Ammonite eteromorfa (Ephamulina arcuata)

Heteromorphic ammonite (Ephamulina arcuata). Shell, ca. 100-113 million years old, Madagascar Fossil 28x24x10 cm Provenance: market (Italy) Conservation status. Surface area: 70%. Conservation status. Support: 70% (gaps, fractures, consolidation) 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 that separate 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 specimen under consideration belongs to the heteromorphic species Ephamulina arcuata, identified by Collignon in 1963, and belonging to the family Anisoceratidae, heteromorphic ammonites of the superfamily Ancyloceratina Turrilitoidea. Members of the family range from the Lower Albian to the Upper Turonian. The family may derive from a member of the Hamitidae. The shells of anisoceratids begin as an irregular helical spiral that typically borders on a single plane with growth. This is usually followed by at least one straight termination. Ribs and tubercles are common features. The fossil dates to the Middle Cretaceous, Albian stage.