By 635 million years ago, the snowball Earth event ended and the oceans were clear of ice. Perhaps, Xiao says, "that prepared the ground for the evolution of complex eukaryotes."
The team examined the black shale rocks because, although they were laid down in less than optimal waters for oxygen-dependent organisms, "they are known to be able to preserve fossils very well," says Shuhai.
"In most cases, dead organisms were washed in and preserved in black shales. In this case, we discovered fossils that were preserved in pristine condition--some seaweeds still rooted--where they had lived."
The conclusion that the environment would have been poisonous is derived from geochemical data, "but the bedding surfaces where these fossils were found represent moments of geologic time during which oxygen was available and conditions were favorable," says Xiao.
"They are very brief moments to a geologist, but long enough for the oxygen-demanding organisms to colonize the Lantian basin and capture the rare opportunities."
The research team suggests that the Lantian basin was largely without oxygen, but was punctuated by brief oxic episodes that were populated by complex new life forms.
Those life forms were subsequently killed and preserved when the oxygen disappeared.
"Such brief intervals need high-resolution sampling for geochemical analysis to capture the dynamic and complex nature of oxygen history in the Ediacaran Period," says lead paper author Xunlai Yuan. The Ediacaran Period is the last geological period immediately preceding the Cambrian Period.
Proving that hypothesis awaits further study.
The rocks in the study region are deposited in layered beds. The nature of the rock changes subtly, and there are finer and finer layers that can be recognized within each bed.
"We will need
|Contact: Cheryl Dybas|
National Science Foundation