402 REPTILES xv. 6-7 



which includes Sphenodon, the lizards and snakes, and the Archo- 

 sauria, including the crocodiles, dinosaurs, pterosaurs, and the an- 

 cestors of birds. It is not known if the archosaurs were derived from 

 primitive lepidosaurs such as * Yonngina, or whether the two groups 

 arose independently from cotylosaurian ancestors. 



Order *Eosuchia 



The earliest lepidosaurs belong to the order Eosuchia. The best 

 known of these, * Youngina (Figs. 229 and 230), was a lizard-like 

 creature, found in the Upper Permian of South Africa, and retaining 

 many cotylosaurian features, for instance, teeth on the palate and no 

 opening between the bones of the snout (antorbital vacuity). The 

 two fossae at the back of the skull immediately show the affinity with 

 other diapsids. Little is known of the post-cranial skeleton. The fifth 

 metatarsal does not show the hooked shape that is found in other 

 diapsids and also in Chelonia. It is difficult, however, to ascribe very 

 great weight to this single point, as against the general features of the 

 skull, which indicate that * Youngina could have given rise to the later 

 two-arched reptiles and, by loss of the lower margin of the lower 

 temporal fossa, also to the lizards and snakes. *ProIacerta, from the 

 Lower Trias, shows how this may have come about; it is so like 

 * Youngina that it is classed as an eosuchian, but there is a gap in the 

 lower temporal arch, suggesting that the animal may have been near 

 the ancestry of lizards. 



7. Order Rhynchocephalia 



Sphenodon ( = Hatteria), the tuatara of New Zealand, is the oldest 

 surviving lepidosaurian reptile; it still remains in essentially the 

 eosuchian condition. Very similar Mesozoic fossils (e.g. *Homoeo- 

 saurus from the Jurassic) show the continuity of the type (Fig. 221). 

 Among the many primitive features that this race has preserved un- 

 changed for 200 million years are the two complete temporal fossae 

 (Fig. 231), the well-developed pineal eye (the pineal foramen is 

 marked in the early diapsid fossil skulls), and the amphicoelous verte- 

 brae with intercentra. Sphenodon, alone of surviving reptiles, has no 

 copulatory organ. The large wedge-shaped front teeth are among the 

 few specialized characters. 



The tuatara was once widespread throughout New Zealand but 

 became much reduced and in danger of extinction. Recently rigid 

 conservation measures seem to have allowed it to recover in numbers 

 on some small northern islands. The animals are up to 2 feet long and 



