Abstract
Archaeological and palaeontological excavations frequently produce large quantities of highly fragmentary bone. These bones can help to answer questions regarding past environments and human and animal lifeways via a number of analytical techniques but this potential is limited by the inability to distinguish individual animals and generate sufficiently large samples. Using stable carbon and nitrogen isotope values of bone collagen (δ13C, δ15N), we present a metric to identify the number of isotopically distinct specimens (NIDS) from highly fragmented faunal assemblages. We quantified the amount of intra-individual isotopic variation by generating isotopic data from multiple elements from individual animals representing a wide variety of taxa as well as multiple samples from the same skeletal element. The mean intra-individual variation (inter-bone) was 0.52‰ (σ = 0.45) (Euclidean distance between two points in isotopic bivariate space), while the mean intra-bone variation was 0.63‰ (σ = 0.06). Using archaeological data consisting of large numbers of individual taxa from single sites, the mean inter-individual isotopic variation was 1.45‰ (σ = 1.15). We suggest the use of 1.50‰ in bivariate (δ13C, δ15N) space as a metric to distinguish NIDS. Blind tests of modelled archaeological datasets of different size and isotopic variability resulted in a rate of misclassification (two or more elements from the same individual being classified as coming from different individuals) of < 5%.
Highlights
Fragmented faunal remains at archaeological sites present challenges when attempting to determine the minimum number of individuals (MNI) within past animal populations
Wherein fragmentary remains have been sampled for stable isotope analysis and AMS radiocarbon dating, researchers have identified samples to a taxon through inferences based on their stable carbon (δ13C) and nitrogen (δ15N) isotope compositions (Corbett et al, 2008; Gorlova et al, 2015; Szpak et al, 2019)
To distinguish what level of isotopic variation represents values beyond those expected from intra-individual comparisons, we recommend using the upper limit of the 2σ range of the mean intra-individual isotopic distance calculations, 1.50‰ as the number of isotopically distinct specimens (NIDS) metric
Summary
Fragmented faunal remains at archaeological sites present challenges when attempting to determine the minimum number of individuals (MNI) within past animal populations This is noteworthy in the context of analyses involving these remains such as radiocarbon dating, ancient DNA, and stable isotope analysis that require the sampling of discrete individuals. Wherein fragmentary remains have been sampled for stable isotope analysis and AMS radiocarbon dating, researchers have identified samples to a taxon through inferences based on their stable carbon (δ13C) and nitrogen (δ15N) isotope compositions (Corbett et al, 2008; Gorlova et al, 2015; Szpak et al, 2019) In these contexts, the MNI is almost certainly a significant underestimation of the true number of distinct individuals and the MNI may be 1 even with a large number of individual specimens (NISP). In order to do so, it is necessary to understand the amount of variation in δ13C and δ15N that exists within individuals and how this variation compares to that which exists among individuals
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