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the growth of larger animals. These, in turn, required stronger structural sup-
ports. Skeletons also evolved as a response to an incoming wave of fierce
predators. Paradoxically, most of these predators were soft-bodied and there-
fore not well preserved as fossils.
Soft-bodied organisms living before the arrival of shelled animals at the
beginning of the Cambrian had a great difficulty entering the fossil record.
Animals with soft body parts decayed rapidly upon death. Therefore, only
traces of their existence remain, such as imprints, tracks, and borings. These
include fossil impressions of soft-bodied animals found in the Ediacara Hills
of southern Australia that date from about 670 million years ago.
The Ediacaran fauna spawned from adaptations to highly unstable con-
ditions during the late Precambrian. Overspecialization to a narrow range of
environmental conditions caused a major extinction of Ediacaran species
around 540 million years ago at the very beginning of the Cambrian explo-
sion. Marine animals that survived the die out differed markedly from their
Ediacaran ancestors and participated in the greatest diversification of new
species the world has ever known.
At the dawn of the Cambrian, 125 million years following the appear-
ance of the Ediacaran fauna, most of which were evolutionary dead ends, the
seascape abruptly changed with the sudden arrival of animals with hard skele-
tal parts. Most phyla of living organisms appeared almost simultaneously, many
ofwhich had their origins in the latter part of the Precambrian. Body styles
that evolved in the Cambrian largely served as blueprints for modern species,
withfew new radical body plans appearing since then.
When skeletons evolved, the number of organisms preserved in the fos-
sil record jumped dramatically. All known animal phyla that readily fossilized
appeared during the Cambrian, after which the number of new classes
decreased sharply. Fossils became abundant at the beginning of the period
because of the development of hard body parts that fossilize by replacement
with calcium carbonate or silica, rapid burrowing to prevent attack by scav-
engers and decay by oxidation, long periods of deposition with little erosion,
and large populations of species.
THE AGE OF TRILOBITES
The Cambrian is best known for a famous group of invertebrates called trilo-
bites (Fig. 46). Early in the Cambrian, a wave of extinctions decimated a huge
variety of newly evolved species. The mass extinction eliminated more than
80 percent of all marine animal genera and is numbered among the worst in
Earth history. It coincided with a drop in sea level that followed continental
collisions. The die-offs paved the way for the ascendancy of the trilobites,
69CAMBRIAN INVERTEBRATES