A B 



Figure 6-58. Pectoral girdle of Latimeria. A, lateral view; B, medial 

 view of left half of girdle. (After Millot and Anthony, \958) 



girdle to the posterior margin of the cranium, lateral to the 

 exoccipital bone. The cleithrum is much reduced, while the 

 clavicle extends from near the dorsal end of the cleithrum to 

 near the ventral midline. The cartilaginous scapulocoracoids 

 are small and joined across the ventral midline of their 

 clavicular processes. 



Of particular interest is the fact that the scapulocoracoid 

 articulates with and is partly supported by a massive rib 

 attached to the occipital segment of the head. Articulation 

 is through a cartilaginous process of that rib. Whereas this 



cranio] bone 



exoccipital 

 ligament 



uprocleithrum 



cleithrum 

 pectoral fin 



-cranial rib 

 scapulocoracoid cartilage 



Figure 6-59. Pectoral girdle of a dipnoan, Protopterus. The ventro- 

 anterior end of the girdle has been pulled outward. 



head rib is very large, the first two ribs following it are not 

 ossified in Proloplerus; the first trunk rib is that of the third 

 segment behind the head. In Neoceratodus these first spinal 

 ribs are present but smaller than the head rib. 



The choanate fishes all agree in having a fin with an ex- 

 tensive fleshy lobe. Possession of such a lobe is described by 

 the term Sarcopterygii (from Greek rarte— flesh; pteryx or 

 ptervgos—vi'ms,, fin, feather). The choanates are effectively 

 identified by this term as well as by having internal nares. 

 In view of the lack of agreement as to whether the dipno- 

 ans have internal nares, Sarcopterygii is perhaps a better 

 term. 



The crossopterygian fin {krossoi or krossotos—iringe or 

 fringed) is generally assumed to be ancestral to the five- 

 fingered (pentadactyle) limb of the tetrapod. The fins of 

 rhipidistians are not well known; generally they are repre- 

 sented by that of Saunptems or Euslhenopteron. The actinistian 

 can be represented by Latimeria, while the dipnoan Neocera- 

 todus has perhaps the closest approach to the ancestral type 

 (thickest, shortest lobe) of that group. 



The common feature of these fins is the fanned arrange- 

 ment of parts from a single basal element (Figure 6-60). 

 The skeleton of the fin of Neoceratodus appears to be an 

 attenuation comparable in some ways to that observed m 

 flippers of reptiles and mammals. The lack of a ray-sup- 

 ported fin in the dipnoan agrees with this conclusion. 



In Neoceratodus we see a central axis for the fin with rays 

 extending out on the preaxial and postaxial sides. This con- 

 dition has been described as a biserial archipterygium. 

 Gegenbaur assumed that the archipterygium (arc/i!— first or 

 primitive), as exemplified by Neoceratodus, was the primitive 

 type. In contrast, the fin of the rhipidistian is almost a uni- 

 serial archipterygium; only reduced postaxial rays are pres- 

 ent. The central axis of the rhipidistian fin is not so well 

 defined as that of the lung fish. The latter is most likely spe- 

 cialized in the direction of a flipper rather than a fin. 



Derivation of the tetrapod limb from a fish fin, or ichthy- 

 opterygium, is most easily achieved by starting with the fin 

 of a rhipidistian. Several descriptions of the way in which 

 this might have happened have been published (Figure 

 6-61). It does not seem worthwhile to speculate on how the 

 archipterygium was converted to a cheiropterygium or 

 cheiridium {cheir— hand; iffmn— diminutive suffix) since sev- 

 eral possible paths could be followed in this conversion. 



Pelvic appendage The pelvic limb and its girdle in choa- 

 nates are variable just as was the pectoral (Figure 6-62). 

 The girdle consists of bilateral elements with a dorsolateral 

 projecting iliac process, a medial ischial process, and a for- 

 ward pubic process. The girdle of the dipnoan is peculiar 

 in that the two parts are fused at the midline. Thus there 

 is a resemblance to the amphibian; this similarity, when 

 taken along with the great reduction of the cleithrum and 

 the lengthening of the clavicle in the pectoral girdle, gains 

 some importance. 



1 80 . THE VERTEBRATE BODY SKELETON 



