THE STORY OF OHIO'S ROCKS 



25 



The gastropods were exceedingly varied and included long-spired forms as well as the 

 curious bellerophontids (see fig. 334) and several kinds of limpets. 



Both nautiloid and ammonoid cephalopods are present in Pennsylvanian strata. The 

 ammonoids expanded rapidly during Pennsylvanian time, foreshadowing their culmination in the 

 Mesozoic. 



On the other hand, Pennsylvanian trilobites are rare. Ohio's Upper Pennsylvanian rocks 

 are non-marine; therefore we cannot expect to find trilobites in them. In other parts of the 

 world, fewer and fewer trilobites survived as Pennsylvanian time progressed, an indication of 

 their eventual extinction in Permian time. 



No great changes are to be noted in the fishes of Pennsylvanian time. They were not 

 abundant in the seas but swarmed in fresh water, where they were represented by sharks, 

 ganoids, and other bony fishes. 



Amphibians were abundant in the Pennsylvania swamps (Fig. 25). Some of them reached 

 large size - ten feet or more - but they were unlike the living amphibians for they had a bony 

 covering over the skull which gave them their name, stegocephalians (meaning roof -skulled). 



The first true reptiles have been found in Pennsylvanian rocks but they were puny beside 

 the giant amphibians for they were only one or two feet long and lizard-like in appearance. 



Where the water of the coal swamps was reasonably fresh or even brackish, it was popu- 

 lated by a characteristic group of freshwater clams with thin shells ( Naiadites , fig. 313, and 

 other genera) which are found in great numbers in some Pennsylvanian shales. More often than 

 not, they are accompanied by clam-like arthropods ( Leaia and Estheria) and ostracodes by the 

 thousand. A few kinds of freshwater snails have also been found in the Pennsylvanian. 



Tree stumps are sometimes found in Pennsylvanian rocks. They look so much like tree 

 stumps of the present day that some geologists had the idea that they might have served then, 

 as they do now, as hiding places for small animals. The suspicion proved correct and these 

 ancient tree stumps, when broken apart, sometimes reveal skeletons of ancient lizards and 

 amphibians, fossil land snails, spiders, and millipedes which had died within their shelter 

 (fig. 26). 



Long before the end of Pennsylvanian time the sea had withdrawn from Ohio. The seas 

 advanced and retreated unevenly over the state; in general each advance was a little less than 

 the preceding one, and the land was slowly lifted up higher and higher. The climate became 

 drier and coal-swamps occupied smaller and smaller areas. They continued into the Permian 

 but after a time they disappeared entirely and their place was taken by forests of hardier plants 

 able to endure a more rugged climate. 



Life in the Coal Forest 



