Carr, R. K., 1995a. - Biological Sciences

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mechanism balanced an anterior muscular insertion (improved in-force) with improved gape. For
a given mass of muscle, a more distal placement and increased velocity associated with enlarged
gape are potentially in conflict (inferognathal velocity is dependent in part on the force available
for mandibular acceleration which reflects muscle fiber organization and rotational inertial effects).
It is not clear what constitutes MILES' (1969) concept of elongation for the lower jaw.
The ossified "blade" does not extend from the articular to the occlusal surface in all arthrodires.
Elongation may be used to describe the lengthening of the "blade" to reach the articular or to
describe the relative increase in length for the entire lower jaw (articular to s.ymphysis). It appears
that both forms of elongation have occurred among arthrodires. A complete "blade", present
from the articular to the occlusal region, appears to be a synapomorphy of eubrachythoracid
arthrodires and Homostius (CARR, 1991). A visual inspection of the relative lengths between the
lower jaw (or the length between the quadrate on the postsuborbital plate and the position of
the posterior superognathal on the suborbital) and a longitudinal measure for the head shield
(e.g. orbit to glenoid condyle) suggests that pachyosteomorph arthrodires have developed an
elongated lower jaw. Finally, MILES' summary does not explain the presence of forms with a
reduced blade restricting muscular insertion posteriorly (e.g. Hadrosteus, Fig. 12F).
In a rotational gnathal system (Eq. 1; ALEXANDER, 1968), the out-moment is equal to the
in-moment.
Fo Lo = Fi L; (Eq. I)
Fo = ( F; L; ) / Lo (Eq. 2)
More effective out-force application (Eq. 2) can be achieved in a number of ways.
(1) The out-force moment arm (Lo) can be reduced either by applying forces only to posterior
aspects of the occlusal surface or by shortening the inferognathal (as seen in Mylostoma,
DENISON, 1978, Fig. 79, or in Oxyosteus, STENSIO, 1963, PI. 55, Figs. 4, 5).
(2) Muscle force (F;) can be increased by increasing the mass of the muscle (GANS & DE
VREE, 1987; GANS & GAUNT, 1991). Addition of muscle mass is constrained by restrictions on
muscle packing (i.e. available space and fiber orientation) and metabolic costs.
(3) Finally, out-forces can be affected through modification of inferognathal shape wi th
development of a coronoid process (as noted by MILES, 1969, and among placoderms seen only
in Brachyosteus dietrichi, Fig. 12D). This either accommodates phylogenetic shifts in the angle
between lines of muscle action and lever action or simply provides increased area for the insertion
surface. MILES' (1969) proposal - a simple anterior shift of muscle insertion (increased L i) ­
fails to recognize that an increase in moment arm is paid for by a reduction in muscle force
(GANS, 1988; GANS & GAUNT, 1991). Placement of the adductor muscle closer to the joint does
minimize the rotational inertia (GANS, 1988).
An alternative to MILES' hypothesis (elongation of inferognathals to increase in-force moment
arm and gape) includes (1) inferognathal elongation to increase gape, (2) elongation and
reduction of inertial effects to increase closure velocity (note that power increases with increased
muscle force, but not with changes of in-force moment arm), and (3) ossification of the inferognathal
plate, from the articular to the symphysis, to stabilize and strengthen the lower jaw. Since
power does not increase and out-forces decrease with the increase of jaw length (out-force moment
arm), other mechanisms for enhancement of the bite might be predicted based on feeding
strategy. Development of an ossified blade provides a strengthened and stable insertion for a