EXCEPTIONAL PRESERVATION OF MIDDLE CRETACEOUS (ALBIAN) THECAMOEBIANS (ARCELLACEANS): EVOLUTION OF A TRANSIENT LACUSTRINE ENVIRONMENT

Thecamoebians (Arcellaceans) are testate Rhizopods that are ubiquitous in most freshwater to slightly brackish environments and have a test that is either self-secreted or made by agglutinating foreign particles using organic cement. Due to their test composition thecamoebian tests have a poor preservation potential through deep geologic time. Although some specimens have been found in Mesozoic and early Paleozoic sediments; limited preservation prevents detailed identification beyond the genus level resulting in a fragmented geologic record. Late Albian sediments from a regionally-variable shale deposit within the Dakota Formation (Nebraska, USA) have revealed exceptionally well-preserved thecamoebian tests that allow identification to the specific and even "strain" level. Identified species include abundant Difflugia oblonga, D. protaeiformis, Pontigulasia compressa, and Lesquereusia sp.. Previous regional paleoenvironmental research on the Dakota Formation suggests that during the late Albian a large extensive braided river system drained the North American craton into the Western Interior Seaway, westward of the Appalachian Basin. The recovered thecamoebian assemblage from the shale unit indicate deposition in a transient, quiescent lacustrine environment suggesting the presence of an oxbow lake setting, pond, or shallow lake transiently present in the ancient river system floodplain. These paleoenvironmental interpretations based on the recovered thecamoebians are consistent with similar Holocene environmental reconstructions.