Cultural Influences on the Differential Decomposition of Animal Bone

April M. Beisaw, RPA

Presented Before the 2000 Meeting of the Society for American Archaeology, Philadelphia, Pennsylvania

The Site

The Thomas/Luckey Site (SUBi-888) is a Late WoodlandÐOwasco site on the Chemung River in Ashland, New York. The comprehensive site report characterizes the faunal assemblage as follows: "Faunal data from the Thomas/Luckey Site were meager. The majority of the animal remains preserved at the site consist of very small fragments of calcined bone. Little can be gleaned from these fragments". I disagree.

The Study

Before cooking, mammals are skinned, birds defeathered, and fish may or may not be scaled. Inedible or undesirable portions such as heads and tails may be removed prior to or after cooking. These elements as well as those remaining after consumption are discarded, often into fires or trash pits, decomposition beginning immediately thereafter. These cultural practices of food preparation and disposal dictate the immediate environment in which faunal remains decompose (for example: flesh may be interred with cranial elements and charcoal with odorous fish bone). To determine the extent of cultural influences on the paucity of faunal remains at the Thomas/Luckey site, I experimented with the differential decomposition of uncooked (or natural), cooked, skeletonized, and burned carcasses from three common animal classes available to the inhabitants of the site: mammals, fish, and birds. These taphonomic experiments address two questions: 1) Do cooked animals decompose at a different rate from uncooked animals, and if so what affect does the degree of burning of bones have on their rates of decomposition? 2) Do skeletonized animals decompose at a different rate from animals which have their flesh and organs intact, and if so, can analysts differentiate culturally deposited animal remains from naturally deposited animal remains?

Design

To hold all factors constant, except animal class and pre-burial conditions, I buried various animal carcasses at the Thomas/Luckey site for a period of one year, holding constant the variables of climate, sediment composition, and duration of decomposition as well as allow for direct applicability of the results to the archaeological site. Due to the time limitations, a variation of the experiment was devised in an attempt to accelerate diagenesis while still holding all other factors constant. This experiment essentially eliminates the slow decomposition cycle of the winter months by keeping the experiments at room temperature for the year of internment. As this time span is still far from that of a typical archaeological site, a compositional study of the Thomas/Luckey faunal assemblage served as a test case for the results of the differential decomposition experiments. Experiments were conducted using squirrels (Sciurus carolinensus), pigeons (Columbia liva), and panfish (Lepomis gibbosus). These species were chosen so as to approximate equal body sizes (between species). By holding these biological and environmental factors constant, the impact of animal processing and disposal on decomposition can be assessed.

Field and Laboratory Experiments

Each animal was prepared, and a portion of the animal was removed for the accelerated diagenesis experiment. The remaining carcasses were then placed in plastic containers, half filled with soil. Each container was then topped off with soil and interred at the Thomas/Luckey site for 1 year. All field containers were open on one side to allow for the movement of insects and other small animals and were perforated on the bottom to allow for the movement of rainwater through the containers. The specimens for the accelerated diagenesis experiment were treated in a similar fashion but were kept in sealed containers and indoors at, or above, room temperature for approximately ten months. The soil in each container was kept moist by periodic additions of water during which the containers were allowed to air out to allow for oxygen exchange and soil drying.

Data Analysis and Results

I excavated the field samples by hand. All bones were separated from the soil, dried, cleaned, weighed, and measured. During the excavation of the laboratory experiments, I noted that the soil in all containers, except for those with the uncooked squirrel and pigeon, remained fairly dry. The uncooked squirrel was also the only container in which flesh or fur survived, albeit as an odorous black sludge. Bone from this experiment also displayed a dark and waxy appearance. Bone brittleness was measured by applying pressure to each bone in a tensiometer device. All specimens subject to this test were compared to a reference bone that did not undergo the diagenesis experiment. These experiments revealed a significant difference in the brittleness of squirrel femurs. The femora that were interred with flesh were able to withstand much greater amounts of pressure than those which were skeletonized prior to internment. The same trend was not observed with the uncooked versus skeletonized pigeon bones. However, the tibiotarsus of a bird is not completely covered in flesh as is the squirrel femur. This suggests that the simultaneous decomposition of flesh is responsible for the hardening of the uncooked squirrel femurs. All of the pigeon femora and tibiotarsi exposed to diagenesis appear to be less resistant to breakage than those that were not interred. Neither the outdoor nor the indoor decomposition experiments seemed to produce differential decomposition within the pigeon experiments. Fish vertebrae from all but the burned sample proved to be stronger than the pigeon femur/tibiotarsi in all cases. For this analysis, one single fish vertebra was used where possible, except for the burned fish where three vertebrae fused together. Despite this extra mass, the burned vertebrae crushed as soon as tension was applied. Carbon specks were all that remained which, if found at an archaeological site, would not even be identifiable as bone. Fish remains also displayed other evidence of differential decomposition. Despite the fact that the uncooked and cooked conditions of both the indoor and outdoor experiments were interred with approximately equivalent amount of scales, the uncooked field experiment recovered over 200 scales, while only 30 were recovered from the cooked field condition. A more dramatic difference can be seen in the analysis of the corresponding lab experiments where the cooked condition yielded no scales at all. These results suggest that the practice of cooking fish with scales intact may result in a total or near total destruction of fish scales. Cooking unscaled fish and subsequent disposal of unconsumed bones into a fire may totally eliminate fish remains from the archaeological record.

Conclusions

First, the effect of cooking on the preservation of animal bones varies among taxa. While cooking seemed to help preserve the squirrel bone, it had little affect on the bird bone, and increased the effects of decomposition on the scales of the fish specimen. The burning of bones also affected decomposition differentially. Again, the squirrel remains were the least affected and the fish remains were all but destroyed by burning. Since disposal of trash by fire is a common cultural practice, many faunal assemblages may be biased towards naturally versus culturally deposited remains. Second, the only taxon that seemed to be greatly affected by the presence/absence of flesh was the mammal (squirrel) where the presence of flesh seemed to increase the resiliency of the bones. The effect of flesh on the bird and fish bone was minimal. The results from this experiment suggest that naturally deposited bones and bones from undesirable and inedible portions are more likely to survive in the archaeological record than are culturally deposited and desirable faunal remains. While these experiments were conducted in the Northeast, a region characterized by acidic soils and extreme temperature fluctuations, the results demonstrate the biases inherent in faunal assemblages in all regions.

Discussion

Along with these diagenesis experiments, a standard faunal analysis of 2 years of excavations at the Thomas/Luckey site was completed. Faunal data from the Thomas/Luckey Site was meager only in the respect of its total size, 165.12 grams. Contrary to the site report, the majority of the animal remains preserved at the site consist of very small fragments of unburned and hardened dark reddish brown bone. This reddish brown bone may represent fire hardened bone or, in light of the diagenesis experiments may in fact represent bone which decomposed in the presence of flesh in which case further analysis of this bone may provide important information on butchery practices. Small amounts of bird and fish bone were also identified although no charred or calcined fish were identified. Given the results of the diagenesis experiment, the riverine location, and the finding of fish net weights at the site, the lack of fish remains suggest that the inhabitants of the site may have disposed of their fish remains in fires. Approximately half of the faunal remains were recovered from one feature, which contained unburned, charred, and calcined bone of mammals and birds. Pigeon, squirrel, and rabbit were among the identified species in this feature. Along with information regarding the consumption and preparation of small game, the faunal analysis of this feature alone suggests that animal waste generated by the inhabitants of the Thomas/Luckey site was deposited in central locations. Future excavation may locate other features that also contain a similarly high density of faunal material, further contesting the belief that soil acidity of the northeast renders many faunal assemblages invaluable. (The soil pH at the Thomas Luckey site is only slightly acidic at 6.0).

Reference

Beisaw, April M. 1998 Differential Preservation and Recovery: Taphonomy of Bone Preservation at the Thomas/Luckey Site, Ashland, NY. Unpublished M.A. thesis, Department of Anthropology, Binghamton University.