions: the masseter superficialis ( ), the masseter lateralis ( ), and masseter medialis or profundus ( ). In primitive forms, the superficial masseter arose near the anterior end of the toothrow (a bump that serves as the point of origin in many rodents is termed the masseteric tubercle) and inserted along the lower part of the mandible at the rear. The lateral masseter arose from the midportion of the zygomatic arch and also inserted along the bottom of the lower jaw towards its rear. The medial masseter was small, originating along the inside of the middle of the zygomatic arch and inserting on the mandible near the end of the molar toothrow. This condition is called and in living rodents, is seen only in aplodontias.
By moving the point of origin of parts of the masseteric musculature anteriorly, rodents gain both mechanical advantage and additional range of movement of the lower jaw. This change in the origin has happened in at least three different ways.
The morphology of the area of insertion of the masseter on the lower jaw also differs among groups of rodents. In the angular process, which receives much of the the masseter, arises more or less in a line with the rest of the jaw -- that is, it originates in the same vertical plane that also includes the alveolus (socket) of the incisors. In a jaw, the origin of the angular process is distinctly lateral to this plane, and the angular process often appears to be flared laterally. The coronoid process is usually reduced in hystricognathous forms.jaw, the
Rodent phylogeny and classification has been the focus of many disagreements, some of which continue. Our current best guess about the phylogeny of rodents, however, suggests that the difference in pattern of insertion of the masseter (sciurognathy vs. hystricognathy) reflects a deep phylogenetic division. Here, we follow Carleton (1984) in recognizing two suborders, the Sciurognathi and Hystricognathi. The myomorph and sciuromorph taxa are included in the Sciurognathi, while most hystricomorphs are in the Hystricognathi.
Phil Myers (author).