ANCIENT ANIMALS 421 



almost from end to end of the body. Each leg had an outer gill branch, an inner 

 walking and swimming branch, and a jawlike spiny base. Apparently a trilobite 

 could chew from stem to stern, crawling over its food and passing a continuous 

 stream of food particles forward along the row of jaws to the scooplike mouth. 

 The average trilobite was 2 or 3 inches long, though a few giants reached a length 

 of 2 feet. Most of them were scavengers that crawled over the sea floor, but a few 

 were swimming plankton feeders, and others became eyeless burrowers that fed 

 upon mud. The peak of trilobite dominance was reached in the late Cambrian, 

 but the group remained important until Devonian times and then dwindled to 

 final extinction during the Permian. 



We can scarcely comprehend the immensity of even 1 million years; 

 this Cambrian fauna lived some 450 million years ago, and the most 

 remarkable thing about it is that it was essentially modern in aspect. Its 

 species were fewer and on the whole much simpler than those of today, 

 but all belonged to phyla which still exist, and with the possible exception 

 of the chordates no phyla not present in the Cambrian have since ap- 

 peared. Post-Cambrian evolution has been largely an elaboration, a 

 branching out, and a replacement of old by new, within the main evolu- 

 tionary patterns laid down during or prior to the early Paleozoic. 



In tracing the later history of animals we cannot even attempt to 

 describe the changing faunas, period by period, nor to follow more than a 

 few of the many evolutionary lines. Instead we shall have to confine our 

 attention to selected groups that illustrate the general course of evolu- 

 tionary change, with particular reference to the chordate stocks which 

 include man's ancestors. 



The record of life during the two periods following the Cambrian is 

 still almost wholly that of the seas. The Ordovician fauna was more 

 diversified and abundant than the Cambrian, and additional groups 

 appeared in Silurian time. Trilobites and brachiopods continued to exist 

 in large numbers, and graptolites, colonial corals, bryozoans, crinoids, 

 starfishes, sea urchins, barnacles and eurypterids joined the older groups 

 of invertebrates. Sometime during the Silurian the first land plants 

 appeared, and some of the arthropods became land dwellers at about the 

 same period. The most significant event in the animal world was the 

 appearance of the first vertebrates in late Ordovician time. 



The early vertebrates. The oldest known members of the phylum 

 Chordata were small, fishlike creatures, with a heavy armor of bony 

 plates and scales that gives them their name ostracoderms (meaning 

 "hard-shelled"). Fragments of their armor have been found in late 

 Ordovician rocks, and the group became common in the Silurian period. 

 They are true vertebrates, though very primitive ones, and must have 

 had a long line of simpler chordate ancestors of which no trace has yet 

 been found. Of the many theories about the origin of the chordates, that 



