Geology Reference
In-Depth Information
spend the first part of their lives (as eggs and larvae) in the water. By the Devonian peri-
od, insects and plants had colonized the land, providing food resources for amphibians
when they ventured out of the water.
The discovery of a fossil called
Tiktaalik roseae
provided scientists with a
missing link between water-dwelling and land-dwelling animals.
Tiktaalikroseae
is
informally called a “fishapod” because it had characteristics of both lobe-finned
fish and four-legged animals (called
tetrapods
).
Scientists think that early amphibians developed limbs to help them move around the
swampy, shallow water environments of the middle Paleozoic. Amphibians became most
diverse and abundant in the late Paleozoic as they spent more time outside the water. It
appears that they gave rise to the next major animal group in earth history: the reptiles.
Adapting to life on land: The reptiles
Reptiles didn't begin to dominate the earth until the Mesozoic (see Chapter 20), but they
evolved and established themselves — conquering the land for vertebrates — in the late
Paleozoic.
In order to live on land, animals had to develop certain characteristics that al-
lowed them to be away from water. Amphibians lived the first stage of their lives
in the water and had to return to water to lay their eggs. With the appearance of
an amniote egg, reptiles no longer needed water the way amphibians did. An
amni-
ote egg
is an egg with a yolk sac inside it that provides nutrients to the developing
embryo so that when it hatches from the egg, the animal is well past the larval
stage and doesn't need to live in water.
Early reptiles included the
pelycosaurs,
such as the Dimetrodon fossil pictured in this
topic's color photo section. These animals had large ins along their backs. Paleontolo-
gists think the fins may have helped them control their body temperature. Reptiles are
cold blooded,
which means they have no internal way of warming themselves up (unlike
warm-blooded animals, who regulate their own body temperature). But the pelycosaurs
and their later relatives, the
therapsids,
may have begun to develop methods of body
heat regulation that were precursors to the way mammals control their body temperat-
