9.3: Modes of Fossil Preservation

Unaltered remains

An amazing diversity of fossils exist in a relatively unaltered state. While there may be some compression of the fossil material due to the burden of burial sediments above, or separation of individual shell valves or bones, unaltered remains are skeletal components preserved in their original mineral composition. It is important to have a general sense of what the expected mineralogy of a fossil organism should be to determine whether another mode of preservation could apply to the specimen.

Table \(\PageIndex{1}\)

Common fossil mineralogy	Common organisms
Calcite (\(\ce{CaCO3}\))	Foraminifera, coccolithophores, sponges, tabulate and rugose corals, brachiopods, bryozoans, trilobites, molluscs, echinoderms
Aragonite (\(\ce{CaCO3}\))	Modern (scleractinian) corals, molluscs
Quartz (\(\ce{SiO2}\))	Radiolarians, diatoms, sponges
Hydroxyapatite \(\ce{Ca5(PO4)3}\)	Vertebrate bone and teeth

Figure \(\PageIndex{1}\): Wooly mammoth (Mammathus primigenius) hair.

In some exceptional cases, soft tissues, such as skin, feathers, or fur may be preserved. This type of preservation does not typically occur within sediments, but rather, in materials like amber or glacial ice.

Permineralization

Permineralization occurs when open spaces within porous tissues are filled in with mineral crystals after the skeletal material of the fossil has been buried. Groundwater containing dissolved minerals passes through the buried fossil, and when that water evaporates, minerals grow in the available space. Typical infilling minerals include quartz, calcite, and iron oxides, which are all common cementing minerals for sedimentary rocks. Permineralization is a common mode of preservation for vertebrate bones and wood, but can also occur in some shells. In the case of petrifaction (“petrified” wood), minerals both fill the porous spaces in the plant tissues and replace the organic material of trees. Permineralized fossils tend to maintain a high degree of detail, as they preserve both internal structures and the three-dimensional shape of the original organism. Because crystal growth occurs in the prior open spaces within the skeletal material, permineralized fossils tend to be denser and heavier than their original counterparts.

Figure \(\PageIndex{2}\): Permineralized wood from the late-Cretaceous Hell Creek Formation of eastern Montana. Note the preservation of the fine internal growth rings. Yellow pencil provides a sense of scale; each black bar on the pencil is 1 cm long.

Carbonization

When soft tissues are quickly buried in quiet water, they are subjected to increased pressure and rising temperatures as part of the lithification process. These conditions affect the remaining organic material, resulting in a process that releases chemically volatile compounds. When the volatiles are released, a dark-colored film of chemically stable carbon is left behind. Intricate details of delicate soft tissues may be preserved, including individual cell structure in plants, fish scales, insect wings, and even internal organs.

Figure \(\PageIndex{3}\): Crane fly, Florissant Fossil Beds National Park

Replacement

Figure \(\PageIndex{4}\): Pyritized trilobite, Triarthrus eatoni, Upper Ordovician, New York

Increased pressure and temperature conditions in the deep burial of sediments liberate trapped fluids which contain mobile chemical ions. The fluids interact with buried fossils and may completely dissolve the original mineral or organic material, replacing it with different ions that precipitate from the circulating fluids. Common replacement minerals include silica and pyrite. The decay of organic material requires bacterial support which can assist in reducing sulfur and precipitating the iron sulfide mineral, pyrite. Often, much of the detail of the original organism can be preserved via replacement.

Recrystallization

Recrystallization is an important process that preserves fossils made of aragonite, a common, but chemically unstable mineral over the geologic time. Many marine invertebrates (molluscs, modern corals) have a metabolism that allows them to crystallize aragonite as their skeletal material. Aragonite is a polymorph of \(\ce{CaCO3}\) and it sometimes appears as beautiful “mother of pearl” on the inside nacreous layer of shells. But outside of the organism’s living tissue, aragonite is less chemically stable than calcite, and so it spontaneous recrystallizes to make calcite after burial. This calcite “version” of the shell is more likely to last long enough to enter the geological record and be discovered by geologists. There is no change to the chemical formula of the mineral during recrystallization, just the same atoms reconfiguring into a more stable crystal structure. This preserves the overall shape of the fossil, but recrystallization can destroy some of the finer details as new crystals form.

Figure \(\PageIndex{5}\): Iridescent mother of pearl on nautiloid shell

Internal molds

Figure \(\PageIndex{6}\): Internal mold of snail. Note shell is dissolved away and only infilling sediment remains.

When the interior space of an organism is buried and filled with sediment that is then lithified, the interior shape is preserved as an internal mold. Imagine making a sand castle at the beach, filling a plastic bucket with wet sand, then inverting it onto the ground and lifting the bucket away. The shape of the inside of the bucket is preserved though the bucket itself is gone–that’s an internal mold. In geology, this mode of preservation is usually found in bivalves or brachiopods, or snails that have a single open chamber where sediment can get in and work its way into the entire space. This mode of preservation can reveal lovely impressions of internal features like muscle scars, but since most molluscs are more readily identified using external ornamentation, loss of the original shell can make things tricky for paleontologists.

In some cases, small cavities inside a shell may remain after sediment is washed into the shell. The sediment sinks to the bottom of the cavity and leaves a pocket of water above. Over time, these pockets can later close up with precipitated minerals, potentially serving as geopetal structures. Geopetal structures are “up indicators”, and give geologists a good sense of which way was up when the bed was deposited, which can be helpful telling the stories of sedimentary layers that have been deformed by tectonic processes. In the image below, examine the difference in sizes of the infilled calcite crystals in the area outlined in pink versus the area outlined in orange. Which way was up?

Figure \(\PageIndex{7}\): Geopetal structures in Reynolds Limestone

External molds

Figure \(\PageIndex{8}\): External mold of a bivalve from the Logan Formation, Lower Carboniferous, Ohio

An external mold is an impression of an organism’s outside shape. This type of preservation commonly occurs when a fossil is pressed into soft sediment. The original hard parts of the organism may then be transported away by currents, or after burial and lithification, the original hard parts may dissolve away. When external molds are discovered, the observer sees a version of the exterior of that organism, but it is not an exact replica because it shows negative relief. Negative relief means that on the impression, raised areas represent indentations on the original fossil, while depressed areas represent raised areas. If you have ever walked along a beach, turned, and examined the footprints you left behind, these are negative impressions of your feet. If that sand could lithify instantly, it would represent an external mold of your foot.

Figure \(\PageIndex{9}\): Dinosaur trackways, Dinosaur Ridge, Colorado.

Fossil trackways, a type of trace fossil, are a type of external mold. Some very famous dinosaur tracks are preserved in this way. Most often, external molds are created by hard shells or bone. However, some external molds of dinosaur skin have been discovered, meaning that even soft tissues can be preserved via this mode under the right conditions.

Another method of creating external molds is through bioimmuration. Bioimmuration comes in several forms, but consists of the creation of an external mold through the encrustation of other organisms on exterior surfaces.

Figure \(\PageIndex{10}\): Bioimmuration of a soft-bodied organism, Catellocaula (star-shapes) on bryozoan

For example, barnacles often encrust boats and if you were to remove the wood from the barnacle, an impression of the wood would remain on the bottom of the barnacle. Sometimes, whole successions of external encrusters will attach themselves. These can certainly include arthropods such as barnacles, but also encrusting corals, brachiopods, sponges, and many more organisms just looking for a place to attach. Bioimmuration as a form of external mold only happens when the original hard substrate dissolves, rots, or is otherwise removed, leaving behind the encrusting organisms and, very often, a history of successive encrustations that give important glimpses into unique paleoecological situations.

Figure \(\PageIndex{11}\): External mold (left) of a trilobite body fossil in a concretion

Casts

Figure \(\PageIndex{12}\): Sue, the most complete (and famous) Tyrranosaurus rex fossil at the Field Museum, Chicago.

Casts are replicas of organisms, essentially recreating their original shape. First, an organism must be preserved as an external mold, or perhaps open burrows preserved as trace fossils. The impressions or other cavities are then filled in with sediment, producing a three-dimensional form that mimics the form of the original fossil. Imagine if you took some clay and pressed it into the external mold of the trilobite above. Once hardened, you could pop that trilobite out and have a cast of the original fossil, mimicking its original form, and presented in positive relief. Some details are typically lost when a cast is created, but can still provide critical information about the organism. Interestingly, many fossil skeletons on display at museums are primarily casts. This doesn’t mean the fossils are fakes since the casts are created based off the original fossils. Casts are used for many reasons: the original fossils can be hundreds of pounds of permineralized bone making it a challenge to mount the specimens, the originals may be fragile and require a controlled environment for preservation, it is a way to share specimens among museums, or the fossils may be currently be under study.

Modes of Fossil Preservation - Quiz

Exercise \(\PageIndex{1}\)

What mode of preservation is this?

a. Internal mold

b. External mold

c. Cast

Answer

b. External mold

Exercise \(\PageIndex{2}\)

The opalized belemnite (mollusc) in this photo is from the Coober Pedy mines in Australia. Opal is a form of silica (quartz mixed with water). What mode of preservation is this?

a. Replacement

b. Permineralization

c. Recrystallization

Answer

a. Replacement

Exercise \(\PageIndex{3}\)

What mode of preservation is this?

a. Carbonization

b. Unaltered hard parts

c. Permineralization

Answer

a. Carbonization