What were ammonites' lives like? Isotope study reveals clues.

Using isotopic analysis, researchers have revealed new ecological data about ancient mollusks.

In medieval Europe, ammonite stones were believed to have divine powers. In Nepal, Hindus have long interpreted them as manifestations of Vishnu.

To Jocelyn Sessa, they’re a window to the past.

At Mississippi’s Owl Creek Formation, researchers are using isotopic analysis to reconstruct the habitats of these ancient mollusks. Armed with new data, they hope to piece together details about prehistoric climates. Their study, which was published Monday in PNAS, was led by Dr. Sessa, a paleontological fellow at the American Museum of Natural History.

Ammonites were a group of marine mollusks, closely related to the octopus and squid of today. They first appeared in the Devonian period, more than 400 million years ago, and group persisted until the Cretaceous-Paleogene extinction event that ended the Mesozoic period. Thanks to their sturdy calcium composition, ammonites' distinctive shells, most of which were simple spirals, tend to be well-preserved and quite common.

And because of their ubiquity, ammonites are considered excellent index fossils. Geologists and paleontologists alike use them as reference points when dating rock layers. But while the “when” is abundantly clear, relative little is known about “how” these mollusks lived.

“When I first started this study, I was surprised to realize how little is known about ammonites' ecologies,” Sessa says. “In general, this is because ammonites are extinct, and their modern relatives – squids, octopus, and Nautilus – exhibit a variety of migrational behaviors.”

But thanks to the unusual conditions at the Owl Creek Formation, this knowledge gap is closing. Unlike many other fossil sites, which preserve just the ammonite specimens, Owl Creek is home to other well-preserved marine fossils. Sessa and colleagues performed oxygen and carbon isotopic analysis on these organisms, as well as several types of ammonites. By comparing the chemical makeup of these fossils, they could determine the depth of ammonite habitats and make inferences about their ecosystem generally.

Researchers found that baculite and scaphite ammonites were chemically similar to benthic organisms, which are found exclusively on the sea floor. Sphenodiscid ammonites, by comparison, had chemical compositions associated with plankton – these mollusks probably lived closer to the water’s surface.

“The unique aspect of the Owl Creek outcrop, and of this study, is that all of these organisms were found together – one locality, same stratigraphic horizons. So we are reconstructing the temperatures of one water mass,” Sessa says. “Other studies have analyzed the oxygen isotopic composition of ammonites and related it to their depth habitat, but those studies have not had co-occurring benthic and planktic organisms to constrain the temperatures of a water column.”

According to Sessa, this data could spur valuable paleoecological research.

“Now that the depth habitat of these three families of ammonites has been established, in the future these taxa could be used to provide temperature estimates of particular water masses,” Sessa says. “The sphenodiscids [provide] estimates of nearshore, surface waters, and the baculites and scaphites of the near bottom. Given their commonness, these taxa could contribute a lot of new paleo-temperature data.”

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