Large Kranaosphinctes Ammonite With Fossil Mollusk

Alongside trilobites and dinosaurs, ammonites are among the most iconic fossils. Ammonites swam the seas freely during the Jurassic and Cretaceous Periods, evolving rapidly and leaving abundant examples in the fossil record. Ammonites were cephalopods, a taxonomic class that includes today's nautilus, octopus, and squid. They used a long, tubular organ known as a siphuncle to fill or empty their chambers with water and thus move up or down in the water column. Apart from the siphuncle, the animal lived solely in the outermost chamber.

The outermost layer of ammonite shells was made up of aragonite, a type of calcium carbonate, which provided strong protection and allowed ammonites to fossilize much more readily than animals that either had a soft shell or lacked a shell. In the case of the Kranaosphinctes offered here, its aragonite shell has developed a beautifully nacreous and pearlescent outer layer.

Kranaosphinctes lived during the Late Jurassic Period, approximately 161 to 154 million years ago. Because ammonites rapidly evolved into new species and their shells were prone to fossilization, they have been crucial for scientists interested in dating the Earth's layers: finding the same species of ammonite in layers of soil thousands of miles apart implies that those layers are from the same time period in Earth's history.