Middle Jurassic period, Callovian stage c. 162 million years ago, Dun-sur-Auron, France, private collection from Netherlands.
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 c. 201 million years ago and the Cretaceous Period ended c. 66 million years ago. The ammonites became extinct at the end of the Cretaceous, at roughly the same time as the dinosaurs disappeared. Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (octopuses, squid and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species (Nautiloidea). The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Gaius Plinius Secundus (AD 23/24 – 79), called Pliny the Elder, called fossils of these animals ammonis cornua ("horns of Ammon") because the Egyptian god Ammon (Amun) was typically depicted wearing rams' horns. Often, the name of an ammonite genus ends in -ceras, which is Greek (for "horn" (κέρας).
Ammonites' growing shells typically formed into a flat spiral, known as a planispiral, although a variety of shapes did evolve over time. Shells could be a loose spiral or tightly curled with whorls touching. They could be flat or helical. Some species would begin growing their shell in a tight spiral but straighten it out through later growth phases. The ammonite’s shell was divided into chambers separated by walls known as septa (sing. septum). The chambered part of the ammonite shell is called a phragmocone. It contains a series of progressively larger chambers, called camerae (sing. camera). Only the last and largest chamber, the body chamber, was occupied by the living animal at any given moment. As it grew, it added newer and larger chambers to the open end of the coil. A thin living tube called a siphuncle (Lat. sīphunculus, meaning "little siphon") passed through the septa, extending from the ammonite's body into the empty shell chambers. Through a hyperosmotic active transport process, the ammonite emptied water out of these shell chambers. This enabled it to control the buoyancy of the shell and thereby rise or descend in the water column. Scientists believed that ammonites, like modern cephalopods, had soft body tissue with tentacles attached to their heads for catching prey. Fossil evidence indicates they had sharp, beaklike jaws to snare prey such as plankton, crustaceans, and other ammonites.
Macrocephalites is a genus of the stephanoceratoid ammonite family Macrocephalitidae, diagnostic of the Callovian stage of the Middle Jurassic. Three subgenera, Dolikephalites, Kamptokephalites, and Pleurocephalites are recognized in addition to Macrocephalites itself, with Indocephalites tentatively included as the fourth. Macrocephalites itself has a strongly ribbed, involute, subglobose to discoidal shell with a somewhat small open umbilicus, compressed whorl section, higher than wide, and rounded venter.
Lovely ammonite fossil. Good shape. Age-related wear. Dirt and dust. Size c. 4,0cm x 4,5cm x 2,5cm. Weight c. 74g.