Introduction
To understand the general features of the functional morphology of the jaw apparatus of rodents one must understand one thing: rodents gnaw. Although rodents are the most speciose order of mammals and remarkably diverse, including aquatic, arboreal, arid, and subterranean forms, all rodents have retained a suite of features that are a mechanical complex for gnawing. To put this another way, there is at once tremendous diversification and extreme conservation of characters in the jaw apparatus of rodents.
On the one hand, there is wonderful diversification of form in the structures that compose the jaw apparatus. Almost all of the over 2200 species of extant rodents (Carleton and Musser, 2005) and the hundreds of extinct species known from fossils can be identified on the basis of the morphology of the cheek teeth alone. And, traditionally, the rodents have been grouped into three sub-orders distinguished largely on the basis of characteristics of the jaw adductor muscles and other features of the masticatory apparatus.
The three classic sub-orders [Sciuromorpha (squirrels), Myomorpha (rats and mice), and Hystricomorpha (porcupines and the South American caviomorph rodents) (see Wood, 1955, and Landry, 1999, for excellent reviews of the literature)] were defined by characteristics of the jaw apparatus, including differences in the position and architecture of the masseter and zygomaticomandibularis muscles. An additional, fourth, suborder of rodents, the Protrogomorpha, was defined by Wood (1937). Protrogomorph rodents are supposed to represent the primitive condition of rodent masticatory muscles in that they lack the sciuromorph and hystricomorph expansions of the masseter and zygomaticomandibularis, respectively. Although it has been many years since the Sciuromorpha, Hystricomorpha, and Myomorpha sensu stricto were considered to be monophyletic groups (Wood, 1965) and it is clear that each of these major grades of the rodent masticatory apparatus has evolved more than one time, the variety of forms of the jaw apparatus within the Rodentia is striking.
On the other hand, in spite of all of this morphological diversity, all rodents have a suite of morphological features that form a mechanical complex for gnawing.