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GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions Vol. 58 (2008), Pages 695-698

EXTENDED ABSTRACT: Inferring the Late Eocene Paleoenvironment from the Population Structure of Ocala Limestone Echinoids

Justin M. Miller

Department of Geology, University of Georgia, 210 Field St., Athens, Georgia 30602

EXTENDED ABSTRACT

The goal of this short document is to show the effectiveness of assessing echinoid population structure to infer paleoenvironmental conditions. Previous work has shown that irregular echinoid test morphology provides clues to their paleoenvironment (Nichols, 1959; Carter et al., 1989). However, inferring environmental conditions based upon the population structure of the echinoids has not been as well developed (Neraudeau, 1991). Understanding the paleoecology of the different echinoid groups is crucial to identifying the environmental conditions they occupied. Spatangoid echinoids (heart urchins) are particularly sensitive to environmental changes and are known to be reliable indicators of the paleoenvironment. The Upper Eocene Ocala Limestone exposures in southwest Georgia offer an unusual opportunity to examine and compare contemporaneous populations. Two limestone exposures north of Albany, Georgia, are 1 km (0.6 mi) apart, and are assumed to be contemporaneous (Fig. 1). One site is located behind a dam on the Flint River and the other is behind a spillway on Muckalee Creek. In order to document the echinoid population, a 1 m2 (11 ft2) quadrat was placed along a 15 m (49 ft) transect line at 1 m (3 ft) increments (Fig. 2). Five parameters were counted within each quadrat, and should provide a solid assessment of the total echinoid population (Table 1). For the purposes of this study, echinoid fragments were also counted as they have been documented to be more prevalent than complete tests and serve as reliable echinoid population indicators (Nebelsick, 1992). Regular and cassiduloid echinoids are grouped together in this study since they are known to occupy similar environments. They are generally more robust than spatangoid echinoids.

Of the five characters assessed within each quadrat, four were shown to differ significantly between the two localities (Table 1). The number of spatangoid test fragments proved to be the only parameter not significantly different between the two sites. The Flint River Dam locality is characterized by fewer echinoid spines, non-spatangoid fragments, and non-spatangoid tests per square meter compared with the Muckalee Creek locality. However, the Flint River site produced a higher species diversity of echinoid fragments and complete tests, especially in regards to the deeper burrowing spatangoids. The Muckalee Creek outcrop produced evidence for three echinoid species (one regular; two cassiduloids), while the Flint River outcrop produced fragments or tests of seven species, of which over half are spatangoids. A t-test was performed on one species of echinoid (Rhyncholampas conradi) found commonly at both sites; it indicated that specimens found at Muckalee Creek are significantly longer (p = 0.0008) than those found at Flint River.

The abundance of regular echinoids and shallow burrowing cassiduloid echinoids, coupled with the lack of complete spatangoid tests, indicate that the Muckalee Creek site was possibly in a higher energy area, nearer to the paleoshoreline, in comparison with the Flint River locality. Environmental conditions at Muckalee Creek favored echinoids that were robust and could withstand the highly energy. The current or wave energy at this site may have prevented the thin tested spatangoids from either occupying or being preserved unbroken at this location. The size distribution of R. conradi could be an indication that the juveniles occupied habitats that were less affected by currents (Flint River) and as size increased were able to withstand the higher current energy of the Muckalee Creek site. This study illustrates that echinoid diversity, taphonomy, and size distribution can be used to determine paleoenvironmental conditions.

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