File:Paraspirifer bownockeri (pyritized fossil brachiopod) (Silica Formation, Middle Devonian; quarry in Lucas County, Ohio, USA) 12.jpg

English: Paraspirifer bownockeri Stewart, 1927 - pyritized fossil brachiopod from the Devonian of Ohio, USA (view of dorsal valve)

Brachiopods are sessile, benthic, filter-feeding, marine invertebrates. They first appear in Cambrian rocks and were abundant in Earth's oceans throughout the Paleozoic. They were also common in Mesozoic oceans, but are scarce in modern oceanic biotas. Brachiopods have two shells, called valves, that are usually calcareous (made of calcite - CaCO3 - calcium carbonate). Each shell of a brachiopod is bilaterally symmetrical, unlike each shell of a bivalve (clam).

This fossil brachiopod's shell is now composed of pyrite ("fool's gold" - FeS2, iron sulfide), a preservation style referred to as replacement. The specimen comes from the Middle Devonian Silica Formation (also known as the Silica Shale) in northwestern Ohio. Pyritized fossils are relatively common in the unit - pyrite can replace shells or it can be a coarsely-crystalline coating atop the calcareous shells.

The most common fossil replacement minerals are quartz, pyrite, and apatite. Quartz is SiO2 (silica) - fossils that have been quartz-replaced 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. Replacement by apatite (calcium phosphate) is called phosphatization.

Other minerals have also been found replacing minerals - many 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. (List mostly from info. in Hartzell, 1906 and Klein & Hurlbut, 1985)

Classification: Animalia, Brachiopoda, Articulata (a.k.a. Rhynchonelliformea), Spiriferida, Hysterolitidae

Stratigraphy: Silica Formation, Givetian Stage, upper Middle Devonian

Locality: quarry in the Sylvania area, far-northern Lucas County, northwestern Ohio, USA