Paleontological and paleoecological significance of the oldest highly productive Upper Cretaceous (lowermost Maastrichtian) bonebed of Haţeg Basin (western Romania; Densuş-Ciula Formation)

Recent extensive fieldwork in the Densuş-Ciula Formation in Haţeg Basin has led to the discovery of several important high-diversity bonebeds. Among the excavated locations, site K2 is by far the most significant, as based on its stratigraphical position it is considered the oldest known (earliest Maastrichtian) highly diversified vertebrate site in the entire Haţeg Basin, and thus provides a good starting point for paleofaunistic, paleoecological and biostratigraphic comparisons with other similar sites across the Transylvanian area. During this study, detailed sedimentological, palynological, invertebrate- and vertebrate paleontological investigations were conducted to reconstruct the former paleoenvironment and the different depositional processes that allowed the formation of this productive bonebed. More than 800 vertebrate fossils were collected from an approximately 4.75 m2 area of the bonebed horizon of site K2 representing at least 17 species including fish, amphibians, turtles, squamates, crocodyliforms, dinosaurs, pterosaurs and mammals, ranking this site among the most taxonomically diverse ones within the basin. The sedimentological investigation points towards a lacustrine depositional environment in which a high-diversity, multitaxic, multidominant mixed assemblage was accumulated on a flood-related delta due to a sudden drop in transport energy. Based on its stratigraphical position, site K2 represents the oldest vertebrate site within the Haţeg area and suggests a remarkable large-scale faunal stability on the Haţeg Island during the Maastrichtian. The dominant elements of the local fauna were already present in the earliest Maastrichtian, and no significant differences in faunal composition can be detected between this oldest and other, younger vertebrate assemblages of Haţeg Basin, at least at the level of higher taxa. Furthermore, just as the faunal composition, the dominance spectrum of the different taxa has not changed significantly among the Maastrichtian sites of Haţeg Basin.