Fossil Plants 



497 



lacked hard tissues which would resist bacterial action until 

 prints and casts were made. Furthermore, the rocks of the early 

 Paleozoic and preceding periods have been metamorphosed by 

 being subjected to great pressure by overlying rocks and by heat 

 due to crushing, faulting, and warping of the earth's crust. 

 Even though there had been a fossil record in them, it wauld 

 have been erased by the changes that have occurred during the 

 millions of years that have elapsed since the rocks were deposited. 



Importance of fossils in tracing relationships. In spite of the 

 fragmentary character of the record, hundreds of species have 

 been found and they have been of great importance in establish- 

 ing the relationships between some of the phyla of plants. Con- 

 ditions during the Carboniferous period were such that plant 

 remains are very abundant in coal seams and in the shales 

 associated with coal deposits. The rocks of the Carboniferous 

 and succeeding periods have not been so greatly modified, and 

 they have accordingly yielded many fossils. Nevertheless, 

 there is as yet very little geo- 

 logical evidence concerning the 

 origin of the conifers and flow- 

 ering plants. Until fossils of 

 the ancestors of these groups 

 are discovered, there is no 

 satisfactory basis for explain- 

 ing their origin from the seed- 

 less plants, or their relation- 

 ships to the known plants of 

 earlier ages. 



The fossil record. The dia- 

 gram on the next page shows in 

 a general way the occurrence 

 of the larger plant groups and 

 some of the probable relation- ^ ^ ., . . . . ,, , 



^ riG. 309. Fossil imprints of tern-like leaves 



ships. The diagram shows very in a rock of the Carboniferous period. 



