THE STORY OF OHIO'S ROCKS 



19 



(fig. 18) abounded; they are scorpion-like in appearance, except for the tail, which does not 

 bear a sting. They may be ancestral to the scorpions. 



Primitive plants were beginning to invade the land surfaces of the globe during late 

 Silurian time. None of them was large and they were probably localized in certain areas of the 

 globe. Their remains have been found in Australia and Scotland. The land was almost unin- 

 habited by animals also for the only record of a Silurian land animal is of a rare scorpion of 

 which there are only four known specimens from the Upper Silurian of Scotland. 



DEVONIAN TIME: THE RISE OF THE FISHES. 



Ohio was land during early Devonian time. No rocks of this age have been preserved 

 here. The Middle Devonian seas covered most of the state except the low islands in its 

 western third (fig. 19). The seas brought with them 

 marine animals from the south, north, and northwest. 

 During this time the seas were shallow and clear, bor- 

 dered by low lands from which little sediment .vas being 

 washed, and apparently they were warm, if not subtropi- 

 cal, judging by the abundance of corals in them. Coral 

 reefs flourished over parts of the state and they are now 

 preserved in our Devonian rocks. 



As might be expected, the sediments deposited in 

 these seas are limestones and shales (Figs. 20a and 20b). 

 The topmost ones are thick beds of black shale with a 

 high carbon content in which fossils are few and of a 

 peculiar character; they are floating and swimming forms 

 but there are almost no bottom dwellers. 



The Devonian limestones of Ohio are particularly 

 pure and some of them, for example the Columbus lime- 

 stone, are thick-bedded and suitable for building stone. 

 In the early days of settlement in the state, the limestones 

 were used as building stone; the state Capitol in Columbus 

 is built of Devonian Columbus limestone and so are many 

 of the fine early stone houses of central Ohio. The lime- 

 stones are used for lime in great quantities, and also for 



Fig. 19 Middle Devonian lands and seas 



crushed stone and agricultural lime wherever they can be quarried profitably. 



The Middle Devonian seas of Ohio swarmed with innumerable animals whose fossils can 

 be gathered literally by the bushel from some parts of the Columbus and Delaware limestones 

 and the Silica formation (fig. 21). Their most remarkable feature, as compared with Silurian 

 fossils, is the presence of great numbers of fish fragments in the "bone beds" (see p. 74) of 

 the Columbus and Delaware limestones. Some of these fishes reached great size, for example 

 the placoderm Dinichthys with a length of 25 feet; others were of more familiar aspect, for 

 example the shark Cladoselache found in our Upper Devonian black shales. During the Devon- 

 ian the fishes had invaded the seas successfully and many different groups evolved during that 

 time. Notable among these were the lungfish and their relatives which gave rise, during 

 Devonian time, to the Amphibians. 



The invertebrates were even more numerous than the fishes in the Devonian seas of Ohio. 

 Even the tiny Foraminifera were represented and they may have been much more numerous 

 than has been so far suspected (see Stewart and Lampe, 1947). Sponges are not conspicuous 

 in the Devonian .of Ohio but spectacular finds of glass sponges have been made locally. The 

 corals were among the most abundant of the inhabitants of the Devonian sea . Worms were 

 undoubtedly present in much greater numbers than the record indicates; only a few kinds, 

 which had a limy tube, have been preserved. These tubes are either coiled or straight and 

 are found attached to brachiopods and other shells. 



