MATTHEW, CLIMATE AND EVOLUTION 2T5 



members, when readapted to the marsh or littoral conditions, an advan- 

 tage which enabled them to supersede the autochthonous dwellers in those 

 conditions. Per contra, there have not been a succession of invasions of 

 the dry land by the vertebrate inhabitants of swamp and sea-coast. Once 

 established on dry land, the primary groups of dry-land reptiles held 

 their own and evolved and expanded into higher types and greater variety, 

 but they were not recruited, so far as the evidence shows, by new invasions 

 from the swamp and aquatic fauna. 



DINOSAURIA 



The dinosaurs appear to be primarily a dry-land adaptation (properly 

 speaking, two distinct but parallel adaptations) of the primitive rep- 

 tiles. ^'^ Their most obvious adaptive characters lie in the long limbs and 

 swift-running gait and the general parallelism to the ratite l)irds. As 

 such, the conditions of life would tend to greater activity and higher de- 

 velopment and enable them, when they reinvaded the swamps during the 

 epochs of great swamp-extension, to reach greater size and dominance. 

 It is these readaptions that are chiefly known to us and are apt to give 

 the idea that the dinosaurs were distinguished by gigantic size and mass- 

 ive proportions. In fact, these are no more typical of the order as such 

 than the whale, hippopotamus and elephant are fairly typical of the 

 mammals as such. There must have been multitudes of small dinosaurs, 

 mostly inhabiting the upland, a smaller number living among the swamps 

 and marshes, but we know comparatively little about them. Some notion 

 of their numbers and variety in the Triassic is gained from the innumer- 

 able footprints spread over the Triassic shore-deposits of the Connecticut 

 Eiver. But of all this multitude, we have actual remains of only two or 

 three types. The Compsognathus skeleton of Solenhofen is, perhaps, an 

 example of the small light-limbed upland dinosaurs of the Jurassic; 

 HaUopus and PodoJcesawus are perhaps fairly representative of their 

 Triassic ancestors. The Jurassic sauropods, while highly specialized for 

 aquatic life and river-bottom wading, yet retain a few features indicative 

 of former land life. One of these is the long limbs, which it would seem 

 must have been acquired on land. Another is the fact that the knee bends 

 forward as it does in all other dinosaurs, while in reptiles primarily am- 

 phibious the knee bends outward and the limbs are short. The elbow of 

 the Sauropoda, on the other hand, bends outward, as in reptiles generally, 

 not backward, as it does in primarily quadrupedal land animals, and this 



»« F. VON HuENE : Geol. u. Pal. Abh., N. F., Bd. xill, s. 22-38. 1914 ; Neues Jahrb., 

 Beil. Bd. xxxvii, s. 577-587. 1914. 



