the Cambrian explosion, when almost every major group of animals
abruptly appeared in the fossil record. About 1 billion years ago, echino-
derms and chordates went their separate ways. This suggests that a slow-
paced course of evolution was well under way half a billion years prior to
the Cambrian explosion.
The brachiopods (Fig. 50), also called lampshells because of their resem-
blance to ancient oil lamps, were once the most abundant and diverse organ-
isms. More than 30,000 species are cataloged in the fossil record. The prolific
brachiopods ranged from the Cambrian to the present but were most abun-
dant in the Paleozoic and to a lesser extent in the Mesozoic. The appearance
of brachiopod fossils in the strata indicates that seas of moderate to shallow
depth once covered the area.The brachiopods had two saucerlike shells called
valves that fitted face to face and opened and closed using simple muscles.
Brachiopods filtered food particles through opened shells that closed to
protect the animals against predators. More advanced species, including bra-
chiopods called articulates, had ribbed shells with interlocking teeth that
opened and closed along a hinge. Many brachiopods are excellent index fos-
sils for correlating rock formations throughout the world.They are important
as guide fossils and are used to date many Paleozoic rocks.
Figure 49Crinoids were
a dominate species in the
middle and late Paleozoic
and are still in existence
today.CAMBRIAN INVERTEBRATES