Geology Reference
In-Depth Information
shallow marine and reef-building organisms would dominate the earth for millions of
years.
The extensive shallow sea environments of the early Paleozoic provided a variety of
niches for organisms to adapt to. While some organisms had exoskeletons, none had yet
developed internal skeletons, so all the sea creatures were
invertebrates:
animals lacking
an internal skeleton.
Other than the trilobites, other marine invertebrates of the early Paleozoic included
reef-building creatures similar to those that build reefs in warm, shallow seas today.
Many of the invertebrate groups that appeared in the Cambrian have relatives alive
today. Sponges, corals, sea urchins and starfish, clams, barnacles, squid, and insects are
all related to the Cambrian invertebrates.
Some invertebrate fossils can be used as
guidefossils
or
index fossils:
fossils of
organisms that lived in a wide variety of places but only for a short time (geologic-
ally speaking). They're useful in determining the age of rock layers they are found
in.
For example, while trilobites were common throughout the 300 million years of the Pa-
leozoic, another organism called an
archaeocyathid
existed only during the 60 million
years of the Cambrian period. Archaeocyathids were reef-building organisms that lived
on the sea floor and went extinct at the end of the Cambrian. Finding a trilobite fossil in
a rock tells you that rock layer formed sometime during the Paleozoic. Finding an ar-
chaeocyathid fossil indicates that the rock layer containing these fossils was created
during the more specific Cambrian period within the Paleozoic era.
After the archaeocyathids went extinct, other animals took over the role of reef building,
including early forms of corals, sponges, and echinoderms (early ancestors to starfish
and sea urchins). But reef builders were not the only creatures inhabiting the shallow
seas. While it is impossible within the scope of this topic to describe all the diverse Pa-
leozoic invertebrates, I describe a few of the most fascinating and important ones next.
Eurypterids
In the Silurian period, an arthropod called a
eurypterid
appeared (see Figure 19-2). Some
eurypterids looked like large scorpions with frighteningly large pincers, and others were
just frighteningly large. Eurypterids achieved something that most marine invertebrates
of the Paleozoic did not: They successfully moved into and adapted to freshwater habit-
ats. Fossils of eurypterids from the late Paleozoic are found in diverse habitats (freshwa-
