•Taphonomy is the study of the incorporation of living things into the sedimentary record.
•Various factors (taphonomic filters) control the likelihood that a given body might become a body fossil.
•Diagenesis (chemical modification of the fossil material after deposition) results in varying types of preservation.
•Lagerstätten are localities of exceptional concentration or preservation of fossils.
"During each year, over the whole world, the land and the water have been peopled by hosts of living forms. What an infinite number of generations, which the mind cannot grasp, must have succeeded each other in the long roll of years! Now turn to our richest geological museums, and what a paltry display we behold!"
--Charles Darwin, Chapter IX "On the Imperfection of the Geological Record", On the Origin of Species by Means of Natural Selection First Edition
Fossils as Sedimentary Particles, Burial & Taphonomic Filters:
Taphonomy: study of incorporation of living things into the sedimentary record
Taphonomic processes include necrolysis (the break up of organisms after death), biostratinomy
(the burial process itself), and diagensis (the post-burial transformation of the
Many differences between biocenosis (life assemblages) and thanatocenosis (death
assemblages) as revealed by studies of Aktuopaläontologie (taphonomic studies based on comparisions to natural or
experimental examples of modern organisms being incorporated into sediment). Some differences include:
Preservation potential of organism
Preservation potential in substrate
Diagenetic effects after burial
All the above represent taphonomic filters. Different organisms thus have different potential for fossilization.
Hard parts vs. no hard parts
Single hard parts (e.g., gastropods & cephalopods) vs. two hard parts (e.g.,
brachiopods & bivalves) vs. many well-connected parts (e.g., many arthropods & echinoderms)
vs. many parts connected only by soft tissue (e.g., vertebrates, holothurians)
Microscopic to sediment-sized to immense
Lived in erosive environments (e.g., mountains) vs. depositional environments
Lived in accessible vs. inaccessible environments (e.g., continental shelves vs.
In the case of vertebrates, it is common for the different individual bones (which have
different hydrodynamic properties) to be transported different distances for the initial point
of death: Voorhies groups.
Plants a special case: different organs (leaves, stems, trunks, fruit, flowers, seeds,
pollen, etc.) are only very rarely preserved together. Each part generally given its
own species name!
Autochthonous vs. Allochthonous fossils or fossil assemblages: in their original
spot (in situ) vs. transported. Some autochthonous fossils might be in situ reefs;
or they might be only vertically transported (sank from above).
Allochthonous fossil assemblages requires transport; therefore, imprint of transportation
processes on the assemblage.
Diagensis: Modes of Fossilization:
Unaltered: simple burial, some weathering. Becomes rarer (for stochastic
reasons) further back in fossil record.
Permineralized: very common mode
Pore space is filled in with ground water: some dissolved minerals precipitate in
pores (probably some contribution by bacterial activity)
Common minerals found in permineralized fossils: silica, calcite, phosphates (rarer include uraninite)
Original hard parts remain, but extra material added to pores
Recrystallization: very common in calcareous fossils. After burial, calcite
crystals reorder and grow into each other. Original mineralogy remains, but structure is
Replacement: grades from permineralization
Partial to complete replacement of crystals of one mineralogy with another, controlled
by hard part material and by dissolved material in ground water
Bone (hydroxylapatite) can be replaced with uranium-bearing minerals, for instance
Common forms of replacement: silicification; pyritization;
phosphatization (often high phosphate apatite)
Carbonization: organic material is "distilled" under pressure. Many volatiles
lost: carbon film left behind. Mode of preservation of coal. Also preserves soft tissues
of various animals and plants. Bacterially controlled.
Fossil Site Analysis:
Most of the discussion above primarily concerns the history of single individuals. However,
much information can be gained by looking at the totality of the fossils from a single horizon at
a single site. This data might include:
Taxonomic diversity: Number of taxa represented
Relative abundances: How many of each taxon is represented.
For macrofossils, this
might be calculated as Minimum Numbers of Individuals (MNI):
For example, for bivalved organisms, counting all the left valves, and all the right valves. The larger
number represents the MNI
For vertebrates, counting the number of particular common preserved bones, and finding which gives
the MNI (for example, if there is only one skull, three left humeri, and twelve right femora, we know that there
were at least 12 individuals)
For microfossils, might calculate relative abundances both as percentages and total counts
from a unit volume of sediment
Orientation of Fossils: Indicates something of the environment of deposition during the assimilation of
the fossils into the rock record. Randomly oriented fossils may indicate autochthonous deposit; oriented long bones,
tree trunks, and similar fossils will give indication of directionality of flow (and thus likelihood that the
deposit is allochthonous and/or at least slightly disturbed)
Can plot the orientations by means of a rose diagram
German mining jargon for "motherload" or "bonanza". However, usually used outside Germany for
what is properly "Fossil-Lagerstätten".
Adolph Seilacher proposed two major types of Fossil-Lagerstätten:
Konzentrat-Lagerstätten: anomolously high amounts of fossil material
Possibly by decreased rate of sedimentation
Possibly by increased rate of organism reproduction ("bloom")
Possibly by increased rate of organismal death
Condensation deposits (decresed rate of sedimentation)
Placer deposits (hydrodynamic concentration by currents, eddies)
What is generally thought of by the term "Lagerstätten"
Often requires anoxic bottom conditions (so no scavenging), quiet water (so bodies are
not disturbed), rapid burial (to reduce possibility of mechanical destruction of material)
Sometimes preserves not only hard parts, but impressions and/or carbonizations of soft parts;
possibly even mineralized soft parts
Stagnation deposits (autochthonous conditions of anoxia, low currents, etc.)
Obrution deposits (assemblage is transported into such conditions)
Conservation traps (amber, for example)
Lagerstätten represent extremely important window into the past:
Taxa (such as soft bodied organisms) about which we wouldn't otherwise know
Morphologies (again, soft tissue) about which we might not know
Some attributes of soft tissue preservation:
Typically bacterially mediated
Generally begins VERY quickly (diagensis starts within hours of burial)
Rate of burial, salinity, and organic content of sediment and organisms will determine
which new (authigenetic) minerals form to preserve soft tissue:
These include pyrite, carbonates (calcite and siderite), phosphates, and silica
Pyrite: rapid burial, low organic content, normal or near-normal salinity, and sulphate
must be available
Carbonates: rapid burial, high organic content. For low salinity, siderite forms; normal salinity
Phosphates: low rate of burial, high organic content. For low salinity, vivianite; normal
Three common modes of soft tissue preservation by authigenic minerals:
Permineralization: rare, and only phosphates. Most effective of "hard" soft tissues, like cellulose
Mineral coats: most common. May be phosphate, carbonate, or pyrite.
Tissue casts: rapid stabilization of sediments through diagensis prior to lithification
of rock as a whole. Often as nodules and concretions. May be siliceous or calcareous.
Some Famous Lagerstätten
Burgess Shale (Mid-Cambrian, British Columbia, Canada)
and related earlier Cambrian (Chengjiang, China;
Sirius Passet, Greenland) to Ordovician (Fezouata, Morocco)) sites:
Environment: obrution deposits from limestone escarpment into deeper water by turbidity current
Preservation: carbonization enhanced by pyrite (some films are actually calcium aluminosilicates!),
preserving soft tissue of many taxa
Significance: demonstrates high taxonomic diversity and morphological disparity of Cambrian
forms which would not have otherwise been preserved. Did not probably represent an unusually
diverse community compared to other Cambrian sites: instead, shows us what "typical" Cambrian
faunas actually were like (i.e., they weren't all trilobites!!)
Environment: deltaic environment between coal swamp and epeiric sea
Preservation: soft tissues preserved as carbonized films in ironstone nodules and
Significance: shows diversity of soft bodied organisms in both freshwater and marine
enivronments. Some of the oldest (and only Paleozoic fossils) of many modern taxa lacking
hard parts, and some of taxa whose phylogenetic affinities are far from certain
Preservation: carbonized films of the soft tissues around skeletons of marine vertebrates
Significance: preserves anatomical details otherwise undetectable from skeletons of ichthyosaurs (dorsal
fins, caudal fin, etc.). Because of high abundance of pregnant females, suggests that this was
a birthing region.
Preservation: impressions of soft tissues, as well as undisturbed bony and other hard part
Significance: shows diversity of marine organisms around the time of the origin of "modern"
style invertebrates. Also, high diversity of pterosaurs (flying reptiles). Locality for the
early feathered dinosaur (basal bird) Archaeopteryx lithographica.