vendobionts, after the Vendian period, the final stage of the Precambrian,
evolved a unique solution to the problem of growing large bodies. They
simply employed networks of tubes, such as blood vessels, to transport nutri-
ents and oxygen to individual cells.The fossils show no openings for ingest-
ing food and eliminating wastes. They probably absorbed oxygen and
nutrients directly from seawater or harbored symbiotic algae that converted
sunlight into energy. They showed no obvious internal digestive or circula-
tory systems, which apparently did not fossilize. Nevertheless, some animals
did leave what appear to be fecal pellets as a testament to an advanced diges-
tive system.
The extremely flattened bodies of the Ediacaran faunas maximized the
ratio of surface area to volume, enabling the efficient absorption of nutrients
and oxygen and the collection of light for symbiotic algae. The algae lived
embedded within the tissues of the host animals, which offered protection from
predation in return for nutrients and the removal of waste products. These
adaptations served well for the prevailing marine conditions of the late Pre-
cambrian, when shallow seas were poorly supplied with nutrients and oxygen.
The Ediacaran faunas spawned from adaptations to highly unstable con-
ditions during the late Precambrian. Overspecialization to a narrow range of
environmental conditions, however, brought about a major extinction of Edi-
acaran species around 540 million years ago at the very doorstep of the “Cam-
brian explosion,” a mass diversification of species. The rapid evolution of
multicellular animal life therefore ushered in the Cambrian period. Marine
animals that survived the die out were markedly different from their Ediacaran
ancestors and participated in the greatest diversification of new species the
world has ever known.The descendants of the Ediacaran faunas split into the
two great lineages of modern life, which include the protostomes such as mol-
lusks, annelids, and arthropods, and the deuterostomes such as the echino-
derms and chordates,the phylum where humans belong.
BANDED IRON FORMATIONS
Mineral deposits of the Proterozoic are bedded or stratified, as opposed to the
vein deposits of the Archean. Iron, the forth most abundant element in Earth’s
crust, was leached from the continents and dissolved in seawater under reduc-
ing (deoxidizing) conditions. When the iron reacted with oxygen in the
ocean, it precipitated (became undissolved) in vast deposits on shallow conti-
nental margins. Alternating bands of iron-rich and iron-poor sediments gave
the ore abanded appearance, thus prompting the name banded iron forma-
tion or BIF. These deposits, mined extensively throughout the world, provide
more than 90 percent of the minable iron reserves.
PROTEROZOIC METAZOANS