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 organic material).

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

• 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. oceanic basins)

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.

• 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 lost.
• 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)
  • But there are some rarer examples
• 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
  • Major categories:
    • Condensation deposits (decresed rate of sedimentation)
    • Placer deposits (hydrodynamic concentration by currents, eddies)
    • Concentration traps (holes, pits, caves)
• Konservat-lagerstätten: unusually well-preserved fossils
  • 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
  • Major categories:
    • 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 favors calcite
  • Phosphates: low rate of burial, high organic content. For low salinity, vivianite; normal salinity, apatite
• Three common modes of soft tissue preservation by authigenic minerals:
  • Permineralization: rare, and only phosphates. Most effective of "hard" soft tissues, like cellulose and chitin
  • 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!!)
• Hunsrückschiefer (Early Devonian, Rhineland, Germany):
  • Environment: stagnation deposit, muddy offshore
  • Preservation: pyritization of soft tissues (can be seen best in X-rays)
  • Significance: anatomical details of soft tissues of Devonian bottom dwellers
• Mazon Creek fauna of Francis Creek Shale (Pennsylvanian, Illinois):
  • Environment: deltaic environment between coal swamp and epeiric sea
  • Preservation: soft tissues preserved as carbonized films in ironstone nodules and concretions
  • 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
• Holzmaden (Lower Jurassic, Swabia, Germany)
  • Environment: deepwater black shale
  • 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.
• Solnhofen Lithographic Limestone (Late Jurassic, Bavaria, Germany)
  • Environment: anoxic shallow lagoons in warm sea
  • Preservation: impressions of soft tissues, as well as undisturbed bony and other hard part fossils
  • 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.
• Jehol Group
  • Environment: lacustrine deposit with volcaniclastic sediment
  • Preservation: carbonized films of soft tissues
  • Significance: abundant preservation of flowers, insects, vertebrate integument (including mammalian fur, reptilian scales, and dinosaurian feathers).

Last modified: 30 August 2022