Researchers Discover Ammonites Made Homes at Ancient Methane Seeps

Posted on April 16, 2012

Deepsea expeditions have revealed that methane seeps are teeming with life. Ancient methane seeps were also sources of refuge for ancient lifeforms in the Late Cretaceous. Scientists have found that ammonites - an extinct type of shelled mollusk closely related to nautiluses and squids - once made homes in the unique environments surrounding methane seeps in the seaway that once covered America's Great Plains (you can find out more about ammonites here and here). The above image shows the variety of invertebrate fossils collected from the ancient methane seep studied for this research. The findings, led by scientists at the American Museum of Natural History, was published online in the journal Geology.

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.

Neil Landman, lead author of the Geology paper and a curator in the Division of Paleontology at the American Museum of Natural History, says, "We've found that these methane seeps are little oases on the sea floor, little self-perpetuating ecosystems. 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."

The researchers are investigating a 74-million-year-old seep with extremely well-preserved fossils.

Landman says, "Most seeps have eroded significantly over the last 70 million years. 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."

The ammonites were once thought to be passersby at the seep. But after analyzing the abundance of isotopes of carbon, oxygen, and strontium, the researchers discovered the ammonites spent their entire lives at the seep.

Landman says, "Ammonites are generally considered mobile animals, freely coming and going. 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."


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