GEOL2301: Palaeoecosystems

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, just occasionally, its discovery as a fossil. The process of taphonomy is typically broken down into two phases: biostratinomy describes the history of an organism from its death through sedimentary re-working to its final burial, whereas diagenesis relates its history between burial and exhumation.

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.

• What will a burrowing bivalve, such as the model pictured here, look like 10 million years from now?

• Perhaps its burrowing position will keep it from being swept away, and the surrounding mud might even keep its valves together and allow it to be fossilized and discovered. Its soft parts (i.e. anything but the shell) don't stand much (really any?) chance of surviving.

  But much, much more likely is that the shell will be totally annihilated.

  The rest of the session will introduce you to the processes involved.

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.

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.

• What sorts of processes might affect shells whilst they are in the TAZ?
• I mention transport; burrowing; chemical destruction; sediment deformation; erosion. Can you think of more?
• What diagenetic processes might affect shells when they are below the TAZ?
• I mention metamorphic deformation, insertion of veins, and replacement / remineralization. Can you think of more?

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) – original living communities – into death assemblages (thanatocoenoses) which may bear little relation to original communities.

• How might taphonomic processes distort our reconstruction of macroevolutionary patterns?
• Beside the obvious effects of the amount of rock available and the extent of metamorphosis / reworking, think about how groups with different properties (mineralogy, propensity to disarticulation, substrate niche) might have different preservation potentials. How well would we be able to track diversification of different groups in different settings?

• I use the term "life assemblage" strictly, to refer to the full set of organisms that actually lived together. Some A-level text books use the term more loosely, to mean "a death assemblage that is quite similar to the life assemblage".

Predilection for preservation

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, graptolite periderm, 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.

Biomineralization typically employs one of three major minerals:

• 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.
• Calcium phosphate (apatite) is the principal component of the vertebrate skeletons and the shells of inarticulate (lingulid) brachiopods.
• Silica forms the skeletons of sponges and certain plankton, and is the biomineral of choice for land plants.

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.

• Use your mouse to mark the region of eH–pH space in which each mineralogy is stable
• CaCO₃ Pyrite Silica Apatite Organic Carbon ×
• Reset the canvas by double-clicking the white ×.

• Which form of calcium carbonate is most likely to persist through geological time?
• Aragonite
• Low-magnesium calcite
• High-magnesium calcite
• Aragonite is metastable; input energy and it'll recrystalize. Mg atoms introduce irregularities and thus strain into the CaCO₃ crystal lattice; the more Mg is involved, the less stable the calcite will prove.
• Which common skeletal biomaterial will be most likely to survive diagenesis in an acidic, reducing environment?
• Pyrite
• Carbonate
• Organic carbon
• [Calcium] carbonate dissolves quickly in acid. Pyrite is stable – but it's not a widespread biomineral. (Search for the Scaly-Footed Snail to find a curious exception.) Organic carbon survives well in these conditions: picture the yukky leaf matter at the bottom of an oft-stagnant pond.

Time-averaged assemblages

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. 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.

• Why might a species appear in a life assemblage, but not a death assemblage?
• This low preservation potential may reflect composition (lack of durable hard parts), ecology (preservation potential of infaunal vs. epifaunal taxa? Succeptibility to end up in a predator's gut?), transportability (buoyancy?)… what else?
• Why might a species appear in a death assemblage, but not a life assemblage?
• A species may be a temporary resident (seasonal abundance? migration?) or may be transported in from elsewhere. Time-averaging may reflect a community that preceded a long-term change (outcompeted pioneer species? local extinction?)
• How can one be confident that ecological interpretations are not simply taphonomic biases?
• It's tricky! If a reconstruction matches a bias that is known to occur (e.g. dominance of suspension feeders in biomass counts?), then we might treat the conclusion with a little suspicion. Organisms themselves might give indirect evidence: for example, apedunculate brachiopods might struggle to stay put in a high-energy environment; low-energy environments may struggle to support a highly tiered community of suspension feeders. It's definitely worth evaluating whether a quantitatively derived reconstruction is consistent with what you know and can infer about the actual organisms that are preserved.

Taphonomic gain

Although taphonomy serves to destroy a particular category of biological information, it may also provide information regarding the depositional environment. High concentrations of highly abraded shelly debris may suggest a restricted input of terrigenous clastics. 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. The survival of certain types of tissue can establish the nature of pore-water chemistry. 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.