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FIGURE 1. Ventral, dorsal, posterior, and longitudinal-sectional views of the fossil brachiopods Laqueus rubellus, Terebratula terebratula, Pseudoatrypa sp., and Platystrophia ponderosa . Scale bars equal 1 cm.

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FIGURE 2. Example of a shell sample ( Platystrophia ponderosa) sectioning through the plane of symmetry, producing two identical halves. Both halves were analyzed for their spiral deviations.

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FIGURE 3. Example of shell ( Laqueus rubellus) digitization and spiral fitting. 1, The brachiopod shell was digitized by clicking the outline in an anterior-ward direction, starting at the umbo. 2, The digitized outline was saved as (x,y) coordinates, which were used to plot a biological outline in R. 3, R code was used to fit a perfect logarithmic spiral to the shell outline. 4, The deviations from a perfect spiral were represented in a spiral deviation graph, with green points representing spiral maxima and red points representing spiral minima.

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FIGURE 4. Spiral deviations from the dorsal and ventral valves of specimens one ( 1), two ( 2), and three ( 3) of the species Laqueus rubellus .

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FIGURE 5. Spiral deviations from three specimens of the species Terebratula terebratula.

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FIGURE 6. Spiral deviations from three specimens of the species Platystrophia ponderosa.

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FIGURE 7. Spiral deviations from three specimens of the species Pseudoatrypa sp.

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FIGURE 8. Example of two fossil brachiopod species, Rafinesquina sp. (left) and Pseudoatrypa sp. (right), which generated problematic spiral deviation graphs. 1, Ventral view 2, Longitudinal-sectional view 3, Spiral fitting 4, Spiral deviations graphs.

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FIGURE 9. Comparison of spiral deviation graphs when the dorsal valve of Terebratula terebratula specimen is fit to one (left) and two (right) spirals. A and B represent locations on the graphs that change drastically when two spirals are fit to the biological outline instead of one. Minimum A becomes more distinct when two spirals are used whereas the opposite occurs for minimum B.

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