Biology (Holt)

Vertebrates

A is an animal with a backbone—vertebrates are the ani-
mals most familiar to us. Humans are vertebrates, and almost all other
land animals bigger than our fist are vertebrates as well.

Fishes

According to the fossil record, the first verte-
brates were small, jawless fishes that evolved
in the oceans about 530 million years ago.
Jawed fishes first appeared about 430 million
years ago. Jaws enabled fishes to bite and
chew their food instead of sucking up their
food. As a result, jawed fishes were efficient
predators. A fossilized example of a jawed
fish is shown in Figure 15.Fishes soon came
to be among the most abundant animals in the seas, and for hun-
dreds of millions of years the sea is where vertebrates stayed. Fishes
are the most successful living vertebrates—they make up more than
half of all modern vertebrate species. After nearly 200 million years
of living in the sea, fishes have become uniquely adapted for success
in water. Major changes had to occur in fish body organization,
however, before some descendants of fishes became capable of liv-
ing on land.

Amphibians

The first vertebrates to inhabit the land did not come out of the sea
until 370 million years ago. Those first land vertebrates were early
amphibians. Amphibians are smooth-skinned, four-legged animals
that today include frogs, toads, and salamanders.
Several structural changes in the bodies of amphibians occurred
as they adapted to life on land. Amphibians had moist breathing
sacs—lungs—which allowed the animals to absorb oxygen from air.
The limbs of amphibians are thought to have derived from the bones
of fish fins. The evolution of a strong support system of bones in the
region just behind the head made walking possible. This system of
bones provided a rigid base for the limbs to work against. Because
of their strong, flexible internal skeleton, the bodies of vertebrates
can be much larger than those of insects. While amphibians were
well adapted to their environment, a new group of animals more
suited to a drier environment evolved from them.

vertebrate

SECTION 3Life Invaded the Land 267

Appearance of flowering plants

CRETACEOUS PERIOD

200 ••••• 180 160 140 120 100 •

JURASSIC PERIOD CRETACEOUS PERIOD

Figure 15 Fossilized fish

skeleton.This fish skeleton

clearly shows the backbone,

the structure that is character-

istic of all vertebrate animals.