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A Kinematic Model for the Upper-Jaw Protrusion in Some Labrinae (Pisces, Perciformes)

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image of Netherlands Journal of Zoology
For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

This paper, which is the last in a series of four articles (ef. VAN HASSELT, 1978, 1979a, b) on the morphology and the kinematics of the jaw apparatus with respect to feeding in the labrid fishes Labrus berggylta Ascanius, Crenilabrus melops (L.), and Ctenolabrus rupestris (L.), deals with the completion of a theoretical kinematic qualitative model incorporating explosively rapid and wide opening of the mouth and designed by the application of a deductive methodology. In previous papers three structures were shown to be essential: transverse axes of jaw rotation, a point-type coupling between the upper and lower jaws, and a medial symphyseal line of articulation. Here, the consequences of the optimalization of the position and movement of the articulating symphyseal line in this model are discussed. On the basis of the lower-jaw structures functioning in the closing of the protruded mouth, deductive reasoning indicates a specific protrusive translation of the premaxilla and symphyseal line relative to the neurocranium. As described for the basic model, the maxilla remains functional in upper-jaw rotation and jaw coupling. The structures necessary to effectuate and limit this protrusion are derived, and the articulation profiles between the bony elements in the upper-jaw region are explained by application of the kinematics of rolling spheres and planes. Preference was given, for a number of reasons, to point-type contacts that change position over the protruding bony elements. This permits participation of successive parts of the elements in movement transmission, support, and forces. Thus, two bony elements and the area of their contact move relative to each other. In this situation, the establishment of a stationary centre of rotation between these three elements is shown to be impossible. With each element considered a specific part of a moving body with undefined extension, successive positions of the centre form a (continuous) set in each body. The three sets roll over each other, their common point of tangency being the instantaneous centre of rotation. Curved sets whose roll is derived by the smallest possible number of specifications and conditions are provided by spheres and planes, i.e., circles and straight lines for coplanar motion in one plane. Each bony element, and the point of contact as well, now are a specific part of the plane of the circle or line. When rolling occurs, the successive positions of a point-type contact form the profile of the articular surface on each bony element. Relative to a given amount of rotation between the bony elements, short profiles are considered particularly suitable, and should glide along each other smoothly without sudden changes in the directions of the glide or the required forces. To realize these conditions, a suitable configuration of rolling circles and lines must be chosen and a suitable position for the point of contact in the plane of the corresponding rolling circle or straight line. To describe rotational movements between two bony elements, the most appropriate configuration proved to be the one describing what is called cardioidal-elliptical motion, in which two circles, one with a radius twice as long as that of the other, roll over each other, the smaller circle within the larger. This combination gives typically harmonic oscillatory movements. This basic configuration was used for the composition of the configuration of rolling upper-jaw elements. In this complete linkage, a number of ligaments and the insertion of the long tendon of the A1 muscle proved to be functional in limiting, guiding, or conducting the rolls. Flexible cartilaginous discs at articulating contacts and the flexible rostral cartilage of the premaxilla gave good absorption of small disturbing movements at the contact. In addition, the three-dimensional shape of the articulation surfaces was designed particularly for the maxillo-vomerine joint. Some quantitative aspects of the mechanism underlying the linkage of rolling elements and the influences of the structures on each other are discussed. The model seems to have wide applicability to explain the various jaw structures in fishes.

Affiliations: 1: (Department of Morphology, Zoology Laboratory, University of Leiden, The Netherlands


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