ANCIENT ANIMALS 



425 



and sharks is due to loss; bone was not acquired but merely retained by 

 the ancestors of the bony fishes and terrestrial vertebrates. 



We have just mentioned the eurypterids (Figs. 25.3 and 27.6) as enemies of the 

 ostracoderms. Eurypterids were aquatic arachnids descended from trilobitelike 

 ancestors, and included some of the largest, most active, and successful animals 

 of the early Paleozoic. Their elongated bodies consisted of a small cephalothorax 

 bearing compound eyes and a series of jointed appendages, and a segmented 

 abdomen which was broad anteriorly and narrow posteriorly like that of a scor- 

 pion. As in all arachnids, the first pair of appendages was pincerlike, whereas in 

 the Crustacea, which they somewhat resembled, the first appendage is always 

 feelerlike. 



Most of the eurypterids were less than a foot in length, but some of the later 

 types were large; Pterygotus, of the Devonian, reached a length of 9 feet and was 



Fig. 27.6. Two types of Silurian eurypterids. Left, Eurypterus; rij^lit , Eusarcus. Compare 

 Fig. 25.3. (Courtesy Rochester Museum.) 



probably the largest arthropod that ever lived. The habits of eurypterids were 

 probably much like those of modern hunting crabs, and most of them had swim- 

 ming paddles as well as walking legs. The Ordovician eurypterids were marine, 

 but most of the later ones were inhabitants of fresh waters. They reached their 

 maximum development in the Silurian and Devonian periods and became extinct 

 in the Permian ; but they left descendants in the form of the terrestrial scorpions, 

 spiders, and other arachnids. Primitive scorpions found associated with euryp- 

 terids in Silurian strata were long accepted as the first air-breathing animals but 

 are now known to have been aquatic; land-dwelling scorpions are first certainly 

 known from the Carboniferous. 



The Devonian fishes. Corals, sea lilies and cephalopods were increas- 

 ingly prominent groups in the Devonian seas, and brachiopods continued 

 abundant, though snails and bivalves were few and the trilobites were on 

 the decline. The dominance of invertebrates, however, was past; hordes 

 of fishes lived in the seas and fresh waters of this period and left such 

 abundant fossil remains that the Devonian is often called the Age of 



