spiracular plates 

 V 



suprobranchials 



.epibranchial I, II, III 



hyomandibula 

 symplectic 

 quadrate 

 ceratohyal posterior 



articular area of 



Meckel's cartilage 



prearticular 



anterior and posterior spleniol angular 



Figure 4-31 . Visceral skeleton, mandible, and endocranium of Eust/ienopteron. (After Jarvik, 1 954) 



which in turn articulated with the anterior aspect of an an- 

 terior copula. No basihyal is Icnown but there was a long 

 sublingual rod lying below the position of a basihyal. Below 

 the anterior copula was a midline, deeply keeled, urohyal. 

 The branchial arches probably numbered five. The first 

 arch had hypobranchial, ceratobranchial, epibranchial, 

 and fused pharyngo- and suprabranchial segments. The 

 pharyngobranchial articulated with the basis cranii medial 

 to the vestibular fontanelle and the suprabranchial with a 

 process medial to the tip of the pterotic wing. The second 

 arch was similar, except that the suprabranchial was smaller 

 and attached to a slight dorsal process on the anterior 

 end of the epibranchial. This suprabranchial attached to the 

 base of the otic section just above, and anterior to the aortic 

 groove. The third and fourth arches lacked suprabranchials 

 and articulated below with a posterior copula. Fifth cerato- 

 branchials were probably present. 



Porolepiform crossopferygian The porolepiform skull is 

 fairly well known only for Holoptychtus flemtiig (Figure 4-32). 

 Only fragments of Porolepis have been found, but these con- 

 stitute the earliest remains of crossopterygians. Within this 

 group there is variation in the openings of the nasal cap- 

 sule. Species of Porolepis had two outer openings and an in- 

 ternal choana. Holoplychius had only a single outer opening 

 in the position of the posterior opening oi Porolepis. In terms 

 of the snout both genera agreed in having the nasal capsules 



widely separated by bilateral internasal cavities, between 

 which was a median septum. These cavities opened ventrally 

 between the widely separated vomers. 



The skull table or roof differed in that the temporal canal 

 passed through what appears to be the postparietal. On the 

 basis of the canal, this was probably the supratemporal 

 which had extended medially to exclude the postparietal, 

 just as it does in Polypterus. Anterior to the hinge line was 

 a parietal. The pineal or parietal organ has pitted the 

 underside of a small ossicle in the skull rooi oi Holoplychius 

 anterior to the parietals. The large intertemporal is not 

 included in the anterior part of the roof; it bears a canal or 

 outlet tube to the surface in Holoplychius but not in Glyplo- 

 lepts or Porolepis. 



The cheek differs in that there are three canal-bearing 

 units between the jugal and the lower jaw. The distal one 

 is usually identified as the preopercle, the other two, and 

 some irregular bones below them, as the squamosals. It is 

 more probable that the so-called preopercular unit, which 

 extends down behind the quadratojugal, overlaps the lower 

 jaw somewhat, and extends back over the operculum 

 slightly, is missing in the osteolepiform. The preopercular 

 bone of the osteolepiform lies above the quadratojugal in 

 the position of the second squamosal of Holoplychius or 

 Glyptolepis. The cheek of the porolepiform is thus better de- 

 scribed as having a single squamosal, and two preopercu- 

 lars, the more distal of which is lost in osteolepiforms. The 



96 • HEAD SKELETON OF OTHER TETRAPODS AND CHOANATES 



