Fossilization
• I. Possess hard parts
• II. Get buried
• III. Become preserved over the long term
– Permineralization - petrification, precipitation of minerals in
skeletal pores.
– Recrystalization- mineral reorganization of shell material,
destroys original microstructure.
– Replacement- substitution of a mineral substance, simultaneous
substitution-Pyrite, Calcite, Dolomite, Iron Oxides, Silica
– Carbonization - thin film of carbon residue due to volatilization of
H 2 , O2 , N 2
– Molding and casting - Negative impressions and positive images
in sediment.
• IV. Avoid dissolution
• V. Become exposed over the long term. Tectonic
uplift essential
• VI. Get discovered
TAPHONOMY
• Processes that occur between the death of an
organism and its subsequent burial in the sediment
are termed biostratinomy.
• Generally, this includes the decomposition and
scavenging of the animal's soft parts, and at least
some amount of post-mortem transport.
• Such things as the amount of shell breakage and the
concentration of shells in layers often indicate the
level of water energy and post-mortem transport.
• For example, the shell-hash or coquina has
experienced a significant amount of shell breakage
and probably post-mortem transport suggesting
deposition in high energy environments; whereas,
intact articulated plant remains suggest little or no
post-mortem transport and deposition in a very low
energy and oxygen-free environment.
The field of Taphonomy as it relates to steps in transformation from
living organisms to fossils
Death
Preservation- Unaltered Soft Parts
• 1. Remains of mammoth and rhinoceroses in frozen
tundras of Siberia and North America.
• 2. Insect exoskeletons and minute appendages have
been preserved in amber, the hardened resin of ancient
coniferous trees. (The Jurassic Park scenario)
• 3. In Poland an Ice Age wooly rhinoceros was well
preserved in asphalt and the La Brea Tar Pits of Los
Angeles contain many Pleistocene animals.
• 4. In semi-arid South America, parts of mummified
ground sloths have been preserved in caves.
• 6. Algae and bacteria have been reported preserved in
Precambrian chert.
Preservation- Unaltered Hard Parts
• 1. Calcite (CaCO3 is the most abundant original skeletal
material found in fossils
• 2. Aragonite (CaCO3) is preserved in the shells of some
corals and molluscs. Rare in late Paleozoic fossils,
absent from older ones, since aragonite tends to invert
into the more stable calcite or dissolves.
• 3. Tricalcium Phosphate (Ca3(PO4)2) is a chemically
resistant mineral found unaltered in vertebrate bones,
some arthropods and brachiopods.
• 4. Opal (SiO2H2O) is an amorphous hydrated silica
preserved in latter geologic time in some Protozoans and
sponges. It is unstable.
• 5. Chitin is an organic compound that is resistant to
bacterial action and is not readily altered. It is found in
many arthropods and graptolites
Permineralization
• (Petrification) process whereby porous
skeletons are made denser and heavier by
precipitation of minerals by ground water
in the open spaces or pores of the
skeletons. E.g. calcite, silica and
glauconite
Recrystalization
• Refers to the process whereby the shell material
undergoes reorganization that partly or
completely destroys the original microstructure
• This is a common phenomenon in shells which
were originally aragonite and/or calcite (both
forms of calcium carbonate- CaCO3).
– Examples, both of which are now calcite, include a
gastropod which was originally aragonite and a
brachiopod which was originally calcite
Replacement
• Refers to mineral substances being substituted for the
hard parts (sometimes even soft part) of organisms,
probably by simultaneous solution and precipitation.
– Pyrite (FeS2) frequently replaces shells: pyritization.
– Calcite may replace silica, calcite and aragonite:calcification.
– Dolomite (CaMg(CO3)2) may replace calcite or aragonite shells:
dolomitization.
– Replacement may be done by iron minerals such as hematite or
limonite.
– Silica (chalcedony and quartz) commonly replaces calcareous
skeletons: silicification.
– Other carbonates (siderite, rhodochrosite) may also replace
shells, though less commonly: calcification.
Carbonization
• Involves a decrease in volatile constituents
of tissues and chitin such as hydrogen,
oxygen and nitrogen, leaving a thin film of
carbon. E.g. graptolites, plant remains
Molds and Casts
• Skeletons are frequently found completely
dissolved away by meteoric and groundwaters,
leaving molds (negative impressions) of their
internal or external surfaces in the sediments
enclosing them.
• Casts are formed when sediment or mineral
precipitates fill these molds. A cast is a positive
image identical in symmetry to the original. As a
general rule, if a shell has been dissolved away
you most likely have a mold
Trace Fossils
• Tracks, trails and borings: imprints,
burrows or tubes made by organisms are
common.
• Coprolites are fossil excreta.
• Gastroliths: smooth rounded pebbles
found in the stomachs of dinosaurs, an aid
to digestion.
• Artifacts: stone tools or weapons of
humanoids
Pseudofossils
• Inorganic structures that may resemble an
organism or part thereof or a trace or
imprint of one. E.g. dendrites (manganese
oxide), banded flint, desiccation cracks,
tool marks, liesegang rings (rhythmic
precipitation in a gel), soft-sediment
deformation structures