Ammonite (Lytoceras sp.)

Ammonite (Lytoceras sp.) Shell, about 160 million years old, Madagascar Fossil 38x30x16 cm Provenance: market (France) Conservation status. Surface area: 70%. Conservation status. Support: 85% (consolidation, gaps) Ammonites were cephalopods - mollusks characterized by bilateral body symmetry divided between head and tentacles - with a spiral shell. They are closely related to the living coleoids, i.e., octopus, squid and cuttlefish, although in appearance they are more reminiscent of Nautilus. The first ammonites-more properly ammonoid-appeared during the Devonian (419.2-358.9 million years ago). The last species disappeared during or soon after the Cretaceous-Paleogene extinction event, also called K-T extinction (about 66 million years ago), in which three-fourths of animal and plant species disappeared and among other things all non-winged dinosaurs and most quadrupeds weighing more than 25 kg. 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 geological periods. Their fossil shells usually take the form of planispirals, although some helical and non-spiral spiral forms, so-called heteromorphic ammonites, have been found. The name "ammonites," was invented by Pliny the Elder, who noted their similarity to ram's horns (he coined the term "Ammonis cornua," "horns of Ammon," because the Egyptian god Ammon was typically depicted with ram's horns). Ammonites are distinguished by their septa, the partitions separating the chambers of the phragmocone, by the nature of the sutures at the point where the septa join the outer shell wall, and generally by the siphon. The genus Lytoceras, to which the specimen on auction belongs, existed during most of the Jurassic and Cretaceous: these cephalopods were carnivores capable of swimming and rapid movements. Lytoceras shells are evolved, round or square in cross-section, covered with rippled growth lines or ribbing and may have slight constrictions on the inner forms. Some have fine striations and parallel furrows running longitudinally along the sides. Lytoceras-whose species are many and difficult to distinguish-have been found worldwide, particularly in Western Europe, Morocco, Madagascar, South Africa and the United States. The fossil on auction is from Madagascar, probably from ammonite-rich deposits in the southern Morondava Basin and southwestern Madagascar that accumulated in the Jurassic Oxfordian stage between 163.5 and 157.3 million years ago. A similar specimen of Lobolytoceras bukmanni was presented by René Hoffmann and Helmut Keupp in 2008 ("A giant member of the genus Lobolytoceras BUCKMAN, 1923 (Ammonitina, Lytoceratinae) from the Oxfordian of SW Madagascar," N. Jb. Geol. Paläont. Abh. 2008, vol. 250/1, p.53-64, Stuttgart, October 2008, published online 2008).