The dirty dozen: taxonomical and taphonomical overview of a unique ankylosaurian (Dinosauria: Ornithischia) assemblage from the Santonian Iharkút locality, Hungary

Ankylosaurian fossils are usually standard elements of Cretaceous continental vertebrate localities; however, bone-yielding horizons including more than one individual are extremely rare. Here, we present a unique assemblage of 12 partial, articulated or associated ankylosaurian skeletons and thousands of isolated bones and teeth discovered from the Santonian Iharkut vertebrate locality, western Hungary. Collected from an area of 600 m2 and from a single bone bed, this material is one of the richest ankylosaurian accumulation worldwide. The 12 skeletons are not monospecific but mostly based on the pelvic armour composition: six of them are from Hungarosaurus, two are referred to Struthiosaurus and four can be assigned to Nodosauridae indet. Sedimentological and taphonomical examinations revealed a single mass mortality event as the cause of the death and accumulation of these quadruped animals that are described here. The ankylosaur assemblage from Iharkut suggests at least a temporarily gregarious behaviour of these animals and also shows that Hungarosaurus and Struthiosaurus might live in the same moist habitat or at least preferred relatively close environments.