fused or quite fused in the region of the acetabulum (Figure 

 6-52). There is a large obturator fenestra, but the ends of 

 the ischium and pubis do not join to close this gap. The 

 pubis is pierced near the acetabulum by an obturator fora- 

 men. The ilium is blade-like, nearly vertical in Sphenodon, 

 sloping down and forward in Tupmamhis. 



In the primitive reptile or amphibian, the pubis and 

 ischium were broad plates, jointed to one another across the 

 ventral midline; the ilium was a dorsally directed plate of 

 variable shape. The pubis was pierced by an obturator 

 foramen. 



In the pelycosaur line (Ophiacodon and Dimetrodon), the 

 obturator fenestra lies near the acetabulum, the bones meet- 

 ing ventromedial to this fenestra. In the bulk of the reptiles, 

 the obturator fenestra appears to have formed by a gradual 

 separation of pubis and ischium, first near the midline, then 

 with progressive increase in size until the fenestra ap- 

 proached the acetabulum. The medioventral ends of the 

 pubis and ischium may now secondarily approach each 

 other. Since these types of obturator fenestrae appeared late 

 in time, they serve to mark the two lines of reptiles. 



Extremes of the reptile type are observed in the crocodil- 

 ian and in the bird. The pelvis of the bird has an antero- 

 posteriorly drawn-out ilium with the pubis and ischium di- 

 rected posteroventrally. The obturator fenestra is a long, 

 thin fissure between these latter bones; the anterodorsal end 

 of the fenestra serves as the obturator foramen. In the primi- 



tive bird the ischium and ilium are not connected poste- 

 riorly, but in more advanced types this has been achieved. 



The pubis of the alligator is not perforated by an obtura- 

 tor foramen, and it resembles the marsupial bone of the 

 monotreme or marsupial, perhaps having much the same 

 function as that bone. It does not participate in the forma- 

 tion of the acetabulum, whereas that of the bird does. 



The pelvic limb parallels the pectoral limb, but in the 

 lizard or Sphenodon it shows even greater reduction in the 

 number of tarsals. The centrales are missing as is the first 

 tarsal in Sphenodon and the first and second tarsals in Tupi- 

 nanibis or Iguana. In the lizard or Sphenodon, tarsal IV is 

 much enlarged and V is missing. The fifth metatarsal is 

 peculiar in that it appears to be fused with its distal tarsal 

 or has extended into the position of that tarsal. This pecu- 

 liarity is generally typical of living reptiles and was used by 

 Goodrich to identify the true reptiles as opposed to the 

 pelycosaur-mammal line. In the Eosuchia a fifth distal tar- 

 sal may be present (Youngina) or absent (Tangasaitrus) and 

 the fifth metatarsal is not hooked. In the pelycosaurs a fifth 

 metatarsal is present in the early forms, lost in the later 

 ones, but the pattern of the mammal is already indicated. 

 The fifth metatarsal is never hooked. 



The phalangeal formula is usually the same as for the 

 manus: 2-3-4-5-4. In the alligator, the fifth digit is reduced 

 to its metatarsal; its formula is 2-3-4-5-0. 



In the bird, the number of digits is reduced to four, the 



ischium 



A ARCHERIA 



B 



ERYOPS 



C OPHIACODON 



D DIMETRODON 



obturator foramen / <-:^- 



ilium 



schium 



CYNIDIOGNATHUS 



SPHENODON 



ALLIGATOR 



Figure 6-52. Pelvic girdles of amphibians and reptiles as seen in lateral view. A, Archeria, 

 an anthracosaur; B, Eryops; C, D, the pelycosaurs, Ophiocodon and Dimetrodon; E, Cynidiognothus, 

 a therapsid; F, Sphenodon, G, Alligator. (A, B, C, D, E after Romer, 1955) 



176 



THE VERTEBRATE BODY SKELETON 



