Histotaphonomic Signatures of Thermally Altered Human Skeletal Remains: Implications for Archaeological Interpretation
ABSTRACT
Interpretations of burial sites from past populations have previously relied on archaeological artifacts, body position, and gross morphological skeletal analysis to reconstruct funerary practices. Recently, microscopic analysis of biotic and abiotic inclusions in bone have resulted in broad assumptions about the antemortem treatment of human remains, including whether the bioerosion is of endogenous and/or exogenous origin. To contextualize the diagenesis present in bone, researchers have developed indices to quantify histotaphonomic features including overall destruction (OHI, GHI), birefringence (BI), cracking (CI), and color changes due to burning (HI). Quantification of the histotaphonomy of bone also allows researchers to determine if the bone is preserved well enough for the application of histological methods, such as age-at-death estimation, which can contribute to the osteobiography of the skeletal remains. However, burned bone found at cremation sites may complicate these analyses if thermal alterations obscure histological structures. Though many studies have experimentally tested the impact of burning on bone, most have used excised bones, which presents a very specific example of the effect of burning on excarnated remains. The aim of this research is to test the histotaphonomic effects of thermal alteration on six fleshed human bodies using the indices listed above. One preburn sample of bone from the femur, sixth rib, and metatarsal was collected prior to burning, and the antimere was removed after the experiment, if recoverable (N = 33). These results show that the presence of body tissue and the amount of time the body is burned likely have the greatest impact on bone histological preservation. None of the remains showed evidence of biotic bioerosion, which was expected from previous research that suggests putrefaction in the early postmortem period contributes to microfocal destruction that can be observed soon after death.
1 Introduction
Archaeological investigations of burial sites have traditionally focused on the analysis of grave types, goods, historical records, isotope analyses, and gross morphological skeletal characteristics to make interpretations regarding the life histories of past populations (Nielson-Marsh et al. 2007; Knüsel 2010; Crevecoeur, Schmitt, and Schoep 2015; Katzenberg and Waters-Rist 2018; Robb et al. 2019). In the past few decades, researchers began to use histotaphonomy to make inferences about the burial practices of archaeological populations that would otherwise not be possible with gross morphological methods alone (Squires et al. 2011; Cambra-Moo et al. 2018; Hollund, Blank, and Sjögren 2018).
Previously, archaeological interpretations of burial events have been documented as either burial or cremation, resulting in an oversimplified interpretation of funerary practices (Knüsel and Robb 2016). In an attempt to provide more detailed information about these cultural behaviors, researchers have begun to incorporate analyses of diagenetic transformations of the microstructural and chemical properties of bone (Portmann et al. 2022) to infer hidden complexities in mortuary history (Bell, Skinner, and Jones 1996; Booth 2016; Booth and Madgwick 2016; Hollund et al. 2012; Parker Pearson 1999; Redfern 2008; Madgwick, 2008, 2010; Smith et al. 2007; White and Booth 2014).
Primarily, histological investigations into the degree of bone degradation, type of microfocal destruction (abiotic or biotic), and determination of whether the destruction is a result of gut bacteria from the early postmortem period (endogenous/enteric), environmental surroundings (exogenous), or combined external and internal factors are thought to help contextualize the postmortem experience of human remains (Booth 2016; Booth and Madgwick 2016; Turner-Walker 2019; Papakonstantinou, Booth, and Triantaphyllou 2020; Emmons et al. 2022; Végh et al. 2021; Lemmers et al. 2020; Mavroudas et al. 2023; Schotsmans et al. 2024). However, occurrences of fragmented burned bone are common in archaeological contexts and often limit the osteobiographical information that can be recovered from such sites (Hanson and Cain 2007; Squires et al. 2011).
The final extent of thermal damage, whether fully or partially cremated, varies across archaeological contexts and will inevitably impact the interpretation of the remains (Thompson 2015). As the sequence of burial events, including whether the body was burned prior to burial, shortly after burial, or long after burial, is of primary interest to researchers (Duday 2009; Lemmers et al. 2020; Rincón-Jaramillo and Gómez-Mejía 2024), it is important to understand the impact soft tissue presence and the stage of decomposition at the time of thermal alteration has on bone preservation (Lemmers et al. 2020; Végh et al. 2021).
Although histotaphonomy can provide an array of information about the postmortem context, the problem that archaeologists face is that any physical, chemical, or biological alteration to bone that obscures histological structures could prevent the application of established histological methods to reconstruct the archaeological record, including differentiating human from nonhuman bone, estimating age-at-death, or even interpreting the taphonomic circumstances described above (Mavroudas et al. 2022).
Previous researchers have attempted to standardize the use of histotaphonomy for archaeological interpretation through the development of indices so that cross-context comparisons can be made and limitations to available methods can be predicted (Hollund et al. 2012; Hedges, Millard, and Pike 1995; Millard 2001; Jans 2005). However, in archaeological contexts in which thermal alteration is inferred, there is a lack of understanding of how that thermal change may affect the utility of these indices. Additionally, experimental research that tests the assumptions of these observations using realistic burning scenarios is scarce (Mavroudas et al. 2022).
1.1 Thermal Alteration to Bone Microstructure
The initial observations of histological changes in bone from experimental thermal alteration were conducted in 1941 by Forbes, in which he describes the general obstruction of osteon lamellae over time with burning and the retained visibility of osteocyte lacunae post burning. Since then, few papers have detailed the relationship between burning temperature and time on the histological structures of human bone (Bradtmiller and Buikstra 1984; Neson 1992; Cattaneo et al. 1999; Hanson and Cain 2007; Fernández Castillo et al. 2013). Additionally, real-world application of the findings by Forbes and others is limited because previous experimentation was conducted using disarticulated skeletonized elements rather than articulated fleshed remains. The use of whole-body experimentations such as those used in the scenarios by Mavroudas et al. (2022), Galloway, Pope, and Juarez (2024), and Schotsmans et al. (2024) may more accurately replicate perimortem contexts.
Although histological investigations into thermal effects on bone most often include observations of the size and shape of skeletal microstructures such as osteons and Haversian canals (Bradtmiller and Buikstra 1984; Mavroudas et al. 2022), researchers have also explored the differences in color changes related to temperature (Hanson and Cain 2007; Squires et al. 2011). Hanson and Cain (2007), for example, investigated histological color changes associated with burned bone in an attempt to associate burning temperatures with color to predict the extent of preservation. Their results showed patterned color changes as well as an increase of cracking of the bone that corresponded to an increase in burning temperature. Squires et al. (2011) used a heat index (HEI) to quantify the observed burn color and remaining collagen within the cross sections of their samples. Like most of the previously discussed experiments, however, this research was conducted directly on excised bone samples, which would be applicable to archaeological scenarios in which the soft tissue was removed or allowed to decompose before thermal alteration or cremation was performed. For archaeological scenarios in which whole, fleshed bodies are believed to be thermally altered, the interpretations from these sites should consider the limitations of the aforementioned research and the affect that the presence of flesh may have on bone color and fracturing.
1.2 Early Postmortem Changes to Bone Microstructure
Previous research has discussed the possibility that microfocal destruction in bone microstructure begins during the fresh/early stage of decomposition (Bell, Skinner, and Jones 1996; Galloway 1997; Trueman and Martill 2002; Jans et al. 2004; Booth and Madgwick 2016; Brönnimann et al. 2018), when the bony structures of the thorax and the abdomen are first exposed to the decomposing gut microbiome (Wescott 2018). This vein of research is based on the assumption that the microorganisms primarily responsible for the bacterial diagenesis of bone are also the driving force of putrefaction (Child 1995a, 1995b).
The patterns of microfocal destruction expected to present postmortem include modifications known as “Wedl” tunneling and “non-Wedl” tunneling. Diagenesis identified as Wedl tunneling was previously thought to be caused by fungi but now is considered the result of cyanobacterial microorganisms from aquatic environments (Turner-Walker et al. 2023; Eriksen et al. 2020). Bacteria has been the primary culprit of other named types of non-Wedl features described as budded, lamellar, or linear longitudinal tunneling (Hackett 1981; Turner-Walker 2019; Papakonstantinou, Booth, and Triantaphyllou 2020). Identification of these features may allow for very specific inferences to be made about peri- and postmortem conditions. For example, some authors suggest the presence of Wedl tunnels in histological cross sections can indicate whether an infant was stillborn or lived for a short period of time before death (Booth, Redfern, and Gowland 2016; Booth 2016). Others suggest methods of body disposal, manipulation, and other specific mortuary practices can be inferred from purported endogenous histotaphonomic features (Papakonstantinou, Booth, and Triantaphyllou 2020; Booth, Bricking, and Magdwick 2024; Reid et al. 2024).
Recently, researchers have begun to question the assertion that diagenetic changes occur early in the decomposition process, which complicates the assumptions made by bioarchaeologists when describing the taphonomy associated with funerary practices of past populations. These studies assert that microorganisms found in soil and other exogenous burial components, such as coffin materials and local water sources, are the primary contributions to the appearance of microfocal inclusions (Bell, Skinner, and Jones 1996; Jans et al. 2004; Turner-Walker 2012; Kontopoulos, Nystrom, and White 2016; Kendall et al. 2018; Brönnimann et al. 2018; Turner-Walker 2019; Turner-Walker et al. 2023).
In the past couple of years, researchers with access to willed-body donation programs have focused on developing realistic, rigorous studies aimed at uncovering the nuances of diagenetic presentation in human bone. Mavroudas et al. (2023) tested multiple burial and surface deposition scenarios, whereas Schotsmans et al. (2024) examined microstructure from long-term surface depositions. Neither study found a direct relationship between putrefaction and microfocal destruction. Analysis of histotaphonomic alteration to bone in the early postmortem period from controlled, realistic decomposition scenarios is necessary to clarify the origins and meaning of diagenetic changes in archaeological interpretations.
1.3 Quantifying Taphonomic Change in Bone Microstructure
Bioarchaeologists utilize bone degradation indices for comparable results across research teams and contexts (Hollund et al. 2012; Hedges, Millard, and Pike 1995; Millard 2001; Jans 2005). Documenting the degree of destruction to histological structures is useful for tempering taphonomic interpretations from histological material that could otherwise lead to sweeping explanations of the postmortem experience for certain types of bioerosion.
The histological index established by Hedges, Millard, and Pike (1995) was the first scoring system to examine the effects of biotic agents such as bacteria and fungi in bone microstructure. With a scale of 0–5, it allows investigators to quantify the degree of cortical destruction at the histological level. A score of 0 indicates that no original histological features are identifiable (other than Haversian canals), and a score of 5 indicates that there is no evidence of destruction to the bone microstructure. This system became known as the Oxford Histological Index (OHI) and was later slightly modified by Millard (2001) as the index most used by histotaphonomic researchers today (Table 1). In 2012, Hollund et al. developed a new measure of cortical destruction named the General Histological Index (GHI) (Table 2). Similar to the OHI, the GHI is also scored on a scale of 0–5 but allows the practitioner to include abiotic artifacts such as cracking, infiltrations, and color change.
| OHI | Approx. % of intact bone | Description |
|---|---|---|
| 0 | < 5 | No original features identifiable, other than Haversian canals |
| 1 | < 15 | Small areas of well-preserved bone present or some lamellar structure preserved by pattern of destructive foci |
| 2 | < 50 | Clear lamellate structure preserved between destructive foci |
| 3 | > 50 | Clear preservation of some osteocyte lacunae |
| 4 | > 85 | Only minor amounts of destructive foci, otherwise generally well preserved |
| 5 | > 95 | Very well preserved, virtually indistinguishable from fresh bone |
| GHI | Approx. % of intact bone | Description |
|---|---|---|
| 0 | < 5 | No original features identifiable, except that Haversian canals may be identifiable |
| 1 | < 15 | Small areas of well-preserved bone present, or the lamellate structure is preserved by the pattern of destructive foci |
| 2 | < 50 | Some well-preserved bone present between destroyed areas |
| 3 | > 50 | Larger areas of well-preserved bone present |
| 4 | > 85 | Bone is fairly well preserved with minor amounts of destroyed areas |
| 5 | > 95 | Very well preserved, similar to modern bone |
To document the loss of collagen over time, Jans et al. (2002) developed the birefringence index (BI), which measures the intensity of birefringence throughout the sample as viewed through polarized light on a scale of 0–1: 0 indicates no birefringence, 0.5 indicates reduced birefringence, and 1 indicates full birefringence (Table 3). To document the destruction of bone, Jans (2005) also developed the cracking index (CI), which measures the number of fractures observed throughout the cortical bone cross section across several fields of view (Table 4).
| 0 | No birefringence |
| 0.5 | Reduced birefringence |
| 1 | Perfect birefringence |
| Microcracking | Small cracks within the perimeter of a given osteon |
| Cracking index | A measure of the percentage of cracked osteons in one field of view |
Another measure of bone taphonomy, known as the HEI, is a bone color and collagen content scoring system created by Brönnimann et al. (2018) (Table 5). This system was adapted from two different scales: A color scale described by Squires et al. (2011) based on evidence that bone color can infer burning temperature (Munro, Longstaffe, and White 2007; Walker, Miller, and Richman 2008) and observations of thermal alteration to collagen fibers and crystal structures documented by Fernández Castillo et al. (2013). The original Squires table had three categories (less intensely cremated, intensely cremated, and completely cremated) with prescribed percentages of cross-sectional destruction and associated temperature ranges. Each category also included an image detailing the degree of destruction, as well as a description of the structure's appearance and other information regarding the structures visible in the section (Squires et al. 2011). In their adaptation, Brönnimann et al. (2018) included scoring intervals ranging from 0 to 3 with 0.5 increments to allow for a more variable description of color changes and collagen degradation.
| HEI | Coloring | Bone microstructure | Mineral structure | Description |
|---|---|---|---|---|
| 0 | — | No change in microstructure (100% preserved) | Collagen fibers well visible | No thermal alteration |
| 0.5 | Weak, orange-brown | No change in microstructure (100% preserved) | Collagen fibers well visible | Probable thermal alteration (uncertain) |
| 1 | Weak, orange-brown | Minor changes in microstructure (> 80% preserved) | Collagen fibers visible; longitudinal microfractures | Weak thermal alteration |
| 1.5 | Distinct, orange-brown | Minor changes in microstructure (> 80% preserved) | Collagen fibers still visible; longitudinal microfractures; first formation of crystalline structures | Moderate thermal alteration |
| 2 | Distinct, brown | Partly destroyed microstructure (< 50% preserved) | Formation of large (cubic) crystalline structures | Distinct thermal alteration |
| 2.5 | Strong, brown-gray | Mostly destroyed microstructure (only Haversian canals visible; < 20% preserved) | Formation of large (cubic) crystalline structures | Strong thermal alteration |
| 3 | Strong, brown-gray | Microstructure destroyed (0% preserved) | The granular structure has completely disappeared | Very strong thermal alteration |
Recently, Végh et al. (2021) noted that color changes due to burning can obscure and mimic features of microfocal destruction. Alternatively, Lemmers et al. (2020) found that thermal alteration from simulated cremations reaching 900°C did not alter the clarity of bioerosion in bone microstructure. The HI will be used as a tool to quantify the visibility of bone prior to the analysis of bioerosion, if present.
The aim of this research is to experimentally test the taphonomic effects of thermal alteration on bone histological structures from fully fleshed human remains using the five established histotaphonomic indices to document: (1) overall degree of destruction (OHI and GHI), (2) birefringence (BI), (3) degree of cracking (CI), and (4) extent of thermal color changes (HI; a new index adapted from HEI as described in the experimental design below). This analysis will provide a realistic expectation of bone preservation from burned remains recovered across various archaeological contexts and provide more information about the origin of diagenetic changes.
1.4 Experimental Design
Six donors from the Forensic Anthropology Center at Texas State (FACTS) were placed in various fire death scenarios at the Forensic Anthropology Research Facility at Texas State (FARF) in San Marcos, Texas. FACTS operates a whole-body donation program wherein individuals or their legal next of kin can donate their bodies for forensic science research and education and complies with all legal and ethical standards regarding the use of human remains for scientific research in the United States (Gocha, Mavroudas, and Wescott 2022). Each of the donors or donors' next of kin consented to traumatic analyses, including thermal alteration, prior to or at the time of donation.
The complete demographic descriptions of each donor are shown in Table 6. Locations of placement included a sport utility vehicle (n = 1; D1), temporary building structures referred to as Pods (n = 3; D3, D4, D5), and a pyre set over a pit in the ground (n = 2; D2, D6). Table 7 details these placements as well as the minimum and maximum recorded temperatures for the structures and bodies (descriptions of fire scenarios and images can be found in Mavroudas et al. 2022). Donors 1–5 were placed while in the “fresh” stage of decomposition. Prior to placement at the fire scenes, five of the donors were kept in a freezer between 2 and 9 months and then left to thaw for 72 h before the experiment. Individual D6 was in the “mummified” state of decomposition when placed for the burn experiment (Table 8) (Megyesi, Nawrocki, and Haskell 2005; Wescott et al. 2018). Before this research, D6 was a part of an ongoing longitudinal decomposition study at FARF where the body was placed on the ground surface in a fresh stage of decomposition and allowed to decompose naturally for 62 days. Photographs taken 5 days postmortem show the donor with active bloat in the thorax, abdomen, and limbs with multiple maggot masses throughout the body. At the time of burning, D6 had a thin layer of mummified skin stretched across the skeleton and some dry abdominal soft tissue. Inclusion of the mummified donor allowed for the potential to observe any thermal changes to biotic microfocal destruction, which should occur in the early postmortem period before mummification. Including D6 also allowed for observations of differences in burning patterns between fresh and mummified remains.
| Donor | Biological sex | Age at death (years) | Height (cm) | Weight (kg) | Placement | Cause of death |
|---|---|---|---|---|---|---|
| D1 | Female | 85 | 157 | 34.5 | Supine, back of car | Respiratory failure |
| D2 | Female | 81 | 165 | 65.8 | Supine, pyre/pit | Alzheimer's |
| D3 | Female | 74 | 164 | 56.7 | Supine, couch in Pod 1 | Coronary artery disease |
| D4 | Female | 61 | 167 | 47.6 | Supine, bed in Pod 2 | Lung cancer |
| D5 | Male | 65 | 173 | 65.8 | Supine, bed in Pod 3 | Pneumonia |
| D6 | Female | 67 | 158 | 43.5 | Supine, pyre/pit | Pulmonary disease |
| Donor | Placement | Burn time (min) | Structure | Mouth middle | Mouth side | Sixth rib | Elbow | Thigh | Shin | Calf | Foot | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| D1 | Max | Supine, back of car | 42 | 1116 | 802 | 795 | 1273a | 880 | 1054 | 1390a | 959 | 1385a |
| Mean | 416 | 392 | 401 | 353a | 363 | 427 | 447a | 455 | 464 | |||
| D2 | Max | Supine, pyre/pit | 53 | 1037 | 549 | 667a | 956 | 924 | 718 | 823a | 314 | 988a |
| Mean | 207 | 104 | 163a | 222 | 287 | 196 | 322a | 103 | 377a | |||
| D3 | Max | Supine, couch in Pod 1 | 32 | 880 | 821 | 861 | 324a | 308a | 440a | 931 | 886 | 781 |
| Mean | 193 | 166 | 187 | 44a | 43a | 38a | 193 | 199 | 138 | |||
| D4 | Max | Supine, bed in Pod 2 | 32 | 833 | 479 | 458 | 532a | 83a | 30a | 106 | 731 | 827a |
| Mean | 152 | 72 | 69 | 50a | 27a | 20a | 31a | 72 | 25a | |||
| D5 | Max | Supine, bed in Pod 3 | 22 | 1070 | 1035 | 1072 | 26a | 452 | 46a | 756 | 684 | 818 |
| Mean | 251 | 206 | 198 | 24a | 65 | 31a | 111 | 73 | 144 | |||
| D6 | Max | Supine, pyre/pit | 53 | 1080 | 936a | 925a | 1074 | 1014 | 1045 | 1021 | 1015 | 1063 |
| Mean | 353 | 250a | 144a | 317 | 333 | 491 | 385 | 512 | 312 | |||
- a Indicates thermocouple failure, incomplete dataset.
| Donor | No. of days at FARF preburn | Stage of decomposition at placement | No. of days at FARF postburn |
|---|---|---|---|
| D1 | N/A | Fresh | 58 |
| D2 | N/A | Fresh | 58 |
| D3 | N/A | Fresh | 59 |
| D4 | N/A | Fresh | 59 |
| D5 | N/A | Fresh | 59 |
| D6 | 62 | Mummified | 0 |
Once the donors were in place, thermocouples were inserted in the bodies to capture temperature changes near the bones and teeth during burning. These locations include: the elbow (cubital fossa), rib cage (near the midshaft of the sixth rib), thigh (deep, near the midshaft of the femur), shin (near the midshaft of the tibia), calf (also near the midshaft tibia), foot (near the fifth metatarsal), and two in the mouth (one in the midline of the oral cavity, the other laterally in the oral vestibule between the gums and cheek). The placement of thermocouples in anatomical regions allows for the comparison of variation in bone color changes with burn temperature and time in each location. Thermocouples were also placed within each structure to capture temperature fluctuation during burning. The fires were set, and burn times were established with assistance from local fire departments. All fires were extinguished with water through built-in sprinkler systems and/or water applied via firehose.
Prior to burning, three 1–3 cm bone segments were retrieved from the left side of each donor, including the midshaft femur, midshaft sixth rib, and fifth metatarsal (known as the “preburn” sample). These sampling locations were selected due to their inclusion in standardized methods for histological analysis (Gocha, Robling, and Stout 2018) and their varying distances from the gut microbiome. After burning, the same bones from the contralateral side were collected when available (known as the “postburn” sample) and processed according to the same procedures.
In order to quantify the thermal alteration seen in the cortical bone samples, the five previously described taphonomic indices (OHI, GHI, BI, CI, and HI) were applied to each sample cross section in both the pre- and postburn groups. Using live imaging on a Leica DM6M microscope, each cortical bone slide was evaluated for taphonomic changes using standard applications of OHI, GHI, BI, and CI. A simplified version of HEI, more similar to that described by Squires et al. (2011), was applied to analyze only microscopic color changes on a 0–3 scale (the mineral/collagen component was excluded). Additionally, the descriptors “Black” and “White” were included in score of 3, and very strong brown was removed from score of 3 as it was determined those colors were covered under score 2. Neither “Black” nor “White” was listed in either of the former tables, but the colors were observed in this analysis (Table 9). As this index was modified from Squires et al. (2011) and Brönnimann et al. (2018) tables, it will be referred to as “HI” for the remainder of this paper.
| HI | Color | Description |
|---|---|---|
| 0 | — | No thermal alteration |
| 1 | Weak, orange-brown | Weak thermal alteration |
| 2 | Distinct, brown | Distinct thermal alteration |
| 3 | Black, white | Very strong thermal alteration |
Two-tailed t-tests were used to evaluate the differences in index scores between the pre- and postburn samples by element. Additionally, two-tailed Spearman's correlation tests were conducted to assess the relationship between all index scores and temperature for each of the thermocouple locations. A Bonferroni correction was applied to control for the multiple tests and ensure robust results (p ≤ 0003).
2 Results
2.1 Thermocouple Readings and Sample Collection
An antimere from each element was available for postburn sampling except for the rib and metatarsal of D6 and the metatarsal of D1. Near complete incineration of individual D6, the mummified donor who was burned on the pyre in the pit, prevented postburn sampling for all elements except a small fragment of the femur. Additionally, only a rib and femur were available for sampling from individual D1, the donor who was burned in the vehicle. The thermocouple data showed that the overall structure temperature was the highest for the vehicle, which reached 1116°C, but the lowest max temperature was recorded in Pod 2 at 833°C. All available thermocouple data are shown in Table 7.
2.2 Histotaphonomic Index Results
Two-tailed paired t-tests were used to determine if there were differences in index score between the pre- and postburn sample. Index scores for all elements by donor are presented in Table 10. For individual and pooled elements, no significant differences in taphonomic indices were observed in the femur, ribs, or metatarsals.
| Donor 1 | Donor 2 | Donor 3 | Donor 4 | Donor 5 | Donor 6 | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Element | Index | Pre | Post | Pre | Post | Pre | Post | Pre | Post | Pre | Post | Pre | Post |
| Femora | OHI | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 |
| GHI | 5 | 3 | 5 | 4 | 5 | 4 | 5 | 5 | 5 | 5 | 5 | 1 | |
| BI | 1 | 0.5 | 1 | 0.5 | 1 | 0.5 | 1 | 1 | 1 | 1 | 1 | 0 | |
| CI | 0.11 | 0 | 0.02 | 0.12 | 0.13 | 0.11 | 0.02 | 0.05 | 0.19 | 0.06 | 0.18 | 0.18 | |
| HI | 0 | 3 | 0 | 2 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 3 | |
| Ribs | OHI | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | — |
| GHI | 5 | 4 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | — | |
| BI | 1 | 0.5 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0.5 | 1 | — | |
| CI | 0.13 | 0.36 | 0.18 | 0 | 0.12 | 0.02 | 0 | 0.05 | 0.05 | 0 | 0.29 | — | |
| HI | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | — | |
| Metatarsals | OHI | 5 | — | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | — |
| GHI | 5 | — | 5 | 2 | 5 | 5 | 5 | 4 | 5 | 5 | 5 | — | |
| BI | 1 | — | 1 | 0 | 1 | 1 | 1 | 0.5 | 1 | 1 | 1 | — | |
| CI | 0 | — | 0.02 | 0.49 | 0 | 0.05 | 0.02 | 0.24 | 0.04 | 0.15 | 0 | — | |
| HI | 0 | — | 0 | 3 | 0 | 0 | 0 | 3 | 0 | 0 | 0 | — | |
The results for each index are presented in the following paragraphs. Descriptions of the burning details, including placement, duration of burning, available postburn elements, days of decomposition, and thermocouple readings for various elements were described in a previous manuscript and have been reiterated in Tables 6–8 for context (Mavroudas et al. 2022).
2.2.1 OHI
No significant changes in OHI were observed between pre- and postburn samples for any of the donors because there was no biotic destruction observed. As a result, no donor scored lower than OHI = 5, very well preserved, virtually indistinguishable from fresh bone.
2.2.2 GHI
There were no statistical differences in GHI scores between pre- and postburn samples. All individuals had a preburn GHI score of 5, very well-preserved bone, for every element in the sample. Deviations from the preburn score were observed for individual D1 in both the postburn femur (GHI = 3) and the postburn rib (GHI = 4). Changes in GHI for individual D2 were observed in the postburn femur (GHI = 4) and in the metatarsal (GHI = 2), changes in GHI for individual D3 were observed in the postburn femur (GHI = 4), changes in GHI for individual D4 were observed in the postburn metatarsal (GHI = 4), and changes in GHI for individual D6 were observed in the postburn femur (GHI = 1). No changes in GHI were observed for individuals D5.
2.2.3 BI
There were no statistically significant differences in BI between the pre- and postburn samples. Changes in BI from score 1, perfect birefringence, were observed for individual D1 in the postburn femur (BI = 0.5) and the postburn rib (BI = 0.5), in individual D2 for the postburn femur (BI = 0.5) and for the postburn metatarsal (BI = 0), in individual D3 for the postburn femur (BI = 0.5), in individual D4 for the postburn metatarsal (BI = 0.5), in individual D5 for the postburn rib (BI = 0.5), and in individual D6 for the postburn femur (BI = 0).
2.2.4 CI
There were no significant differences between the pre- and postburn CI index scores for any elements. Interestingly, whereas some donors showed increases in cracks between pre- and postburn elements, many donors displayed fewer cracks in the postburn analysis (Table 10). It is possible that these preburn cracks are artifacts of sample preparation. Postburn samples showed a combination of cracks that could have resulted from either thermal alteration or sample preparation, or a combination of both causes, especially because the section may have less collagen, resulting in weaker bone postburn. It appears CI is cross section dependent in this sample and could not be used as a reliable indicator of postburn changes. Therefore, further discussion of index changes will not include the CI variable.
2.2.5 HI
Images of cross sections of femora, ribs, and metatarsals with thermal alteration as identified using the HI are included in the supplementary material for this manuscript (Figures S1–S3) (Mavroudas et al. 2022).
There were no statistically significant changes in HI scores between pre- and postburn samples. All individuals had an HI score of 0 for every element in the preburn sample. After burning, individuals D1, D2, D3, and D6 all had higher HI scores (HI = 3, 2, 1, and 3; respectively) in the femora. The rib of individual D1 showed an increase in HI score (HI = 1), and the metatarsals of individuals D2 and D4 had increased HI scores as well (HI = 3 for both donors).
2.2.6 Spearman's Correlation Coefficient for Index Score and Temperature
No statistically significant relationship between index score and temperature were observed. The highest max temperature recorded for a donor with index score changes was 1385°C in the foot of individual D1, whereas the lowest recorded temperature for a donor with index score changes was 324°C in the rib of individual D3. The max temperatures for regions where index scores changed in each donor are reported below for reference.
Individual D1 burned for 42 min and had index changes for GHI, BI, and HI in the rib where the temperature reached 1273°C. There were also index changes for BI and HI in the femur where the temperature reached 1054°C. The highest recorded temperature for individual D1 was in the foot at 1385°C. The metatarsal was not able to be recovered from this individual.
Individual D2 burned for 52 min and showed index score changes for GHI and BI in the femur where the temperature was 718°C. There were no index changes in the rib where the temperature reached 956°C. And, finally, there was change in BI and HI where the temperature reached 988°C; respectively. Overall, the highest recorded temperature for individual D2 was 1037°C, which was recorded in the thermocouple placed in the structure (car).
Individual D3 burned for 32 min and showed index changes for BI and HI in the femur where the temperature reached 440°C in the thigh. There were no index changes in the rib where the temperature reached 324°C and no changes in the metatarsal of D3 where the temperature reached 781°C in the foot. The highest recorded temperature for individual D3 was 880°C, which came from the thermocouple placed within the structure (Pod 1).
Individual D4 burned for 32 min and did not record any index changes in the femur where the temperature reached 1045°C or in the rib where the temperature high was 532°C. In the metatarsal, index changes were observed for GHI, BI, and HI where the temperature reached 827°C in the foot. The highest recorded temperature for individual D4 was 833°C, which came from the thermocouple placed within the structure (Pod 2).
Individual D5 burned for 22 min and showed no index changes in the femur, but there were changes in BI for the rib; however, the thermocouple for both elements failed, so the temperature range in this region was not recorded. No index score changes were recorded in the metatarsal of individual D5 where the temperature of the foot reached 818°C. The highest recorded temperature for individual D5 was from the thermocouple placed in the side of the mouth, where it reached 1072°C.
Finally, D6 burned for 52 min. The HI score changed in the femur where the thigh reached a temperature of 1045°C. The highest recorded temperature for individual D6 was 1080°C, which came from the thermocouple placed within the structure (pyre/pit).
3 Discussion
Overall, there were no significant index score changes between pre- and postburn samples when all donors and elements were examined individually or when they were pooled.
The lack of recovery for the metatarsals precluded use of postburn samples for D1 and D6. Individuals who experience fire trauma are often discovered in what is known as the “pugilistic position,” or a boxer-like posture, due to the contraction of the muscles of upper and lower limbs including the fixed flexion of the phalanges and metacarpals/metatarsals of the hands and feet (Harding, Márquez-Grant, and Williams 2024). This position made it difficult to recover the metatarsals in this study. As a result, metatarsals could not be recovered from D1 and D6 due to the extent of damage to the skeleton in the vehicle and the pyre, respectively. The pugilistic position also caused the femora of D1 and D6 to fracture, contributing to the thermal alteration (color change) identified in the microstructure of those two elements. Femora that remained intact (D2–D5) contained microstructure that was better preserved.
Spearman's correlation coefficient results suggest that temperature is not a good indicator of the potential for taphonomic signatures to be present in bone histomorphology. When HI scores were compared to body region temperatures, the results of Spearman's test indicated an increase in HI score with an increase in temperature in the sixth rib, thigh, and shin. However, when these scores are compared to the temperature highs for each element, there is not a clear association between temperature and HI score. This result might be due to the small sample size and lack of accurate temperature data for individuals with thermocouple failure. Failed thermocouple temperatures in Table 7 represent the highest temperature recorded prior to failure. These temperatures were included in the data if the recorded temperature was greater than the lowest successfully recorded temperature in which the thermocouple did not fail (106°C).
The presence of soft tissue appears to be the most important factor in predicting the extent of taphonomic change to bone postburn. None of the ribs recovered after burning showed significant taphonomic changes, which may be due to the presence of thoracic soft tissue at the time of burning. Although the portion of the rib adjacent to the posterior and lateral thorax may not be as well protected by soft tissue as the medial aspect, multiple layers of deep fascia and muscle may protect the ribs from direct exposure to fire. It is likely that body size plays a role in how much of the rib is left vulnerable to thermal alteration. The importance of this protection may be demonstrated by D6, the mummified individual. Although this donor still maintained some dry soft tissue in the abdomen, the ribs were exposed prior to burning and the sixth rib was subsequently not recoverable.
Color changes were only observed on the cutaneous surface of one rib (D1). As this is the anterior facing region, it is logical that it would be affected prior to the pleural surface of the rib. The superior surface of this rib was not available for sampling or scoring due to thermal destruction. The inferior and superior surface of the rib of individual D2 was also not available as a part of the sample; however, that individual does not show any color changes on the remaining cutaneous and pleural surfaces.
The femora were affected by burning to a greater degree than the rib but were less impacted by burning than the metatarsals as indicated by the lack of recovery of the metatarsals due to fire damage. In life, the femur is nestled deep in the thigh, surrounded by a thick layer of superficial fascia and fat as well as some of the largest muscles in the human body. Therefore, it would seem that the femur is better protected by soft tissue than the ribs. However, based on the pattern of thermal damage, it is likely that the femur was exposed to fire and smoke by way of the lateral muscular septum, an anatomical space that divides the lateral and posterior thigh muscle compartments. This space extends from the superficial fascia to the linea aspera of the femur, leaving the bone vulnerable to fire exposure as the muscles contract and the compartments separate throughout the burning process.
There does not seem to be a discernable pattern between the thermal damage to the metatarsals. This is surprising, considering the difference in soft tissue thickness between the dorsal and plantar surfaces of the foot. Two postburn metatarsals were not recoverable (D1 and D6). The metatarsal is the most superficial of all the bony elements sampled as it is surrounded only by a thin layer of fascia on its dorsal surface, though it is better insulated on the plantar surface by a relatively thick layer of fat, muscles, and plantar fascia. The differences in preservation between the ribs, femora, and metatarsals suggest that realistic burning experiments lead to varying results in taphonomic presentation that may depend on the amount of soft tissue present on the body at the time of burning.
After the presence of soft tissue, length of burn time is also an important factor when considering the presence of taphonomic changes in bone histomorphology in archaeological contexts. Donor 5, which showed only a half point decrease in BI index score for the rib but no changes for any of the other elements, was only burning in Pod 3 for 22 min before the fire was extinguished. This is the least amount of time for any of the burned structures. Donor 2 and individual D6 were burned side by side on the same pyre for a total of 53 min, the longest burn time of all the structures. Although all elements were recovered from D2 after the burn, there were changes in GHI, BI, and HI for multiple elements. Only the femur was recoverable for D6. It is likely this degree of destruction is due to the difference in soft tissue (wet vs. mummified) between these donors as discussed above. The next longest burn time was 42 min and was recorded in the back of the vehicle where only the femur and rib were recovered from D1. The burn temperature of the vehicle was the highest of all the structures (1115.71°C), which, combined with the long burn time, likely contributed to the degree of bone destruction. Both the femur and rib showed GHI, BI, and HI score changes between pre- and postburn elements. This burn time is 10 min longer than the time recorded for D3 and D4 in Pods 1 and 2, respectively, where all elements were recovered. Only the femur of individual D3 showed index changes, and the metatarsal showed taphonomic changes in individual D4.
The only index that did not show any impact of thermal alteration on the cortical bone was OHI. This index measures the degree of taphonomic change due to biotic factors such as bacteria and fungi. None of the six donors presented with biotic changes in either the preburn or postburn analysis. The lack of OHI changes in this study is consistent with previous histotaphonomic studies at the same facility (Mavroudas et al. 2023). These results contradict research that indicates biotic material will present histologically in early-stage decomposition, also known as the early postmortem period, a process each of these donors entered immediately after death (Bell, Skinner, and Jones 1996; Booth 2016).
Although the donors were burned, four of the six retained some abdominal soft tissue (D2–D5). These individuals were left to continue decomposing on the ground surface for 58–59 days until the samples were removed. If microfocal destruction begins shortly after death, the authors might expect to find evidence of diagenesis in these donors at the time of sampling; however, no biotic changes were observed. Therefore, this study is in agreement with recent research that suggests the infiltration of gut bacteria into bone is a more complicated process than previously thought and that burial context, including soil content and length of time before exhumation, plays a significant role in microfocal destruction (Papakonstantinou, Booth, and Triantaphyllou 2020; Turner-Walker et al. 2023; Bricking 2023; Mavroudas et al. 2023; Booth, Bricking, and Magdwick 2024; Schotsmans et al. 2024). These factors will be the focus of future research.
According to Booth, Chamberlain, and Pearson (2015), mummification can affect the presence of osteolytic bacteria if putrefaction dried too quickly to allow the bacteria to leach into the bones. The mummified donor in this study (D6) experienced bloat over the course of 62 days prior to dehydration of the skin. If the origin of diagenesis is endemic and discoverable shortly after death, the donor should have displayed evidence of bioerosion despite mummification. Additionally, White and Booth (2014) and others (Kontopoulos, Nystrom, and White 2016; Fernández-Jalvo et al. 2010) have suggested that the activity of “skeletonizing insects” can impede the progress of diagenesis by speeding up the process of decomposition.
Végh et al. (2021) suggested that burning can conceal and/or mimic diagenetic processes such as Wedl tunneling, but this was not observed in the present sample as the majority of bone microstructure from the available samples was not disrupted. Lemmers et al. (2020) found that bioerosion (if present) will still be visible in bone burned with or without flesh at temperatures at or around 900°C, the temperature at which they considered the excised bone sample to be cremated. This study agrees with the findings by Lemmers et al., and future research will stage burial scenarios using fleshed remains.
4 Limitations of the Study
It is possible that the context of deposition contributed to the lack of biotic microfocal destruction present in this study. Five of the donors were burned while in the fresh state of decomposition, and, after burning, the bodies were laid on the surface at FARF for 58–59 days (Table 8). This scenario is different from some archaeological contexts where remains found to have bioerosion are buried beneath the surface for many years prior to discovery and analysis. It is likely that the type of soil, degree of saturation of the soil, and length of time of deposition have an impact on the amount of diagenesis present in the bone (Maurer et al. 2014; Reid et al. 2024; Turner-Walker et al. 2023). The present study does not include buried remains where underground skeletal material would have soaked in putrefaction for extended periods of time. In the future, it would also be worth observing if there is a link between the presence of bioerosion and the season of death, which affects the time it takes a body to mummify as well as the amount of invertebrate activity present throughout the decomposition process. However, the recent study by Schotsmans et al. (2024) indicates surface burials across multiple seasons show no difference in the presence of bioerosion.
5 Conclusion
This research aimed to contextualize the use of commonly applied histotaphonomic indices within realistic burning scenarios that more accurately replicate perimortem burning of the adult human body. Although many bioarchaeological investigations do not include histology due to hesitancy to use destructive methodologies, this research shows that burned skeletal elements found fragmented in situ may be viable for histological analysis because many elements examined here displayed little to no taphonomic destruction.
Bone destruction due to bacteria and fungi, scored using OHI, was not present in this sample and should not be considered necessary evidence that an individual experienced a natural process of decomposition. Destruction of bone due to abiotic factors (fire damage) was frequently identified in this sample and changed the bone, according to HI scores. Burning also had an impact on the GHI and BI. Rather than the temperature of the fire, it is likely that burning time and moisture content of soft tissue play a greater role in the taphonomic changes observed in bone histomorphology after exposure to realistic burning scenarios. The combination of long burn times, high temperatures, and the presence of dry tissue do appear to lead to greater skeletal destruction with the least amount of recoverable bone. However, in this study, even when small fragments of bone are recovered, as in the case of cremains, the histological structure is mostly intact and available for observation of diagenesis and other analyses.
Because this study did not find significant microfocal destruction from abdominal decomposition, future studies should test the impact of burial depth and amount of time beneath the surface prior to and after burning as those factors could prove to be more significant contributors to microfocal destruction. Additional research will consider the condition of the skeleton before the burn occurs (fleshed, mummified, or skeletonized) and the body size of the individual in order to properly understand the circumstances surrounding death and the postmortem treatment of archaeological skeletal remains.
Acknowledgments
The authors would like to thank the donors and their families whose generous gift made this research possible as well as the Forensic Anthropology Center at Texas State and the Collin County Fire Investigators Association for access to the fire death scenarios. The authors would also like to thank the anonymous reviewers for their helpful revisions.
Disclosure
The authors have nothing to report.
Conflicts of Interest
The authors declare no conflicts of interest.
Open Research
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
