164 INTRODUCTION TO EVOLUTION 



bers of foraminiferans and crinoids. Crinoids reached the peak of their 

 development in the seas of this period. Brachiopods having long spines 

 radiating from their shells were also characteristic of these seas. Trilobites, 

 on the other hand, were rare. Their continued decline may be explained 

 in part by the presence of some 300 species of sharks having flat, "pave- 

 ment" teeth adapted for crushing the shells of molluscs and arthropods. 

 Fossils of terrestrial life of the time are relatively few in deposits re- 

 maining to us. Skeletons of small, salamanderlike amphibians have been 

 found, as have casts of amphibian footprints. 



Pennsylvanian Period 



The great coal deposits remain to us as memorials of the Pennsylvanian 

 period. Vast expanses of lowland were for long periods of time but slightly 

 elevated above sea level, and hence were perennial swamps. In these 

 swamps, encouraged by a mild climate, flourished luxuriant plant growth 

 whose carbon later became fossilized as coal. A Pennsylvanian forest 

 would have looked strange indeed to modern eyes. Deciduous trees, the 

 type most familiar to us, were lacking, as were true conifers, although a 

 forerunner of the latter having bladelike leaves was found. The least 

 strange plants would have been the ferns, though we are scarcely accus- 

 tomed to the sight of ferns with fronds 5 or 6 feet long and trunks 50 feet 

 high. Otherwise the plant life was quite unlike anything which meets our 

 eyes. The largest trees, and among the most common ones, were the scale 

 trees, so cafled because the surface of the bark had a pattern resem- 

 bling the pattern of scales on a snake's skin. The patterning was produced 

 by scars left by the bases of closely set leaves. Trunk diameters of 6 feet 

 and heights of 100 feet were found. Vast canebrakes of scouring rushes, 

 similar to their modern relative Equisetum ("horsetails") but reaching 

 heights of one hundred feet, added to the luxuriant plant growth destined 

 for conversion into coal. 



Insect life flourished. Predecessors of the Pennsylvanian insects are still 

 unknown. Future discoveries may help to fill this gap in our knowledge, re- 

 vealing the ancestry and early evolution of the group. Most of the Penn- 

 sylvanian insects were of archaic types not now living, though one struck 

 a distinctly modern note: the cockroach (Fig. 8.21 ). Although cockroaches 

 constitute only about 1 percent of modern insect faunas, they formed about 

 60 percent of insects living in Pennsylvanian times. Some reached a length 

 of 4 inches. They were strikingly similar to their modern descendants in 

 structure. It is a remarkable fact that while some animals are undergoing 



