. Chordate morphology. Morphology (Animals); Chordata. vomer porethmoid maxillary articulati( itercolari notic epiotic. parospnenoid exoccipital basioccipitol Figure 5-15. Semidiagrammotic sketches comparing the head skeletons of Lepisosteus, to the left, and Am/a, to the right. A and B, medial views of suspensorium and operculum; C and D, medial views of posterior ends of mandibles; E and F, lateral views of endocronio. The general opinion is that the ancestral gnathostome had a dermal cover of many small individual scales or plates and that, through fusion of these, each of the several lines
. Chordate morphology. Morphology (Animals); Chordata. vomer porethmoid maxillary articulati( itercolari notic epiotic. parospnenoid exoccipital basioccipitol Figure 5-15. Semidiagrammotic sketches comparing the head skeletons of Lepisosteus, to the left, and Am/a, to the right. A and B, medial views of suspensorium and operculum; C and D, medial views of posterior ends of mandibles; E and F, lateral views of endocronio. The general opinion is that the ancestral gnathostome had a dermal cover of many small individual scales or plates and that, through fusion of these, each of the several lines of gnathostomes (amphibian, choanate, and actinopterygian) evolved its own head cover of plates. The process also in- volved reduction in the number of ossification centers, each plate tending to have only a single center. The differences in pattern between these lines can then be ascribed to the independent processes of plate formation, while the similari- ties must be due to parallel solutions to the problems of form- ing a head shield of plates, problems involving similar head shapes and makeup in the several groups, as well as similar functional needs. This parallelism suggests that the basic pattern observed in these groups is functionally controlled. The idea that the roof pattern is functionally controlled is shaken by the observed loss of this pattern in the dipnoans and in some of the actinopterygians. The alternative view, and the one most strongly suggested by the evidence, is in- heritance of a common pattern, or partial pattern, by the several lines. In comparing the three basic types—tetrapod with choa- nate fishes with actinopterygian, the question becomes one of whether there is more inheritance involved in one case than the other. Surely, as choanates, the crossopterygian should be closer to the amphibian but its cranial pattern does not indicate this. The snout of the crossopterygian is essentially an unbroken shield or a mosaic of small irregular plates as
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