Historical Geology Understanding Our Planet\'s Past

conditions, reef building intensified, forming thick deposits of limestone that
were laid down by prolific, lime-secreting organisms.

MASS EXTINCTION

Throughout Earth history, massive numbers of species have vanished in sev-
eral short periods (Fig. 127). During geologically brief intervals of perhaps a
few million years, mass extinctions in the ocean have eliminated half or more
ofthe existing families of plants and animals. Devastation of this magnitude
resulted from radical global changes in the environment. Drastic changes in
environmental limiting factors, including temperature and living space on the
ocean floor, determined the distribution and abundance of species in the sea.
Many episodes of extinction coincided with periods of glaciation, and
global cooling had a major effect on life. The living space of warmth-loving

Figure 127The number

of families through time.

The large drop at the end

of the Permian indicates a

major mass extinction.

PERMIAN REPTILES

Quaternary

Oligocene

Paleocene

Cretaceous

Carboniferous

Devonian

Ordovician

Cambrian

Quaternary

2

65

230

570

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

0 50 100 150

Number of families

Million years before present

200 250 300

Cambrian