630 



ORGANIC EVOLUTION 



Fig. 25-7. This is a reconstruction of the ocean floor during the Cretaceous Period in the region of Tennessee (U. S.), 

 showing the abundance of invertebrate life. Although none of these species is alive today, it is not difficult to 

 recognize them as mollusks. The two squid-like forms in the upper right certainly are very similar to our present- 

 day forms. The large coiled ammonite in the lower right is very much like the chambered nautilus of today. The 

 numerous cephalopods with shells resembling cones are all extinct in any form. Our present-day clams and snails 

 seem to be well represented on the floor of this ancient ocean. 



the Chordata, during the ensumg periods 

 and note some of its ramifications in estab- 

 Hshing subgroups. In this brief survey we 

 should gain some understandins; of how 

 evokition has gone on in the past, which 

 will prepare us for the next chapter where 

 we shall consider how it is going on today. 



The story of vertebrates 



The vertebrates, both past and present, 

 constitute the most important animals in 

 the phylum Chordata. Since the prochor- 

 dates have left no fossil remains, we are 

 forced to ignore them and study the history 

 of the vertebrates alone. 



The Chordates undoubtedly had their 

 beginnings in the late Cambrian because 



they are well established as vertebrates in 

 the Ordovician, well on their way toward 

 becoming the most important of all groups 

 of animals (Fig. 25-8). There are eight 

 classes represented, of which four are 

 swimmers and four possess legs adapted to 

 movement over solid surfaces. Of the 

 classes of swimmers, all but the placoderms 

 have living representatives today. The jaw- 

 less fishes ( cyclostomes ) had rather mod- 

 est beginnings and all but died out in the 

 late Devonian. In recent times they have 

 shown signs of increasing in numbers and 

 varieties as indicated by considerable num- 

 bers in the oceans. Some have also invaded 

 fresh water. The placoderms had the first 

 true jaws which were hinged to the skull. 



