Jaw muscles of new world squirrels

Abstract

The jaw, suprahyoid, and extrinsic tongue muscles are described for eight species of New World squirrels, spanning more than an order of magnitude in body mass. Anatomical differences are discussed in the light of body size, natural history, and phylogeny. The relative sizes of different muscles, their orientations, and the shapes and positions of their areas of attachment vary but show few trends in relation to body size. The anatomical differences are likewise not readily explained by the mechanical requirements of the animals' diets, which are similar. The most marked anatomical differences occur in Sciurillus (the pygmy tree squirrel), as well as those genera--Glaucomys (the flying squirrel) and Tamias (the chipmunk)--that are taxonomically most distinct from the tree squirrels. Sciurillus is noteworthy for its unusually small temporalis and an anterior deep masseter that is oriented to assist in retraction of the jaw. Tamias has a more vertically oriented temporalis and greater inclination in the anterior masseter muscles than the other squirrels, features that may be associated with its large diastema and relatively posteriorly situated cheek teeth, which in turn may relate to its having cheek pouches. Our results form a valuable database of information to be used in further studies of functional morphology and phylogeny.

Functional Anatomy of Incisal Biting in <i>Aplodontia rufa</i> and Sciuromorph Rodents – Part 1: Masticatory Muscles, Skull Shape and Digging

“…masseter posterior' [Woods and Howland, 1979]. In recent studies of A. rufa and sciuromorphs [Ball and Roth, 1995;Thorington and Darrow, 1996], there are no indications that the authors made a distinction between posterior fibers of the lateral masseter, ZM, and this third muscle, the posterior masseter (PM). Even though the orientation of the PM is very different in hystricomorphs compared to A. rufa and sciuromorphs, the positions of the origin and insertion are very similar.…”

“…The posterior fibers are a horizontally oriented band that originates on the ventral surface of the posterior root of the zygomatic arch and runs anteriorly to join the fibers of the zygomatic portion of the temporalis. The posterior portion of the ZM (PZM) is probably homologous to the PZM or m. masseter medialis, pars posterior of other authors [cf., Tullberg, 1899;Von Toldt, 1905;Stark and Wehrli, 1935;Ball and Roth, 1995;Thorington and Darrow, 1996], but these authors may also have included all or part of what is called the PM in the present study.…”

“…Since the ZM is usually called the 'deep masseter' in humans and other primates, the use of the term 'deep masseter' [Ball and Roth, 1995;Thorington and Darrow, 1996] for fibers that originate on the lateral surface and ventral border of the zygomatic arch is confusing, so this muscle will be called the 'lateral masseter' following Woods and Howland [1979;Tullberg, 1899].…”

“…To simplify comparisons between rodents and other mammals, 'ZM' will be used in the present study. Comparative anatomists have used this term for many years [e.g., Parsons, 1894;Von Toldt, 1905;Stark and Wehrli, 1935;Becht, 1953;Turnbull, 1970;Offermans and de Vree, 1989;Ball and Roth, 1995;Thorington and Darrow, 1996].…”

Functional Anatomy of Incisal Biting in <i>Aplodontia rufa</i> and Sciuromorph Rodents – Part 2: Sciuromorphy Is Efficacious for Production of Force at the Incisors

“…Ball and Roth's [1995] and Velhagen and Roth's [1997] comparative studies of the masticatory apparatus in squirrels (and outgroups, including Aplodontia ) do not provide a mechanical analysis of incisal biting.…”