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THE CAMBRIAN EXPLOSION
The Cambrian was an evolutionary heyday for species, featuring a 10-million-
year rapid evolution of the first complex animals with exoskeletons.The Cam-
brian witnessed the first appearance of nearly all animal phyla in the fossil
record. After 3 billion years of unicellular life, a short term of intense creativity
was followed by more than 500 million years of variations on anatomic themes
established at the beginning of the Cambrian. So many new and varied life-
forms came into existence that the age is depicted as the Cambrian explosion.
The Cambrian explosion was the most remarkable and puzzling event
in the history of life.The early Cambrian witnessed the disappearance of soft-
bodied Ediacaran faunas and the rapid proliferation of shelly faunas (Fig. 45).
The biological proliferation peaked about 530 million years ago, filling the
ocean with a rich assortment of life. Seemingly out of nowhere and in bewil-
dering abundance, animals appeared in an astonishingly short time span
cloaked in a baffling variety of exoskeletons.
Figure 44 A syncline in
weathered shale of the
lower Cambrian Rome
Formation, Johnson
County,Tennessee.
(Photo by W. B. Hamilton,
courtesy USGS)67CAMBRIAN INVERTEBRATES