modern animals.The proliferation of these new organisms, representing nearly
every major group of marine species, set the stage for the evolution of the
progenitors of all animal life on Earth today.
The Ediacara Formation in South Australia’s Ediacara Hills contains
fossil impressions of highly curious creatures.The radiation of Ediacaran fau-
nas was abrupt, caused by unprecedented environmental changes. Some Edi-
acaran fauna were giants in their day, reaching 3 feet in size and ranging in
shape from spoked wheels and miniature anchors to corrugated and lettuce-
like fronds. The Ediacaran behemoths provide the first evidence of complex
life on the planet, which was previously populated by microscopic, single-
celled organisms.
The complex life-forms abruptly came into existence around 600 mil-
lion years ago not long after the last major episode of late Precambrian glacia-
tion, possibly the most severe in Earth history. The emergence of the
Ediacaran fauna was closely linked to profound changes in Earth’s physical
environment.This included the breakup of continents and the increased oxy-
gen that made possible the evolution of large animals.
Some 30 species of these simple, beautiful creatures have been identified
from fossil impressions in rock from 20 sites scattered around the world. Many
impressions were left by animals apparently related to modern jellyfish, corals,
and segmented worms. Others appear to represent arthropods, annelids, and
possibly even echinoderms. The bodies of some forms had threefold symme-
try unlike any seen in modern organisms.
Figure 35The
archaeocyathans built the
earliest limestone reefs.PROTEROZOIC METAZOANS