Paleoecological Implications of a Holocene Fossil Assemblage: Lower Rio Grande, Cameron County, Texas

A Late Holocene fossil site from extreme southern Texas consists of invertebrate remains dominated by terrestrial, freshwater and brackish water mollusks. Sparse plant remains were recovered. Analysis of the origin of this heterogeneous fossil biota indicates that a brackish marsh was periodically inundated by freshwater runoff. The presence of a marsh clam not known living in the area today is significant; the fossil site is reconstructed to have been a brackish marsh habitat at elevation of 3.6 meters above mean sea level. A brackish marsh at this elevation may indicate high sea levels, although existence of nontidal brackish marshes are known from the lower Texas coast. Changes in river flow and seasonal distribution of local precipitation, and/or regional runoff, are postulated to explain the existence of a saline marsh in an area where this biotype is extremely rare today.