Peruvian adjustable macramé necklace with ammonite and fossil tooth

Ammonoids (Ammonoidea), or better known as ammonites, 1 are a subclass of extinct cephalopod mollusks that existed in the seas from the Middle Devonian (about 400 million years ago) to the late Cretaceous (66 million years ago) 2 Thanks to their rapid evolution and worldwide distribution, they are excellent guide fossils for rock dating and have made possible the elaboration of biozone successions of great biostratigraphic precision.The characteristics that a guide fossil must have are: first, that they are of a species that lived for a short period of time; second, that the species evolve rapidly; third, that they are geographically widespread and, lastly, that they are abundant.zize8 x 3