REPTILES, BIRDS, AND MAMMALS 



439 



primitive mammals, and the shell-less, yolkless egg of the higher mam- 

 mals (including man) has been derived from it. Perfection of the amniote 

 egg freed the ancestral reptiles and all of their descendants from de- 

 pendence upon water for reproduction and thus opened the way for 

 complete conquest of land environ- 

 ments by the higher vertebrates. 



ommon 



embryo 



ollontois 



yolk-sac 



Fig. 28.2. Diagram of the shelled amniote 

 egg, which freed vertebrates from depend- 

 ence upon water for reproduction. (Redrawn 

 from Romer, Man and the Vertebrates, by 

 permission University of Chicago Press.) 



THE MESOZOIC REPTILIAN 

 RADIATION 



By Permian times the evolu- 

 tionary radiation of the reptiles was 

 well under way, and almost every 

 known reptilian order had appeared 

 before the end of the Triassic period. 

 This rapid expansion was accom- 

 panied by most diverse specializa- 

 tions for different modes of life, and 

 there exists no more striking 

 example of adaptive radiation than 

 that furnished by the Mesozoic reptiles. We may begin our account by 

 defining the five great groups into which the numerous orders of reptiles 

 may be assembled. 1 



The five reptilian stocks. All reptiles, living and extinct', may be 

 classified in a broad, general way on the basis of skull design, as follows: 



1. Anapsida. Skull roof solid like that of the stegocephalians, without any open- 

 ings behind the eye. Includes the extinct "stem reptiles" (cotylosaurs) and the 

 turtles. 



2. Synapsida. Skull roof perforated by a lower opening behind the eye, 

 bounded above by the postorbital and squamosal bones. Includes the Permian 



1 Unfortunately we shall have to use quite a number of unfamiliar names in this 

 account of the reptiles. This cannot be helped but is not so bad as might appear at 

 first sight. For one thing, most of them contain the root saiir, which simply means 

 reptile. Omitting a few which have no good translation, here is a brief list with mean- 

 ings: ankylosaur, "reptile with hooks (spines)"; archosaur, "ruling reptile"; Brachio- 

 saurus, "arm reptile"; Brontosaurus, "thundering reptile"; Ceratopsia, "of horned 

 aspect"; cotylosaur, "reptile with hollows (cupped vertebrae)"; dinosaur, "terrible 

 reptile"; ichthyosaur, "fish reptile"; mosasaur, "reptile first found in the Meuse river 

 valley in France"; Ornithischia, "with birdlike hips"; phytosaur, "plant reptile" 

 (a misnomer due to false interpretation of the first fossils of these crocodilelike ani- 

 mals); plesiosaur, "near reptile"; pterosaur, "winged reptile"; Saurischia, "with 

 reptilian hips"; stegosaur, "roofed reptile"; Triceratops, "of three-horned aspect"; 

 Tyrannosaurus, "tyrant reptile." The "apsid' 7 part of Anapsida, Diapsida, etc. means 

 "opening," and these terms refer to the number and position of the openings in the 

 skull. 



