390


Journal of Paleontology 92(3):388–397


Figure 2. Actinopteria boydi from the Middle Devonian of New York: (1) PRI 73120, #4 Easement Road, LV, internal mold, GIV-1, length 2.7 cm; (2) PRI 73121, Oran Gulf, LV, internal mold, GIV-1, length 2.9 cm; (3) PRI 73122, Oran Gulf, LV, external mold, GIV-1, length 3.1 cm; (4) PRI 73123, Oran Gulf, RV, internal mold, GIV-1, length 2 cm; (5) PRI 73124, Deep Run Gully, LV, internal mold, GIV-3, length 5 cm; (6) PRI 73125, Nickel Middle, LV, external mold, GIV-1, length 2.8 cm. Scale bars = 1 cm.


Actinopteria boydi’s valve interior, including the taxonomi-


cally important traits of musculature and dentition, are poorly known. Additionally, delicate auricles, although well known, are seldom completely preserved, leaving the outline of the shell disk and sculptural elements as the primary diagnostic traits to identify species and morphological variants. Here we use A. boydi’sdisk outline to quantify morphological variation that is difficult to capture by traditional height and length measurements of the entire skeleton. Of the examined material, 171 valves were sufficiently well preserved to be used for this morphometric landmark study. Although Actinopteria boydi is slightly inequivalve, its


margins do not overlap or gape but coincide with the commis- sural plane. Thus, both right and left valves can be pooled into one dataset; images were mirrored prior to digitization (anterior to left). Each specimen was oriented with its commissural plane parallel to the plane of the image, avoiding distortion of the shell outline. Digital images were taken with a Canon EOS Rebel


digital camera and the resulting images were converted into .tps files using the tpsUtil software package (Rohlf, 2004). The image order was randomized prior to landmarking to prevent bias due to batch processing of similarly preserved specimens; landmarks were placed using the tpsdig software (Rohlf, 2005). We defined a set of seven landmarks describing the outline of


the shell disk, including three type I landmarks placed on biologi- cally homologous features of the shell and four type II landmarks describing curvaturemaxima of the disk outline (Fig. 3; Hennessy et al., 2005). The preservation of the material limited the number of usable specimens for an outline or curve analysis (Fig. 2); to collect an adequate number of data points, we limited our landmarks to reliably reproducible landmarks describing a heptagon of the shell


disk, providing biologically relevant information on the variation of its morphology. The chosen landmarks, additionally, captured the position of auricles and byssus in relation to the shell body, adding information beyond the outline of the shell disk. Although inconsistent preservation of intact posterior and


anterior auricle extremities prevented their landmarking, the chosen landmarks include junction points of these skeletal elements with the shell disk. Landmarks were placed solely by one author to avoid the introduction of a collector bias into the dataset. In cases of insufficient outline preservation, the largest complete growth line was used for landmarking. Landmark data were examined using MorphoJ software


(Klingenberg, 2011); Cartesian landmark coordinates were scaled to centroid size, rotated, and translated through a Generalized Procrustes Analysis (GPA), eliminating size, location, and rotation effects before further analysis (Rohlf, 1990, 1999; Bookstein, 1991; Slice, 2001). The superimposed coordi- nates were then examined in a principal component analysis (PCA) to generate a series of vectors that summarize variation and covariation of the disk shape, allowing us to observe and interpret its variation (Zelditch et al., 2012). Canonical variate analyses (CVA) of the superimposed coordinates were used to describe and analyze and the difference between material assigned to predefined groups (stratigraphy and facies) (Zelditch et al., 2012). Thin-plate spline visualizations (Bookstein, 1991) further provided graphic descriptions of shape transformations represented by the vectors resulting from the PCA. The landmark data were analyzed to determine whether


trends in shape can be discriminated across temporal and environmental gradients. Specimens were categorized by matrix


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