Nagel-Myers et al.—Devonian bivalve ecophenotypic flexibility
significantly different from those of the silt (PC 4, p=0.03) and fine silt samples (PC 1, p=0.02). On average, this mud-morph
develops broader, circular shell disks with longer posterior sulci than the specimens from coarser sediments (Fig. 5.4). Skaneateles and Ludlowville formations (Table 1) include
comparable numbers of specimens from the three lithofacies, but the Moscow Formation includes more specimens from the muddy facies. The overrepresentation of one facies in the youngest samples might have biased the data toward morpho- logies more prevalent in the muddy facies. A CVA analysis of the differences among specimens
grouped by stratigraphic position and facies relative to within- group variance (Fig. 6) demonstrates no clear separation of groups. Comparing stratigraphic units, CV 1 axis comprises 80% and CV 2 axis 20% of the variation; examining facies groups, CV 1 represents 59% and CV 2 41% of the variation. Most of the difference among groups can be found in develop- ment of the posterior sulcus and its orientation in relation to the hinge line. Skaneateles and Ludlowville formation specimens occupy much of the same morphospace. Material from the Moscow Formation is more variable, encompassing much of the range of both Skaneateles and Ludlowville formations (Fig. 6.2). Comparing the different lithofacies groups, the CVA
Table 2. P-values of Mann-Whitney pairwise comparison of samples grouped by stratigraphic units using PC 1 and PC 2 values; asterisk denotes significant p-value.
PC 1 PC 2 PC 3 PC 4
Skaneateles/Ludlowville Ludlowville/Moscow Skaneateles/Moscow <0.001*
0.81 0.40 0.11
0.15 0.87 0.10 0.36
<0.001*
0.02* 0.70 0.12
Table 3. P-values of Mann-Whitney pairwise comparison of samples grouped by lithofacies (PC 1 through PC 4 values); asterisk denotes significant p-value.
PC 1 PC 2 PC 3 PC 4
mudstone/fine siltstone mudstone/siltstone fine siltstone/siltstone 0.02*
0.21 0.13 0.28
0.18 0.09 0.36
0.03*
0.06 0.63 0.19 0.08
395
expresses variation of the outline from more circular (CV 1) to more retrocrescent (CV 2) as the main difference among the three lithofacies (Fig. 6.1). Shell disks of specimens from the
muddy facies are, on average, rounder, whereas specimens from fine silt and silt vary along the CV 2 axis, with fine silt samples being more retrocrescent than those from the silt facies.
Discussion
It has been postulated that coordinated stasis is not only expressed in the taxonomic composition of a particular marine benthic community, but is also reflected in lineage-level morphological stasis (Brett, 2012). For example, Lieberman et al. (1995) found morphological fluctuation in two brachiopod lineages rather than directional morphological change; our data support these results. Our results show that specimens from the Ludlowville Formation develop, on average, shell disks that have a slightly wider posterior sulcus and are anteroposteriorly shortened compared to older specimens from the Skaneateles Formation. The Moscow variant generally develops more circular specimens with longer posterior sulci, resulting in a slightly greater difference between the youngest samples and the rest. Despite these differences, the overall variability in the Actinopteria boydi lineage is generally not great. We found a statistical difference between the oldest Skaneateles Formation and the Ludlowville and Moscow samples, but these are limited to PC 1, which accounts for only 32% of the variation. Despite this difference, no distinct morphotypes distinguishing these two stratigraphic levels can be observed in the PCA analysis (Fig. 5). The variation captured is mostly driven by the greater variability of Ludlowville specimens’ outlines, but is not great enough to establish distinct morphological groups from the examined stratigraphic units. The difference between Skaneateles and Moscow forma-
tions found in PC 1 and PC 4 seems to be an expression of the high proportion of mud morphs included in the Moscow mate- rial. Although only 1.9% of Skaneateles specimens are from this lithofacies, mud morphs comprise 41.4% of the Moscow specimens (Table 1). The overrepresentation of the distinctly different variant (see below) combined with a small sample size
Figure 6. Canonical variate analysis based on shape variations, grouped by facies (1) and stratigraphic position (2), all showing 90% confidence ellipses of population mean. fs = fine silt; GIV-1 = Skaneateles Formation; GIV-2 = Ludlowville Formation; GIV-3 = Moscow Formation; s = silt; m = muddy.
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