Fossil ammonite from the Jurassic of Germany.

This fossil is from the famous Solnhofen Limestone, a deposit that includes soft-part preservation. The number one most famous fossil on Earth came from Solnhofen - Archaeopteryx, which is literally half-reptile, half-bird (creationists hate that fossil). The Solnhofen is a lagoon deposit that has marine, nonmarine, and marginal marine organisms, including animals and plants.

Ammonites are common & conspicuous fossils in Mesozoic marine sedimentary rocks. Ammonites are an extinct group of cephalopods - they’re basically squids in coiled shells. The living chambered nautilus also has a squid-in-a-coiled-shell body plan, but ammonites are a different group.

Ammonites get their name from the coiled shell shape being reminiscent of a ram’s horn. The ancient Egyptian god Amun (“Ammon” in Greek) was often depicted with a ram’s head & horns. Pliny’s Natural History, book 37, written in the 70s A.D., refers to these fossils as “Hammonis cornu” (the horn of Ammon), and mentions that people living in northeastern Africa perceived them as sacred. Pliny also indicates that ammonites were often pyritized.

Ammonites typically have a planispirally coiled, calcareous shell. Sometimes, the nacreous aragonite ("mother-of-pearl") is still preserved, as seen in some examples elsewhere in this photo album.

The dark brown-colored, calcareous structures in the living chamber of the fossil ammonite shown above are aptychi. They are rarely found in living position - they are often discovered isolated and were originally interpreted and described as fossil bivalve shells. Aptychi likely functioned as opercula that could be retracted to protect the animal inside the shell, or they were used as part of the ammonite's jaw.

From museum signage:

Fossils from the quarries of Solnhofen in southern Germany reveal the rich variety of life that thrived in and around Late Jurassic seas.

The ancient environmental conditions at Solnhofen resulted in the remarkable preservation of fossils representing hundreds of Late Jurassic plant, invertebrate, fish, reptile, and bird species. A series of shallow, tropical lagoons extended across southern Europe during this time. Reefs and rock outcrops sheltered these waters from the action of waves and tides. When organisms died in these quiet lagoons, they often sank to the bottom to be buried by fine-grained sediments. The water at the bottom of many Solnhofen lagoons contained little or no oxygen. These conditions not only spared many carcasses from damage or decomposition by bacteria and other scavengers, but also helped to preserved delicate soft-tissue features such as skin, scales, and feathers in exquisite detail.

Classification: Animalia, Mollusca, Cephalopoda, Ammonoidea, Ammonitina

Stratigraphy: Solnhofen Limestone, Upper Jurassic

Locality: Bavaria, southern Germany

See info. at: <a href="https://en.wikipedia.org/wiki/Ammonoidea" rel="nofollow">en.wikipedia.org/wiki/Ammonoidea</a> and <a href="https://en.wikipedia.org/wiki/Ammonitina" rel="nofollow">en.wikipedia.org/wiki/Ammonitina</a> and