Ammonites were spiral-shelled marine mollusks that drifted throughout the Mesozoic seas until the Cretaceous–Paleogene mass extinction event. Despite how abundant they are in the fossil record, we don’t know that much about their ecology. Now, researchers trying to reconstruct their habitats reveal where in the water column these ubiquitous fossils preferred to live. The findings, published in Proceedings of the National Academy of Sciences this week, might help researchers reconstruct ancient climate.
Because ammonites are diverse and their shells preserve so well, these sea creatures are an important index fossil for dating rock layers. For example, the 35-million-year-long record of Upper Cretaceous deposits in the U.S. Western Interior Seaway has been divided into 66 ammonite zones. At the upper Maastrichtian Owl Creek Formation in northern Mississippi, three common ammonite families – Baculitidae, Scaphitidae, and Sphenodiscidae – co-occur with other well-preserved organisms living at various depths, from planktonic (surface dwelling) critters to benthic (bottom dwelling) ones. This area provides a rare opportunity to constrain depth habitats of ammonites by comparing isotopic compositions of all the co-occurring groups of organisms: Shell chemistry, specifically oxygen isotopes, records the temperature of the surrounding water.
To recreate the water column profile and determine where ammonites lived, a team led by Jocelyn Sessa from the American Museum of Natural History performed isotopic analyses on ammonites as well as planktonic and benthic protists called foraminifera. Of the 553 Owl Creek Formation mollusk specimens they studied, the team ended up with 234 isotopic measurements.
Baculites and scaphites, they found, have similar chemical compositions as benthic organisms – which means they lived close to the sea floor. The third family of ammonites, the sphenodiscids, was more closely associated with planktonic foraminifera and those from the warmer half of the benthic environments. That means they lived near the sea surface. Relative to baculites and scaphites, sphenodiscids were rare and poorly preserved: Their shells likely reached Owl Creek by drifting seaward after death.
Being able to determine where different types of ammonites lived – using the oxygen isotopes of their shells – may help researchers reconstruct past climate. "Ammonites haven’t been used much in temperature reconstructions because we didn't know whether they were surface dwellers, bottom dwellers, or perhaps even both if they swam through different water masses," Sessa explains to IFLScience. "Now that the depth habitats of these three groups have been established, they could be used to provide temperature estimates of particular water masses in the future."