Laboratoire de Zoologie Mammifères et Oiseaux—MNHN, 55 rue Buffon, 75005 Paris, France
Many experiments have been conducted in the field of taphonomy in order to understand theeffects of some agents and the associated pattern and process of bone modifications. Areview of these experiments and the patterns of bone modifications is presented here. Numer-ous experiments focus on peculiar points of bone accumulation, mainly to gain knowledgeabout human habits and practices, but the action of some taphonomic agents has also beenexplored, among them, diagenetic processes. The striking feature of all these works are thatthey are localized, performed on non-statistical samples and without repeatability. Very fewexperiments combine different agents to fit more closely to the complexity of most fossil boneaccumulations.
KEYWORDS: TAPHONOMY, BONE MODIFICATION, DIAGENESIS, EXPERIMENTATION
INTRODUCTION
Taphonomy is the study of the laws of burial and fossilization and, by extension, of thepalaeoecological biases resulting from the passage of an individual of a living community to afossil one. Experimentation is a scientific method based on the systematic use of experiment inorder to verify hypotheses. Like other historical sciences, taphonomy relies on uniformitarianassumptions for linking the present to the past. The patterns obtained from comparisons canlead to the formulation of hypotheses allowing us to elaborate models or protocols of experi-ments that need to be discussed and/or tested by experimentation and simulations, to reproducesome past patterns and recover some processes. Young (1989) argued that, under certain condi-tions, modern data replication experiments allow us to make generalizations about the past thatare more powerful than the generalizations normally allowed by analogical reasoning. Experi-mentation has been used sporadically since the beginnings of taphonomy, but not systematic-ally. This discipline provides materials for this type of research, and here I will try to presentsome information on how experimentation has been used in taphonomy.
Taphonomic processes are rather complex and still not yet fully understood. Among the mainprocesses of bone concentration and alteration are digestion (for small mammals), transport,weathering, soil corrosion, trampling, burning and diagenesis (mineralization, changes incomposition) (see Fig. 1). All of these processes are known to leave characteristic patterns ofalteration and fragmentation on bone surfaces (Fernández-Jalvo et al. 2002), but the mech-anisms by which they operate cannot be studied without comparisons or experimental work.Moreover, the taphonomic agents that are responsible for the observed processes are not alwaysidentified.
Here, we present a comprehensive, but not exhaustive, review of the literature of the differ-ent experimental works and the way in which they contribute to our knowledge of fossilizationprocesses and site formation.
Figure 1 A summary of the pre-burial, burial and post-burial processes leading to bone fossilization.
EXPERIMENTAL TAPHONOMY OF PRE-BURIAL PROCESS
Many processes lead to the formation of fossiliferous sites, and they are generally categorizedas either pre-burial or post-burial (Fig. 1). Taphonomic modifications may occur at all stages,from the living animal to the fossil. Experimental work may be carried out on different environ-ments and materials to check the importance or effects of various physical or biological agents.
Cooking and burning
One of the tasks confronting archaeologists and palaeoanthropologists that is relevant totaphonomy is to distinguish between the ‘natural’ and ‘cultural’ origins of an accumulation—this constitutes the field of archaeozoology. Differential preservation of mammalian bones atarchaeological sites leads to hypotheses concerning the natural or cultural origin of accumula-tions, and especially human consumption or use. It is easy to experiment into human activity,and this is one of the earliest fields of experimentation. For example, Lartet (1860) experi-mented with stone tools on fresh bones, to compare with prehistoric bones and interpret human
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activity. Martin (1907–1910) experimentally broke mammal bones in an attempt to discern thecriteria that denoted a human fracture agent.
Some studies on heating, burning and boiling have analysed bone frequencies and aspectsafter experiments (Buikstra and Swegle 1989; Lubinski 1996), while other studies haveexamined bone chemistry and structure after these experiments (Shipman et al. 1984; De Niroet al. 1985; Richter 1986; Currey 1990; Grupe and Hummel 1991; McCutcheon 1992). Morerecently, Roberts et al. (2002) tried to characterize through experiments the effects of cookingon both inorganic and organic elements, as well as their interface. There are indications that firecontrolled by humans occurs around 2 Ma ago and is quite frequent at archaeological Holoceneand late Pleistocene sites. Fire is well recognized in sediments, but it is more difficult toappreciate on bone surfaces. This topic of archaeology has received much attention, but most ofthe experiments concern the physico-chemical signatures of burnt bones (Shipman et al. 1984;Villa et al. 1986; Brain and Sillen 1988; Walters 1988; Nicholson 1993; Sillen and Hoering1993; Stiner et al. 1995; Taylor et al. 1995; Wu et al. 1999). Other studies have observed theaspect of the burnt bones after stripping the flesh, or after drying or fragmenting them (Thurmanand Willmore 1981; Knight 1985; Gifford-Gonzalez 1989; Johnson 1989). Other protocolshave shown the difference between anthropic and natural fires (Bellomo and Harris 1990;David 1990; Bellomo 1991, 1993), while others have paid attention to the potential survival ofburnt bones (Walters 1988; David 1990; Stiner et al. 1995). The latter authors show that theseburnt bones are more fragile to diagenesis and trampling, while Gilchrist and Mytum (1986)concluded with the hypothesis that recrystallization that occurs when bones are not formed ofcalcinate reinforces the strength of the bone. Differences in colours and fragmentation, exfolia-tion and cracking are current patterns observed on burnt bone surfaces (Fig. 2 (a)). Costamagnoet al. (1999) have shown experimentally that bones may have been used as fuel in Palaeolithictimes.
Butchery, marrow extraction and percussion
At archaeological sites, bone surfaces show various marks that result from man’s activity, suchas percussion or butchery marks (Shipman and Rose 1983). These activities lead to somespatial patterns of distribution and fragmentation of bones, which generally allow us to interprethuman strategies of consumption and the type of site (Isaac 1967). However, especially inPlio-Pleistocene times, it may be difficult to distinguish between a natural Hyaena den and afloor occupation or a butchery site. In order to help discrimination, some authors have madeexperimental cut marks with different tools (Denys 1985; Walker and Long 1977), as well asobserving the results under the scanning electron microscope (SEM) (Shipman 1981) or throughbinoculars (Blumenschine et al. 1996). Many experiments on disarticulation processes withvarious techniques and tools can be found in the archaeological literature.
Decay and disarticulation
The role played by micro-organisms in the decomposition and disarticulation of bone is wellknown, but their effects on the bone surface are not yet well established. Many aspects ofdecay and disarticulation have been described after monitoring carcasses in the field in variousenvironments (Weigelt 1927; Toots 1965; Hill 1980; Blumenschine 1986; Potts 1988) but, forthis aspect of taphonomy, few works have attempted experimental approaches, with controlledparameters for time, carcasses and the environment.
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Figure 2 Pre-burial patterns and related experiments. (a) Burned bone, Matjies River Shelter, South Africa, layerP (×12). (b) Digestion on the tip of a fossil incisor of rodent (Pech Crabit, Oligocene) (×40). (c) Light to moderatedigestion on a rodent femur from a spotted eagle owl pellet (×20). (d) A detail of digestion (×70). (e) The distal partof a humerus after acid + pronase attack (×12) (after Denys et al. 1995). (f) The proximal head of a humerus afteracid + pronase attack (×42) (after Denys et al. 1995).
Simple experiments on decay, scavenging and disarticulation in a temperate fluvial settinghave been carried out by Bickart (1984), who placed carcasses of birds and pigs on a streamflood plain in Maryland for one year. Disarticulation took from 13 days to six months, andmany of the bones were damaged by the activity of scavengers. Micozzi (1986) observedthe effects of freezing, thawing and mechanical injury on the degradation and separation of
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articulatory joints, while Janaway (1990) tested the effects on decay of the burial environmentof inhumation graves. Davis and Briggs (1998) placed carcasses—some protected by metalcages and some unprotected—in two field sites in southern Florida, to compare their relativerates of decay and disarticulation, as well as their weight loss, in order to interpret fossils fromsome famous lägerstatten deposits. They found similar sequences of morphological decay inboth environments (brackish swamp and marine embayment) and they used this experiment toproduce a predictive equation of weight loss versus elapsed time from death to stabilization ofthe carcass for fossil birds.
Another experiment, on two shot adult impala carcasses from South Africa, has been con-ducted over a few months. Different sub-regions of the carcasses were suspended on Acaciatree branches and treated once with insecticides. Daily visits allowed observation of the state ofdecay and the activity of large vertebrate scavengers and necrophagous insects (Ellison 1990).This work confirmed that the colonization of carcasses by insects is consistently related to thestate of decay, but for the two carcasses the timing and duration of the decay sequences weredifferent due to different scavengers (civet on one, serval on the other).
Digestion
Digestion occurs in medium to small vertebrates. It has been shown to act as a selective andsequential process. Enamel from rodent teeth is first etched, and then dentine and bone (Fig. 2(b)). On teeth and bones, it leaves characteristic traces that may be used efficiently to recognizethe origin of a predator at a fossil site (Andrews 1990; Fernández-Jalvo and Andrews 1992;Fernández-Jalvo et al. 1998). Generally, epiphyses and salient angles of bones are more stronglyaffected than diaphyses (Figs 2 (c) and (d)). Digestion can be easily simulated by using HClsolutions at various pH values, as in the stomachs of raptors, and some previous experimentshave shown the effects on the surface (Fernández-Jalvo 1992) and on the structure (Boyde et al.1978). However, as far as bone is concerned, we showed that to simulate the enlargement ofosteoblasts and the polishing effects of digestion, the acidity of the solution is not a sufficientfactor. The addition of enzymes was more effective than pH value and duration time (Denyset al. 1995; Figs 2 (e) and (f)). Human digestive effects have been tested by Crandall and Stahl(1995), who analysed faeces containing remains of digested skinned, eviscerated and segmentedinsectivores. Examination of skeletal element proportions, breakage and digestive damage showedthat human digestion produces category 5 predator modification according to Andrews’ classi-fication (1990), showing similar patterns to mammalian carnivore digestion. Another experimenton human digestion, and also that of pigs and dogs, was made on fish bones by Jones (1984,1986, 1990).
Experimental owl feeding by Dodson and Wexlar (1979) has allowed us to quantify, forthe first time, the bone loss and fragmentation due to digestion. Several captive owls werefed on house mice (Mus musculus), and this allowed the authors to develop a methodology ofcalculation of bone loss related to digestion and to shed light on differential preservationamong owls and bones. Similar experimental work has been carried out by Hoffman (1988)on a larger sample of predators with the same type of food (adult mice) and, similarly, quant-itative criteria were found allowing us to distinguish patterns of bone fragmentation in thefunctioning of raptor species. However, the latter author distinguished only between nocturnaland diurnal raptors, and failed to include in his results the smaller bones (vertebrae, metapodials,phalanxes and ribs), which may provide more evidence of discrimination, or of different preysizes.
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Gnawing
Many mammals are known to collect skeletal elements in their dens or in their territories forvarious purposes, one of which is to supplement calcium and phosphate in their diets, the otherbeing to prevent excessive growth of incisors. If squirrels, murids, rodents and shrews, as wellas some ungulates, have been recognized to behave in this way, the most effective actors arethe hystricids. Gnawing is characterized by the presence of multiple parallel striations thatmatch the width of the incisors, and by the removal of the articular ends of diaphyses (Brain1980; Maguire et al. 1980; Andrews 1990; Laudet and Fosse 2001).
Experimental feeding of captive porcupines (Hystrix indica spp.) in Israel has been con-ducted by Rabinovitch and Horwitz (1994) in order to verify some hypotheses of porcupinegnawing on middle Palaeolithic bones from Israelian sites. A previous study was made incaptive Hystrix africae-australis from the Cape region by Duthie and Skinner (1986), forphysiological research into the causes for such behaviour.
Lichens, algae and fungi
Lichens, algae and fungi are very abundant in nature and seem to have effects in degradingsome bone tissue structure (Fernández-Jalvo 1992; Fernández-Jalvo et al. 2002). Experimentalwork is currently being undertaken in this field (Fernández-Jalvo pers. comm.).
Weathering
Modifications produced by exposure of bones on the ground for various periods of time havebeen described in detail in different tropical environments by Behrensmeyer (1978) and Tappen(1994, 1995). Different stages have also been recognized both for large and small mammals(Andrews 1990) in the temperate wet climate of Wales (UK). Progressive cracking, splittingand exfoliation may lead to complete and more or less rapid bone destruction. A camel skeletonhas been monitored by Andrews (1995) in the United Arab Emirates. The animal died in 1984and this author observed the progressive dispersal of the skeleton until 1994. After 10 years ofaerial exposure, bones remaining at the surface reached stages 1–2 of Behrensmeyer, but buriedbones were protected from surface weathering (Fernández-Jalvo et al. 2002). On the Welshhills, a long-term monitoring project in Rhulen (Wales, UK) has allowed observation of theprocess of bone dispersal, burial, weathering, trampling, scavenging and predation (Armour-Chelu and Andrews 1996; Andrews and Armour-Chelu 1998) in a temperate environment.The skeletons of approximately 150 sheep, cows, horses, foxes, badgers and small mammalswere left undisturbed in the place of death at altitudes between 300 and 400 m in woodland,moorland and grassland environments. Initially, Andrews and Cook (1985) showed rapid dis-persal of bones and those in wet and sheltered conditions showed evidence of soil corrosionsimilar to those provided by carnivore-gnawing. After 12 years in Wales, some bones may bedestroyed without any of the signs of surface weathering shown by Behrensmeyer (1978) intropical environments. Other Welsh bones exposed without vegetation cover reached weatheringstage 1 of Behrensmeyer’s classification after 10–12 years and stage 2 by 19 years at the earliest).Burial of skeletons occurs for some bones after 3 years of exposure, but for others may takedecades.
In order to characterize the effects of frost on bones, Guadelli and Ozouf (1994) haveconducted two series of experiments. The first consisted of putting long bones, short bones,
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teeth and antlers through daily freeze–thaw cycles between –5°C and +12°C for four years(1012 cycles) in a cold chamber. The second consisted of the creation of an experimental siteincluding five identical squares, with flint and faunal remains left in the South Alps at 3169 maltitude. From laboratory work, these authors concluded that bone fragmentation is not depend-ent on water content, differences in fragmentation being a function of the bone type. Thefragmentation pattern is very irregular. In the natural site, no conclusions have been reached atthe present time.
Trampling
Bones remains lying on the ground for a long period of time can be trampled by cattle or largemammals. This results in breakage or burial in the sediments, and in some gravel or sandy soilsfriction against sediments may leave some shallow, subparallel scratches on the bone shafts(Fig. 3 (a)). In order to look at some parameters of the soil and to prove the effects of tramplingon mammalian bones, experiments have been conducted by Courtin and Villa (1982), Fiorillo(1989), Behrensmeyer et al. (1986) and Olsen and Shipman (1988). These experiments havedemonstrated that trampling can produce scratch marks and have elicited criteria to distinguishthem from those produced by stone tool cut-marks. Fiorillo (1989) also described the effectsof trampling on the spatial arrangement of bones on different soil surfaces, by comparing theinitial disposition of experimentally oriented, aligned bones before and after the passage of acattle herd of 165 heads. This led to a randomization of the orientation of the bones, and ahigher frequency of trampling marks in sandy soil. In wet sediment, bones sank vertically,indicating the importance of the substrate for this type of bone modification.
Abrasion
In order to observe the different types of abrasion, and also in an attempt to understand somekinds of water transport action and processes, various experiments have been conducted. Largebones at various stages of weathering and fossilization have been abraded in a rotary tumblerfor fixed periods, and with varying sizes of silt, sand, gravel and pebbles, for small mammals(Korth 1979) with or without water, and for larger mammals (Andrews 1990). Weathered boneswere found to be abraded more rapidly than unweathered ones (Andrews 1995), and fossilbones moved faster than fresh and weathered ones (Fernández-Jalvo 1992). Breakage was alsorelated to the degree of weathering. The degree of rounding was found to be related to the clastsizes as well as breakage. Water action produces smooth, polished surfaces on bones (Figs 3 (b)and (c)), by contrast with bones that show weathering or corrosion.
Re-modelling bone surfaces of sub-adult macaque cranial bones have been subjected to waterabrasion, brushing, rubbing, exfoliation and chipping in order to characterize the micromorpho-anatomy of bone surfaces (Bromage 1984) and to distinguish this from possible interference dueto taphonomic modifications. Some effects of air-polishing on bone and teeth have also beendescribed by Boyde (1984) and d’Errico et al. (1984).
Transport
Some fossil bone accumulations result from accumulations in river channels or in deltaicenvironments. Transport by water has been studied experimentally by Voorhies (1969), Dodson(1973) and Boaz and Behrensmeyer (1976), who first showed that this process induces sorting
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Figure 3 Burials and related experiments. (a) Parallel striation characterizing trampling on a Holocene rodentfemur (×177). (b) The contrast between rounding and polishing (right) after water transport and soil corrosion (left)effects on the same skeletal element (Cainotherium phalanxes) at the Pech Crabit site (Oligocene) (×12).(c) A polished phalanx resulting from water transport (×30). (d) Bone corrosion by acid soil pH on anarchaeological fossil rodent femur (×15) (Monte di Tuda site, Corsica, Holocene). (e) An experimentallyaltered rodent distal femur after immersion at pH 2.35 for 21.5 h (×20). (f) Root marks on a Holocene rodentfemur diaphysis (×35). (g) A detail of the previous photograph (×88).
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of bones Behrensmeyer (1975) described the behaviour of bones in water as a function of theirsize and density. She elaborated a method of using the quartz-equivalent of bones as a predict-ive value. Similar experiments have been carried out by Korth (1979) on small mammals. Thelast author also undertook experiments concerning the transport of owl pellets by water, andmeasured the distance and time of flotation and disintegration. Some other flume and riverexperiments in natural conditions are described by Hanson (1980), who attempted to modelsome aspects of water transportation and its associated deposit processes. In order to test theeffect of water on articulated bones, Coard and Dennell (1995) conducted flume experiments ina water stream. The main result of their study was that articulated bones displayed a greatertransport potential and that, in one species, the rate of movement was significantly faster thanwhen disarticulated.
EXPERIMENTAL TAPHONOMY OF BURIAL AND POST-BURIAL PROCESSES
Burial is a major process in fossilization and site formation, because it concerns the transitionfrom the biosphere to the lithosphere. It may be defined as the covering of faunal remains withsediment, which can be either mineralogical or biological, such as sand or vegetation, respect-ively (Lyman 1994, 406). It has been shown that this passage is not unique and sometimes mayoccur many times but, generally, better preservation is related to rapid burial, as shown experi-mentally by Meyer (1991) and probably with the absence of reworking and re-elaboration(Briggs 2001).
Soil corrosion, bacteria and fungal action
In addition to digestion, bones and teeth may be corroded by the chemical attack of the naturalacid soils, which produce relief-abraded bone surfaces, especially on epiphyses (Fig. 3 (d)).Experimental work (Fernández-Jalvo 1992; Fernández-Jalvo et al. 2002), has also shown thathighly alkaline sediments (pH 9–14) also produce alterations such as a surface exfoliation ofbone and dentine called ‘desquamation’, while enamel is not affected. Fernández-Jalvo et al.(2002) have shown that acid attack is very quickly effective (after 1 min) on human teethimmersed in 0.1 M HCl (pH ~ 1.7), while teeth immersed in 1 M KOH (pH ~ 14) showedmodifications only after 48 and 72 h.
In order to simulate different soil chemical conditions, small mammal carcasses were buriedat Rhulen, Wales, UK, in a peat bog with pH = 4.0, another sample was left for four years in acontainer with a little water, and a third one was placed in a solution derived from carbonatitevolcanic rock with pH = 6.0 (Andrews 1995). As a result, the bones in acid soil were etched,while the bones left in alkaline conditions showed little modification (Andrews and Armour-Chelu 1998). Other experiments have been conducted by immersing bones in solutions atvarious pH values, to simulate the pH of acid soils. At pH = 2.35, after 21.5 h some very lightmodifications occur (Fig. 3 (e)). After five days at pH 5, no significant traces of corrosion werefound in these bones (Denys et al., unpublished).
The burial of complete cadavers of various domestic animals, from the size of a rat to Bos(fresh or boiled), in a range of different soil types under controlled pH and drainage regimes ina temperate environment over seven years showed that almost all soft tissues disappeared, butfur, leather and hoofs survived in acidic soils (Nicholson 1996). The rate of bone loss andmodification could not simply be related to the soil pH and/or the inferred Eh. Bone modifica-tion varied greatly between animals of similar size and in geographically adjacent soils of
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similar pH and drainage. The state of the remains prior to burial played a role in acceleratingdiagenesis for boiled specimens.
Child (1995) found than across a range of 26 micro-organisms isolated from the soil andinoculated into fresh excavated sterilized archaeological bones, only 17% possessed collagenaseand then were able to use collagen as their sole nitrogen source. For this author, the soilchemistry had a greater influence on the types and microbial and fungal abundances of populationsof bones than the other way round.
At Overton Down, UK, an experimental earthwork was set up by archaeologists in 1961(Jewell 1963) in order to investigate fungal attacks, invertebrate attack, bone surface altera-tion and degradation of cooked bones. Part of the earthwork was excavated in 1993 and twobones were examined by Armour-Chelu and Andrews (1994). One sheep distal metapodial wasboiled for 1 h before burial in 1961 and the second showed surface degradation due to percola-tion of acid ground water. Additionally, fungal attacks had left multiple small-scale grooves onboth bones, and the experimental cut-marks, made during preparation of the bone prior toburial, were still visible. The experiment will continue for the next 32 years. At the site ofDraycott, Somerset, UK, two other experiments were undertaken in 1977 and are described inAndrews (1995). A single cow skeleton was naturally dispersed at the entrance to the cavesystem, providing a trampling pattern described by Andrews and Cook (1985). Few root markswere observed; nor was there either breakage or insect damage. The second experiment con-cerned some amphibian bones recovered inside the Draycott cave and then left exposed outsidefor 16 years. SEM observations showed incipient weathering and root and micro-organismattack (Andrews 1995).
Concerning some of the praecocial diagenesis aspects related to burial, one experimentattempted to define the speed of humic uptake in the collagen and the effects of differentchemical treatments to remove these humic acids (Van Klinken and Hedges 1995). Anotherdetermined the survival of albumin in experimentally buried fragmented bovine and humanbones in a suburban garden exposed to rainfall for a 26-month period (Cattaneo et al. 1995).The rate of survival was lower in those bones than in intact complete human bones that were4000 years old. The authors concluded that bone integrity is important for protein conservationand exposure of marrow and trabecular bone to water is the most significant factor for thesediagenetic changes.
Roots
Bone surfaces are currently affected by more or less deep sinuous crests that result from rootprinting. These roots may destroy the bone by splitting and increase the porosity by enlargingthe osteoblasts (Figs 3 (f ) and (g)). Experimental work is in progress (Fernández-Jalvo pers.comm.).
Bioturbation
Soil agents such as burrowing mammals are well known to disturb soil levels and are suspectedto be responsible for bone dispersion, especially at archaeological sites. Less well known is therole of earthworms as bioturbation agents, which was experimentally demonstrated by Armour-Chelu and Andrews (1994), and has allowed us for the first time to elicit criteria by which torecognize earthworm activity in palaeosoils. The stratigraphic consequences of such activity areimportant, because earthworms are able to move some bones 30 cm in depth.
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Diagenesis
Once they are buried, bones are affected by new physico-chemical agents that may produceimportant changes in their mineral and organic structures, and in their chemical composition.In order to understand some of the numerous parameters that occur during diagenesis, someexperimental work has been undertaken. Water exchanges, soil pH and pressure may play animportant role in conditioning bones for early diagenesis. Similarly, the previous modificationsoccurring on bone may already have induced irreversible changes of bone structure and com-position. Time also plays an important role in diagenesis, and it is more difficult to deal with itin experiments, although the action of time is sometimes simulated by heating bones (Von Endtand Ortner 1984). However, some recent work on bone incorporation of trace elements showsthat time may not be the relevant factor, because some recently buried bones are modified morestrongly than older bones (Dauphin et al. 1994, 1999; Fernández-Jalvo et al. 1998).
Experimental dissolution of synthetic apatites at varying pH have allowed measurementof the changes in bone crystallinity using infrared spectrometry on the phosphate ν4 bandsplitting factor (Sillen and Le Gros 1991). Another similar experiment (Nielsen-Marsh andHedges 1997) showed that changes in the crystallinity of bone apatite occur independently ofthe degree of bone mineral dissolved and depend strongly on the ionic composition of the acidsolution.
Collins et al. (1995) have simulated the chemical degradation of ancient collagen, whileWaite et al. (1997) have studied DNA stability during artificial diagenesis.
EXPERIMENTS AND SITE FORMATION, TIME AVERAGING
Site formation is complex, as demonstrated by various kinds of taphonomic synthesis on fossilassemblages, and is very rarely a monophase process. Re-elaboration, mixing and reworkingprocesses may constitute a sequence of events through time that is called the taphonomichistory of a site. Only catastrophocoenosis may be regarded as a unique event of formation,called ‘time-specific assemblages’ by Behrensmeyer (1982). More frequently, either open-air orkarstic sites represent a more sequential history, with many phases of bone accumulation–dispersal episodes, and mixing may be frequent: according to Behrensmeyer (1982), these areeither ‘time- or spatial-averaged sites’.
Only one original experiment (TRAVEX; Carbonell et al. 1996) has attempted to reproducethe physical, biological, chemical and geological processes that may occur, simulating theenvironmental conditions and taphonomic and geomorphological modifications of a travertinerock-shelter site (Abric-Romani of Barcelona, Spain). The main physico-chemical features(temperature, humidity, pH and alkalinity) and rock-shelter structures were simulated in inde-pendent trays to observe their respective effects. The results of this experiment have not yetbeen published.
DISCUSSION AND CONCLUSIONS
The examination of the experimental taphonomy literature shows that nearly all aspects arecovered by various experiments, except for some roots action. Globally, archaeologists havehad a more experimental approach than other disciplines. Some process are now well known,while some—such as decomposition at various levels, decay, human predation, gnawing, rootetching, insect action, bioturbation, reworking, bacterial and fungal attacks—are poorly studied.
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The experimental work remains timely and is generally not replicated. The samples used for theexperiments are in most cases too small to be statistically significant, and the protocols are notalways detailed in the published papers. These experiments have yielded information on physicalagencies and allow us to replicate patterns resulting from processes. In the field of diagenesis,the problem of time is generally solved by heating, which may not be the most appropriatemethod, bearing in mind the number of parameters that are involved in diagenetic processes,such as soil composition, bacteria, geomorphological transformations of soils, compaction,chemical modification of bones and soils, and climate, amongst others.
Very few experiments are devoted to successions of processes such as those occurring duringsite formation. It has been well demonstrated that each site may yield results that are due to itsown specific sequence of processes, and that these may lead to superimposed patterns of bonedegradation and chemical composition changes, between which it may then be difficult todistinguish (Denys et al. 1997a,b).
Moreover, to my knowledge there are at present no experiments on spatial or time averagingprocesses, even though their importance in the reconstruction of the taphonomic history of asite is fundamental. In this field, simulations of a mixture of elements may be the best way totest various hypotheses.
However, by limiting the number of variables in test situations, experimental taphonomy inthe laboratory helps to isolate factors that are important in the formation of fossil assemblages,and to build models to test the effects of these factors in natural situations.
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