Historical Geology Understanding Our Planet\'s Past

Evaporite deposits generally form under arid conditions between 30
degrees north and south of the equator. However, extensive evaporite deposits
are not currently being formed,suggesting a comparatively cooler global cli-
mate. The existence of ancient evaporite deposits as far north as the Arctic
regions implies that either these areas were once closer to the equator or the
global climate was considerably warmer in the geologic past.
The Devonian year was 400 days long and the lunar cycle was about
30.5 days as determined by the daily growth rings of fossil corals. Paleomag-
netic studies indicate that the equator passed from California to Labrador and
from Scotland to the Black Sea during the Devonian and Carboniferous (Fig.
107). The ideal climate setting helped spur the rise of the amphibians that
inhabited the great Carboniferous swamps.
After covering the marine and terrestrial life-forms of the Devonian, the
next chapter follows the evolution of the first amphibians of the Carbonifer-
ous period.

Figure 107The

approximate positions of

the continents relative to

the equator during the

Devonian and

Carboniferous periods.

DEVONIAN FISH

NORTH

AMERICA

SOUTH

AMERICA

AFRICA

EURASIA

Equ

ator