Ammonite jaws provide a window into ancient climate

When I was searching for ammonite fossils last year, I tended to concentrate on searching out the distinctive coiled shapes that were the shells of these once-abundant marine invertebrates. Other parts of the animal that lived inside that shell could be fossilized as well, but most of us would never notice such small bits. One of those tiny parts sometimes found in ammonite fossils is the animal’s jaw. These jaws have now given scientists a peek into the temperature conditions of Cretaceous North America. Eighty million years ago, the interior West was warmer than the Gulf Coast, Isabelle Kruta of the American Museum of Natural History in New York City and colleagues report January 27 in PLOS ONE.

Scientists have previously used the oxygen in fossil ammonite shells to determine temperatures long ago. Most oxygen comes in the form of eight protons and eight neutrons, making the isotope oxygen-16. A smaller amount of oxygen is present in the form of oxygen-18, which has 10 neutrons and eight protons. The ratio of oxygen-16 to oxygen-18 changes with temperature. That ratio is preserved in certain minerals, such as aragonite and calcite, found in some fossils and can be used to determine the temperature at which that organism had lived.

But ammonite shells aren’t always preserved well enough to allow such analyses. In the new study, the researchers wanted to figure out whether they could use another part of the animal, the jaw, instead. They couldn’t assume that the jaw recorded temperatures in the same way, though. The jaw formed separately from the shell; it is secreted by different tissue and is composed of a different mineral.

The team began with a living relative of ammonites — the chambered nautilus (Nautilus pompilius). The researchers collected eight specimens from Vanuatu and calculated the temperature of their environment from the oxygen ratio preserved in their shells and their jaws. The two temperatures were off by about a degree and a half, but they were consistent with the temperature of the water where the animals live.

The researchers then turned their attention to fossil ammonites. These were not the typical coiled ammonites that many amateur fossil collectors might be familiar with. Instead, they belonged to the genus Baculites, which looked more like an unrolled nautilus. The fossils came from two sites, in South Dakota and Alabama, that correspond to approximately the same time period in the Cretaceous, 80 million years ago. At that time, both sites were covered by ocean.

When the ammonites were living, the South Dakota site’s water temperature was around 27.5° C, and the Alabama site’s was about 22.4°C, the researchers calculated from the ammonite jaws.

Not every fossil can reveal such information, however. The researchers point out that only those that have well-preserved features that contain their original mineral structure are suitable to such endeavors. They’re still fun to find.