THE EVIDENCE FOR EVOLUTION 793 



an inner walking or swimming branch. Most trilobites were only two 

 or three inches long but the largest was about two feet. They reached 

 their peak of importance in the late Cambrian and then dwindled and 

 became extinct in the Permian. 



Evolution since the Cambrian has been characterized by the elab- 

 oration and ramification of the lines already present, rather than by 

 the establishment of entirely new forms. The original, primitive mem- 

 bers of most lines were replaced by more complex, better adapted ones. 

 The Ordovician seas contained, among other forms, giant cephalopods, 

 squidlike animals with straight shells 15 to 20 feet long ancl a foot in 

 diameter. The Ordovician seas were apparently quite warm, for corals, 

 which grow only in warm waters, lived as far north as Ontario and 

 Greenland. The first vertebrates, the jawless, limbless, armored, bottom- 

 dwelling fishes called ostracoderms, appeared in the Ordovician. These 

 lived in fresh water and their bony armor may have served as a defense 

 against their chief predator, the carnivorous, giant arachnids called 

 eurypterids. Two important events of the Silurian were the evolution of 

 land plants and of the first air-breathing animals, primitive scorpions. 



The evolution of the vertebrates, from ostracoderms to placoderms, 

 cartilaginous and bony fishes, amphibians, reptiles, birds and mammals 

 has been traced in Chapters 22 to 24. The Devonian seas contained 

 corals, sea lilies and brachiopods in addition to a great variety of fishes. 

 Trilobites were still present but were declining in numbers and im- 

 portance. The first land vertebrates, the amphibians called labyrintho- 

 donts, appeared in the latter part of the Devonian; this period also 

 saw the first true forests of ferns, "seed ferns," club mosses and horsetails 

 and the first wingless insects and millipedes. 



The Mississippian and Pennsylvanian periods are frequently 

 grouped together as the Carboniferous, for during this time there flour- 

 ishecl the great swamp forests whose remains gave rise to the major coal 

 deposits of the world. The earliest stem reptiles appeared in the Penn- 

 sylvanian and from these there evolved in the succeeding Permian period 

 a group of early, mammal-like reptiles, the pelycosaurs, from which the 

 mammals eventually evolved (Fig. 35.2). 



The Permian period was characterized by widespread changes in 

 topography and climate. The land began to rise early in the period, so 

 that the swamps and shallow seas were drained, and the Appalachian 

 Revolution that ended the period, together with widespread glaciation, 

 killed off a great many kinds of animals. The trilobites finally dis- 

 appeared and the brachiopods, stalked echinoderms, cephalopods, and 

 many other kinds of invertebrates were reduced to small, unimportant, 

 relict groups. 



Alesozoic Era. The Mesozoic era, which began some 225,000,000 

 years ago and lasted about 150,000,000 years, is subdivided into the 

 Triassic, Jurassic and Cretaceous periods. During the Triassic and Ju- 

 rassic most of the continental area was above water, warm and fairly 

 dry. During the Cretaceous the Gulf of Mexico expanded into Texas 

 and New Mexico, and the sea once again overspread large parts of the 

 continents. There were great swamps from Colorado to British Columbia 



