Although museum
drawers may seem to overflow with fossils, paleontologists have long known that
the record of life preserved in the rocks is incomplete. Just how incomplete
the fossil record is, though, has remained a colossal question. Recently, paleontologists
have been working on ways to fill in gaps in life’s diversity record, and
some researchers are finding that climate change — including greenhouse
gas warming — may play a pivotal role in preserving fossil assemblages.
Last summer, researchers uncovered the
remains of several dinosaurs in this bonebed in northern Madagascar that appear
to have perished during drought — just one example of a climatic link to
extraordinary fossil preservation. Image courtesy of Raymond Rogers.
Understanding fossil preservation — with a focus on exactly what happens
to an organism after its death and until its discovery as a fossil — is
important for figuring out potential biases in the fossil record, says Raymond
Rogers, a geologist at Macalester College in St. Paul, Minn. “Preservation
is what gives us a record,” but natural processes can act to distort the
record, he says.
“We all know the record is biased,” says Peter Allison of Imperial
College London, who co-chaired a session on preservation biases at the annual
meeting of the Geological Society of America (GSA) in Salt Lake City, Utah,
in October. “Just go out to any beach and collect shells,” he says.
The beach may seem to be teeming with hard-bodied organisms living in the shells,
but modern biology says that soft-bodied organisms comprise two-thirds of the
community living there. The beach’s tendency to preserve hard-bodied organisms
would present a significant bias in the fossil record.
Factors that affect preservation include timing, location, the durability of
the organism’s remains, natural disasters and climate. “Certain environments
harbor greater diversity than others, and climate will control the distribution
of these environments,” says Christopher Noto of Stony Brook University
in New York, who presented at the GSA meeting. Climate, Noto says, may also
control preservation patterns.
In the Late Jurassic dinosaur record, for example, fewer fossils have been found
at higher latitudes of the time — a warmer, wetter environment with higher
biological activity — than at lower latitudes where the climate was drier.
That pattern, however, may indicate a bias rather than real conditions, Noto
says: Tropical regions are some of the worst environments for preservation,
despite supporting some of the highest diversity of plant and animal life in
the world. Thus, to understand the fossil record and the diversity at any given
time, “we need to gain insight into conditions” at the time and “match
them up with the fossil record,” he says.
Greg Retallack of the University of Oregon in Eugene, also presenting at the
GSA meeting, suggested that climate, and specifically global warming, may have
contributed to fossil preservation. Retallack scanned through a database of
500 million years’ worth of fossils and found 41 individual episodes of
“exceptional preservation” of organisms such as fish, crustaceans,
insects and starfish scattered throughout the world.
Retallack took those 41 locations, including well-known and well-dated sites,
such as the 508-million-year-old Burgess Shale in Canada and the 123-million-year-old
Liaoning site in China, and compared them to climate records over the same time
periods. He found that during each peak in the fossil record, there was a corresponding
spike in atmospheric carbon dioxide and methane levels, and a spike in precipitation
levels and temperatures. It appears, he said, that “greenhouse episodes”
may have occurred simultaneously with the “mass kills.”
However, the question remains which came first: whether the greenhouse gas episodes
preserved the fossils, or whether the episodes simply killed many life forms
and thus left more to be preserved. “With a mass kill, there’s obviously
a better chance of preservation because scavengers are overwhelmed, and they
can’t completely recycle or consume” the bodies, Rogers says.
The dinosaur record from Madagascar, for example, has shown a strong correlation
to climate, he says. In a 15-meter-thick section of rock that contains several
distinct dinosaur bonebeds, Rogers and colleagues also found evidence of recurring
severe droughts followed by extreme rainy periods that promoted burial —
a link between climate conditions, mass death and preservation.
While Noto and Retallack pose “interesting ideas,” Allison says, more
data is needed on how preservation and biases changed through time. Until researchers
have that information, he says, refining the record of life’s diversity
over time will be difficult.
Megan Sever
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