Only a few years ago, the expression "Precambrian fossils"
would have been considered almost a contradiction in terms.
At the beginning of the Cambrian era, 600 million years ago, there
was a veritable explosion in the fossil record. Every major branch
of modern animal life was present at the start of the Cambrian era
except vertebrates. Geologists even define and identify the beginning
of the Cambrian era by this sudden increase in volume of the fossil
Prior to 600 million years ago the fossil record quickly shrinks
to almost nothing. Jellyfish and other softbodied marine life from
900 million to 600 million years ago are known from deposits in
Australia and a few other places.
Much of the trouble is that these creatures are soft-bodied. Jellyfish
do not preserve well in the fossil record in comparison with shellfish.
Part of the Cambrian explosion is not a sudden burst of life, but
a sudden increase in the use of hard, protective materials such
as shells and body armor.
No matter what the reason, most paleontologists only a few years
ago considered that little was to be learned from the fossil record
prior to this Cambrian population explosion.