The scapulocoracoid ossification probably has always had 

 scapular and procoracoid centers, although these sometimes 

 become so closely associated (as indicated by Ascaphus) as to 

 appear to be one. In the heavily ossified forms, such as 

 Pleroplax, or in the early reptile Petrolacosaurus, these two 

 centers are lost in the fused element of the adult. Young 

 specimens would probably show these centers. 



The occurrence of three bones in this girdle suggests the 

 origin of a new center posterior to the glenoid fossa. The oc- 

 currence of three centers in several types of early reptiles 

 suggests a parallel origin in the different groups. The two- 

 center type has been carried down to the present in frogs, 

 living reptiles, and birds, while the three-center type has 

 come down through the synapsids to the mammals. 



The pectoral limb of living salamanders is quite simple 

 in its structure; this can be viewed as secondary simplifi- 

 cation since it goes along with reduction in the number of 

 digits. Reduction has gone slightly further in Necturus than 

 Cryplohranchus with the loss of the fifth distal carpale (Figure 

 6-54). The digital formula of both is 0-2-2-3-2. The frog 

 has the radius and ulna fused and the carpals highly modi- 

 fied. The salamander and frog agree in having only four 

 digits, the first having been lost. 



The presence of four digits (or less) in the forelimb ex- 

 tends far back in the history of these groups and into the 

 earliest known amphibians: the branchiosaurs and micro- 

 saurs. Eryops, a large and presumably modified type, appears 

 to have had only four digits in the manus (Figure 6-55). 

 Holmgren (1949) suggested that the four-digit forelimb of 



pubic cartiloge 



sacral rib 



A B 



Figure 6-54. Manus, A, and pes, B, of Cryptobronchus. 



A B 



Figure 6-55. Pelvic girdle of Cryptobronchus as seen in lateral. A, 

 and ventral, B, views. 



the amphibian evolved independently from the archiptery- 

 gium. It is generally assumed that the four-digit limb was 

 derived by loss of the fifth digit. 



The original number of digits was apparently five, with 

 an additional prepollex extending down from the radiale 

 (as in Eryops) and a postminimus, perhaps represented by 

 the pisiforme. The basic five-digit plan is shown by Sey- 

 mouria and the early reptiles. 



Pelvic appendage The pelvic girdle of living amphibians 

 consists of two ossifications and an extensive and forward 

 extending cartilaginous plate; attached to this is a separate 

 anterior process, the epipubis (Figure 6-55). Above the ace- 

 tabulum is the ilium, while below and behind it is the is- 

 chium. A pubic ossification is lacking. In fossil forms, the 

 pelvis includes a pubis, which in its form is similar to that 

 of the primitive reptile. In the early amphibians the three 

 units tend to be fused, and suture lines are obscured or 

 lacking. 



The pelvic limb of Cryplohranchus (Figure 6-54), and many 

 salamanders, has been conservative in its structure; it has 

 retained five digits. There are only four in Necturus. The 

 carpals fan out from the end of the fibula in a way sugges- 

 tive of the speculations relative to the origin of the ancestral 

 limb from the archipterygium of the fish (Figure 6-61). 



In fossil amphibians, five digits are known for the pes 

 (foot) of the rhachitomous type Trematops (Figure 6-56). 

 Again in Trematops we see the archipterygial pattern but 

 note that there are several centrales rather than the single 

 one of Cryptobronchus, also there is a distinct prepollex. The 

 carpals and tarsals of some amphibians are ossified. 



General observofions Whereas the manus (hand) of the 

 amphibian is modified as compared with that of the primi- 

 tive reptile, the pes is less modified than that of the reptile. 

 In the primitive captorhinid reptile (Figure 6-56), three ele- 

 ments coalesce— the intermedium, tibiale, and a proximal 

 centrale— to form the astragalus. Although this appears to 

 be the phylogenetic origin of the astragalus, in ontogeny it 



178 • THE VERTEBRATE BODY SKELETON 



