Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
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mechanism balanced an anterior muscular insertion (improved in-force) with improved gape. For<br />
a given mass of muscle, a more distal placement and increased velocity associated with enlarged<br />
gape are potentially in conflict (inferognathal velocity is dependent in part on the force available<br />
for mandibular acceleration which reflects muscle fiber organization and rotational inertial effects).<br />
It is not clear what constitutes MILES' (1969) concept of elongation for the lower jaw.<br />
The ossified "blade" does not extend from the articular to the occlusal surface in all arthrodires.<br />
Elongation may be used to describe the lengthening of the "blade" to reach the articular or to<br />
describe the relative increase in length for the entire lower jaw (articular to s.ymphysis). It appears<br />
that both forms of elongation have occurred among arthrodires. A complete "blade", present<br />
from the articular to the occlusal region, appears to be a synapomorphy of eubrachythoracid<br />
arthrodires and Homostius (CARR, 1991). A visual inspection of the relative lengths between the<br />
lower jaw (or the length between the quadrate on the postsuborbital plate and the position of<br />
the posterior superognathal on the suborbital) and a longitudinal measure for the head shield<br />
(e.g. orbit to glenoid condyle) suggests that pachyosteomorph arthrodires have developed an<br />
elongated lower jaw. Finally, MILES' summary does not explain the presence of forms with a<br />
reduced blade restricting muscular insertion posteriorly (e.g. Hadrosteus, Fig. 12F).<br />
In a rotational gnathal system (Eq. 1; ALEXANDER, 1968), the out-moment is equal to the<br />
in-moment.<br />
Fo Lo = Fi L; (Eq. I)<br />
Fo = ( F; L; ) / Lo (Eq. 2)<br />
More effective out-force application (Eq. 2) can be achieved in a number of ways.<br />
(1) The out-force moment arm (Lo) can be reduced either by applying forces only to posterior<br />
aspects of the occlusal surface or by shortening the inferognathal (as seen in Mylostoma,<br />
DENISON, 1978, Fig. 79, or in Oxyosteus, STENSIO, 1963, PI. 55, Figs. 4, 5).<br />
(2) Muscle force (F;) can be increased by increasing the mass of the muscle (GANS & DE<br />
VREE, 1987; GANS & GAUNT, 1991). Addition of muscle mass is constrained by restrictions on<br />
muscle packing (i.e. available space and fiber orientation) and metabolic costs.<br />
(3) Finally, out-forces can be affected through modification of inferognathal shape wi th<br />
development of a coronoid process (as noted by MILES, 1969, and among placoderms seen only<br />
in Brachyosteus dietrichi, Fig. 12D). This either accommodates phylogenetic shifts in the angle<br />
between lines of muscle action and lever action or simply provides increased area for the insertion<br />
surface. MILES' (1969) proposal - a simple anterior shift of muscle insertion (increased L i) <br />
fails to recognize that an increase in moment arm is paid for by a reduction in muscle force<br />
(GANS, 1988; GANS & GAUNT, 1991). Placement of the adductor muscle closer to the joint does<br />
minimize the rotational inertia (GANS, 1988).<br />
An alternative to MILES' hypothesis (elongation of inferognathals to increase in-force moment<br />
arm and gape) includes (1) inferognathal elongation to increase gape, (2) elongation and<br />
reduction of inertial effects to increase closure velocity (note that power increases with increased<br />
muscle force, but not with changes of in-force moment arm), and (3) ossification of the inferognathal<br />
plate, from the articular to the symphysis, to stabilize and strengthen the lower jaw. Since<br />
power does not increase and out-forces decrease with the increase of jaw length (out-force moment<br />
arm), other mechanisms for enhancement of the bite might be predicted based on feeding<br />
strategy. Development of an ossified blade provides a strengthened and stable insertion for a