PRE-CAMBRIAN AND PALEOZOIC ERAS 153 



Ostracods, tiny crustaceans enclosed in bivalved shells, first appeared in 

 this period. 



The most portentous occurrence in the Ordovician was the appearance 

 of the first vertebrate members of Phylum Chordata. Bony scales have 

 been found in deposits of this period, 

 indicating the presence of armored 

 vertebrates. We have no further knowl- 

 edge concerning the possessors of these 

 scales but they may have been ancestral 

 to ostracoderms, jawless fishes, whose 

 remains are found in Silurian deposits. 



Silurian Period 



During the Silurian period some of 

 the invertebrate groups previously pres- 

 ent expanded greatly in numbers of 

 kinds and of individuals while other 

 groups declined. Of the groups which 

 expanded we may mention the corals, 

 the brachiopods, and the crinoids, all of 

 which were extremely abundant. Grap- 

 tolites, on the other hand, had declined 

 from their abundance in Ordovician 

 seas, and trilobites also were beginning 

 to decline. Some of the Silurian tri- 

 lobites developed quite bizarre shapes 

 and spines (Fig. 8.10). This type of 

 specialization is frequently ascribed to 

 racial "senescence" or "old age." Ap- 

 parently it forms one indication that a 

 group has become highly specialized for 

 a particular mode of life and has corre- 

 spondingly lost that plasticity which 



would enable it to adapt to other modes of life should conditions change. 

 The spines in this instance may have served for protection from predators, 

 if there were any present capable of preying on trilobites. 



The most characteristic invertebrates of Silurian times were the euryp- 

 terids or "sea scorpions" (Fig. 8.1 1 ) . They were a group of arthropods that 

 had made a small beginning in the Cambrian but did not constitute an im- 



FIG. 8.7. A simple crinoid. 



