ENVIRONMENTAL CONDITIONS 411 



discovered by Isaac Lea in 1849, near Pottsville, at about 700 feet (one- 

 quarter of the thickness) from the top of the formation. The impres- 

 sions, handlike in form, were made by an animal whose stride was 13 

 inches and the breadth between the outer edges of the footprints 8 inches. 

 Not long after this the Geological Survey brought to light, about 1,500 

 feet lower in the formation, another species of footprints of much smaller 

 dimensions, and soon afterward two other varieties. 



In 1908 Branson collected in Virginia footprints belonging to four or 

 five species from near the bottom of the Hinton formation. This is in 

 the Upper Mississippian. 25 



Since 1908 Mr. Unger, of Pottsville, Pennsylvania, by careful search- 

 ing of the sections of the Mauch Chunk, admirably exposed near there, 

 has collected a number of slabs showing impressions of footprints, mostly 

 of rather small size and delicately impressed. 



These details show the character and continuity of the amphibian 

 record from the earliest known occurrence at the close of the Devonian, 

 through the Mississippian (Lower Carboniferous) period. In all cases 

 they indicate a sustaining limb holding the body clear of the ground. 

 The foot is primitive, and in the earliest known occurrence, Thinopus, it 

 is possible that a stage is represented before the full development of 

 digits. The feet, in general, show a capacity for either walking or 

 swimming. 



A striking feature of this early record of the amphibians is that it 

 consists not of bones, but of footprints. In the Old Eed Sandstone, in 

 contrast, are horizons of abundant and well preserved fish fossils. Dur- 

 ing the time of deposition of the Upper Old Eed Sandstone the am- 

 phibians must have been in existence, but they are not preserved with 

 the fishes. Apparently, then, they did not at first live under those con- 

 ditions which resulted in the preservation of the fossils of fishes. This 

 suggests that they were not compelled to concentrate into pools by the 

 seasonal droughts, or if so concentrated the} r did not die under those 

 conditions. Neither did they apparently live as yet in the shallow lakes. 

 What, then, was the nature of their environment? 



The first amphibian bones are found in the Edinburgh coal measures 

 of the Lower Carboniferous coal field of Scotland. There the oldest 

 known skeletons, those of Loxomma and Pholidogaster, are approxi- 

 mately contemporaneous with the footprints of the non-coal-bearing 

 Horton formation of Nova Scotia. But the Lower Carboniferous coal 

 measures gave way again to the deposition of red shales and sandstones. 



25 E. B. Branson : Amphibian footprints from the Mississippian of Virginia. Jour. 

 Geol., vol. xviii, 1910, pp. 356-358. 



