COCCOSTEUS 



GEMUNDINA 



BOTHRIOLEPIS 



Figure 2-14. Restorotions of some fossil plocoderms. 



Pteraspis, Drepanaspis, and Birkema (Figure 2-13). Tremataspis 

 differed from Cephalaspis in lacking the horn-like develop- 

 ments of the head margin and in lacking fins. These genera 

 agreed in having the head and trunk largely encased in 

 armor. The tail was covered by scales or plates. The naso- 

 hypophyseal duct opened on the top of the head just Ln 

 front of the eyes, which lay near the midline. Behind the 

 eyes were a dorsal "field" covered by small plates and the 

 openings of the endolymphatic pores. 



Drepanaspis and Pteraspis (Figure 2-13) differed in having 

 the nasohypophyseal opening at the margin of the mouth 

 or inside the mouth. Drepanaspis was a very flattened form 

 apparently of bottom living habit. 



Birkema (Figure 2-13) is an example of the anaspid. It was 

 a laterally compressed fish of small size. It is thought that it 

 foraged along the bottom by more or less standing on its 

 head. This would account for the hypocercal type of tail 

 seen in this group and in Pteraspis. 



Among the fossil gnathostomes, jawed fishes, the arthro- 

 dires are well known, and of these Coccosteus and Dunkelo- 



steus are the best known. Dunkelosteus was large, its head and 

 trunk were covered with bony armor, and the head artic- 

 ulated with the trunk (Figure 2-14). It had paired fins and 

 a long, tapered, isocercal tail. 



Bothriolepis (Figure 2-14) is a peculiar looking placoderm 

 related to the arthrodire. Its ventrally located mouth sug- 

 gests that it was a bottom feeder. Other arthrodires were of 

 many shapes and forms. Geinundina (Figure 2-14) was a very 

 flattened, shark-like form, covered with small bony plates. 



The shark array had many fossil representatives ranging 

 from the Devonian Ciadoselache or the carboniferous Pleura- 

 canthus (Figure 2-15). The former had a cover of fine scales, 

 and the latter was probably naked. 



The acanthodians (Figure 2-15) are sometimes placed with 

 the sharks or with the arthrodires. The fins in this group 

 are of interest in that, with the exception of the caudal, 

 each was formed by a strong spine, behind which was a 

 membrane. The number of lateral fins ranged from two 

 pairs to seven pairs. One or two dorsals were present; the 

 tail was heterocercal. The head and body were covered by 



spine. 



spiracle 



C@ 



pectoral spine 

 ACANTHODES 



branchial openings 



pectoral fin 



spines 

 CLIMATIUS 



PLEURACANTHUS 

 Figure 2-15. External features of some fossil, shark-like creatures as reconstructed. 



FOSSIL VERTEBRATES 



31 



