Does elliptical biface morphology differ as a function of raw material color?
Supplementary Materials
Robert Z. Selden, Jr. 
This process of comparison, of recognising in one form a definite permutation or deformation of another, apart altogether from a precise and adequate understanding of the original “type” or standard of comparison, lies within the immediate province of mathematics, and finds its solution in the elementary use of a certain method of the mathematician (Thompson 1917).
Basis of inquiry
The physical properties associated with lithic raw materials are regularly assumed to be a determining factor that articulates with stone tool morphology. Among those attributes most frequently advanced to account for differences between sites and assemblages are raw material size and mechanical flaking properties. Quantitative analyses of raw material color can provide a consistent, and replicable, means of assigning material to raw material color groups. To evaluate whether elliptical biface morphology differs as a function of raw material color, each elliptical biface from 41AN13 was assigned to a raw material color group, then analyzed using the tools of geometric morphometrics. Results demonstrate that elliptical bifaces differ significantly in shape, but not size, by raw material color group. This finding supports the interpretation that extralocal producers—and local Caddo users/knappers—conditioned elliptical biface shape not based upon nodule size or mechanical flaking properties, but on the basis of raw material color.
Keywords
Caddo; lithics; computational archaeology; archaeoinformatics; museum studies; digital humanities; non-Western art history, ovoid biface, Galt biface, Jowell knife, Jowell knives, Jowell biface, Nikolas knife, lithics, parallel-oblique flaking
Analysis overview
Elliptical bifaces comprise an understudied category of Caddo material culture. The elliptical bifaces from 41AN13 (Jowell Farm) were purchased by the Texas Archeological Research Laboratory (TARL) at The University of Texas at Austin on April 13, 1933, and the site is located near Kickapoo Springs Village, south of Frankston, Texas in Anderson County. The bifaces were excavated from a Caddo burial by a local farmer (George H. Adams), and of the 35 bifaces reportedly acquired by TARL when the collection was purchased, 18 could be located, and 13 of those are complete. Due to the dearth of locally-available raw materials of suitable size and quality needed to produce the elliptical bifaces (Selden Jr et al. 2021, fig. 2), an assumption also made for Gahagan bifaces from the ancestral Caddo region (Selden Jr., Dockall, and Shafer 2018; Selden Jr., Dockall, and Dubied 2020), the elliptical bifaces are thought to have been imported through trade and/or exchange. This study asks whether raw material color may have been a determining factor impacting elliptical biface morphology.
knitr::include_graphics('./img/map.png')
Seriation
Temporally, 41AN13 is among the five sites—others include 41AN21, 41AN26, 41CE6, and 41CE12—that Kleinschmidt (1982, 213–20) analyzed when he built upon the work of Cole (1975) to propose the Allen Phase in northeast Texas. Pulling directly from Kleinschmidt’s (1982, 220) Cemetery Data and Tentative Chronology table, a new seriation was generated for the Allen Phase sites. For the purpose of revising the seriation, the Poynor Engraved and Patton Engraved types were each collapsed into a single column, and any row or column included in the Allen Phase section of Kleinschmidt’s (1982) table with a sum of zero was omitted.
knitr::include_graphics('./img/seriation.jpg')
Color
Both sides of the 13 elliptical bifaces from 41AN13 were scanned at 1200 dpi using an HP ScanJet G4050. Images were subsequently transferred to a transparent background in Photoshop, and all labels and ink were deleted in preparation for analysis using the colordistance package in R (R Core Development Team, 2023; Weller and Westneat 2019). A histogram binning method was used to group similar colors in the RGB color space, and pairwise distances between histograms were computed using earth mover’s distance, yielding three distinct raw material color groups (Rubner and Tomasi 2001). Since one cluster included only a single biface (81), it was dropped from the analysis.
knitr::include_graphics('./img/fig2-2-01.jpg')
The elliptical bifaces were coded as ColorGroup A and ColorGroup B. To test whether the color groups identified by the clusters differ in color, the color distance matrix was exported, joined with the categorical data, then analyzed using a permutational multivariate analysis of variance (perMANOVA) in the vegan package, where the color groups were found to differ significantly (permutations = 10,000; Rsq = 0.54754; Pr(>F) = 0.0011). A non-metric multidimensional scaling ordination (NMDS) was used to illustrate the color ranges for elliptical bifaces from ColorGroup A (more gray) and ColorGroup B (more brown). The same elliptical bifaces were subsequently used in a geometric morphometric analysis to assess whether the morphology of elliptical bifaces differs between raw material color groups.
knitr::include_graphics('./img/nmds-elliptical.png')
Geometric morphometrics
Prior to landmarking, elliptical bifaces from 41AN13 were oriented with the most heavily retouched edge at top right. The landmarking protocol uses three landmarks; two horizontal tangents (top/bottom), and the third placed at the furthest extent of the edge bearing the heaviest amount of retouch. Landmarks and semilandmarks were applied in R using the StereoMorph library (R Core Development Team, 2023; Olsen and Westneat 2015). Some bifaces include multiple areas of heavy retouch at the top and bottom of the same lateral edge, while others include one retouched edge at top right and another at bottom left, similar to alternately beveled bifaces.
To identify which edge was most heavily retouched, we employed a modified approach to the flaking index initially developed by Miller (2007). The modified approach uses counts of flake scars from each edge in the two most heavily worked areas, paired with a measure of edge length (inclusive of curvature - this is not a linear metric). The number of flake scars was then divided by the length of the worked edge, with the heaviest worked edge identified by the greater value.
Generalized Procrustes Analysis
Landmark data were aligned to a global coordinate system (Bookstein et al. 1999; Gunz, Mitteroecker, and Bookstein 2005; Kendall 1981, 1984; Slice 2001), achieved through generalized Procrustes superimposition (Bookstein 1986; Rohlf and Slice 1990; Rohlf 1999) in R using the geomorph and RRPP packages (Adams and Collyer 2015; Collyer and Adams 2018; Adams et al. 2017; Adams and Otarola-Castillo 2013; Baken et al. 2021). Procrustes superimposition translates and rotates the coordinate data to allow for comparisons among objects, while also scaling each biface using unit-centroid size—the square root of the sum of squared distances from each landmark to the specimen’s centroid (Chapman 1990; Dryden and Mardia 1998; Gower 1975; Rohlf and Slice 1990). The geomorph package uses a partial Procrustes superimposition that projects the aligned specimens into tangent space subsequent to alignment in preparation for the use of multivariate methods that assume linear space (Dryden and Mardia 1993; Kent and Mardia 2001; Rohlf 1999; Slice 2001).
knitr::include_graphics('./img/gpa.png')
Principal Components Analysis
Principal components analysis (Jolliffe 2002) was used to visualize shape variation among the elliptical bifaces, and the scatterplot represents the dispersion of shapes in tangent space (Adams, Rohlf, and Slice 2013; Bookstein 1991; O’Higgins and Jones 1998; Rohlf and Marcus 1993). Shape ranges described by each principal axis are commonly visualized using thin-plate spline warping of a reference image (Klingenberg 2013; Sherratt et al. 2014).
knitr::include_graphics('./img/pca.png')
Procrustes ANOVA
To assess whether shape and size differed by ColorGroup, Procrustes ANOVAs (Goodall 1991) were run that enlist effect-sizes (zscores) computed as standard deviates of the generated sampling distributions (Collyer, Sekora, and Adams 2015). A residual randomization permutation procedure (RRPP; n = 10,000 permutations) was used for all Procrustes ANOVAs (Adams and Collyer 2015; Collyer and Adams 2018), which has higher statistical power and a greater ability to identify patterns in the data should they be present (Anderson and Ter Braak 2003). Results demonstrate that elliptical bifaces differ significantly in shape (RRPP = 10,000; Rsq = 0.31811; Pr(>F) = 0.0473), but not size (RRPP = 10,000; Rsq = 0.00831; Pr(>F) = 0.7793) by raw material color group.
Summary of Findings
Even if raw material color groups are not found to differ in terms of geochemistry, results clearly demonstrate that two discrete elliptical biface shapes articulate with two distinct raw material color groups, indicating a previously unreported Caddo preference related to local lithic technology. All of the elliptical bifaces are thought to have been produced with Edwards chert; however, color (more/less gray/brown) could have been selected for in advance of manufacture, with bifaces of different colors crafted to meet with specific cultural constraints related to shape. Thus, in terms of lithic technological organization, these findings demonstrate that elliptical biface shape was conditioned by raw material color, where one group is comparatively slimmer and browner, while the other is wider and grayer. This study provides evidence for two discrete, contemporary, and sympatric Caddo knapping communities delimited by macroscopic production attributes, and posits differential production-based intention among Caddo knappers.
With regard to function, elliptical bifaces may have occupied a ceremonial role in the Serpent/Snake Dance (Howard 1955), in which symbolic implements were heralded to convey elements of the lower world (Gadus 2013). Similar bifaces are depicted in a shell cup recovered from the Craig Mound at Spiro (Phillips and Brown 1978-1984), where both Hero Twins can be seen grasping an elliptical biface. Given the extant differences that occur in elliptical biface shape and color, these implements may have been meant to depict one or more specific species. The amount of retouch along the edges suggests that the bifaces experienced regular use, raising the question of whether the associated colors and shapes may have articulated with specific tools employed to dispatch or prepare offerings for a particular species or group of snakes. Experimental use-wear studies may aid in clarifying whether or not elliptical bifaces were employed for such tasks.
knitr::include_graphics('./img/spiro.png')
Acknowledgments
My thanks to the Caddo Nation of Oklahoma, the Caddo Nation Tribal Council, Tribal Chairman, and Tribal Historic Preservation Office for permission and access to NAGPRA and previously repatriated collections. I also extend my gratitude to Lauren Bussiere at the Texas Archeological Research Laboratory for her assistance with access to the elliptical bifaces. Thanks also to John Harman for access to the DStretch plugin for ImageJ, to John E. Dockall, David H. Dye, Harry J. Shafer, Hiram F. (Pete) Gregory, Timothy K. Perttula, Christian S. Hoggard, and David K. Thulman for their comments and constructive criticisms on the ongoing analyses of Caddo bifaces, to David L. Carlson for guidance regarding the seriation, and to Emma Sherratt, Kersten Bergstrom, Dean C. Adams, and Michael L. Collyer for their constructive criticisms, general comments, and suggestions throughout the development of this research program.
Funding
Components of this analytical work flow were developed and funded by a Preservation Technology and Training grant (P14AP00138) to RZS from the National Center for Preservation Technology and Training (NCPTT), and additional grants to RZS from the Caddo Nation of Oklahoma, National Forests and Grasslands in Texas (15-PA-11081300-033) and the United States Forest Service (20-PA-11081300-074). Funding and logistical support to analyze the bifaces from 41AN13 was provided by the Heritage Research Center at Stephen F. Austin State University.
Data management
Reproducibility—the ability to recompute results—and replicability—the chances other experimenters will achieve a consistent result—are two foundational characteristics of successful scientific research (Leek and Peng 2015). The analysis code associated with this project can be accessed through this document, is available in the GitHub repository (https://github.com/seldenlab/elliptical.bifaces.1), and digitally curated on the Open Science Framework (DOI: 10.17605/OSF.IO/2CHFE). The reproducible nature of this enterprise provides a means for others to critically assess and evaluate the various analytical components (Gray and Marwick 2019; Peng 2011; Gandrud 2014), which is a necessary requirement for the production of reliable knowledge.
Reproducibility projects in psychology and cancer biology are impacting current research practices across all domains. Examples of reproducible research are becoming more abundant in archaeology (Marwick 2016; Ivanovaite et al. 2020; Selden Jr., Dockall, and Shafer 2018; Selden Jr., Dockall, and Dubied 2020; Selden Jr et al. 2021; Selden Jr. 2022), and the next generation of archaeologists are learning those tools and methods needed to reproduce and/or replicate research results (Marwick et al. 2019). Reproducible and replicable research work flows are often employed at the highest levels of humanities-based inquiries to mitigate concern or doubt regarding proper execution, and is of particular import should the results have—explicitly or implicitly—a major impact on scientific progress (Peels and Bouter 2018).
Colophon
This version of the analysis was generated on 2023-05-31 05:10:03.370105 using the following computational environment and dependencies:
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Head: [3eaed5b] 2023-05-31: <edit index>References
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