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their sides, signifying they had somehow escaped the terrifying grip of Anom-
alocaris.Several trilobite species evolved long spines that might have served as
protection against Anomalocaris attacks.
Most Burgess Shale species abruptly went extinct at the end of the Cam-
brian.Although many mass extinctions and proliferations of marine organisms
have occurred since, no fundamentally new body styles have appeared during the
past 500 million years. After the late Cambrian extinction, only a few archaic
forms survived to the middle Devonian. Had they prevailed and produced
descendants, Earth would now be graced by an entirely different set of life-forms.GONDWANA
Near the end of the Precambrian, all landmasses assembled into the superconti-
nent Rodinia.The continental collision resulted in environmental changes that
had a profound influence on the evolution of life. No broad oceans or extreme
differences in temperature existed to prevent species from migrating to various
parts of the world. Between 630 and 560 million years ago, the supercontinent
rifted apart, and four or five continents rapidly drifted away from each other.
Most continents huddled near the equator, which explains the presence of
warm Cambrian seas.The continental breakup caused sea levels to rise and flood
large portions of the land at the beginning of the Cambrian. The extended
shoreline might have spurred the explosion of new species, with twice as many
phyla living during the Cambrian than before or since.
Many more experimental organisms were in existence than at any other
time in Earth history, none of which have any modern counterparts. OneFigure 57Anomalocaris
was a fierce predator of
trilobites.
82
Historical Geology