Biology (Holt)

Arthropods

By 100 million years after their first union with fungi, plants had cov-

ered the surface of the Earth, forming large forests. These land plants

provided a food source for land-dwelling animals. The first animals to

successfully invade land from the sea were arthropods. An

is a kind of animal with a hard outer skeleton, a segmented body, and

paired, jointed limbs. Examples of arthropods include lobsters, crabs,

insects, and spiders, like the one in Figure 13.Biologists think a type

of scorpion was the first arthropod to live on land.

A unique kind of terrestrial arthropod—the insect—evolved from

the first land dwellers. Insects have since become the most plenti-

ful and diverse group of animals in Earth’s history. The success of

the insects is probably connected to their ability to fly. Insects were

the first animals to have wings. Flying allowed insects, like the

dragonfly shown in Figure 14,to efficiently search for food, mates,

and nesting sites. It also led to partnerships between insects and

flowering plants. The oldest known fossils of flowering plants are

from about 127 million years ago, but flowering plants may be

much older than that.

arthropod

266 CHAPTER 12History of Life on Earth

Figure 13 An arthropod.
This marbled spider is a
member of the phylum
Arthropoda, which includes
about 1 billion billion (10^18 )
individuals in about 1.5 million
described species.

CARBONIFEROUS PERIOD PERMIAN PERIOD TRIASSIC PERIOD

 

Age(in Millions of Years Ago)

Third mass

extinction

Fourth mass

extinction

The first dinosaurs and mammals

300 ••••• 280 260 240 220 200 •

Figure 14 Swamp
320 million years ago.
Forested swamps were
dominated by tall, seedless
canopy trees and shorter
tree ferns. Dragonflies had
wingspans of more than
1 m (about 3.25 ft).