Animals and Plants of Past and Their Records 613 



is little land today which has not at some time been below the level of the 

 sea, sometimes repeatedly. This explains why we may find fossils of 

 former marine organisms even on high mountains today. Records of 

 past animals and plants frequently suggest certain climatic conditions, 

 such as moisture and temperature, which have existed at certain periods 

 in the earth's history. Certain types of fossilized plants present in cer- 

 tain strata give us a good idea of the type of vegetation and necessary 

 climatic conditions at the time when such plants were placed in these 

 forming strata. 



The study of fossil animals and plants is important because they often 

 include the ancestors of modern species. In addition, the data secured 

 from such fossils often explain relationships of present animals and 

 plants. In some instances the ancient types serve to connect groups of 

 organisms which today seem to have no direct connections. A study of 

 fossil animals and plants also reveals that the race history (phylogeny) 

 can be accurately traced. A study of the stratigraphic successions of 

 fossil animals and plants gives much information in regard to the pro- 

 gressive developments of these animals and plants of the past, as well as 

 suggests present and future progressive developmental tendencies. This 

 study naturally would be much easier and more complete if unbroken 

 and perfect records of fossil organisms could be procured. This is not 

 possible so that these data must be interpreted accordingly. 



Many interesting facts about ancient plants have been ascertained 

 from a study of their fossil records in geologic rocks of the past. One 

 reason for studying plant fossils is to secure a complete picture of the 

 relationships between living organisms of today and their ancestors. 

 The earliest known plants were very simply constructed. Age by age, 

 more and more complex types appeared as shown by the study of the 

 proper strata of the earth (Fig. 320). Before the Paleozoic era the 

 only plants of which we find good records are the bacteria and blue- 

 green algae. The most ancient land plants were the ancient spore plants 

 from the Devonian period of the Paleozoic era. During the succeeding 

 Carboniferous period there appeared large, complex, treelike ferns. The 

 earliest seed plants (the seed ferns) occurred during the late Devonian 

 and early Carboniferous periods. These primitive ferns belonged to 

 the gymnosperms and became extinct in the Mesozoic era. Angiosperms 

 are first found in the Cretaceous period of the Mesozoic era. They were 

 dominant in this period and have retained this position ever since. 



A knowledge of fossil plants is necessary for an understanding of the 

 classification of plants. Formerly, the classification of plants was based 

 on living forms, but as the knowledge of fossil plants increased, the clas- 



