Taphonomy is the study of processes by which organic remains and traces are incorporated into the fossil record. The term is derived from Greek roots: taphos, meaning burial, and nomos, meaning law. Taphonomy is the subdiscipline of paleontology and archaeology predominantly concerned with the characteristics and context of fossil remains, in which the challenge of deciphering information from ancient remnants is considerable. The rigors of death, transport, and burial severely modify and degrade organic remains to cause information loss: loss of original chemistry, loss of original size and shape, and loss of biological context. Only rarely do geologic processes stem this loss and preserve the remains of an individual or assemblage as a fossil. By then, the richness and diversity of a living ecosystem has become a few bones or shells scattered along a thin layer of silt-stone. By measuring, mapping, and understanding the way dead vertebrate, invertebrate or plant remains behave on this transition from biosphere into lithosphere, taphonomists help clarify natural biases of the fossil record and oppose the entropy of information invoked by death and time.
History
The origins of taphonomy lie in the development of paleontology during the late 15th century, when Leonardo da Vinci recorded concern that marine fossil deposits found at elevations far from the coast could not be explained by the preponderant theory of biblical deluge. Questions about what agents and mechanisms could explain the mortality, assemblage, and preservation of such fossil deposits formed the basis of taphonomical investigation, as it became recognized that a better understanding of these processes could provide more accurate answers about ancient life and environments. Archeology and forensic science were also challenged by postmortem processes to focus, in archaeology’s case, on prehistoric hominid reconstructions, and in forensic sciences, to discern the time and cause of death for legal reasons. The fusion of these aspects with concepts of physical biology and sedimentology occurred in 1940 when the Russian paleontologist Efremov coined taphonomy as “the study of the transition (in all its details) of animal remains from the biosphere into the lithosphere.”
Process and bias
Taphonomic studies are based upon observing the abundance, distribution, and modification attributes of fossils. Data collection starts with the mapping, measuring, and identification of all elements in context with enclosing sedimentology. The taxa present may be identified and any evidence of sexual and age distributions considered, with a view to later calculating the relative abundance and representation of different species.
The spatial relationships and orientation of fossil elements can be critical to the interpretation of paleohistories. The alignment of elements (or a whole carcass) can demonstrate factors from the time of death or burial such as stream flow direction and rate of deposition. Further information about the taphonomic history may be deduced by combining the spatial data with detailed lithological study. In the case where an accumulation of bones with vertical orientations could derive from a high-energy hydraulic event or be the result of biological factors like trampling around a waterhole, the structure of sedimentary layers surrounding the bones will indicate the correct paleohistory.
Further evidence can be found directly on the fossilized elements, most of which show modification and damage from different stages of their life-and-death cycle. By studying characteristics of the damage, such as shape, size, and position on the element or carcass, the most likely explanation or taphonomic path may be resolved. Pathological and traumatic damage is sustained during life and is evidenced by diseased tissue, natural fractures, and healed bone growth. Postmortem damage may result from many factors including predation, scavenging, trampling, and transport, which may all leave signs upon the hard tissues, such as tooth marks and erosions. Postburial damage is sustained due to the compaction of burial in sediment or diagenetic dissolution, which may leave characteristic fractures.
The correct identification of such fossil evidence can be greatly assisted by studying the taphonomic path of modern organisms. For example, when reasoning why there is a preferential preservation of males over females in Pleistocene deposits of fossil Irish elk, it may be of great help to study the factors affecting seasonal mortality in extant elk herds. Other types of actualistic experiments may include the agents of decomposition, comparing the effects on fresh organic tissue with those of fossil counterparts. Conclusions must allow for differences when comparing environments across great timespans in order to minimize the degree of bias that will inevitably develop.
Our view of the quality and quantity of biota in ancient environments can also be biased due to organisms having different preservation potential. Mortal processes tend to destroy soft-bodied organisms and preferentially preserve those with hard shells or strong skeletal structures. This causes hard-bodied organisms to be overabundantly represented in the fossil record, skewing our data with which to reconstruct accurate paleoecologies. The effect can apply equally within fauna or marine environments, in which the movement of shell material by waves or currents will affect the eventual composition of the fossil assemblage. Small or thin shells may be abraded away while larger, more robust shell material may be concentrated. Another factor for taphonomists to consider is that only those remains of organisms inhabiting a depositional environment will tend to be buried and fossilized, compared to those from erosional or nondepositional areas. These sorts of factors explain the paucity of taxa in some fossil assemblages, and why fossils of terrestrial vertebrates are rare compared to marine invertebrates. In extremely rare cases a mass mortality event will capture a census assemblage where all of the contemporaneously living members of a community are preserved together. These exceptional deposits are termed Fossil-Lagerstätten, coined from a German mining word meaning mother lode. These deposits are treasured for their greater, more complete sample of biota that provides less biased evidence with which to study paeloecologies.
Mortality
Death is an inevitable part of the cycle of physical elements through a biologic host, and the modes of death may be coarsely classified into dying or being killed. Although the actual causes of mortality are many and include death by predation, drowning, dehydration, starvation, and hypothermia, amongst so many others, it remains difficult to ascertain with confidence the cause of death in any long-dead organism from its fossilized remains. Any direct evidence of fatal cause is usually requisite in the soft tissues, which rarely endure into fossilization. Most evidence of attritional mortality is ambiguous. Even in the rare fossils in which a vertebrate forms the stomach contents of another, it is challenging to determine if the consumed organism was killed in the act of predation or was in fact scavenged after an earlier unrelated mortal event. Catastrophic events can be more suggestive by providing an apparent sedimentological record with which to analyze mortal cause. Dinosaurs in Mongolia were fossilized in the act of fighting or still perched upon a nest of eggs, demonstrating how rapid the sand-slides were that entombed them, and thereby showing the likely cause of death to be suffocation.
Biostratinomy
At the instant of death, a host of biological and physical processes will modify and degrade an organism’s remains until the potential protection of burial. The study and understanding of these processes and their effects on taphonomic histories is called biostratinomy. The first agent of change is usually necrolysis, the process of decomposition and putrefaction.
This removes the soft tissues that normally bind bodily elements together, allowing them to disarticulate. Other agents that can effect disarticulation are predators and scavengers, including ancient man whose tool use during butchering operations created unique marks and fractures on bony elements. Necrolysis and disarticulation also have the effect of exposing a greater amount of the remains to weathering factors such as saturation, desiccation, and temperature changes, as evinced by characteristic surface textures, oxidation, flaking, or fractures. The consumption of remains by predators and scavengers will often result in further disarticulation and scattering, but not the total loss of material. The resulting fecal deposits are termed coprolites; these can provide valuable information of dietary behavior, while elements that have been consumed and passed can be deduced from characteristic corrosion of surfaces.
Most organic remains undergo some form of transport between death and incorporation into the fossil record, either by physical or biological processes. Due to the inherent nature of sedimentation, hydrodynamic transport and sorting of remains is often a major agent of dispersal or accumulation. In the case of vertebrates, it is common for the different individual bones, which have different hydrodynamic properties, to be transported different distances from the initial point of death. Plants too are only very rarely preserved in situ with all their different components attached: leaves, stems, trunks, fruit, flowers, and seeds. An organism’s remains believed to be in situ are termed autochthonous; whereas if the remains have moved they are allochthonous. Because allochthonous fossil assemblages require transport, there will usually be evidence of the transport process such as abrasion or an obvious contrasting environment. A vertebrate carcass, bloated with the gases of necrolysis, can float away from the site of death to travel great distances on ocean currents before descending to the sea floor. Fossilized remains of dinosaurs, found wholly within marine sediments, indicate the carcasses were transported over 60 kilometers from their nearest terrestrial habitat. But not all hydraulic agents of transport have the effect of dispersing remains. Rivers can concentrate scattered elements to form rich bone-beds in point bar deposits, and wave action in marine settings may concentrate invertebrate remains to geological proportions.
There can also be active biological agents of accumulation to consider in taphonomic study. Extant porcupines in Africa have been observed collecting up to thirty-eight bones each year, accumulating them in each lair to gnaw on for minerals and to control the length of ever-growing incisor teeth. This type of taphonomic history may be indicated in an accumulation of fossil bones where flat-bottomed grooves on bones are prevalent.
Diagenesis
As remains are entombed, usually in sediment but sometimes in amber, they become isolated from bio-stratinomic modifications, but then begin a new stage of different processes acting to modify their original mineralogy and texture even further. This post-burial physical and chemical alteration of remains is termed diagenesis. After deposition, organic tissue is compacted as it is buried beneath successive layers of sediment. Hard tissues will be broken, the resulting fractures combining with those of previous processes to obscure the true taphonomic histories. Compaction also causes plastic deformation that distorts morphological measurements, and in some cases causes misidentification of elements or even taxa.
As the organic tissue and any associated bacteria come into contact with ascending or descending groundwater, the remains become a focal point and catalyst for geochemical reactions of enrichment or dissolution. It is common for the organic tissue to be dissolved away altogether, leaving only casts and molds in the sediment to preserve a shape or outline. Carbonization occurs where the original remains are lost as volatiles, leaving behind a film of mostly carbon that can preserve soft tissue outlines and is a common mode of plant preservation. When the surrounding groundwater contains favorable elements in solution, processes of mineralization may take place. Minerals like phosphate, pyrite, quartz, and calcite may precipitate into the pore spaces and replace the original tissue structures. In exceptionally rare cases, anaerobic bacteria may continue activity for some time after burial, their metabolism releasing gases that initiate rapid precipitation at the microscopic sites of consumption. In this way, soft tissues such as skin and internal organs are preserved as three-dimensional, mineralized replacements.
The mineralizing processes of diagenesis may assist the long-term preservation of the remains by replacing relatively soft bone minerals with those more resistant to crushing and, much later, erosion. From this aspect, diagenesis and other mortal processes may be seen to provide information of the past rather than destroy it, enabling material to be discovered for the taphonomist to ponder.
References:
- Behrensmeyer, A. K., & Hill, A. P. (Eds.). (1988). Fossils in the making: Vertebrate taphonomy and paleoecology. Chicago: University of Chicago Press.
- Donovan, S. K. (Ed.). (1991). The processes of fossilization. London: Belhaven Press.
- Lyman, R. L. (1994). Vertebrate taphonomy. Cambridge: Cambridge University Press.
- Martin, A. J. (2001). Introduction to the study of dinosaurs. Malden, MA: Blackwell Science.
- Mountain, M., & Bowdery, D. (Eds.). (1999). Taphonomy: The analysis of processes from phytoliths to megafauna. Canberra: ANH Publications, Australian National University.
- Shipman, P. (1981). Life history of a fossil: An introduction to taphonomy and paleoecology. Cambridge, MA: Harvard University Press.
