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."
|Contact: Kendra Snyder|
American Museum of Natural History