Nagel-Myers et al.—Devonian bivalve ecophenotypic flexibility
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Figure 1. Geographic and stratigraphic distribution of examined material (based on Brett et al., 2010).
material in this study is assigned to A. boydi. This paper pro- vides a brief discussion of the taxonomy of this species (see below), which, although not an exhaustive treatment, can serve as preliminary guide to comparisons of similar taxa in North America.
Geologic setting
The material for this study comes from the Middle Devonian Hamilton Group of central and western New York (Fig. 1). These marine deposits accumulated in the Appalachian foreland basin that formed parallel to the Acadian mountain belt (Lash and Engelder, 2011). This orogeny, coupled with high sea levels during the Middle Devonian, resulted in much of present-day central and western New York being covered by an epeiric sea (Brett and Baird, 1996). The stratigraphic and facies framework of the Hamilton Group has been thoroughly documented (for a review, see Cooper, 1929, 1935; Brett and Baird, 1985, 1996; Brett, 1986; Batt, 1996; Brett et al., 2010). These strata are predominantly composed of an eastward-thickening wedge of clastic marine sediments with a minor carbonate component, which transitions eastward to the coastal- and fluvial-dominated depositional sequences of the Catskill Delta complex (Cooper, 1935; Ettensohn, 1985). The Hamilton Group comprises five formations: Union Springs, Oatka Creek, Skaneateles, Ludlowville, and Moscow formations. Together with the overlying Tully Formation, these units span ~5.0−5.5Myr based on recent cyclostratigraphic calibration (Ellwood et al., 2011). The faunal assemblages found in these deposits are typical Paleozoic communities dominated by moderately diverse, sessile, epifaunal suspension feeders (e.g., Ivany et al., 2009).
Table 1. Number of specimens from examined lithofacies within each stratigraphic unit.
Muddy Fine Silt Silt
Total
Skaneateles Fm. Ludlowville Fm. Moscow Fm. Total 2
108
24 82
3 5
22 30
12 15 2
29
17 44
167 106
Material and methods
Our material of Actinopteria boydi was collected from 16 localities across central and western New York (Fig. 1). Four additional specimens had only vague locality but detailed stratigraphic data. The material comes from three stratigraphic levels: the Skaneateles (GIV-1), Ludlowville (GIV-2), and Moscow (GIV-3) formations spanning ~3–4Myr (Fig. 1; Table 1; Brett et al., 2010). These units comprise depositional sequences characterized by transgressive, highstand, and falling-stage systems tracts (Fig. 1; Brett et al., 2010). Each of the four formations represents approximately a third-order cycle of sea-level change lasting ~1–1.5Myr (Brett and Baird, 1996; Brett et al., 2009). The transgressive system tracts are capped by silty mudstones and siltstones (Brett et al., 2010); all samples examined in this study represent material sampled from these ‘caps’ of the depositional cycles (Brett et al., 2010). All specimens are internal, external, and compositemolds (Fig. 2) and samples from field collections were supplemented by museum specimens from the Paleontological Research Institution, the New York State Museum, and the American Museum of Natural History. Detailed descriptions of Linsley localities are available in his thesis (Linley, 1986).
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