Ischyromys douglassi: Morphometric analysis of an anatomically protrogomorphous Rodent, and its implications for the evolution of the group.
[sciencythoughts.blogspot.com]
Several adaptations enable gnawing in Rodents. They possess a single pair of ever-growing incisors in the upper and lower jaws, a reduced number of cheek teeth, a long diastema separating the incisors from the cheek teeth, and differentiation of the masseter muscle into three separate components to control jaw movement: the superficial masseter, deep masseter, and zygomaticomandibularis. The variation seen in the arrangement of the masseter tissues varies across Rodents and falls into four morphologies: sciuromorphy, hystricomorphy, myomorphy, and protrogomorphy. These differences were first discussed by George Robert Waterhouse in 1839, and then formally described by Johann Friedrich Brandt in 1855. Protrogomorphy describes a condition where the origin of the masseter is limited to the zygomatic arch, a condition typically observed in non-Rodents. Anatomical protrogomorphy is evident in many fossil Rodents, but among living Rodents only occurs in Mountain Beavers, Aplodontia, and some and some Mole Rats, Mathyergidae. The other three arrangements are characterised by divisions of the masseter extending anteriorly to varying degrees. In sciuromorphy the origin of the deep masseter extends anteriorly from the zygomatic arch, past the infraorbital foramen, onto the rostrum. The rostrum features a correspondingly broadened and angled margin of the zygoma and lateral rostrum, called the zygomatic plate, which in some taxa (e.g. Geomyids, Sciurids) approaches the dorsal and anterior margins of the skull. In hystricomorphy the deep masseter remains restricted to the ventrum of the zygomatic arch and the zygomaticomandibularis extends onto the rostrum, passing through an enlarged infraorbital foramen along its course. Myomorphy is a combination of both sciuromorphy and hystricomorphy; both the deep masseter and zygomaticomandibularis spread anteriorly, the latter through the infraorbital foramen and the former onto the zygomatic plate.