PALEOECOLOGY OF THE MIDDLE CRETACEOUS WALNUT FORMATION, BELL COUNTY, TEXAS

During the Cretaceous period (144-66 million years ago), sea levels were high enough to flood much of North America, with continuous ocean connections from the Gulf of Mexico to the Arctic Ocean. Rocks of this age in central Texas contain a diverse fauna of invertebrate fossils. An outcrop of the Middle Cretaceous Walnut Formation in the city of Harker Heights, in Bell County, Texas was measured and collected for fossils. Three separate samples were washed, sorted, identified to the lowest taxonomic level possible and counted to recover over 800 individuals. A single, recurring paleocommunity was found throughout the section, dominated by the bivalves, Exogyra texana and Protocardia texana, the echinoid, Heteraster and the gastropod, Tylostoma. While most taxa and individuals are preserved as casts and internal molds, the large (up to 89 mm max length) oyster bivalve, Exogyra texana has the original skeletal material preserved. Over 300 individuals of this species were separated for analysis of bioimmurations on the left valve (over 50%) and xenomorphisms on the right valve (over 20%). In the otherwise soft, muddy sediment, Exogyra larvae would settle on any hard substrate to ensure survival. Bioimmured substrates are all skeletal remains of other organisms, including in rank order: gastropods, bivalves and corals.