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 biocoenosis (life assemblages) and thanatocoenosis (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.

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 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
• 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

An interactive website about geologic time.

The Geologic Timescale.

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Last modified: 12 January 2007