THE ORDOVICIAN PERIOD 



91 



our knowledge of land plants and animals is very scant. The 

 scarcity of land organisms may have been due to prevalent oceanic 

 conditions not favorable for fossilization, though it is also likely 

 that land plants and animals had not progressed far or become 

 very abundant so early in the history of the earth. Because of 

 the unusual abundance and diversity of invertebrate animal forms 

 in such an ancient fossiliferous system, a fuller discussion will be 

 devoted to these forms than 

 in succeeding chapters. 



Plants. — In this period, 

 as well as in the Cambrian, 

 plant life of very simple 

 types at least must have 

 been abundant to serve as 

 a direct or indirect food 

 supply for the myriads of 

 animals. Various fossil sea- 

 weeds (marine Algae) are 

 definitely known, especially 

 in the Trenton series and 

 younger Ordovician shales 

 (Fig. 48). The rather im- 

 perfect record is doubtless 

 due to the fact that such very simple forms were unfavorable for 

 fossilization. Definite knowledge of land plants is lacking, though 

 a few rather doubtful higher Cryptogams are said to have been 

 found in Europe. In view of the abundant land flora of the De- 

 vonian, it seems more than probable that land plants existed as 

 early as the Ordovician, and some may yet be discovered. 



Protozoans. — Both Foraminifers and Radiolarians must have 

 been common in the seas, because in some places many fossil forms 

 have been found. As in the preceding period, many forms without 

 shells almost certainly existed. 



Porifers. — Sponges were more abundant and diversified than 

 in Cambrian time, and some were of large size. Mostly only those 

 Sponges which secreted skeletons were favorable for fossilization. 



Coelenterates. — Hydrozoans were abundantly represented by 

 the Graptolites (Fig. 49), and in fact the Ordovician may be said to 

 have been the period of culmination of this remarkable, long 

 extinct group of animals. They are so abundant and varied in 



Fig. 48 



Ordovician Seaweeds, Callithamnopsisfruc- 



ticosa. (After Ruedemann). 



