The paleoecology and taphonomy of a Santonian-Campanian (Upper Cretaceous) dinosaur-bearing vertebrate locality from Bulgaria: A window into an underexplored part of the Late Cretaceous European Archipelago

The Upper Cretaceous European vertebrate fossil record has improved significantly in the past three decades but there still remain chronostratigraphic and geographic gaps, which obscure our understanding of the paleobiogeography and evolution within the insular environments of the Late Cretaceous European Archipelago. Recently, a new vertebrate locality of late Santonian–early Campanian age was discovered in westernmost Bulgaria, promising to fill some of these gaps. Here, we use a multidisciplinary approach involving palynology, paleontology and paleohistology to investigate aspects of the paleoecology and taphonomy of this new locality and to provide preliminary information on its taxonomic contents. Palynomorph data shows that the flora was dominated by angiosperms of the Normapolles group with subordinate presence of ferns and only rare gymnosperms. The association of the pollen taxa Krutzschipollis crassus and K. spatiosus supports latest Santonian to early Campanian age for the vertebrate-bearing strata. The floral composition and especially a number of fern spore humidity indicators imply the existence of a generally humid subtropical climate, with some seasonal droughts. Using palynofacies analysis, we infer a coastal, proximal shelf to oxidated deltaic or lagoonal depositional environment for the examined sedimentary succession. The locality has so far yielded 250 vertebrate specimens collected from eight strata. There are at least seven clades present, including lamniform sharks, lepisosteid gars, amphibians, turtles, crocodylomorphs, ornithopod and titanosaur dinosaurs, and possibly pterosaurs. Semi-aquatic and aquatic animals dominate the assemblage. Most common are turtles (about 30% of the sample), followed by dinosaurs. Skeletal elements are disarticulated, isolated and mostly fragmentary. Fossils are not sorted by size. Many of the fossil bones show signs of abrasion and bioerosion, both micro- and macroscopically. Paleohistological data reveal that all sampled dinosaur bones belong to subadult or adult individuals. We interpret the site to be an attritional assemblage. Taxonomic comparisons with other well-known Santonian to Maastrichtian fossil assemblages from Central (Hungary and Austria) and Eastern Europe (Romania and Serbia) indicate similarities with both the Santonian Iharkút-Ajka vertebrate fauna of Hungary and the younger Haţeg Island fauna of Romania, although additional material and more precise taxonomic identification of the Bulgarian fossils is needed. Our work presents the first more in-depth look at life on land in this currently underexplored part of the Late Cretaceous European Archipelago.