Ammonites lived during the Paleozoic and Mesozoic eras, from about 400 million years ago to 65 million years ago. Along with other fossilized creatures, their presence in layers of rock shows where ancient seas covered the earth. Because they were so widespread and evolved so many different forms rather rapidly, ammonites can also act as “index fossils” to determine the age of rock layers. Scientists can examine the features of an ammonite shell to determine when that animal appeared in the course of ammonite evolution, and thus how long ago it lived. Finding the same species of ammonite in rocks from different places indicates that the rocks are the same age, even if they are located hundreds or thousands of miles apart. Many species are only found in rocks from specific periods of time. In this way, ammonite fossils serve as “time keepers” - and have more than just beauty to offer.
In the prehistoric seas lived a squid-like creature whose fossilized shells are prized by collectors - and they offer clues about the Earth’s history.
Ammonites are an extinct group of cephalopods, the class of animals that includes modern-day octopus, squid, cuttlefish and nautilus. Similar to the nautilus in appearance, ammonites were predators that had curled shells made up of many gas-filled chambers that provided flotation. The soft-bodied animal lived in the large outer chamber and was able to grow by moving forward when it added a new chamber. These creatures were quite diverse in shell features as well as size, with shells varying from 1 inch to a massive 9 feet in diameter.