PLEURACANTHUS 



CLADOSELACHE 



Figure 6-67. Pectoral girdles and fins of fossil sharks. A, P/euraconthus; B, C/odose/ache. (A after 

 Goodrich, 1930; B ofter Dean, 1909) 



the point of attachment is high on the centrum. The para- 

 pophysis may then act as the point of attachment of both 

 dorsal and ventral ribs, as it does in the salmon. It might be 

 assumed that originally there were separate points of at- 

 tachment for each of these ribs, the parapophysis serving 

 the dorsal rib and the hemapophysis serving the ventral rib, 

 as in Polypterus. Such a view finds no support in the fossil re- 

 cord; the parapophysis appears to be the only constant 

 process whether it serves dorsal or ventral rib or both. The 

 hemapophysis has apparently become associated with the 

 ventral ribs in Polypterus. Certainly, the ventral ribs are re- 

 lated to the hemal arches in the tail in fishes. In the teleost 

 the elongated parapophysis appears to be modified into that 

 arch; the dorsal rib has similar relationship in the tetrapod. 



Another problem is defining parapophysis as opposed to 

 rib. This can only be solved by assuming that the point of 

 articulation is a variable, mechanically determined thing 

 and that the parapophysis of an amphibian is Cjuite differ- 

 ent from that of a mammal. 



The various groups of vertebrates can be characterized 

 by their ribs. The Chondrichthyes have dorsal ribs or no ribs 

 (Hvdrulagus). Actinopterygians in general have well-devel- 

 oped ventral ribs and less-developed or vestigial dorsal ones. 

 In addition, many teleosts have intermuscular bones distinct 

 from the ribs. 



The choanates appear to have had both types of ribs; 

 pleural ribs are developed in dipnoans and actinistians but 

 replaced in rhipidistians by dorsal ribs. However, the dorsal 

 ribs of the rhipidistian have acquired a diapophysis con- 



nection with the neural arch. This suggests that Naef 's as- 

 sumption of the compound nature of the tetrapod rib may 

 apply (Figure 6-73). It is possible that the ventral head is 

 the remnant of the ventral rib, while the rib body is of the 

 dorsal type with a secondarily acquired new capitular head 

 on the neural arch. In the actinistian, it could be assumed 



cerototrichia 



spine 



# — basal radials 



membrane 



A ACANTHODES 



spme 



B CLIMATIUS 



Figure 6-68. Pectoral fins of acanthodians. A, Aconf.Soc/es (Lower 

 Permian); B, C/imotios (Lower Devonian). (After Watson and Westell) 



188 • THE VERTEBRATE BODY SKELETON 



