Body-size evolution and biogeographic history in the Late Cretaceous European ‘archipelago’ enlightened by the first Italian multi-individual dinosaur locality

The latest Cretaceous Adriatic Carbonate Platform (AdCP) system in the paleo-Mediterranean area stands as one of the most complex and debated topics related to the evolution of land vertebrates in the area surrounding the Tethys Sea. Italy holds the sole Late Cretaceous dinosaur-dominated site of the AdCP, namely the Villaggio del Pescatore locality (VdP). The VdP site was discovered thirty years ago in the Late Cretaceous-Paleogene beds exposed near Duino Aurisina, northeastern Italy, and produced, among fish, crustacean and plant remains, an exquisitely preserved skeleton of the diminutive hadrosauroid Tethyshadros insularis. The type specimen was originally described as a pygmy taxon, showing peculiar adaptations connected to insular conditions. This taxon is here reexamined based on additional specimens from the type locality, making the site the first, multiindividual dinosaur locality of Italy. Sole representative of the latest Cretaceous paleo-Mediterranean archipelago, T. insularis offers unmatched data to infer environmentally-driven body-size trends in non-avian dinosaurs. Histological analyses indicate juvenile features in the type specimen, whereas a second, larger individual, was approaching somatic maturity. Consequently, we revised the former description of the holotype, documenting the morphological variation in this taxon, and highlighting the ontogenetically variable characters. Finally, we investigated the phylogenetic position of Tethyshadros using a phylogenetic comparative framework, which combines ancestral state reconstruction and multiple regime Ornstein-Uhlenbeck models. Such approach enables to evaluate whether the evolution of body-size was following a significant and accelerated trend of reduction in this clade, in order to further test the interpretation of this taxon as an ‘aberrant’, insular dwarf.