first (the hallux) being directed backward. Three meta- 

 carpals and a number of tarsals are fused into a tarsome- 

 tatarsus. The situation observed in the bird is approached 

 in some of the bipedal dinosaurs. 



Amphibia 



Pecfofol appendage The pectoral girdle of Crypiohranchus 

 and Neclurus is quite similar (Figure 6-53). It is largely 

 cartilaginous with a distinct anterior process and overlap- 

 ping procoracoid plates. There is a scapular ossification 

 which reaches down to the glenoid fossa but not beyond it. 

 In some salamanders, the scapula extends beyond the fossa 

 to include the supracoracoid foramen. It does not extend 

 much behind the glenoid fossa in any. There is a large 

 supracoracoid cartilage above the scapula. There are no 

 dermal elements involved. 



The frog in contrast has a remnant of the cleithrum, 

 which secondarily extends perichondrally over the outer 

 surface of the suprascapular cartilage, and a clavicle, which 

 is associated with a clavicular cartilage anterior to the large 

 supracoracoid fenestra. The so-called omosternum prob- 

 ably represents a converted remnant of the interclavicle, 

 which is now associated with a calcified cartilaginous mass. 



The scapular ossification lies above and anterior to the 

 glenoid fossa with the large suprascapula above it. The pro- 

 coracoid ossification extends ventromedially from the glen- 

 oid fossa; it lies behind the supracoracoid fenestra. The pro- 

 coracoid cartilages fuse at the midline in the Bullfrog in a 

 "firmisternal union." In some anurans these cartilages 

 overlap posteriorly and are joined only anteriorly. When the 

 procoracoid cartilages overlap, the omosternum is entirely 



cartilaginous or, as an extreme, it is missing {Ascaphus). The 

 sternum of the Bullfrog is ossified, while in those frogs, 

 which have the procoracoids overlapped, it is cartilaginous. 

 The evolutionary sequence is usually described as from arcif- 

 eral (with the procoracoid cartilages overlapped) to firmi- 

 sternal — actually the reverse seems to be indicated both 

 within the frogs (more united, and ossified, in the Bullfrog 

 than Ascaphus) and in terms of amphibians in general. In 

 the early amphibians the procoracoid bones or the procora- 

 coid areas of the scapulocoracoids were probably firmly 

 connected to the broad interclavicle; with reduction of the 

 interclavicle the procoracoids were extended by cartilages 

 which tended to overlap. 



In the anthracosaur Pleroplax (Watson, 1926), the most 

 primitive amphibian known — primitiveness being based on 

 the connection between the girdle and the skull as in the 

 fish, the girdle has a series of dermal elements and a single 

 scapulocoracoid ossification. This girdle contrasts strongly 

 with that of Brancluosaurus, in which the clavicle and clei- 

 thrum are reduced to splints and only a scapular ossifica- 

 tion, lying above the glenoid fossa, is known to occur. 



Consideration of the pectoral girdle in amphibians indi- 

 cates that there has been a continual reduction of the 

 dermal components. The broad dorsal expansion of the clei- 

 thrum has been replaced by the cartilaginous supracora- 

 coid. Reduction of the interclavicle resulted in loss of direct 

 contact with the scapulocoracoid ossifications and was ac- 

 companied by the overlapping of the extensions of the pro- 

 coracoid cartilages. It is also suggested that in the frogs, the 

 interclavicle has been replaced by a cartilaginous omoster- 

 num, which in the Bullfrog is perichondrally ossified, on its 

 ventral surface. 



clavicle. 



supracleithrum 



_postclerthrum 

 _cleithrum 



scapulocoracoid 



interclavicle 

 PTEROPLAX 



ligament 



nerves and blood vessel: 



scapulocoracoid 



omosternum- interclavicle 



DESMOGNATHUS 



Figure 6-53. Pectoral girdles of amphibians as seen in lateral view. A, Pteroplox, an anthracosaur 

 (after Watson, 1 926); B, Crypfobranchus; C, Desmognathus; D, Bullfrog. 



BILATERAL APPENDAGES 



177 



