water and half the time on dry land. It was dog sized, with a broad, flat, fish-
like head and a tail topped with a small fin, apparently used for swimming. It
developed a sturdy rib cage to hold up its internal organs while on land and
crawled around on primitive legs with seven toes on the hind limbs. Amphib-
ians also possessed six and eight digits on their feet, indicating the evolution
of early land vertebrates followed a flexible pattern of development. However,
no terrestrial vertebrates evolved a foot with more than five true digits.
Neither acanthostega nor ichthyostega could do much more than wad-
dle around on land.Their upper arm bones had a broad, blobby shape ill suited
for walking.Their hind limbs splayed out to the side and could not have eas-
ily held up the body. The backbones were looser than those of terrestrial
tetrapods and were similar to those of fish, which offered less support on land.
A small amphibianlike animal called microsaur was less than 6 inches
long, a mere midget compared with its aquarian counterparts. It was among
the first four-legged vertebrates to crawl onto the land more than 300 million
years ago.The animal had uniquely shaped spine bones and a simplified skull,
with just one bone instead of three. The skull was attached to the first verte-
bra of the spine with a pair of rodlike bones, which limited head rotation only
to up-and-down movements. This feature is also found in the large, more
primitive amphibians that predated microsaur.
The weak legs of the early amphibians could hardly keep their squat
bodies off the ground, making them slow and ungainly.The amphibian tracks
are generally broad with a short stride.The animal walked with a clumsy gait.
Therefor, running to attack prey or escape predators was simply not possible.
In order to succeed as hunters without requiring speed or agility, the amphib-
ians developed a unique whiplike tongue that lashed out at insects and flicked
them into the mouth.This successful adaption enabled the amphibians to pop-
ulate the land rapidly.
Although the amphibians had well-developed legs for walking on dry
land,the animals apparently spent most of their time in rivers and swamps.
They depended on accessible sources of water to moisten their skins as well
as for respiration and reproduction. They reproduced like fish, laying small,
shell-less eggs. After hatching, the juveniles lived an aquatic, fishlike existence,
breathing with gills. As they matured, the young amphibians metamorphosed
into air-breathing, four-limbed adults.
The early amphibians living during the late Devonian, when vertebrates
were first making a transition from sea to land, spent most of their time in the
water. The necessity of having to live a semiaquatic lifestyle, however, led to
the eventual downfall of the amphibians when the great swamps began to dry
out toward the end of the Paleozoic.The void left by the amphibians was
quickly filled by their cousins the reptiles, which were better suited for a life
totally out of water.
CARBONIFEROUS AMPHIBIANS