Geology Reference
In-Depth Information
The beginning of the Paleozoic is marked by the sudden appearance of a wide variety of
animal forms. In fact, the fossils from this period exhibit all the animal body plans that
exist even today, 540 million years later. (A
body plan
is how an organism's body parts
and growth patterns are organized.) This sudden appearance of complex life in the geo-
logic record is called the
Cambrian Explosion,
and it's the focus of this section.
The Cambrian Explosion has long been defined by the abundance of creatures
preserved in the fossil record at the beginning of the Cambrian period. But it is
possible, even likely, that this sudden appearance of life is a result of the incom-
plete nature of a history told in rocks. A mid-century geologic find in Australia un-
derscores this possibility. In 1946, a series of fossils were discovered in the Edi-
acara Hills of Australia. At the time, the fossil imprints of soft-bodied organisms
were thought to be the remains of extinct jellyfish or marine plants. Since then,
more than 30 different animal types have been identified and described from the
fossils, composing the
Ediacaranfauna.
Many of them have odd body shapes that
are not seen in later Paleozoic (or modern) animals. Scientists are still puzzled as
to whether the Ediacaran fauna represent ancestors to modern animals like jelly-
fish and sea pens, or whether they were a brief, failed evolutionary experiment
with no modern relatives.
Toughen up! Developing shells
In the Cambrian, creatures living in the sea began to grow shells or
exoskeletons.
This
gave them a tremendous advantage and, as you can see today, became a way of life that
has lasted for millions of years. External hard parts provide the following benefits:
Protection from the sun:
During the Paleozoic, immense shallow seas were the
primary habitat for life on earth. Soft-bodied creatures were exposed to the sun's
harmful rays — the same rays you and I avoid with sunscreen and hats. Building
an exoskeleton protects the soft tissues and internal organs of a creature from be-
ing damaged by the sun.
Moisture retention:
Large, shallow water environments sometimes experience an
occasional absence of water — similar to the beach at low tide. Animals that be-
come stranded when the tide goes out will dry out and die unless they have a
shell that retains enough moisture to help them survive until the tide comes back
in.
