The Paleoecology and Biogeography of Ordovician Edrioasteroids

All organisms are subjected to the living and non-living influences of their surroundings. They derive their energy and essential materials, such as sunlight and nutrients, from their environment, sharing their world not only with members of their own species but also with members of other species. These interactions are central to the survival of the organism, forming reciprocating and integrated systems with other members of their environment. Paleoecology uses the fossil record to interpret and reconstruct life habits of past organisms and environments. By examining well-preserved fossil populations we can assess the relationship between the organisms and their surrounding environment, their distribution within their environment, and the nature of their interactions.

Edrioasteroids, an extinct clade of gregarious, obligate-encrusting echinoderm typical of the Late Ordovician, are rarely encountered in the fossil record as their multi-part skeleton rapidly disarticulates post-mortem. Therefore, the discovery of large pavements encrusted by articulated edrioasteroids indicates that obrution, or a sudden input of sediment that smothers the benthic community, occurred. The near instantaneous nature of obrution allows for the examination of a zero-time-averaged census assemblage rather than a time-averaged death assemblage.

This dissertation aims to increase our understanding of the paleoecology and biogeography of Ordovician edrioasteroids in three chapters. The first study examines a carbonate hardground encrusted with four species of isorophid edrioasteroids collected from Upper Ordovician strata near Maysville, Kentucky. Detailed paleoecologic analyses include edrioasteroid age structure, thecal orientation, inter- and intraspecific spatial utilization and settlement patterns, and degree of post-mortem disarticulation. Chapter two examines edrioasteroid paleoecology on Upper Ordovocian shell pavements using a brachiopod shell pavement from Florence, Kentucky and a bivalve shell pavement from Sharonville, Ohio. The results are then compared with those from the Maysville hardground. The final chapter of this dissertation summarizes the paleogeographic distribution patterns of the edrioasteroids during the Ordovician. For this study we collected the geographic distribution data for Ordovician edrioasteroids from published faunal reports and plotted these occurrences on paleogeographic maps with the hope that this information will help better predict localities where additional specimens of Ordovician edrioasteroids may be found.