Research
led by scientists at the American Museum of Natural History shows that
ammonites—an extinct type of shelled mollusk that’s closely related to
modern-day nautiluses and squids—made homes in the unique environments
surrounding methane seeps in the seaway that once covered America’s
Great Plains. The findings, published online on April 10 in the journal
Geology, provide new insights into the mode of life and habitat of these
ancient animals.
Geologic
formations in parts of South Dakota, Wyoming, and Montana formed as
sediments were deposited in the Western Interior Seaway—a broad expanse
of water that split North America into two land masses—during the Late
Cretaceous, 80 to 65 million years ago. These formations are popular
destinations for paleontologists looking for everything from fossilized
dinosaur bones to ancient clam shells. In the last few years, groups of
researchers have honed in on giant mounds of fossilized material in
these areas where, many millions of years ago, methane-rich fluids
migrated through the sediments onto the sea floor.
“We’ve
found that these methane seeps are little oases on the sea floor,
little self-perpetuating ecosystems,” said Neil Landman, lead author of
the Geology paper and a curator in the Division of Paleontology at the
American Museum of Natural History. “Thousands of these seeps have been
found in the Western Interior Seaway, most containing a very rich fauna
of bivalves, sponges, corals, fish, crinoids, and, as we’ve recently
documented, ammonites.”
In
the Black Hills region of South Dakota, Landman and researchers from
Stony Brook University’s School of Marine and Atmospheric Science, the
Black Hills Museum of Natural History, Brooklyn College, the South
Dakota School of Mines and Technology, and the University of South
Florida are investigating a 74-million-year-old seep with extremely
well-preserved fossils.
“Most
seeps have eroded significantly over the last 70 million years,”
Landman said. “But this seep is part of a cliff whose face recently
slumped off. As the cliff fell away, it revealed beautiful, glistening
shells of all sorts of marine life.”
Studying
these well-preserved shells, the researchers tried to determine the
role of ammonites in the unique seep ecosystem. By analyzing the
abundance of isotopes (alternative forms) of carbon, oxygen, and
strontium, the group made a surprising discovery. The ammonites at the
seep, once thought to be just passersby, had spent their whole lives
there.
“Ammonites
are generally considered mobile animals, freely coming and going”
Landman said. “That’s a characteristic that really distinguishes them
from other mollusks that sit on the sea floor. But to my astonishment,
our analysis showed that these ammonites, while mobile, seemed to have
lived their whole life at a seep, forming an integral part of an
interwoven community.”
The
seeps, which the researchers confirmed through oxygen isotope analysis
to be “cold” (about 27 degrees Celsius, 80 degrees Fahrenheit), also
likely attracted large clusters of plankton – the ammonites’ preferred
prey.
With
these findings in mind, the researchers think that the methane seeps
probably played a role in the evolution of ammonites and other faunal
elements in the Western Interior Seaway. The seeps might have formed
small mounds that rose above the oxygen-poor sea floor, creating mini
oases in a less-hospitable setting. This could be a reason why ammonites
were able to inhabit the seaway over millions of years in spite of
occasional environmental disturbances.
“If
a nearby volcano erupted and ash covered part of the basin, it would
have decimated ammonites in that area,” Landman said. “But if these
communities of seep ammonites survived, they could have repopulated the
rest of the seaway. These habitats might have been semi-permanent,
self-sustaining sites that acted as hedges against extinction.”
Isotope
analysis of strontium also revealed an interesting geologic finding:
seep fluids coming into the seaway were in contact with granite, meaning
that they traveled from deep in the Earth. This suggests that the Black
Hills, a small mountain range in the area, already were beginning to
form in the Late Cretaceous, even though the uplift wasn’t fully
complete until many millions of years later.
This
research was supported by the American Museum of Natural History and a
National Science Foundation Research Experience for Undergraduates grant
for two students from Brooklyn College to participate in the field
work.
