Harpoceras Ammonite | Germany

Harpoceras sp.Lower Jurassic (180 MYO)Posidonia ShaleHolzmaden, Germany Ammonite approx. size: 3.25" x 2.5" Plate approx. size: 4" x 3.25" x 0.75" at thickest part   This Harpoceras ammonite from the Posidonia Shale in Holzmaden, Germany has been compressed on black slate and is partially pyritized.   Ammonoids are a group of extinct marine mollusk animals in the subclass Ammonoidea of the class Cephalopoda. These mollusks, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species. The earliest ammonites appear during the Devonian, and the last species vanished in the Cretaceous–Paleogene extinction event. 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. Their fossil shells usually take the form of planispirals, although there were som