cian. Originally, fossil evidence was scant and fragmentary. Little was known of
their appearance or about their evolutionary history. Earlier descriptions dis-
missed agnathans as a headless, tailless mass of scales and plates or confused the
head with the tail, giving it the dubious title of “backward fossil.”
The widespread distribution of primitive fish fossils throughout the
world suggests a long vertebrate record prior to the Ordovician.The first fish
were small mud grubbers and sea squirts that lacked jaws and teeth.The jaw-
less fish whose modern counterparts include lampreys and hagfish had a flex-
ible rod similar to cartilage instead of a bony spine typical of most vertebrates.
These ancient fish were probably poor swimmers and avoided deep water.
They were generally small, about the size of a minnow, and heavily armored
with bony plates protecting the rounded head.The rest of the body was cov-
ered with thin scales that ended near a narrow tail. Although well protected
from invertebrate predators,the added weight forced the fish to live mostly on
the bottom, where they sifted mud for food particles and expelled the waste
products through slits on both sides of the throat that later became gills.
Gradually,the protofish acquired jawsand teeth, the bony plates gave
way to scales, lateral fins developed on both sides of the lower body for sta-
bility, and air bladders maintained buoyancy. Some primitive fish were sur-
prisingly large, up to 18 inches long and 6 inches wide.Thus, for the first time,
vertebrates skillfully propelled themselves through the sea. Fish soon became
masters of the deep.FAUNA AND FLORA
Corals are marine coelenterates attached to the ocean floor (Fig. 63). They
began constructing extensive carbonate reefs in the Ordovician, buildingFigure 62The jawless
fish agnathans were the
progenitors of fish today.
Historical Geology