been separate. These ossifications were joined by a ventral 

 band of perichondral bone. In some acanthodians the 

 mandible ossified as a single unit and bore teeth. The 

 palatoquadrate and mandible had rays extending into the 

 opercular fold. 



The hyoid arch was modified. The epihyal was quite 

 broad (laterally compressed) at its dorsal end, which may 

 have articulated with the posterior end of the cranium, and 

 tapered to its ventral end. The ossification of hyoid, and 

 branchial arches, was subperichondral. The ventral end of 

 the epihyal cartilage articulated with the cartilaginous end 

 of the ceratohyal. The latter was long and slim and abutted 

 anteriorly against a long hypohyal, which in turn articulated 

 with a small median basihyal. There is no evidence of a 

 pharyngohyal. 



There were three to five branchial arches. The more an- 

 terior arches with pharyngo, epi, cerato, and hypo segments; 

 the more posterior arches lacked the pharyngo and perhaps 

 the hypo segments. The pharyngobranchials were long and 

 with distinct suprabranchial processes. 



Genera) observations The association of the acanthodians 

 with the sharks is based upon such features as their denti- 

 tion and the separate outer gill openings. The teeth are not 

 unlike those of other fishes, while the gill openings are not 

 truly comparable to those of the shark. The group is usually 

 described as having a complete mandibular gill slit — a con- 

 dition described by the term aphetohyoidea. In the shark 

 this gill slit is reduced to the spiracle. On the basis of the 

 course of the preopercular canal in Acanthodes, Stensio 

 (1947) assumed that these fishes had a spiracle and were not 



marginal 

 (supratemporotabular ) 



central 

 (postparietal) 

 postorbitol ^ 



(intertemporol) 

 pineal 



(parietal 

 preorbital ^ 



(nasal), 

 rostral 

 (internasal)^ 



(medial rostral 



(lateral rostral)' 

 (vomer 



inferognothal. 

 (dentary) 



aphetohyoideans. Hydrolagus has a scarcely altered hyoid 

 arch, yet lacks even a spiracle. Aphetohyoidean as used here- 

 after should be taken to imply an unmodified hyoid arch 

 not involved in mandibular support. 



Comparison of the shark palatoquadrate with that of the 

 acanthodian gives no better agreement. The ossified parts 

 of the palatoquadrate of this group are like those of the 

 actinopterygian. The mandible is not like the shark's and 

 there is the suggestion of dermal anchorage of the teeth, an 

 anchorage lacking in the shark. The bony development in 

 this group further supports separation from the sharks. 



Arthrodires 



Another group of fishes assumed to have some relation- 

 ship with the sharks is the arthrodires. These are truly 

 placoderms, so named because of the heavy dermal armor. 

 The arthrodires are subdivided into a number of subgroups, 

 some of which are shark-like. The euarthrodires (Dunkelos- 

 teus, Kujdanowiaspis) and the antiarchs (Bothriokpis) will be 

 described although they are not particularly shark-like. 



Euarthrodire In the euarthrodire (Figure 5-22), the cranial 

 roof is thick and the plates closely joined. The roof is sepa- 

 rated by a gap from the cheek and infraorbital bones. On 

 the rear, outer margin of the roof is a knob and a large socket 

 for the joint with the heavy pectoral girdle or trunk armor. 

 The pattern of the cranial roof has been compared with 

 that of the osteichthian fishes (Table 5-2), but it seems best 

 to use names which do not imply any homology with the 

 plates of the Osteichthyes. The cheek was covered by a large 



(extrascapular series 

 preorbital plate 



postorbitol plate. 



rostral plate 



pineal plate 

 pineal body 



preoperclei 



suborbital plate. 



preoperclej or 

 (postspiraculor) 

 marginal, 

 postsuborbital 

 preopercle. 

 articular with 

 dashed outline 



mental ossification 



paranuchal 



(prearticular 



B 



Figure 5-22. A, lateral view of head skeleton of Dunklosteus terrelli; B, dorsal view of D. 

 (B offer Lehman) 



126 



THE HEAD SKELETON OF FISHES 



