The ontogenetic status of a small hadrosauroid dinosaur from the uppermost Cretaceous of Bulgaria, and implications for the paleobiogeography and assembly of European island faunas

Abstract

During the latest Cretaceous hadrosauroids were the most numerous and taxonomically diverse herbivorous dinosaurs on the European island archipelago, including diminutive non-hadrosaurid hadrosauroids and lambeosaurine immigrants from Asia and, possibly, from North America. In recent years the existence of small-bodied adult hadrosauroids (or ‘island dwarfs’) in the Maastrichtian of Romania and Spain has been confirmed by histological study of their bones. The first fossils attributable to Hadrosauroidea discovered in Bulgaria (from the upper Maastrichtian Kaylaka Formation) are also of diminutive size, but it has been unclear whether the bones belonged to small adults or simply to juveniles. Here, we subjected a tibia, fibula, indeterminate long bone diaphyseal fragment, and six indeterminate bone fragments, which are associated with the hadrosauroid hind limb elements and assumed to had been broken off from one or more of them during excavation, to a histological analysis. The analysis shows that the specimens belong to a young, actively growing individual(s), as evidenced by the presence of highly vascularized primary tissues in the outer cortex, secondary remodelling mostly restricted to certain areas of the bone sections, and absence of annual growth lines or an external fundamental system that would signify skeletal maturity. The bone matrix of the periosteal tissues exhibits high amounts of parallel-fibered and lamellar bone, similar to the long bones of other European island-living non-avian dinosaurs, a feature probably linked to lower growth rates. In general, the histomorphology and size of the Bulgarian hadrosauroid correspond well with ‘late juveniles’ and young ‘subadults’ in the osteohistologically well-studied hadrosauroid Maiasaura peeblesorum. The presence of a medium-sized hadrosauroid in the south-eastern part of the Late Cretaceous European archipelago suggests a dispersal route that allowed large-bodied terrestrial fauna to populate emergent islands in the region.