Recently, the UKAFH invited me to write an article for them on fossil preservation. What kind of palaeontologist would possibly decline! [excluding inverts - they don't count]
This is just a nice basic guide to taphonomic processes, designed for people with limited knowledge of geology or palaeontology. Feedback, as always, is welcome! :)
The fossil record is our one and only key to a physical understanding of ancient or extinct life. Over the years a wealth of fossil remains have been uncovered, ranging from the earliest microbial life to the largest eukaryotic animals, and from isotopic signatures to fragments of DNA. These remains of dead organisms are found in two major divisions: as body fossils, where an actual specimen is preserved in some form or as trace fossils, where a particular aspect of an organism’s life is preserved, typically as trackways or burrows.
The process of preservation is termed taphonomy, and can be broken down into three main stages: necrosis, biostratinomy and diagenesis. The death of an organism, necrosis, is the initial stage in preservation, and is related to either trauma or physiology. Biostratinomy refers to the processes from death to post-mortem burial, such as transportation, bacterial decay and potential scavenging. The time taken during this stage is a critical aspect of preservation likelihood. Finally, diagenesis refers to the processes relating to the transformation of sediments into rock, and organisms into fossils. The mode of preservation leading to what we see in rocks is determined at this point, through the interaction of the surrounding sediment chemistry and the recalcitrance of various tissues. During these three stages, numerous factors act to destroy fossils, including microbial decay, predation, and a multitude of biogeochemical processes. In order for a fossil to be preserved, at some point in the taphonomic cycle, one or more of these processes must be arrested. The degree to which taphonomic breakdown is prevented is directly proportional to the degree of preservation attained.
Modes of Preservation
Permineralisation or Petrifaction
This is the most common style whereby soluble minerals in the surrounding sediments and fluids are deposited within interstitial organic pore spaces, leading to a variety of styles of preservation. This is the most common preservation in most invertebrates, organic-walled microfossils and bones. Directly observable using various microscopic methods, this needs to be distinguished from recrystallization and dissolution processes to reconstruct the initial tissue structure.
The most infamous recent occurrence of desiccation-based preservation is the “dinosaur mummy” from Dakota, the aptly named, Dakota. This Upper Cretaceous hadrosaur preserves actual recrystallized tissue remains, including tendons and ligaments and the epidermal microstructure, in amazing detail. The carcass is thought to have undergone rapid burial on the periphery of a sandy river channel, enclosing it in an anoxic environment and significantly enhancing its preservation.
The La Brea Tar Pits in Los Angeles, California are renowned for the immaculate presence of a multitude of Pleistocene-age mammals. They are the products of crude oil seepage, with lighter hydrocarbon phases being siphoned off via fractionation until just sticky tar remains. The predominant hypothesis for the mass accumulations of fossils is that, once an animal was mired, it became the target for packs of predators, who ultimately met the same sticky fate as their intended prey. Little soft tissue is preserved, but the concentration of bones more than compensates. The bones are actually infused by the tar, turning them a dark brown colour. Smaller invertebrates as well as plant macro- and microfossils are also abundant here. The tar creates a completely anoxic environment in which little to no decay can occur.
Amber is the solidified remains of ancient tree sap. Organisms that are unlucky (or lucky?) enough to be preserved in amber create the most intricate and beautifully preserved fossils of all. Featuring prominently as John Hammond’s cane top in Jurassic Park, they deserve pride of place due to the exquisite detail typically preserved. The most famous deposit is the Eocene-age Baltic Amber, which has produced perfectly preserved plants, insects and even small vertebrates. Amber, like tar, entombs organisms within a completely anoxic environment, ceasing all decompositional processes.
This process involves the conversion of organic tissues into a carbonaceous film or residue through either pyrolysis (i.e., thermochemical decomposition) or destructive distillation (anaerobic decomposition), usually as a result of low-grade regional metamorphism. It is the process that converts woody material into coal seams. Typical fossils found preserved like this are graptolites in shales, typically associated with scavenger-free deep-water anoxic environments, as well as marine vertebrate integument (e.g., in the Holzmaden Shale).
Infamous for the occasional Woolly mammoth occurrence in Alaska and Siberia. The conditions lock organisms, complete with integument and flesh, in time. DNA has even been extracted from several specimens and is incredibly useful in accurately retracing pachyderm lineages.
Classic examples where volcanic interaction has led to sites of exceptional taphonomy include the Mistaken Point Biota (Ediacaran, Newfoundland), and the Jehol Biota (Lower Cretaceous, China). The advantages of volcanogenic interaction are two-fold; firstly, they create toxic, anoxic environments, and are typically rapidly deposited creating the perfect preservational scenario. Secondly, they contain radioactive elements which can be used for high-precision radiometric dating, which can be applied by association to intercalated fossiliferous horizons. At the two mentioned sites, episodic ash falls capture and smother local fauna and flora. These are typically found interbedded with thin mudstones and shales, suggestion that they are lakeside communities mixed in with autochthonous benthic fauna. Fauna preserved associated with these ash deposits have a diagnostic opisthotonic neck posture, infamously depicted in the birds and avian theropods of the Jehol fauna, in the classic ‘angel pose’. This is possibly indicative of hypersaline or toxic waters as a cause of death.
The study of trace fossils is known as Ichnology. Trace fossils are the direct result of biological activity and have their own independent taxonomic system. This makes them extremely useful in reconstructing the behavioural palaeoecology of extinct organisms. They can represent anything from nesting sites, to anastomosing series of trackways, and can be preserved as either exogenic (on the surface of a fabric) or endogenic (made within sediments). The preservation potential for trace fossils is typically a function of grain size and depositional facies.
The fossil record is an incredibly biased sample of ancient ecosystems. Scientists estimate that only 15% of the composite species in an ecosystem are typically preserved, and of these, most are those with ‘hard parts’ (e.g., shells, cuticle, bone). There are also biases reflecting the depositional environment (e.g., fluvial, lacustrine, marine, aeolian, volcanogenic), and amount of rock sampled, amongst others, which recently scientists have begun to unravel in the hopes of better determining the controls on preservation through deep geological time, and the effect this has on our understanding of the fossil record and diversity dynamics.
Occasionally, palaeontologists are fortunate enough to come across sites of exceptional preservation known as Lagerstätten (German for ‘storage place’). These represent snapshots in time, and come into two flavours: Konservat-Lagerstätten and Konzentrat-Lagerstätten. The former represents an accumulation of fossils where the detail preserved is on an incredibly intricate level, such that ‘soft parts’ are visible, even to the molecular level. The best known examples of these include the Burgess Shale (Cambrian, Canadian Rockies), and the Jehol Biota (Lower Cretaceous, China). Here, preservation of articulated elements, original labile soft tissues, unaltered mineral compositions and orientations, and even intracellular structure can be preserved, indicating the early termination of diagenetic processes or that early mineralisation sufficiently outpaced degradation. Konzentrat-Lagerstätte, on the other hand, represent unusually high concentrations of fossils, typically representing an in situ community. A classic example of this is the Morrison Formation bone bed (Late Jurassic, North America). Deposits like these typically represent mass mortality events such as flooding.
Until recently, most fossils were interpreted in terms of their macroscopic preservation features. However, with technological advances such as the increasingly commonly used computed-tomography (CT) scanning and scanning-electron microscopy (SEM), sophisticated details about micro-scale preservation in numerous fossils are being recovered. Accordingly, palaeontologists are uncovering more about macro- and micro-scale physical features, as well as physiological, cellular and even sub-cellular processes.
Allison, P. A. and Bottjer, D. J. (2011) Taphonomy: process and bias through time, second edition, New York: Springer
Nudds, J. and Selden, P. (2008) Fossil-Lagerstätten, Geology Today, 24(4), 153-158
Schweitzer, M. H., Avci, R., Collier, T. and Goodwin, M. B. (2008) Microscopic, chemical and molecular methods for examining fossil preservation, Comptes Rendus Palevol, 7, 159-184
Upchurch, P., Mannion, P. D., Benson, R. B. J., Butler, R. J. & Carrano, M. T. (2011, in press). Geological and anthropogenic controls on the sampling of the terrestrial fossil record: a case study from the Dinosauria. In: Comparing the Geological and Fossil Records: Implications for Biodiversity Studies, McGowan, A. J. and Smith, A. B. (eds). Geological Society, London, Special Publication 358: 209-240