Historical Geology Understanding Our Planet\'s Past

The Ediacaran fossils represent a marine life very different from that of

today’s oceans, including feathery fronds, puckered pouches, flattened blobs, and

engraved disks. Many were marked with radiating, concentric, or parallel creases,

whereas others were inscribed with delicate branches.The organisms seemed to

have no heads or tails; circulatory, nervous, or digestive systems; eyes; mouths;

bones; muscles; or internal organs, making classification very difficult.

Ediacarans are found worldwide in rocks that just predate the Cambrian

explosion, when shelly faunas first erupted onto the scene. Indeed, the disap-

pearance of the Ediacarans made the ensuing Cambrian explosion possible.

Only when the Precambrian seas were cleared of primitive life-forms could

more advanced species flourish and diversify. The evolutionary heyday that

marked the beginning of the Cambrian gave birth to most animal phyla that

presently swim, craw, or fly around the globe. Animals appeared for the first

time covered with shells, bearing jaws, claws, and other biological innovations.

The Ediacaran fauna appeared to be a preview of these animals and pos-

sessed body styles or morphologies never seen before or since in the fossil record.

Most, if not all, were not related to modern forms. They were perhaps a failed

ev olutionary experiment in multicellular organisms completely separate from all

known kingdoms of life and were wiped out in a previously unrecognized mass

extinction.Yet some Ediacaran organisms apparently inhabited the planet far

longer than previously thought, surviving well into the Cambrian.

The Ediacaran fossils displayed an unusual body architecture (Fig. 36)

totally alien to anything seen on Earth today. Some Ediacaran faunas called

Figure 36 The late
Precambrian Ediacaran
fauna from Australia.

Historical Geology