hyomandibula 



hyomondibula 



preopercle 



quadrate pterygoid 



metcpterygoid 



quadrotojugal 



interopercle 



ceratohyal posterior 

 ceratohyal anterior 



ceratohyal posteriori ^^^^^^^ jj^^^j 

 eratohyal anterior J 



coronoids 



preorticular 



supraangulor 



preorticular 



coronoids 



articular 



angular \etroarticula 



dentosplenlal 



supraangulor 



articular 



retroarticulor 



angular 



sphenotic 

 posterior orbitosphenoid \ arrow through lateral canal 



epiotic 



anterior orbitosphenoid 

 nasal capsule 



tooth fenestra 



posterior orbitosphenoid 

 anterior orbitosphenoid 

 lateral ethmoid 

 1 



basioccipitol 



premaxilla 

 fused to 

 mesethmoid 



porasphenoid 



prootic IX 

 bosipterygoid process 



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 compared with the relatively uniform pattern of 

 paired bones in the amphibian; therefore, in terms of the 

 dermal bones, only the roof and the sheath of the lower jaw 



118 -THE HEAD SKELETON OF FISHES 



