the end of the period, the first reptiles emerged and displaced the amphibians
as the dominant land vertebrates.
THE AMPHIBIAN ERA
Plants had been greening Earth for as long as 100 million years before the
vertebrates finally set foot onto dry land. Prior to the amphibian invasion,
freshwater invertebrates and fish had been inhabiting lakes and streams. By
the early Carboniferous, the vertebrates had spent more than 160 million
years underwater, with only a few short forays onto the land. Relatives of the
lungfish lived in freshwater pools that dried out during seasonal droughts,
requiring the fish to breathe with primitive lungs as they crawled to the
safety of the nearest water hole.
A common misconception is that land animals evolved from fish and
developed armlike limbs after flopping onto shore. A distant ancestor of the
catfish living 370 million years ago had complex fingerlike bones in the
underside of its fin and spent its life entirely in water.The fish grew to 8 feet
long and weighed up to 200 pounds. It dated from within a few million years
of the beginning of the tetrapods, the first four-legged animals with vertebrae
that were the forebears of the amphibians and reptiles.
The fish had complex eight-fingered, handlike limbs, suggesting that
some fish developed legs first before venturing out onto land.The fingered fin
shows that limb bones could have evolved in fish for use in water long before
being useful on land. The fish probably used its walking fins to help it move
around and hunt in the shallow, weedy streams, giving it a distinct advantage
over other fish. Also, at this time, shallow swamps began to appear. Limbs with
digits would have helped the first aquatic tetrapods travel through the plant-
choked wetlands. Eventually, the fins evolved into paws that its progeny could
use on land.
Amphibious fish (Fig. 108) probably spent little time on shore because
their primitive legs could not support their body weight for long periods,
requiring them to return to the water. Eventually, as their limbs strengthened,
the amphibious fish wandered farther inland, where crustaceans and insects
were abundant. By the middle Devonian, they began to dominate the land and
were especially attracted to the great Carboniferous swamps.
By about 335 million years ago, the amphibious fish evolved into the
earliest amphibians. The tetrapods branched into two groups, with one line
leading to amphibians and the other to reptiles, dinosaurs, birds, and mammals.
Some species had strong, toothy jaws and resembled giant salamanders, reach-
ing 3 to 5 feet in length. A 2-foot-long amphibian with armadillo like plates
rooted in the soil for worms and snails. During the early Carboniferous, the
CARBONIFEROUS AMPHIBIANS