Utilizing Echinoids as Environmental Indicators from the Late Eocene Ocala Limestone

Justin Miller
Department of Geology, University of Georgia, Athens, GA

The echinoid population structure of two Ocala Limestone outcrops near Albany, Georgia was analyzed to determine if an environmental difference existed between these localities. To assess the echinoids at these locations a 1m2 quadrat was placed along a 15m transect line at both sites and complete body fossils, spines, and partial echinoid tests were documented within each quadrat.

The Albany Dam locality (site 1) was characterized by fewer amounts of echinoid spines, fragments, and complete tests per meter squared in comparison with the Muckafoonee Creek locality (site 2). However, site 1 produced a higher species diversity of echinoid fragments and complete tests, especially in regards to the deeper burrowing spatangoids. Site 2 produced evidence for three echinoid species (one regular; two Cassiduloids), site 1 produced fragments or tests of seven species, of which over half are spatangoids. Size class analysis was performed on one species of echinoid (Rhyncholampas conradi) found commonly at both sites and indicate that specimens found at site 2 are significantly larger than those found at site 1.

The abundance of regular echinoids and shallow burrowing Cassiduloid echinoids coupled with the almost complete lack of spatangoid fragments, possibly indicate that site 2 was at or above wave base; nearer to shoreline in comparison with site 1. Environmental conditions at site 2 favored echinoids that were robust and could withstand the high current activity. The currents at this site would have prevented the thin tested spatangoids from either occupying or being preserved entirely at this location. The size distribution of R. conradi could signify that the juveniles occupied habitats that were less affected by currents (site 1) and as size increased were able to withstand the higher current activity of site 2. This study illustrates that echinoid diversity and size distribution can be used to determine environmental conditions.

AAPG Search and Discover Article #90085 © 2008 GCAGS 58th Annual Meeting, Houston, Texas