. Animal biology. Zoology; Biology. 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
. Animal biology. Zoology; Biology. 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 ha
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