356 AN INTRODUCTION TO THE STUDY OF FOSSILS 



a uniform series to one differentiated into incisors, canines 

 and molars (whence the name from Greek anomos, without law 

 + odous {odont), tooth). They are intermediate in character 

 of skeleton between the highest of the amphibian labyrintho- 

 donts and the lowest mammals, the Monotremata. It is prob- 

 able that a primitive anomodont reptile gave rise to the mam- 

 mals. Because of the many resemblances in their skeleton 

 to that of the latter group they were later called Theromorpha 

 (from Greek ther, a wild beast, with a derived meaning of mam- 

 mal, + morphe, form). 



The anomodonts, which are Permian and Triassic in age, 

 vary from the large massive Pariasaurus about ten feet long, 

 with its triangular head, stout limbs and short tail, to the small, 

 probably agile, Galesaurus. The majority, so far found, are 

 from the Upper Pennsylvanian and Lower Permian of the Texas- 

 New Mexico area (Fig. 150) and from the lower part (Permian) 

 of the Karroo formation in South Africa. . . 



Order j, Sauropterygia (Plesiosaurs) 



Extinct, aquatic reptiles (commonly called plesiosaurs), 

 with long neck, small head and two pairs of limbs ; they are 

 thus lizard-like (whence the name from Greek sauros, a lizard, + 

 pterygia, fins). Teeth conical. A pineal foramen is present. 

 They swam at the surface of the water but could doubtless 

 make swift dives. Some attained a length of forty feet. 



Sauropterygians are known from the entire Mesozoic, - 

 Triassic to Cretaceous. In Triassic forms (as Lariosaurus) 

 the limbs are long and rather slender with the normal nun^ber 

 of phalanges to the digits, but in later genera (as Plesiosc,urus 

 of the Jurassic) the limbs are true paddles with the bones short 

 and the number of phalanges very much increased (Fig. 

 151). Their ancestors were thus in all probability land animals. 

 Some specimens of Plesiosaurus exhibit what were apparently 

 embryos within the abdomen and hence must have produced 

 their young alive. 



