ALTERED HARD PARTS OF ORGANISMS
The original hard parts of an organism normally undergo great change after burial. These changes take place in many ways, but the type of alteration is usually determined by the composition of the hard parts and where the organism lived. Some of the more common processes of alteration are discussed below.
[Carbonization]
This process, known also as [distillation] takes place as organic matter slowly decays after burial. During the process of decomposition, the organic matter gradually loses its gases and liquids leaving only a thin film of carbonaceous material ([Pl. 2], fig. 7). This is the same process by which coal is formed, and large numbers of carbonized plant fossils have been found in many coal deposits.
In Texas the carbonized remains of plants, fish, and certain invertebrates have been preserved in this manner, and some of these carbon residues have accurately recorded even the most minute structures of these organisms.
Petrifaction or [Permineralization]
Many fossils have been permineralized or petrified—literally turned to stone. This type of preservation occurs when mineral-bearing ground waters infiltrate porous bone, shell, or plant material. These underground waters deposit their mineral content in the empty spaces of the hard parts making them heavier and more resistant to weathering. Some of the more common minerals deposited in this manner are [calcite], [silica], and various compounds of iron.
[Replacement] or Mineralization
This type of preservation takes place when the original hard parts of organisms are removed after being dissolved by underground water. This is accompanied by almost simultaneous deposition of other substances in the resulting voids. Some replaced fossils will have the original structure destroyed by the replacing minerals. Others, as in the case of certain silicified tree trunks, may be preserved in minute detail.
Although more than 50 minerals have been known to replace original organic structures, the most frequent replacing substances are [calcite], [dolomite] (a calcium magnesium carbonate), [silica], and certain iron compounds.
[Replacement] by [calcareous] material
[Calcareous] [replacement] occurs when the hard parts of an organism are replaced by [calcite], [dolomite], or [aragonite] (a mineral which is composed of calcium carbonate but which is less stable than calcite). The exoskeletons of many corals, echinoderms, brachiopods, and mollusks have been replaced in this manner.
[Replacement] by [siliceous] material
When the original organic hard parts have been replaced by [silica] the [fossil] is said to have undergone [silicification], and this type of [replacement] often produces a very high degree of preservation. This is particularly true of the silicified [Permian] (geologic time scale, [Pl. 1]) fossils from the Glass Mountains in Brewster County. These fossils are embedded in limestone which must be dissolved in vats of acid, and after the enclosing [rock] has been dissolved the residue yields an amazing [variety] of perfectly preserved invertebrate fossils ([Pl. 3]).
Silicified [Cretaceous] fossils have been recovered from the Edwards limestone of central Texas. The silicified [fauna] is restricted to a few scattered localities, each of which may yield many unusually well-preserved fossils.
[Replacement] by iron compounds
Several different iron compounds have been known to replace organic matter. Many Texas limestones contain [fossil] snails and clams which have had their original shell material replaced by iron compounds such as limonite, hematite, marcasite, or [pyrite]. Certain of the [fossiliferous] [Tertiary] sandstones of the Texas Gulf Coast area contain large amounts of [glauconite] which commonly replaced organic material.
In some areas entire faunas have been replaced by iron compounds. Such is the case in the famous “[Pyrite] [Fossil] Zone” of the Pawpaw [formation] (Lower [Cretaceous]) in Tarrant County. The fossils in this part of the formation are very small or “dwarfed” and have been replaced by limonite, hematite, or pyrite. Ammonites, clams, snails, and corals are particularly abundant at this locality.