T
his chapter examines life in the sea and the first vertebrates to come
onto the land during the Devonian period. The Devonian, from 400
to 345 million years ago, was named for the marine rocks of Devon
in southwest England. Rocks of Devonian age exist on all continents and
show widespread marine and terrestrial conditions. The supercontinents
Laurasia and Gondwana began approaching each other, pinching off the
Tethys Sea between them. The wide distribution of deserts, evaporite
deposits, coral reefs, and coal deposits as far north as the Canadian Arctic indi-
cate a warm climate over large parts of the world.
The warm Devonian seas spurred the evolutionary development of
marine species (Fig. 91), including the first appearance of the ammonoids.
These were coiled-shelled cephalopods that became fantastically successful
in the succeeding Mesozoic era. For 350 million years, species of these giant
mollusks roamed the ancient seas. The vertebrates, the highest form of
marine animal life, dominating all other creatures,left their homes in the sea
to establish a permanent residence on land, which by then was fully
forested.Toward the end of the Devonian, the climate cooled, possibly caus-
ing glaciation near the poles. The climate change caused the extinction of
DEVONIAN FISH
THE AGE OF MARINE ANIMALS