clavicle 



procoracoid 



nterclavicle 



A TUPINAMBIS 



interclavicie 



clavicle 



procoracoid 

 iorocotd foramen 



glenoid fossa 

 calcified corocoid cartilage 



B SPHENODON 



clavicle 



calcified corocoid cartilage 



procoracoid 



nterclavicle 



\. IGUANA 

 Figure 6-49. Ventral views of pectoral girdles of Tubinambis, A, 

 Sphenodon, B, and /guana, C, Port of interclavicie of Iguana removed 

 to show cartilaginous extensions of the procoracoid. 



clavicle. Above the scapula there is a cartilaginous supra- 

 scapula, which may bear the clavicular process. 



The cartilage in which the scapula and procoracoid os- 

 sify extends well beyond these bones: dorsally, anteriorly, 

 and ventromedially. Anterior to the procoracoid, the carti- 

 lage is fenetrated; the number, size, and shape of the open- 

 ings is variable. Ventromedially the cartilages overlap, the 

 left above the right or just the opposite. The amount of 

 overlap is small in Sphenodon, considerably more in the 

 lizard Tupmambis (Figure 6-49). The procoracoid ossifications, 

 perforated by the supracoracoid foramen, approach the 

 midline interclavicie but do not reach it. 



The girdle of the alligator differs in lacking the dermal 

 elements and in having simple outlines for the scapula and 

 procoracoid. The latter articulates with the anterior end of 

 the sternum (endochondral). 



The bird, as a modified archosaur, is much like the alli- 

 gator but retains the clavicle. The clavicles of either side 

 are fused ventromedially to form the furcula. The inter- 

 clavicie as a dermal element is lacking, but the keel of the 

 sternum is perhaps a modified remnant of this bone. The 

 scapula is long and blade-like, while the procoracoid is large 

 and ventromedially expanded where it articulates with the 

 sternum. The procoracoid may be notched or perforated by 

 the supracoracoid foramen. The ventromedial angles of 

 these bilateral bones may overlap at the midline. 



Among fossil reptiles there are two kinds of pectoral 

 girdles (Figure 6-48). The two-element type, lacking a true 

 coracoid, is found in the intermediate amphibian-reptile, 

 Seymouna; the three-element type, with both procoracoid 

 and coracoid along with the scapula, in the pelycosaurs 

 {Dimetrodon) and some other early reptiles such as the 

 captorhinids. The presence of a single ossification in the 

 coracoid region of primitive amphibians, and perhaps some 

 of the earliest reptiles (Petrolacosaurus), may have been due 

 to fusion of coracoid and procoracoid. Living amphibians 

 do not help with this question since the salamander has 

 only a "scapular" center, while the frog has only scapular 

 and procoracoid centers, as do some of the actinopterygian 

 fishes. 



The pelycosaur (three-element) type of pectoral girdle 

 can be followed through the therapsid (mammal-like) rep- 

 tiles to the mammals, whereas most of the reptiles, includ- 

 ing all of the living kinds, and birds, are found to stem from 

 the two-element type. This difference between two lines of 

 early reptiles agrees with what has already been observed in 

 the head skeleton and is made more meaningful by the ob- 

 served intermingling of these types in the earliest knovm 

 reptiles. 



The pectoral limb of the reptile is much like that of the 

 mammal but differs in several respects (Figure 6-50). The 

 lizard has one or two centrales in the middle of the carpal 

 area, and there is a sesamoid plate in the tissues below the 

 carpals. A vestigial intermedium is observed in Tupmambis. 

 In Sphenodon there are two centrales and an intermedium. 



174 • THE VERTEBRATE BODY SKELETON 



