to have the largest difference from the ratios of other, more terrestrial theropods, while those of Spinosaurus from Tunisia and Morocco tended to have the least difference, despite the advanced piscivorous adaptations in the skull observed for this genus. The authors suggested that piscivory and semiaquatic habits may explain how spinosaurids coexisted with other large theropods. By feeding on different prey items and occupying a distinct ecological role, a phenomenon that is known as niche partitioning, the different types of theropods would have been out of direct competition. Further lines of evidence have since demonstrated that spinosaurids, especially those within the Spinosaurinae, developed strong adaptations for aquatic environments, such as dense limb bones for buoyancy control; reduction of the pelvic girdle; and elongated neural spines on the tail, likely used for underwater propulsion. Palaeoenvironment and palaeobiogeography Of all the Mesozoic formations in northeastern Thailand, the Sao Khua is the most abundant and diverse in vertebrate fossil discoveries. The Khorat Group yields fossil taxa only of continental origin, with no definitive evidence for marine fossils or sedimentary structures found so far. In 1963, Yoshitsugu Kobayashi of Hokkaido University reported ichthyosaur and plesiosaur teeth from the Sao Khua Formation, but these have now been identified as belonging to Siamosaurus and a crocodilian respectively. The sediments of the Sao Khua Formation, which comprise red clays, mudstones, sandstones, siltstones, and conglomerate rocks, record a fluvial environment dominated by lakes, floodplains, and meandering low-energy rivers. This is consistent with the types of vertebrate fauna present in the formation, which comprise only terrestrial or freshwater animals. Besides Siamosaurus, there were theropod dinosaurs like the metriacanthosaurid Siamotyrannus isanensis, the ornithomimosaur Kinnareemimus khonkaenensis, the megaraptoran Phuwiangvenator yaemniyomi, the basal coelurosaur Vayuraptor nongbualamphuensis, a compsognathid theropod, and indeterminate birds. Theropod eggs with embryos have also been recovered from the formation. There were also sauropods like the titanosauriform Phuwiangosaurus sirindhornae, mamenchisaurids, and indeterminate forms. Sauropod remains are some of the most abundant in the Sao Khua and Khok Kruat Formations. No ornithischian (or "bird-hipped") dinosaur fossils have been found in the Sao Khua Formation, possibly suggesting that they were uncommon compared to saurischian (or "lizard-hipped") dinosaurs. The faunal assemblage also included indeterminate pterosaurs; carettochelyid, adocid, and softshell turtles; hybodont sharks like hybodontids, ptychodontids, and lonchidiids; pycnodontiform fish; ray-finned fishes such as sinamiids and semionotids; and the goniopholidid crocodyliforms Sunosuchus phuwiangensis, Siamosuchus phuphokensis, and Theriosuchus grandinaris. The Sao Khua and Khok