It has rightly been said that the most important question to ask of any archaeological bone assemblage is "What are all these bones doing here?". Unless and until we understand the range of processes by which the assemblage that we recover from excavated deposits derived from a number of formerly live animals, we have little hope of making a fully-nuanced and soundly-based interpretation.
Prehistorians who study the deep past of humanity, dealing in hundreds of thousands of years, are accustomed to undertaking detailed taphonomic analyses of the animal bone assemblages that often accompany the remains of our earliest ancestors. At the other end of the timescale, we are much less likely to undertake such an analysis.
Yet we should, not least because sites such as medieval towns or Iron Age hillforts may yield prodigious quantities of bones from hundreds of contexts, making it necessary to prioritise their study. Obviously, it is essential that the most informative assemblages are given the highest priority, but without a full taphonomic analysis, we cannot state with confidence which assemblages will be the most informative, nor what classes of information different assemblages may yield.
We start by subdividing the taphonomic trajectory of a bone into a series of stages. First the animal dies, and the place and circumstances of its death may markedly affect the chances of any bones surviving in the archaeological record. Next the carcass undergoes transport. In non-cultural situations, the transporting agency may be water, scavengers or simply the trampling feet of passing animals. On settlement sites, transport includes many of the human activities that we seek to understand, such as butchery, food-sharing and trade, and refuse disposal. Transport is followed by exposure, the stage at which the bone is incorporated into a forming deposit. This stage may be very brief, for example if a bucket-full of bone waste is tipped into a cess-pit. More often, this is a period of time during which bones are exposed to a particularly dynamic and destructive range of processes. These may include trampling and sub-aerial weathering, for example in a bone-strewn medieval backyard.
Once the bone is buried, the processes of diagenesis take over, altering the bone chemically. In some burial conditions, diagenesis may lead to complete destruction of the bone, leaving us archaeologists with nothing to study. More often, the bone stabilises, coming into chemical equilibrium with its surroundings. Once that equilibrium has been attained, bone may survive for many thousands of years, unless changes to the deposit, such as erosion and re-exposure, lead to a change of equilibrium. Excavation is just such a change, of course, and it is a common mistake in archaeology to assume that because a piece of bone has survived for millennia in the ground, it will survive for years or decades following excavation.
Taphonomic work at York follows several different directions: