Ammonites lived during the periods of Earth history known as the Jurassic and Cretaceous. Together, these represent a time interval of about 140 million years.
The Jurassic Period began about 201 million years ago and the Cretaceous Period ended about 66 million years ago. The ammonites became extinct at the end of the Cretaceous, at roughly the same time as the dinosaurs disappeared. However, we know a lot about them because they are commonly found as fossils formed when the remains or traces of the animal became buried by sediments that later solidified into rock.
Ammonites were marine animals belonging to the phylum Mollusca and the class Cephalopoda. They had a coiled external shell similar to that of the modern nautilus. In other living cephalopods, e.g. octopus, squid and cuttlefish, the shells are small and internal, or absent.
The use of ammonites in stratigraphy was pioneered in the 1850s by two Germans — Friedrich Quenstedt of Tübingen (1809–1889) and his one-time pupil, Albert Oppel of Munich (1831–1865). Their work was based on the ammonites of the Swabian and Franconian Alb of southern Germany — the eastern extension of the Jura Mountains of France and Switzerland, from which the Jurassic Period takes its name.
Ammonites make excellent guide fossils for stratigraphy because:
- they evolved rapidly so that each ammonite species has a relatively short life span
- they are found in many types of marine sedimentary rocks
- they are relatively common and reasonably easy to identify
- they have a worldwide geographical distribution
Credit: British Geological Survey
Picture Credit : Google
