At Lung Luong, Keo Yen commune, Ha Quang district, Ammonite fossils are found in thin-bedded limestone, which was formed under marine conditions c.260 million years ago (Late Permian).
Ammonites are an extinct group of marine mollusc animals. Being one of the most evolved molluscs, they are more closely related to living octopus, squid or cuttlefish than they are to shelled species such as the living Nautilus. The earliest ammonites appeared during the Devonian (c.420 million years ago), and the last species died out during the Cretaceous-Paleogene extinction event (c.66 million years ago) when some three-quarters of the plant and animal (most notably the dinosaurs) species on Earth went extinct.
Being able to swim fast - moving backwards by ejecting water under great pressure - they were very popular sea creatures. They may have avoided predation by squirting ink, much like modern cephalopods.
The name "ammonite" was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns (typically worn by the Egyptian god Ammon).
Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geologic time periods. Certainly they also indicate marine environment in the distant past where they are presently found.