sheath and constriction of the notochord in the basal plate, 

 notochord reaching dorsal margin of dorsum sellae and pro- 

 jecting from it (at least in younger stages), dorsum sellae 

 forming between ends of parachordals, epihyal not ex- 

 tending up lateral to head vein (no laterohyal involved), 

 presence of orbital margin of cartilages, presence of nasal 

 and labial cartilages, and separate posterior superficial 

 ophthalmic foramen. 



Acanthodians 



The acanthodians are represented by spines and plates 

 in Upper Silurian deposits, but it is not until Devonian 

 strata that much of their structure is preserved. 



The head (Figure 5-21) was covered by small scales with 

 larger plates forming a circumorbital ring. The several gill 

 slits opened separately behind folds; the most anterior fold 

 or operculum was supported by mandibular rays. The 

 endocranium had a large anterior orbit, little or no ethmoid 



region, and a large postorbital oticoccipital section. The 

 endocranium was largely cartilaginous with subperichondral 

 calcification in some species, and in others perichondral 

 ossification shading into subperichondral calcification, which 

 appears to be the more primitive condition. These ossifica- 

 tions included a pair of exoccipitals lying behind the vagus 

 foramen. There was a basioccipital enclosing the dorsal 

 aorta, and in front of this a basisphenoid. A parasphenoid 

 sheathed the ventral surface and extended forward below 

 the orbits. Its anterior end was associated with five small 

 plates. The postorbital process was ossified and the proc- 

 esses of either side were joined by a section of ossified roof. 

 There was a lateral commissure below the orbital process. 

 The palatoquadrate had quadrate, metapterygoid, and 

 autopalatine centers of ossification. It articulated with the 

 postorbital process and with a basal process of the para- 

 sphenoid. The two palatoquadrates were not joined ante- 

 riorly. The mandible consisted of an anterior and posterior 

 ossification joined synchondrally. The articular may have 



endolymphotic foramen 



sphenotic 

 hypophyseal fenestra 

 parasphenoid 



ossicles 

 pectoral girdle 



asioccipital 



occipitospinal 

 '^ nerve foramina 



articulation for 

 otic process of palatoquadrate 



metapterygoid 



occipitospinal nerve foramina 

 X 

 sphenotic 



exoccjpital 



epihyal 



Meckel's cartilage ossifications 



n mandibular splint 



basibranchial I 1 , , ^ratohyal posterior 



ceratohyal anterior 



Figure 5-21 . Head skeleton of Acanthodes sp. A, lateral view of head showing scales, circumorbital 

 plates and operculum; B, lateral view of jaw elements, branchial skeleton, and endocranium; 

 C, lateral view of endocranium and hyoid arch; D, ventral view of endocranium. {After Watson, 1 937} 



GNATHOSTOME FISHES 



125 



