ENVIRONMENTAL CONDITIONS 409 



reasons for believing that the modern dipnoans, more especially Protop- 

 terus and Lepidosiren, have been derived from their Devonian ancestors 

 by a series of retrogressive changes, gradually approximating toward a 

 final scaleless and limbless condition. 17 The eel-like form of Calamoich- 

 thys, one of the two living genera of crossopterygians, illustrates the 

 same tendency. 



It is notable that all the lingering representatives of these two De- 

 vonian groups of fishes which are found so abundantly in the Old Red 

 Sandstone are essentially tropical and live in climates marked by the 

 alternation of wet and dry seasons. It is for just such conditions that 

 the possession of the power of supplemental breathing by means of rudi- 

 mentary lungs adapts these types, and herein they have an advantage 

 over the fishes in which the swim-bladder has become adapted to other 

 uses. These living forms thus offer strong corroborative testimony to 

 the geological evidence that the Old Red Sandstone was laid down chiefly 

 as terrestrial river deposits in a warm climate marked by seasonal dry- 

 ness. Under such environments, in widely separated parts of the earth, 

 these relics of the remote past have persisted to the present. 



ENVIRONMENT OF THE EARLY AMPHIBIANS 



The earliest known evidence of amphibian life is the impression on a 

 slab of sandstone from the uppermost Devonian of western Pennsylvania 

 of one footprint in fair preservation and with it part of another of the 

 same series. The tracks were found in 1896 by the late Professor Beecher, 

 who presented the specimen to the Yale Museum and supplied the infor- 

 mation in regard to its geological position. The animal which made the 

 impressions was named by Marsh Thinopus antiquus. Marsh makes the 

 following statements concerning it: 



"These impressions are of mucli interest, both on account of their geological 

 age and the size and character of the footprints themselves. The one best pre- 

 served is nearly four inches in length, two and a quarter in width, and was 

 apparently made by a left hind foot. On the inner side in front of the heel, a 

 portion of the margin is split off, and this may have contained the imprint of 

 another toe. The other footprint was a short distance in front, but only the 

 posterior portion is now preserved in the present specimen. It is probably the 

 imprint of the fore foot. 



. . . "The geological horizon is near the top of the Chemung, in the upper 

 Devonian. In the same beds are ripple marks, mud cracks, and impressions 

 of rain drops, indicating shallow water and shore deposits. Land plants are 

 found in the same general horizon. Marine mollusks also occur, and one char- 

 acteristic form (Nuculana) is preserved in the footprint slab." 1S 



17 Sur la Phylogenie des Dipneustes. Bruxelles, 1895. 



1S O. C. Marsh : Amphibian footprints from the Devonian. Amer. Jour. Sci., vol. li, 

 fourth series, 1896, pp. 374, 375. 



