Fossil Types



Plants are most commonly fossilized through carbonization. In this process, the mobile oils in the plantís organic matter are leached out and the remaining matter is reduced to a carbon film. Plants have an inner structure of rigid organic walls that may be preserved in this manner, revealing the framework of the original cells. Animal soft tissue has a less rigid cellular structure and is rarely preserved through carbonization. Although paleontologists have found the carbonized skin of some ichthyosaurs, marine reptiles from the Mesozoic Era (240 to 65 million years before present), the microscopic structure of the skin was not preserved.



Another common mode of preservation of plants is petrifaction, which is the crystallization of minerals inside cells. One of the best-known forms of petrifaction is silicification, a process in which silica-rich fluids enter the plantís cells and crystallize, making the cells appear to have turned to stone (petrified). Famous examples of silicification may be found in the petrified forests of the western United States (see Petrified Forest National Park). Petrifaction may also occur in animals when minerals such as calcite, silica, or iron fill the pores and cavities of fossil shells or bones.



Replacement occurs when an organism is buried in mud and its remains are replaced by sulfide (pyrite) or phosphate (apatite) minerals. This process may replace soft tissue, preserving rarely seen details of the organismís anatomy. X-ray scanning of some German shales from the Devonian Period (410 million to 360 million years before present) have revealed limbs and antennae of trilobites (extinct ocean-dwelling arthropods) and tentacle arms of cephalopods (highly developed mollusks) that have been pyritised (replaced by pyrite). Paleontologists have used mild acids to etch the phosphatized fossil remains of ancient fish found in Brazil to reveal structures such as gills and muscles. Although mineral replacement is rare, fossils created in this way are important in helping paleontologists compare the anatomical details of prehistoric organisms with those of living organisms.



Many animal shells are composed of the mineral aragonite, a form of calcium carbonate that breaks down over millions of years to form the more stable mineral calcite. This method of preservation, called recrystallization, destroys the microscopic details of the shell but does not change the overall shape. Snail shells and bivalve shells from the Jurassic Period (205 million to 138 million years before present) and later are still composed principally of aragonite. Most older shells that have been preserved have recrystallized to calcite.


Soft-Tissue Preservation  

The soft tissues of animals are preserved only under extremely unusual conditions, and the preserved tissue usually lasts for only a short period of geological time. In the Siberian permafrost (earth that remains frozen year-round), for example, entire mammoths have been preserved in ice for thousands of years. The remains of the mammothsí last meals have sometimes been preserved in the stomachs, allowing paleontologists to study the animalsí diet.



(Soft-Tissue Preservation)

may occur in hot, arid climates, which can dehydrate organisms before their soft tissue has decayed fully. The skin itself is preserved for only a short time, but the impressions of the skin in the surrounding sediment can be preserved much longer if the sediment turns to rock. Paleontologists have found skin impressions of dinosaurs preserved by this method.




Organic Traps  

Whole organisms may become trapped and preserved in amber, natural asphalt, or peat (decaying organic matter). Amber is the fossilized remaining part of tree sap. When sap first flows from the tree, it is very thick and sticky, so as it runs down the trunk, it may trap insects, spiders, and occasionally larger animals such as lizards. These organisms can be preserved for millions of years with details of their soft tissue, such as muscles and hair-like bristles, still intact.



(Organic Traps)

Natural asphalt (also called tar) is a residue from oil that has seeped to the earthís surface from deposits in the rock below. When an asphalt pit is covered by water, thirsty animals that come to the pit to drink may become trapped in the sticky substance and be preserved. One well-known example of such an area is the La Brea Tar Pits of the Pleistocene Epoch (1.6 million to 10,000 years before present) in Los Angeles, California.


Peat Moss

(Organic Traps)

Animals may also be preserved in peat, although the acidic environment of this decaying organic matter may cause bones to lose their rigidity. Some human remains have been found in peat bogs in Denmark (2000 years old) and England (2200 years old).


Molds and Casts  

Acidic conditions may slowly dissolve away the skeleton of fossil animals preserved in rock, leaving a space where the organism used to be. The impression that is left in the rock becomes a mold. This process commonly occurs in fossil shells where the calcite shell dissolves easily. The impression of the outside of the shell is the external mold. Sometimes the inside of the shell is filled with sediment before the shell is dissolved, leaving an internal impression of the shell called an internal mold. If the space where the shell used to be is then filled with a new mineral, the replica of the shell forms a cast.


Tracks and Trails  (Trace)

When animals walk through soft sediment such as mud, their feet, tails, and other body parts leave impressions that may harden and become preserved. When such an impression is filled with a different sediment, the impression forms a mold and the sediment that fills the mold forms a cast. Molds and casts of dinosaur tracks are relatively common and help paleontologists understand how these creatures moved.


False Fossils  

Minerals can sometimes grow within rocks into shapes that resemble fossils. Dendrite crystals are often mistaken for fernlike fossils. Flint nodules in chalk can look like a variety of different life forms. Mineral concretions in sediments are sometimes mistaken for fossilized eggs. It is only with close study that the true nature of false fossils can be discovered.

Modern animals and plants sometimes become mummified or coated in travertine (calcium carbonate salts from springwater), or they may die while trapped in cracks in older rock strata. These remains are not true fossils, but trapped animals and plants may eventually fossilize with time.

Rock Types


a sedimentary rock that forms either by the accumulation of shells, shell fragments, or coral fragments, or by the crystallization of the mineral calcite from water.



coarse-grained, sedimentary rock consisting of consolidated masses of sand deposited by moving water or by wind. The chemical constitution of sandstone is the same as that of sand; the rock is thus composed essentially of quartz. The cementing material that binds together the grains of sand is usually composed of silica, calcium carbonate, or iron oxide.




common name applied to fine-grained varieties of sedimentary rock formed by the consolidation of beds of clay or mud. Most shales exhibit fine laminations that are parallel to the bedding plane and along which the rock breaks in an irregular, curving fracture. Shales are usually composed of mica and clay minerals, but the grains are so fine that the rock seems to have a homogeneous appearance, and individual minerals cannot be identified without the aid of a microscope. Most varieties of shale are colored in various shades of gray, but other colors, such as red, pink, green, brown, and black, are often present. Shales are soft enough to be scratched with a knife and feel smooth and almost greasy to the touch. All gradations in consistency exist between shales and clay; true shales differ from clays in their lack of plasticity in water.