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

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-102 architectural and physiological aspects of muscle. Recent advances in the understanding of feeding mechanics necessitate a more thorough consideration of the components involved and the potential trade-offs associated with evolutionary modification. Feeding MILES (1969) and SCHAEFFER (1975) discussed major trends in feeding mechanisms within placoderms and gnathostomes respectively. Placoderms possess an autostylic jaw suspensorium (Fig. 11; MILES, 1969) with the hyomandibula (Hm) supporting the submarginal plate (GOUJET, . 1984a, b; contrast YOUNG, 1980, 1986). The palatoquadrate is fused to the dermal cheek (suborbital, SO, and postsuborbital plates, Fig. 11) providing support for the jaw articulation. An adductor mandibulae muscle originates from the palatoquadrate and the medial surface of the suborbital plate (GOUJET, 1984b) and inserts along the lateral surface of the lower jaw (IG, Fig. 11), attaching either to the non-masticatory portion of the inferognathal plate, when present," and/or to Meckel's cartilage (Mk, Fig. 11). The dermal inferognathal plate (Figs. 11, 12) consists of an anterior occlusal region and posterior non-masticatory or "blade" portion. The non-masticatory ossified portion varies in arthrodires (CARR, 1991) from a short ventrally grooved structure capping Meckel's cartilage (Fig. 12A) to a single enlarged lamina medial to Meckel's cartilage (Fig. 12B-G). MILES (1969: 149) considered main brachythoracid trends to have been associated with increasing the gape and "effective use" of gnathal elements (increased inferognathal length, large nuchal gap, functional articulation of the head). What constitutes "effective use" depends upon the role required of the gnathal elements (e.g. durophages increase crushing forces while some piscivores increase closing velocity to let the strike facilitate prey capture). MILES (1969) characterized the inferognathal as a third class lever, arguing that evolution of the brachythoracid feeding FIG. It. - Lateral view of Coccosleus sp., showing autostylic jaw suspension of arthrodires, from GARDINER, 1984. Structures deep to dermal bones are drawn with a dashed outline. Cartilaginous structures are stippled. Vue tal/iraLe de Coccosteus sp., mOn/ranl La sllpension alilostylique des miichoires, d'apres GARDINER, 1984. Les slruclures perichondraLes profondes sonI indiquees par une lrame. Les Slruclures cartiLagineuses sonl en poin/ille.
-103 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
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Page 15 and 16: -97 PRDIGED SIG EMS ElF GIV FRS F
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Page 24: -106 (3) and decreased agility (r
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Page 32 and 33: -114 CARR, R.K., 1991. - Reanalys
Page 34: -116 NORTHCUTT, R.G., & C. GANS,
Page 38 and 39: Lehmanosleus hyperborells Mediaspis
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Page 42 and 43: Asterolepis mnxima FRS Asterolepis

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

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