THE ORDOVICIAN PERIOD 



429 



addition to the changes in the main line of descent many aberrant forms came 

 into existence, but were not long-lived. 



Another important change in the race was brought about when the graptolites 

 became detached from seaweeds and led an independent floating existence, being 



^n^ 





■ 



llllllllllllllllllll 



D/c^oorap *i/$ Tefraarapfus 0/c/y/r?oorcu?tu$ Az.yooorapf'uS 



Fig. 398. — Diagram showing the evolution of one branch of the Graptolitoidea. At 

 first attached to the sea floor, they later became attached to floating seaweeds and 

 finally acquired floats. This change in their mode of life induced important changes 

 in structure, one of which resulted in a reduction in the number of branches. 



buoyed up by "floats" (Figs. 397 F and 398 C, D, E) to which they were attached 

 by threads. 



Towards the end of the race numerous spines appeared on some species, a network 

 of protecting fibers was developed on others, and the colonies became small. They 

 were on the defensive and soon disappeared. The forms which survived the longest 

 were those inconspicuous ones which had remained attached to the sea bottom from 

 the beginning of the race. 



Because of the many progressive changes which the race under- 

 went, and also because the colonies were carried about by currents 

 over the seas of the world, 

 graptolites are excellent fossils 

 for correlating (determining 

 identity of age) widely sepa- 

 rated beds. The simple forms 

 are especially characteristic of 

 the Upper Cambrian and Lower 

 Ordovician. The group in the 

 main became extinct in the 

 Silurian, but a few species lived 

 even into the Carboniferous. 1 



Stromatopora. — An extinct 

 order of organisms known as 

 stromatoporoids were especially 

 abundant as reef builders in the Ordovician, Silurian, and Devonian. 

 They were allied to the corals and consisted of colonies of minute 

 polyps which secreted concentric layers of thin calcareous plates 



^uedemann, R., — Graptolites of New York, Part 2: N.Y. State Museum, Mem. n, 1908. 



A 



Fig. 399. — Ordovician corals : A, Colum- 

 naria halli; B, Streptelasma profundum. 

 (A portion has been removed to show the 

 interior.) 



