Palaeo – Taphonomy

Question 1

What is taphonomy?

Answer 1

• The death of an organism is typically followed by decay, disintegration, and disappearance.
• On rare occasions, this decline is interrupted, and fossilization ensues.
• Taphonomy is the study of all processes occurring after the death of an organism – ending either in total annihilation or, in exceptionally rare cases, its discovery as a fossil.
• The process of taphonomy is typically broken down into two phases:
o Biostratinomy describes the history of an organism from its death through sedimentary re- working to its final burial
o Diagenesis relates its history between burial and exhumation.

Question 2

Taphonomic sequence

Answer 2

Taphonomic processes distort the way that original communities are depicted in the fossil record. To accurately reconstruct original organisms and original ecosystems, the effects of taphonomy must be understood and accounted for. 
Birth
	Ecology
Death
	Biostratinomy
Final burial
	Diagenesis
Recovery
	Interpretation
Reconstruction

Question 3

Taphonomic activity

Answer 3

• Besides the nature of the animal, the history of the carcass affects its preservation potential.
• Whilst it is in the taphonomically active zone – not just the sediment surface, but also such sub-strata as are affected by burrowing, root activity, or sedimentary re-working – a carcass is vulnerable to various forms of information loss.
• Erosion
• Sedimentary events
o Transport, dissolving, desiccation
• Burrowing organisms
Disarticulation: Losing information
• Further distance travelled leads to rapid loss of resolution and disarticulation

Question 4

Below the Taphonomically Active Zone

Answer 4

Even when a carcass is buried below the taphonomically active zone, its fate will be controlled by the chemical, tectonic and geological conditions in which it finds itself:
Diagenetic processes
• Metamorphism
• Chemical Replacement
• Erosion
It is not only individual organisms whose character is changed by taphonomic processes. Taphonomic processes winnow and re-sculpt life assemblages (biocoenoses, if you want the technical term) – which reflect the original living community – into death assemblages (thanatocoenoses) may bear little relation to original communities.

Question 5

Predilection for preservation

Answer 5

• Fortunately, taphonomy is not entirely random; certain factors affect the propensity of organisms to fossilize in a relatively predictable fashion.
• Composition is a primary control on preservation potential – biomineralized elements (such as shells, teeth or bones) or carbonaceous components (such as wood, spores, graptolites or claws) are less likely to decay or be eaten than ‘soft’ tissues such as muscle.
• If the robust components of an organism are few in number and tightly integrated (such as an ammonite shell or a trilobite exoskeleton), then they will be relatively robust to disarticulation – an organism with many, loosely attached components (such as a crinoid or sponge) is likely to disintegrate rapidly, making it difficult to reconstruct the complete original organism.

• Hard to capture an entire ecological picture
o Geological record is a sampling method that does have flaws

Question 6

• Biomineralization

Answer 6

o Calcium carbonate has three principal forms:
 Aragonite is metastable and is readily dissolved or replaced
 High magnesium calcite is more stable and low magnesium calcite is the most robust to diagenetic alteration (found preserved trilobite lenses).
 Calcium phosphate (apatite) is the principle component of the vertebrate skeletons (porous so more easily broken, teeth don’t have this issue) and the shells of inarticulate (lingulid) brachiopods.
o Silica forms the skeletons of sponges and certain plankton and is the biomineral of choice for land plants!
o Also: carbonaceous biopolymers
 Cuticle
 Wood
 Cell walls
Each of these compositions has its own properties, and so will have its own taphonomic idiosyncrasies. One of the most important is the window of pH (acidity) and eH (oxidising/reducing environment) in which they are stable.
A compositionally unstable shell can be subject to a range of fates: metastable aragonite may recrystallize, obliterating the original structure; or dissolution of the original material may leave a void that may later be filled with mud or other minerals. It’s important to distinguish a mould (an impression of the surface of a fossil) from a cast (an infilling of a mould).

Question 7

Taphonomic gain

Answer 7

Information gain about environment based on fossil record
• Besides the obvious mixing and reorientation effects of transportation and the information loss caused by abrasion, dissolution, bioerosion and breakage, the relatively long residence time of biomineralized components allows for time averaging, where a fossil assemblage includes elements of different ages.
• At a short timescale, this can be a powerful tool of information gain: a fossil assemblage may record members of winter and summer residents, and thus provide a more complete indicator of palaeodiversity than a single snapshot.
o Time averaging: Differences between life and death assemblages
• At a longer timescale, time- averaging may record pioneer taxa alongside members of a climax community, allowing a fuller reconstruction of a palaeoenvironment, though perhaps distorting palaeoecological signal.
• Although taphonomy serves to destroy a particular category of biological information, it may also provide information regarding the depositional environment.
o High concentrations of highly abraded shelly debris may suggest a restricted input of terrigenous clastics.
o The extent of breakage, abrasion and transport can illuminate the energy of an environment, whereas the orientation of fossils may help to reconstruct prevailing currents.
o The survival of certain types of tissue can establish the nature of pore- water chemistry.
o And biological taphonomic processes such as bioimmuration and bioerosion (particularly borings or bite-marks) can point to the existence of taxa that are not present as fossils.