5 



The Head Skeleton of Fishes 



The observed similarities in the general pattern of bones 

 in the heads of osteolepiform and actinopterygian fishes was 

 a part of the evidence on which the Class Osteichthyes, or 

 bony fishes, was founded. The validity of this class and the 

 possible origin of the choanates, and therefore the tetrapods, 

 from an actinopterygian-like fish should now be investigated. 



GNATHOSTOME FISHES 



Most fish have bony jaws (i.e. with premaxilla, maxilla, 

 and dentary) like those of the choanates and tetrapods. 

 These fish are placed in the gnathostome division. Jawless 

 fish are called agnaths and are discussed later in this 

 chapter. 



Actinopterygian or ray-finned fishes 



Most of the living actinopterygians are identified as 

 teleosts, or true bony fishes, but there are a few species of 

 chondrostean and holostean fishes that, beneath their spe- 

 cializations, are presumed to retain some of the primitive 

 features of the group. Primitive features are identified in this 

 case as those observed in the early fossil representatives of 

 this group, the palaeoniscoids. 



Only a few kinds of actinopterygians will be described 

 for the purpose of indicating something of their range of 

 variation and thus the potential of the group as ancestors 

 to the higher forms. 



The head skeleton of the actinopterygian is made up of 

 the components listed in Table 5-1. This list is nearly a 

 duplicate of that for the choanate (Table 4-4, p. 92; the 

 differences are critical to the discussion of the nature of the 

 interrelationship. It should be pointed out here that in most 

 descriptions of the fish skull the parietals are called "fron- 

 tals." This use of frontal is defended on the grounds that 

 it is established and that the bones of the fish cranial roof 

 have not been demonstrated convincingly as homologs of the 

 tetrapod bones. It is a premise of this account that there is 

 virtue in retaining the terminology pattern established in 



the previous descriptions. This view is based on the conten- 

 tion that the student's task is made easier with such a pat- 

 tern, and that, since there is already a rich synonymy, 

 which must be mastered for full understanding of the litera- 

 ture, there is no addition to the ultimate load. It may also 

 be the decision of the reader that this usage is justified, in 

 the final analysis, on grounds of some sort of homologv'. 



As a starting point for an analysis of the actinopterygian, 

 the structure of the teleost head skeleton will be examined. 



7"e/eosf To give some flexibility to the description, three 

 species will be considered: the salmon (Figures 5-1, 5-2), the 

 cod (Figure 5-3), and the perch. Of these the salmon is best 

 known both in terms of its adult anatomy and its develop- 

 ment. 



The roof differs in several ways from what has already 

 been observed. The extrascapular series is made up of 

 tubular (sensory canal) bones varying in number from two 

 pairs in the cod to five pairs in the salmon. Since these are 

 closely bound to the skin, they are easily lost in preparing 

 skeletons. A single plate lies in the region of the supra- 

 temporal and tabular of either side; this is called the supra- 

 temporotabular. The intertemporal lies outside the roof 

 and is more easily described as a part of the circumorbital 

 series. There is no parietal foramen. 



Anterior to the parietals are paired nasals, and midline 

 internasal and mesethmoid. The latter is a chondral bone 

 and is lacking in the salmon. The nasals may be well devel- 

 oped or vestigial in the form of canal tubes. The circumor- 

 bital series consists of the intertemporal, two postorbitals, 

 a jugal, an "infraorbital," and a lacrimal. Identification of 

 these units is completely arbitrary since the total number 

 of bones in this series varies. Elops (Figure 5-4) has three 

 lateral rostral units, continuing forward from the lacrimal, 

 and a median rostral, apparently fused with the internasal. 

 E/ops also has two supraorbitals, the salmon one. The ven- 

 tral aspect of the cranium is covered by a midline toothed 

 vomer in front and the parasphenoid behind. The vomer 

 is partly embedded in the cartilage of the rostrum. The 

 parasphenoid has a slim rostral part and spreads over the 

 basis cranii posteriorly. 



105 



