Nagel-Myers et al.—Devonian bivalve ecophenotypic flexibility In the Middle Devonian of New York, Actinopteria
boydi is most likely to be confused with A. decussata or A. subdecussata Hall, 1843. The last two species are described as having a stronger procrescent growth vector and a more clearly defined posterior auricular sulcus than in A. boydi. Our data illustrating the variation of shell disk shape in different facies and the material reviewed for this study suggest that A. decussata and A. subdecussata could be synonyms of A. boydi (see also McAlester, 1962a). A more comprehensive review including all Devonian Actinopteria taxa will further clarify the taxonomic situation of these related species. Actinopteria boydi differs from A. taberi McAlester,
1962b, known from the Frasnian of Alaska (McAlester, 1962b), in being smaller, having more subdued ribs on the LV, a relatively larger anterior auricle, and more retrocrescent shape. Other Upper Devonian forms described from New York by Hall (1883, 1884, e.g., A. delta, A. epsilon, A. zeta, etc.) might prove to be conspecific with A. boydi as predicted by McAlester (1962a).
Results
The PCA analysis of the landmark data shows that 32.4% of variation is summarized by PC 1 and 31.2% by PC 2; PC 3 and
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PC 4 account for 12% and 10.3% of the variance, respectively. Morphological variation captured by PC 1 is an anteroventral expansion of the valve disk (Fig. 4). The posterior portion of the sulcus shortens and the angle between the hinge line and posterior sulcus becomes wider (Fig. 4). The shell variability expressed by PC 2 is characterized by an increase of the anterior shell area relative to the posterior and an overall narrowing of the disk (Fig. 4); PC 3 and PC 4 describe the variation in length of the posterior sulcus and its angle in relation to the hinge line (Fig. 4). Specimens from the Ludlowville Formation are on average
narrower and more retrocrescent in outline than the Skaneateles material (Fig. 5.1). Those from both the Skaneateles and Ludlowville formations exhibit the same distribution in the morphospace defined by PC 3 and PC 4, with more variation present in the Ludlowville samples (Fig. 5.2). The material from the Moscow Formation includes more specimens with sub- circular outlines and is more variable (PC 1 and PC 2; Fig. 5.1). Moscow specimens develop, on average, a longer posterior sulcus (PC 3 and PC 4, Fig. 5.3) than are found in the two older samples. A pairwise comparison of PC 1 through PC 4 values, grouped by their stratigraphic position, confirms these observa- tions (Table 2). For PC 1, specimens from the Skaneateles Formation are significantly different fromthose from the younger Ludlowville and Moscow formations (p<0.001, Table 2).
Figure 4. Thin-plate spline demonstrating the average deformation of shell disk for PC 1 through PC 4 with the consensus shape depicted at the center. Red/hot = positive deformation; blue/cold = negative deformation).
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