File:Michelinoceras aldenense fossil nautiloid (Alden Pyrite Bed, Ludlowville Formation, Middle Devonian; western New York State, USA) (15360914300).jpg

Michelinoceras aldenense pyritized fossil nautiloid from the Devonian of New York State, USA. (4.1 centimeters across)

Nautiloids are squid-like creatures that construct long and slightly tapering or coiled, aragonitic, hollow shells with regularly-spaced internal walls. In the fossil record, the original aragonite shell has usually dissolved away, leaving an internal mold (an impression of the shell's interior). Nautiloids were relatively common components of Paleozoic oceans. Most species had straight, slightly tapering shells, but some had loosely coiled or tightly coiled shells. The entire group is represented in today's oceans by five living species of chambered nautilus, all of which have a coiled shell: Nautilus pompilius, Nautilus macromphalus, Nautilus stenomphalus, Nautilus belauensis, and Allonautilus scrobiculatus.

This is a pyritized internal mold of a straight-shelled Michelinoceras aldenense nautiloid in a pyrite concretion, from the famous Alden Pyrite Bed of New York State.

Classification: Animalia, Mollusca, Cephalopoda, Nautiloidea, Orthocerida, Orthoceratidae

Stratigraphy: Alden Pyrite Bed, Ledyard Shale Member, Ludlowville Formation, Middle Devonian

Locality: western New York State, USA

Replacement is a fossil preservation style involving the crystal structure and the mineral of an organism's hard parts being changed.

The most common replacement mineral is quartz (silica) (SiO2) - fossils that have been replaced by quartz are said to be silicified (silicification). Many silicified fossils have rounded to pustulose structures covering their surfaces. These are called beekite rings, but they're composed of ordinary quartz.

Other common replacment materials include the mineral pyrite (FeS2 - iron sulfide) and calcium phosphate. These replacement styles are called pyritization and phosphatization.

Numerous other minerals have been found replacing minerals - many of them are quite rare. Reported fossil replacement minerals include: anglesite, apatite, barite, calamine, calcite, cassiterite, celestite, cerargyrite, cerussite, chalcocite, cinnabar, copper, dolomite, fluorite, galena, garnet, glauconite, gumbelite, gypsum, hematite, kaolinite, limonite, magnesite, malachite, marcasite, margarite, opal, pyrite, romanechite/psilomelane, siderite, silica/quartz, silver, smithsonite, specular hematite, sphalerite, sulfur, uranium minerals, and vivianite.