Seaweed holdfast, a 600 million-year-old fossil from the Lantian Formation in China. Credit: Zhe Chen and Xunlai Yuan |
Almost
600 million years ago, before the rapid evolution of life forms known
as the Cambrian explosion, a community of seaweeds and worm-like animals
lived in a quiet deep-water niche near what is now Lantian, a small
village in south China.
Then
they simply died, leaving some 3,000 nearly pristine fossils preserved
between beds of black shale deposited in oxygen-free and unbreathable
waters.
Scientists
from the Chinese Academy of Sciences, Virginia Tech in the United
States and Northwest University in Xi’an, China report the discovery of
the fossils in this week’s issue of the journal Nature.
In
addition to ancient versions of algae and worms, the Lantian
biota–named for its location–included macrofossils with complex and
puzzling structures.
In all, scientists have identified some 15 species at the site.
The
fossils suggest that structural diversification of macroscopic
eukaryotes–the earliest versions of organisms with complex cell
structures–may have occurred only tens of millions of years after the
Snowball Earth event that ended 635 million years ago.
Snowball Earth proposes that the Earth’s surface became almost, or completely, frozen at least once during the planet’s history.
The
presence of macroscopic eukaryotes in the highly organic-rich black
shale suggests that, despite the overall oxygen-free conditions, brief
oxygenation of the oceans did come and go, according to H. Richard Lane,
program director in the National Science Foundation (NSF)’s Division of
Earth Sciences, which funded the research.
“So
there are two questions,” says Shuhai Xiao, a geobiologist at Virginia
Tech. “Why did this community evolve when, and where, it did?
“It
is clearly different in terms of the number of species compared to
biota preserved in older rocks. There are more species here, and they
are more complex and larger than what evolved before.”
The rocks were formed shortly after the largest ice age ever, says Xiao, when much of the global ocean was frozen.
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.”
A cluster of fossils from the Lantian area, preserved and unearthed in very high densities. Credit: Zhe Chen and Xunlai Yuan |
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 to sample each layer to see whether there is any difference
in oxygen contents between layers with fossils and those without,” says
co-author Chuanming Zhou.
The
paper is by Xunlai Yuan and Zhe Chen of Chinese Academy of Sciences;
Shuhai Xiao of Virginia Tech; Chuanming Zhou, also of Chinese Academy of
Sciences; and Hong Hua of Northwest University in Xi’an.
The
research was also supported by Chinese Academy of Sciences, National
Natural Science Foundation of China, Chinese Ministry of Science and
Technology, NASA Exobiology and Evolutionary Biology Program, and a
Guggenheim fellowship to Xiao.
