Journal of Paleontology, 92(3), 2018, p. 388–397 Copyright © 2018, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.138


Ecophenotypic variability during times of evolutionary stasis in Middle Devonian Actinopteria (Bivalvia, Pterioidea) from New York


Judith Nagel-Myers,1 Christopher A. McRoberts,2 and Cullen W. LaPointe3


1Geology Department, St. Lawrence University, Canton, New York 13617, USA ⟨jnagel@stlawu.edu⟩ 2Geology Department, State University of New York Cortland, Cortland, New York 13045, USA ⟨christopher.mcroberts@cortland.edu⟩ 3Department of Geology, University of Georgia, Athens, Georgia 30602, USA ⟨cullen.lapointe25@uga.edu⟩


Abstract.—We examine the morphological variation of a Paleozoic pterineid during a time of relative ecological and taxonomic stability in the Middle Devonian Appalachian Basin in central and eastern New York. We discuss the taxonomic status of the Middle Devonian bivalve Actinopteria boydi (Conrad, 1842) and quantify the variability of its shell disk as well as the width and angle of the auricles and sulci of this otherwise character-poor bivalve species using geometric morphometric techniques employing Cartesian landmarks. We compare variants from three strati- graphic levels (Skaneateles, Ludlowville, and Moscow formations) and from different habitats characterized by lithofacies. The phenotypic variation observed in our data does not amount to an overall directional shift in morphology, i.e.,


they constitute reversible changes of morphology in a single variable taxon. Our study finds small-scale variation in morphology that represents evidence for ecophenotypic variation through ~3–4 Myr. Differences in substrate coupled with water energy seem to impact this taxon’s morphology. Although no clearly separated groups can be observed, material from muddy facies develops variants with, on average, rounder and broader shell disks than are found in material from silty facies. This morphology could have increased the flow rate of water channeled over the posterior shell portion thereby improving filtration rate, which is especially beneficial in environments with low water energy.


Introduction


The pterineid bivalve Actinopteria boydi (Conrad, 1842) is an important constituent of the Middle Devonian shallow-water epibenthos of the Hamilton fauna of New York (Fig. 1; Grasso and Wolfe, 1977; Linsley, 1994; Brett et al., 2007a). These cosmopolitan suspension feeders are, like all pterineids, known for their phenotypic flexibility (Tëmkin, 2006; Wada and Temkin, 2008), and their abundant occurrence in the Middle Devonian of central and eastern New York provides an excellent opportunity to examine morphological variability in units characterized by overall long-term stability. Brett and Baird (1995) recognized extended intervals of


concurrent faunal persistence (coordinated stasis) in the Hamilton fauna that are terminated by environmentally induced turnover events (e.g., Brett et al., 2007a; Brett, 2012). Coordinated stasis is characterized by patterns of taxonomic stability within and among temporally separated paleocommunities (for a review, see Brett, 2012). Althoughmore controversial (Ivany, 1996; Patzkowsky and Holland, 1997; Olszewski and Patzkowsky, 2001), studies have also documented ecological stability within particular biofacies of the Hamilton fauna (Brett et al., 2009; Ivany et al., 2009). At the level of species and populations, individual lineages


identified within the Hamilton fauna have yielded important case studies for punctuated equilibrium (e.g., Eldredge, 1974;


A previous error in this article has been corrected. See doi: 10.1017/jpa.2018.24


Pandolfi and Burke, 1989; Lieberman et al., 1995). Periods of small scale morphological fluctuations have been described separating times of rapid speciation (e.g., Eldredge and Gould, 1972; Gould and Eldredge, 1977; Eldredge et al., 2005). The processes behind this stability remain controversial (e.g., Hansen and Houle, 2004; Eldredge et al., 2005; Estes and Arnold, 2007; Futuyma, 2010), but the phenotypic variability of taxa during these intervals has attracted attention as an indicator of the capacity of species to respond to changes in their envir- onment (e.g., Dietl, 2013) and its role in diversification and speciation (e.g., Pfennig et al., 2010). Geometric morphometric techniques employing Cartesian


landmarks have improved the fidelity of quantifying complex morphologies providing for a more robust method of examining patterns of multivariate variation in shape through time (Roth and Mercer, 2000; Webster and Sheets, 2010; Cruz et al., 2012). Using these techniques, we study the phenotypic variation of Actinopteria boydi in the stratigraphic and paleoecological context of the Middle Devonian Hamilton fauna. Only a few recent studies have focused on Devonian pter-


ineids (McAlester, 1962a, 1962b, 1963; Bailey, 1983; Johnston, 1993; Rode, 2004). Antiquated typological species concepts defining many of these species lack diagnostic criteria and often fail to recognize potential morphological variation within natural populations. Departing from Newell and LaRocque’s (1969) classification, we consider Actinopteria Hall, 1884 to be a distinct genus (separate from Ptychopteria Hall, 1883) and the


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