Revision of Romanian sauropod dinosaurs reveals high titanosaur diversity and body-size disparity on the latest Cretaceous Haţeg Island, with implications for titanosaurian biogeography

The Hațeg Basin and surrounding areas in Transylvania, western Romania, have been a hotspot for research on vertebrate faunas of the Late Cretaceous European Archipelago. One of the historically earliest titanosaurian sauropod dinosaurs to be discovered, the ‘dwarfed’ species Magyarosaurus dacus comes from lower Maastrichtian deposits in this basin; however, this species has been neglected, with no modern treatment of its anatomy, taxonomy or phylogenetic affinities. Via detailed anatomical study of historical and undescribed remains, combined with archival data, we identify shared autapomorphies that link multiple partial skeletons. Our analysis of hundreds of specimens (including >20 monospecific assemblages) enables the stabilization of the type species Magyarosaurus dacus. We propose the presence of three additional, but only partly contemporaneous taxa – Paludititan nalatzensis, Petrustitan (‘Magyarosaurus’) hungaricus n. gen. and Uriash kadici n. gen. n. sp. (the latter being amongst the largest known sauropods of the Late Cretaceous European Archipelago). We present a new phylogenetic analysis (152 taxa scored for 570 characters), with implications for broader titanosaurian evolutionary relationships and biogeography: Magyarosaurus is recovered either as a member or a close relative of Saltasauridae; Paludititan has affinities with Lognkosauria, along with the contemporaneous Lohuecotitan; Petrustitan is most closely related to South American early diverging eutitanosaurian taxa; and Uriash also shares affinities with Gondwanan taxa. Our findings strengthen the hypothesis that latest Cretaceous European titanosaurs belonged to Gondwanan lineages that invaded the former area during the Barremian–Albian. We interpret the presence of body-size disparity as either evidence that large-bodied taxa were ecologically excluded from body-size reduction by competition with small-bodied titanosaurs, or that dwarfing occurred stratigraphically earlier among several lineages and the small-bodied titanosaurs on Hațeg Island are the descendants of existing dwarfed ancestors. Finally, we find no indication of a body size-related titanosaurian turnover in the uppermost Cretaceous of the Transylvanian area.