Mammals with a long diastema typically also have dominant masseter and pterygoid muscles

Author: GREAVES, WALTER STALKER

Source: Zoological Journal of the Linnean Society, Volume 153, Number 3, July 2008 , pp. 625-629(5)

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Abstract:

A few orders of mammals contain many individuals with dominant masseter and pterygoid muscles that pull up and forward as they close the jaw. A dominant temporalis muscle that pulls the jaw up and to the rear is the more common condition in mammals. A long toothless region (diastema) is present in almost all mammals with a large masseter/pterygoid complex. The presence of a diastema, when few teeth have been lost and their size has not changed significantly over evolutionary time, implies that the jaws have lengthened, as in horses and selenodont artiodactyls. (A long jaw with a shorter diastema will also form if very long incisors develop as in rodents.) The sum of the forces of all the jaw muscles (represented by an arrow) typically divides the jaw into a posterior, toothless region and an anterior region where the teeth are located. In most mammals, the sum of all the bite forces at the teeth is maximized when the lengths of the projections of these two regions, onto a line perpendicular to the arrow, are in the ratio of 3 : 7. If the tooth-bearing region of the jaws becomes longer over evolutionary time this ratio will obviously be disturbed. A change in the location of some basic bony features of the jaw mechanism could maintain this ratio, but this requires major disruption of the skull and jaws. Alternatively, simply changing the masses of the muscles that close the jaw (smaller temporalis, larger masseter and/or pterygoid, or some combination), so that the lower jaw is pulled up and forward, rather than backward, also maintains the ratio. According to this view, if the jaw lengthens over evolutionary time, the relative sizes of the jaw muscles will change so that the masseter/pterygoid complex will become dominant. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153, 625-629.

Keywords: jaw mechanics; muscle force vector

Document Type: Research article

DOI: http://dx.doi.org/10.1111/j.1096-3642.2007.00278.x

Publication date: 2008-07-01

Mammals with a long diastema typically also have dominant masseter and pterygoid muscles

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