Dinosaurs and the island rule: The dwarfed dinosaurs from Haţeg Island

Islands are fascinating natural laboratories of evolution. One much debated theme among evolutionary ecologists is whether there is an ‘island rule’, the observation that large animals tend to become smaller and small animals larger. Franz Nopcsa was the first, in 1914, to suggest that the latest Cretaceous dinosaurs from Haţeg, Romania were an island fauna, based on its low diversity and apparently unbalanced composition, and the basal position (“primitiveness”) of many of the included taxa within their respective clades. In turn, the small size of the taxa compared to their relatives from other landmasses in conjunction with the proposed island setting were used to support the presence of the island rule and size reduction (dwarfing; nanism) among the Haţeg dinosaurs. In Nopcsaʹs day, palaeontologists had seen the same phenomenon many times in the Pliocene, Pleistocene, and Holocene mammals of the Mediterranean islands. Although often quoted as a key Mesozoic example of the island rule, the supposedly dwarfed Haţeg dinosaurs have never been investigated thoroughly. Here we review a wealth of new data, from tectonics and regional geology to limb proportions and dinosaur bone histology, which support Nopcsaʹs original claim of insularity of the Haţeg fauna. Current evolutionary studies confirm that the island rule applies in many, if not all, modern cases, as well as to the Mediterranean island mammals. Geological evidence confirms that Haţeg was probably an island in the Late Cretaceous, and phylogenetic, ecological, and bone histological evidence shows that at least two of the Haţeg dinosaurs, the sauropod Magyarosaurus and the ornithopod Telmatosaurus, as well as possibly the ornithopod Zalmoxes, were dwarfs by progenesis, a form of paedomorphosis.