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into specialized organs that could focus on a specific object such as prey. Only
a minority of the major animal groups have true eyes, however, with 6 out of
30 phyla having complex eyes capable of providing images. However, possess-
ing eyes is such an evolutionary advantage that species with complex eyes
comprise 95 percent of the animals on Earth.
Invertebrates, supported by external skeletons, were at a distinct disad-
vantage in terms of mobility and growth. Many animals such as crustaceans
shed their shells as they grew, which often made them vulnerable to predators.
One such predator was an extinct giant sea scorpion called eurypterid (Fig.
61), which ranged from the Ordovician to the Permian. It grew to 6 feet long
and terrorized creatures on the ocean floor with immense pincers.
The earliest vertebrates were probably wormlike creatures with a promi-
nent rod called a notochord down the back, a system of nerves along the
spine, and rows of muscles attached to the backbone arranged in a banded pat-
tern. Rigid structures made of bone or cuticle acted as levers. By using flexi-
ble joints, they efficiently translated muscle contractions into organized
movements such as rapid lateral flicks of the body to propel the animal
through the water. Later, a tail and fins evolved for stabilization. The body
became more streamlined and torpedo shaped for speed. With intense com-
petition among the stationary and slow-moving invertebrates, any advance-
ment in mobility was advantageous to the vertebrates.
The oldest known vertebrates were primitive, jawless fish called agnathans
(Fig. 62),which first appeared in the early Ordovician about 470 million years
ago. Remarkably well-preserved remains of these fish were discovered in the
mountains of Bolivia, much of which was inundated by the sea in the Ordovi-
Figure 61The extinct
eurypterids grew to 6 feet
in length.87ORDOVICIAN VERTEBRATES