The earlier Triassic and Jurassic periods were characterized by joined landmasses throughout the planet. By the Cretaceous period, these landmasses began to separate, isolating some species of dinosaurs, and leading to different areas having a variety of new species. For example, the titanosaur sauropods were mostly present in former Gondwana, such as South America, while the ceratopsians and hadrosaurs were found mainly in Laurasia. But many of these fossil interpretations are highly debated. One of the main reasons for the present uncertainty about the overall distribution of dinosaurs is the incompleteness of the known dinosaur fossil record and the fragmentation of many dinosaur fossils found within rock layers.