PARAPSID (AND EURYAPSID) 



SYNAPSID 



MAAAMAL 



ARCHOSAUR (DIAPSID) 



ANAPSID 



TURTLE (ANAPSID) 



BIRD 



DIAPSID 



LIZARD (AND SNAKE) 



Figure 4-27. Evolution of the reptilian skull roof in terms of the temporal fenestroe or the loss of 

 the cranial roof. Outlines of skulls are semidiogrammatic. 



fused postparietals, this type represents an early and primary 

 branch of the reptiles. 



Goodrich (1916) cast some doubt on the idea of the rep- 

 tiles being monophyletic, that is, all known reptiles stem- 

 ming from a single primitive reptilian or prereptilian line. 

 He proposed that the reptilian line giving rise to the mam- 

 mals (this line as known now begins with the pelycosaurs) 

 was distinct from that to which the other reptiles and birds 

 belong, and that their basic difference in aortic arch plan 

 must be traced back to separate amphibian lines. Watson 

 has agreed with Goodrich and has suggested that the struc- 

 ture of the ear region also identifies these two lines. 



It has also been proposed that the captorhinomorphs gave 

 rise to the synapsids, whereas the diadectomorphs gave rise 

 to the turtles and the other reptiles. Since the palatal struc- 

 ture of lizards or birds denies a diadectomorph origin, it 

 must be assumed that the earliest members of this line had 

 a basipterygoid articulated palate. It is simpler to assume 

 at this stage in our knowledge that there were several prim- 

 itive reptilian types, not just two. For example, the eo- 

 suchians (or Millerosauria) are no more modified than the 

 others and could with equal verity be selected as an ances- 



tral type. Why should the captorhinomorphs be selected as 

 ancestors of the pelecosaurs when these groups, as now 

 known, are contemporary? 



General observations on the tetrapods 



The head skeleton of the tetrapod can now be considered 

 in more general terms. There can be little doubt that there 

 was a common ancestral pattern of bones from which the 

 head skeletons of the various living lines have evolved. In 

 each of the three evolutionary lines — mammal, reptile, and 

 amphibian (the bird is only a modified reptile) — there has 

 been a reduction in the number of bones. For example, in 

 each line the postparietals have been reduced in size and 

 fused to each other and to other elements, or lost. Reduction 

 in the number of bones is related to the loss of the outer 

 cranial roof; this loss is associated in part with the freeing 

 of the muscles of mastication. Reduction has also been a 

 biproduct of the reorganization of the head skeleton in 

 response to shortening of the jaws, the development of dif- 

 ferent kinds of kinesis, and the development of a neck. 

 Reduction has been most marked in the living amphibians; 



OTHER TETRAPODS 



91 



