150 INTRODUCTION TO EVOLUTION 



FIG. 8.4. Peripafus, an onychophoran. 



variously estimated to have been of 60 to 100 million years in duration. 

 Comparison of earlier Cambrian fossils with later Cambrian fossils reveals 

 the fact that much evolutionary change occurred during the course of this 

 long period. 



Ordovician Period 



The oceans continued to support varied invertebrate life. Space will per- 

 mit us merely to note changes which occurred in that fauna as time 

 went by. 



The first corals appeared among the coelenterates in the Ordovician pe- 

 riod. We noted above evidence that jellyfishes were present in the Cam- 

 brian period. Apparently, however, no coelenterates in that period de- 

 veloped the ability to secrete calcium carbonate, thereby forming what is 

 for the coral animal at once its skeleton, its apartment house, and its 

 memorial monument. Each of the pits or hollows on the surface of a piece 

 of coral represents the point of attachment of a tiny sea anemone-like coral 

 animal (Fig. 8.5). One generation builds upon the foundations laid down 

 by its predecessors. Hence coral rock, built up at times into great reefs in 

 the ocean, is the result of cooperative action of countless hordes of 

 coelenterates over great periods of time. Much limestone originated in this 

 way. 



Colonial animals called graptolites were a most characteristic feature of 

 the oceans during this period (Fig. 8.6). Small forms without means of lo- 

 comotion, they achieved world-wide distribution through the action of 

 ocean currents, in which they floated. They are now regarded as Hemi- 

 chordates, members of the subphylum to which Balanoglossus belongs 

 (Fig. 5.5, p. 101). Hence they were among the first known representatives 

 of Phylum Chordata. 



Phylum Echinodermata came into prominence at this time. This spiny- 

 skinned tribe was represented only by tiny cystoids in the Cambrian; in the 

 Ordovician crinoids and even a rare starfish appeared. Crinoids or sea 



