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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions Vol. 58 (2008), Pages 895-900

EXTENDED ABSTRACT: Ecospace Utilization in High-Diversity Shallow Shelf Marine Communities of the U.S. Gulf Coastal Plain

Heather L. B. Wall and Linda C. Ivany

Department of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244

EXTENDED ABSTRACT

Diversity may change in two ways—changing the number of species within an ecological group, or by changing the number of ecological groups. This second method has been invoked to explain changes in global diversity (Vermij, 1977; Thayer, 1979; Ausich and Bottjer, 1982, 1985; Bambach, 1983; Bush and Bambach, 2004; Bambach et al., 2007; Bush et al., 2007; among others), but is it responsible for changes in diversity at smaller spatio-temporal scales? The middle to late Eocene has previously been identified as a time of molluscan diversity decline in the U.S. Gulf Coastal Plain (Dockery, 1986; Hansen, 1987, 1992). Here we assess the magnitude of this decline and examine associated changes in ecospace utilization.

Methods

We compiled a database of bivalve, gastropod, and coral abundance data derived from published datasets, unpublished theses, and our own collections. Toulmin (1977) yielded data for all units, Harrison (1994) for the Gosport Sand, Elder (1981) for the Moodys Branch Formation, and Haasl (1993) and Haasl and Hansen (1996) for the Yazoo Formation. We supplemented these existing datasets with our own collections from the upper Lisbon and Moodys Branch formations. Because these formations are largely unlithified, samples are wet or dry sieved through a 2 mm (0.08 in) screen to remove matrix material. This process allows for a high percent recovery of fossil material. Bivalves, gastropods, and solitary corals are identified to species. To obtain abundance data we counted gastropod apices, bivalve umbos, and whole corallites. Bivalve counts are divided in half and rounded up to a whole number to obtain the number of individuals. The resulting combined dataset contains over 70,000 individuals of 546 different species from 188 collections.

To assess changes in diversity, we calculate both alpha and gamma diversity. Here, alpha diversity of a horizon is the average number of species found within each collection from that horizon. Gamma diversity is the total number of species found in all of the samples for a particular horizon. Because we have an abundance of data, we are able to use the subsampling method rarefaction to mitigate the effects of unequal sampling intensity that are unavoidable in a combined dataset. Three hundred individuals were subsampled from each collection, and richness tabulated.

To assess changes in ecospace utilization, we assign each species to a mode of life based on a modified version of the three-dimensional ecospace framework proposed by Bush et al. (2007). The three axes that make up this framework are tiering, motility, and feeding. Tiering refers to location of an adult organism with respect to the sedimentwater interface. It includes such categories as pelagic and shallow infaunal. Motility refers to the ability of the organism to move. This division ranges from freely mobile to immobile attached. Feeding primarily refers to the mechanism of feeding, though the food source is a secondary consideration. In the case of suspension feeders, no delineation is made for those that primarily feed on suspended detritus versus those that feed on zooplankton. However, we can separate organisms that graze primarily on sessile animals versus those that graze on plants. Figure 1 summarizes all possible categories in theoretical ecospace. To determine ecological characteristics of the taxa, we use the Neogene Marine Biota of Tropical America (NMBTA) online database (see NMBTA, 2008), which was described by Jackson et al. (1999). This database primarily relies on extant genera to determine ecological characteristics of Neogene taxa. We base our ecological assignments on these genera when possible, but in many cases we rely on familial similarity. Species are then grouped by their modes of life regardless of their taxonomic relationships. For each formation we calculated the proportional abundance and the proportional species richness of each mode of life for each formation. In addition, we tabulated the number of modes of life represented in each formation.

Figure 1. Theoretical ecospace utilization cube illustrating all possible modes of life (modified after Bush et al., 2007). For example, the gray box represents deep-infaunal, fully-mobile browsers.

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