Taphonomic and paleoecologic investigations of the Late Cretaceous (Santonian) Iharkút vertebrate assemblage (Bakony Mts, Northwestern Hungary)

Abstract

The Iharkút vertebrate locality, an open-pit mine in the Bakony Mountains (western Hungary), has provided a rich and diverse assemblage of Late Cretaceous (Santonian) continental vertebrates. The isolated and associated remains represent 31 different taxa including fish, amphibians, turtles, lizards, pterosaurs, crocodilians, non-avian dinosaurs and birds. Sedimentologic investigations suggest that the Iharkút depositional environment was represented by the floodplain of a very low-gradient river. The 10–50cm thick bonebed of site SZ-6, is the most important fossiliferous layer in the open-pit mine and analysis of this site indicates alternating energy conditions during the bone accumulation, which resulted in fossils of different states of preservation being deposited together. The vertebrate assemblage of site SZ-6 includes three main different subsets with widely different taphonomic history. The characteristics and the preservation mode (high rate of abrasion and the spherical shape) of the “bone pebbles” suggest that this type of the isolated bones was more exposed to abrasion and probably the remains were transported from farther away than the other isolated bones. The second group includes 88% of the Iharkút collection, containing most of the identified isolated bones and teeth, and represents polytypic attritional remains transported and deposited by high density flow during ephemeral flood events. Meanwhile the monospecific ankylosaur skeletal material from Iharkút site SZ-6 may represent a mass death assemblage because seven skeletons of Hungarosaurus were discovered from an area of approximately 400m2 and in many cases close to each other in the same layer. The Iharkút vertebrate assemblage is dominated by bones of aquatic/semi-aquatic animals, while the number of terrestrial animal remains is subordinate. The taphonomic analysis of the ankylosaur material from Iharkút locality further strengthens the previously suggested hypothesis that some of the ankylosaurs preferred wetland habitats (e.g. areas along fluvial systems) while the other two herbivorous dinosaur groups from Iharkút (ornithopods and ceratopsians) were probably living in distal habitats.