PAST DISTRIBUTION OF ANIMALS 



541 



ozoology of the steps in the evolution of animal life on the earth until the 

 process was far advanced. 



586. Animals of the Past. — Of the Protozoa, only fossilized foramini- 

 ferans and radiolarians are found, the latter in rocks of the Proterozoic 

 era and onward, the former in the Cambrian period and onward. Sponges 

 are also known from the Proterozoic. Hydrozoans have been abundant 

 since the Cambrian, when they were represented by a type known 

 as graptolites. Scyphozoans, represented by impressions and molds, 

 have existed since the Cambrian, as have also corals, the skeletons 

 of which are abundant and of great variety in the rocks of all periods since 

 that time. Brachiopods appeared early in the Cambrian, and bryozoans 

 are abundant from the Ordovician onward. There are over 6000 fossil 

 species of brachiopods, but only 160 are 

 known to be living now. 



Starfishes and holothurians date from 

 the Cambrian, and brittle stars and sea 

 urchins from the Ordovician. Crinoid 

 remains represented by stalk sections 

 (Sec. 240) have been found in the Cam- 

 brian, and fragments of crinoids occur in 

 beds of crinoidal limestone from the 

 Ordovician to the Jurassic. Blastoids 

 and cystoids existed throughout the 

 Paleozoic era; they resembled crinoids 

 in many ways but are more primitive. 



Of the mollusks, chitons and scaph- 

 opods have existed since the Ordovician; 

 and pelecypods, gastropods, and cepha- 

 lopods since the Cambrian. Their fossil 

 shells are found in abundance and are 

 widely distributed. The nautiloids, rep- 

 resented today only by the chambered 

 nautilus, were abundant in the Silurian; 

 2500 species have been described. Twice as many species of ammonoids 

 (Fig. 372) are known; these were most abundant in the Mesozoic and are 

 now extinct. The earliest forms resembling squids and cuttlefishes 

 appeared in the Triassic. 



Chaetopods are the only fossil annehds. They are found from the 

 Cambrian onward, though worm tracks have been found in rocks of the 

 Proterozoic era. 



The earliest arthropods were branchiate and marine. Trilobites 

 (Fig. 373) had their origin before the Cambrian, being abundant in the 

 oldest fossil-bearing strata. They were a dominant type in the Cambrian 

 period and disappeared before the end of the Paleozoic. Other crus- 



FiG. 372. — An ammonoid from the 

 lower Jurassic period, resembling in 

 form a chambered nautilus (Fig. 132), 

 but showing a portion of the shell 

 removed to bring into view the com- 

 plexly folded septa that distinguished 

 these forms. 



