the late Devonian more than 370 million years ago, when amphibians were
just beginning to craw out of the sea onto the land.The tree produced spores
and had thick permanent deciduous branches as well as short-lived branches
that dropped off after a couple of years.
The thick branches would have shaded and cooled the streams where
amphibians were rapidly evolving. As plants spread across the globe, the bur-
geoning vegetation drew carbon dioxide out of the atmosphere thereby cooling
the planet while simultaneously adding oxygen to the air and setting the stage for
land vertebrates. At the beginning of the Devonian, land animals would have
been unable to breathe due to the low atmospheric oxygen content. However,
by the end of the period, they would not have had any difficulty breathing.
Freshwater invertebrates and fish inhabited lakes and streams. Freshwater
fish living in Australia around 370 million years ago were almost identical to
those living in China.This suggests the two landmasses were close enough for
the fish to travel between them. Fish had remained the only vertebrates until
around 360 million years ago, with the evolution of the lobe-finned fish such
as the crossopterygians (Fig. 101).The lobe-finned fish had thick, rounded fins
whose bones were crude forerunners of those in tetrapod (four-legged) limbs.
Lobe-finned fish used gills for respiration.They could also breathe with prim-
itive lungs in oxygen-poor swamps or when stranded on dry land. Their
descendants became the first advanced animals to populate the continents.
The ancestors of the amphibians appear to have been the crossopterygians,
the stem group from which all terrestrial vertebrates descended. They grew
upward of 10 feet long and had powerful jaws containing large teeth. The
amphibians began to dominate the land by the middle Devonian and were espe-
cially attracted to the great swamps.When the climate chilled and glaciers spread
over the continents at the end of the period, the first reptiles emerged and began
to displace the amphibians as the dominant land vertebrates.
Figure 101Lobed-finned
fish evolved into the first
tetrapods.DEVONIAN FISH