332 



PRINCIPLES OF ANIMAL BIOLOGY 



the long period of erosion between Proterozoic and Paleozoic destroyed 

 whatever fossils there were. 



Trilobites were even more common in the Ordovician, and brachiopods 

 continued abundant but mostly with shells of lime instead of horn. 

 With them in this period were graptolites, snails (Fig. 277), and others. 

 The first vertebrate animals, the armored ostracoderm fishes, are found 

 in Ordovician but must have existed long before. The following Silurian 

 preserved few fossil fishes, but they must have been present, for that 



group blossomed out extensivel}^ in the 

 Devonian; these two periods are known 

 as the age of fishes (see time scale). 

 Among the invertebrates of these periods 

 were the brachiopods (now at their peak) , 

 trilobites (now on the decline), corals, 

 snails, siliceous sponges, cup corals (Fig. 

 278), and the scorpionlike eurypterids. 



In Mississippian time the crinoids 

 (stalked echinoderms) reached their cli- 

 max (some of the best-preserved ones in 

 Iowa), and declined greatly in the next 

 period. Clams are preserved in Pennsyl- 

 vanian with their actual shells; before 

 this period the shells had dissolved away 

 and the fossils were only casts. The 

 latter period also had many insects, some 

 of them giants having a wing spread of 

 over two feet, also a number of amphibia 

 chiefly of the armored type. The succeed- 

 ing Permian had many of these armored 

 amphibia, but was chiefly distinguished 

 by its great variety of reptiles, some of 

 which had curious bony spines in a sail over the back (Fig. 279). 



The most characteristic invertebrate animals of the Triassic period 

 were the ammonites, the most highly developed group of cephalopods 

 whose evolution is described in a later section. These animals continued 

 through the rest of the Mesozoic era but declined in the Cretaceous. 

 Other invertebrates of the IMesozoic were crinoids, squids, and Crustacea 

 (particularly crabs). The great evolution of the Mesozoic, however, was 

 in the group of reptiles. On the land were the dinosaurs, in the sea the 

 ichthyosaurs (looking like porpoises or sharks) and the four-paddled 

 plesiosaurs, in the air the pterosaurs. Dinosaurs often had curious rows 

 of dor,sal plates, as in the Jurassic Stegosaurus (Fig. 280), or shields and 

 spines as in the Cretaceous Tricoratops (Fig. 281). Some of them were 



Fig. 278. — Fossil cup coral 

 found in Michigan. {From speci- 

 men in the Museum of Geoloyy, 

 University of Michigan.) 



