Forensic Taphonomy: Extrinsic Factors
These include a vast range of factors which influence the state of decomposition and preservation of the body. Furthermore, each taphonomic variable interacts with the others, as well as with those outlined in the previous section. As a result, the effects on decomposition can vary enormously.
Extrinsic factors include:
Depth of burial
Even within the same cemetery, you will find different levels of preservation of burials depending on depth. If a body is buried in a shallow grave, scavengers may still be able to access it. When buried at a greater depth, the body is more protected from physical disturbance, from extremes in temperature, and to a certain extent rainwater.
Presence of a coffin or wrapping
In archaeological cases, the presence of lead coffins has often led to soft tissue preservation, even after several hundred years. This is because sealed lead coffins create anaerobic conditions (lack of oxygen), which inhibit bacterial action. Bodies buried in a coffin will decay slightly differently to burials straight in the earth. Decomposition within a void (created by a coffin) produces subtle differences in bone position compared to a body surrounded by soil. Slight alterations in bone position can therefore provide information on burial practice. even after organic materials such as wooden coffins or shrouds have decayed. The study of these subtle changes to the position of the skeletal elements after burial is called Archaeothanatology and was developed by Henri Duday at the University of Bordeaux.
Soil type and acidity/alkalinity
Acidic soils will demineralise bones. Bones survive better in neutral or alkaline soil. For example, in the North East of England, the soil is quite acidic leading to the destruction of many archaeological bodies. However, there are patches of chalky soil in this region and these create pockets of good skeletal survival.
The image below shows a ‘sand body’ from the famous early Medieval ship burial of Sutton Hoo in the UK. The acidic sandy soil meant that the skeletons of those buried were quickly demineralised, but preserved an organic stain of the body, the outline of which could then be recorded.
In warm conditions decomposition is accelerated, particularly if also humid. This was a problem after the Boxing Day Tsunami because the bodies decomposed very quickly in the hot and humid weather making it difficult to identify victims (listen to the audio file on the Tsunami).
Very cold conditions can preserve bodies very well as this inhibits microbes from decomposing the soft tissues. Bodies in freezing conditions tend not to show any evidence of decomposition. The preserved bodies of climbers that have perished on Himalayan mountain trails following failed attempts to ascend the peaks are a stark example. Another famous example is the well-preserved body of the Ice Man who died approximately 5000 years ago in the Alps (below).
Extreme cold stops bacteria from living and reproducing. This is not to say that freezing doesn’t cause damage to bone, because it does. Freeze-thaw cycles can cause fragmentation of bone as water within the pores expands while frozen, causing damage. During thawing, this then melts and refills with fresh water which then freezes again. Such freeze-thaw processes can speed up decomposition since the freezing process causes the cells of the soft tissues to expand and rupture which means that the decomposition process is faster during the warmer periods. This is an issue in regions which are not permanently frozen.
Water can be very damaging to bones due to its tendency to undergo mineral exchange with surrounding groundwater. Peat bogs have traditionally preserved bodies due to the combination of the acidic oxygen-free (anaerobic) conditions and cool temperature. The soft tissues and even the internal organs and stomach contents of bog bodies have been found preserved even after thousands of years. Nevertheless, the bones are usually completely demineralised because of the acidity.
Bodies Immersed in Water
Bodies are sometimes found in lakes, rivers and the sea. Immersion in water has an influence on the quality and quantity of bone available to anthropologists for analysis. The first thing to note is that water, especially at depth, is cold. While this may be an obvious point to make, it does mean that ‘normal’ decomposition can slow down as a result. In water, physical damage through currents, objects in the water, or animal activity might become more significant. An understanding of aquatic ecology can be helpful though - as Dr Paola Magni explained in her award-winning presentation on the use of barnacles in forensic science.
The movement of human remains in bodies of water can also cause challenges. Generally, bodies are broken down in similar ways, with the limbs and head becoming separated from the torso because of the anatomically weaker joints. Body parts are also moved away from the deposition site to differing extents based on the size and shape of the bones. It is also worth noting that body parts move differently in different types of water. For example, in rivers bodies become fragmented and are moved along the course of the river, whereas in lakes bodies slowly move around working their way into the middle of the lake.
Stones and debris within fast-flowing water such as rivers can cause damage to the bodies and the bone. After struggling with some forensic case work where bones were washed up on a beach, we did some research to explore this further. We found that different sediment types, speed of movement, and date of the bones all have an effect on the damage seen on the skeleton. However, by understanding this, we can now determine more effectively if bones have been immersed in rivers or not by observing the damage left behind. You can read our paper, which was published in the Journal of Archaeological Science: Reports for free here. After clicking the link please click ‘publication’ in the panel on the right.
Plants, including trees and their roots, can cause a great deal of physical damage to bones if in close proximity. Roots laying adjacent to bones can also etch and stain the bone surfaces. Fungi (although not strictly a plant!) can also penetrate bones causing damage and staining. See the picture of the two humeri (arm bones) below. The outer surface of the one at the bottom of the picture has been stained and damaged through plant and fungal activity whilst in the ground. This sort of damage may mask any pathologies present.
Scavenging by animals is one of the most destructive processes to occur to the body after death. There are lots of different variables to consider when it comes to animal activity. Activity is influenced by the time of day, the season, the temperature, whether the body is inside or outdoors, whether it is in an urban, rural or aquatic environment. A wide range of animal species have been found to scavenge deceased human bodies, including mammals, insects, amphibians, reptiles and birds.
The body tends to be scavenged in a consistent order. The sections of the body attacked first include the abdomen and chest, the buttocks and the throat. The disarticulation of the limbs follows, usually with the forelimbs first since they are less securely attached to the body. The vertebral column is usually the last past of the body remaining. The more porous ends of the long bones are often broken so that the animal can access the marrow inside. The shafts can be gnawed to access minerals. Small bones can be swallowed whole.
- Rodents tend to both remove soft tissues and gnaw bone. On bone, they leave parallel flat-bottomed grooves (striae) from their central incisors (see photo below). They have also been seen to gnaw through the ends of long bones in a way that leaves a piece of bone sticking out. The soft tissue damage can mimic entrance wounds or defence wounds.
Rodent gnawing on a long bone (Copyright Durham University)
Birds such as crows (as well as vultures and other birds) will scavenge human remains. Crows tend to damage the eyes primarily (which can be confused as evidence of sadistic murder/ritual etc) although they do pick at the long bones leaving a characteristic ‘fluffy’ or ‘stringy’ pattern to the remaining soft tissue and bones.
Canines tend to focus on the soft tissues, but they do leave distinctive marks on bones, including punctures, pits, scores and furrows. They will also transport bones, and the distance depends on the size and type of the dog and the surrounding environment.
Indoor pets leave slightly different patterns of scavenging to those in the wild. Dogs and cats still focus on the soft tissues but damage tends to focus on the face and head, often ignoring clothed areas. They can eat through the clavicles and ribs in order to access and the eat viscera (which can mask the cause of death).
Underwater animals can have a significant impact on bodies immersed in water. Sometimes damage from fish can be mistaken for peri-mortem trauma (injuries around the time of death), so care needs to be taken when analysing it. For example, a recent report by Dr Agathe Ribéreau-Gayon and colleagues discussed some very unusual lesions on bodies trapped in a plane that had crashed into the sea. Initial thinking was that the damage was due to the impact of the crash, but the nature of the injuries didn’t make sense. After further research, it became clear that the lesions were the result of cookiecutter sharks.
Burrowing animals can displace bones from their original location and the animals may even transport bones away from the graves. This creates difficulties, because when bones are removed from their original context it makes determining whether they are ancient or modern more problematic.
Insects remove the soft tissues causing massive tissue loss. Maggots and flies tend to do the most damage, whereas species like beetles tend to be more predatory and eat other insects. The destruction of the body by insect activity is a useful as indicator of Post-mortem Interval (PMI). It forms the basis of the discipline of forensic entomology.
Can you think of any other extrinsic factors that might have an impact on preservation? If so, then please add comments below.
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