The effect of preservational quality on paleoecologic studies is evaluated quantitatively using fossil collections from a restricted Cretaceous time interval (Haustator bilira assemblage zone) of Maestrichtian age and are from the Eastern Gulf Coastal Plain Province. Statistical comparisons reveal that preservation affects the faunal makeup. Collections in which both aragonite and calcite are well preserved have more taxa than collections of poorer preservational quality and contain faunal elements not found in other collections. These two effects can confound paleoecologic studies of endemism, eurytopy vs. stenotopy, species longevity and other studies of the distribution of organisms in space and time. Collections of good preservational quality may be interpreted to represent strata of high diversity, taxa having the more durable hard parts will appear to be widespread and long ranging, and taxa that are preserved only in collections of the best preservational quality will appear stenotopic, endemic, and rare.--Modified journal abstract.