PRIMORDIAL SHARKS 



167 



known as the Arthrodira, a group of uncertain relationships. 

 They have many adaptations in common with Bothriolcpis, 

 such as the jointed neck, dermal jaws, carapace, plastron, and 

 paired appendages (Acanthaspis). Some authorities regard 

 the Arthrodira as aberrant lung-fishes. Dean, Hussakof, and 

 others regard the balance of evidence as in favor of relationship 

 with the stem of the Antiarchi {Bothriolepis). In the Middle 

 Devonian (the Cleveland shales 

 of Ohio) they attain the formi- 

 dable size shown in the species 

 Dinichthys intermedins (Fig. 48). 

 Like the ostracoderms, these 

 animals are not in the central 

 or main lines of fish evolution 

 but represent collateral lines 

 which early attained a very high 

 degree of specialization which 

 was followed by extinction. 



Primordial Sharks, Ances- 

 tral TO Higher Ver- 

 tebrates 



Fig. 49. A Primitive Devonian Shark. 



(Above.) CJadosclachc, the type of the 

 primitive Devonian shark of Ohio with 

 paired and median lappet fins provided 

 with rod-Hke cartilaginous supports, 

 from which type by fusion the limbs of 

 all the higher land vertebrates have 

 been derived. Model by Dean, Hussa- 

 kof, and Hortcr from specimens in the 

 American Museum of Natural History. 

 (Below.) The interior structure of 

 the lappet fins of Cladoselache showing 

 the cartilaginous rays (white) within 

 the fin (black). After Dean. 



The central line of fish 

 evolution, destined to give rise 

 to all the higher and modern 

 fish types, is found in the typical cartilaginous skeleton and jaws 

 and four fins of the primordial sharks, the primitive fusiform 

 stage of which appears in the spine-finned type (acanthodian, 

 Diplacanthus , Fig. 51) of Upper Silurian time. The relatively 

 large-headed, bottom-living types of sharks do not appear until 

 the Devonian, during which epoch the early swift-moving, 

 fusiform, predaceous types through a partly reversed adaptation 



