440 



THE CHANGING GENERATIONS 



fin-backed reptiles or, "ship lizards" (pelycosaurs), and the Permian and 

 Triassic mammal-like reptiles (therapsids) from which the mammals evolved. 



3. Parapsida. Skull roof perforated by an upper opening behind the eye, 

 bounded below by the postfrontal and supratemporal bones. Includes the 

 fishlike ichthyosaurs and their allies, all extinct. 



4. Euryapsida. 1 Skull roof perforated by an upper opening behind the eye, 

 bounded below by the postorbital and squamosal bones. Includes the extinct 

 marine plesiosaurs and their allies. 



SQ PQ 



Fig. 28.3. The five basic types of reptilian skulls. The anapsid type (center) is the most 

 primitive and that from which the others were derived. Upper left, synapsid type; upper 

 right, parapsid type; lower left, euryapsid type; lower right, diapsid type. PF, postfrontal 

 bone; PO, postorbital bone; SQ, squamosal bone; ST, supratemporal bone. (Redrawn from 

 Colbert, The Dinosaur Book, by permission American Museum of Natural History.) 



5. Diapsida. Skull roof perforated by two openings behind the eye, one upper 

 and one lower, separated by the postorbital and squamosal bones. Includes the 

 scaly reptiles (among which are the lizards, snakes, and extinct mosasaurs), 

 and the archosaurs or "ruling reptiles" of the Mesozoic. The chief archosaur 

 groups are the phytosaurs, crocodiles and alligators, dinosaurs, and pterosaurs, 

 of which only the crocodiles and alligators still survive. The birds, however, 

 came from one of the archosaur stocks. 



The stem-reptiles and the turtles. The first reptiles were anapsids 

 with solidly roofed skulls and sprawling legs — the cotylosaurs, or "stem 

 reptiles," from which all the other reptilian stocks were evolved. This 

 group arose in Carboniferous times from forms like Seymouria and reached 

 its climax in the Permian, when it branched out into a number of varied 

 lines — mostly lizardlike carnivores from 1 to 5 feet in length but some of 

 them large plant-feeding types. One of the latter was as large as an ox and 

 perhaps heavier, with a small head, great spraddled legs, and a massive 



1 Also called Synaptosauria. 



