The median mass of a dinosaur is determined to be 630 kg (1389 lb), or the size of an American bison, based on the largest published and most accurate data set to date. Most dinosaurs seem to have stayed very small (0–60 kg [0–132 lb]) or grew very large (1081–56,000 kg [2383–123,459 lb]), with less species in the medium-sized range. The dinosaurs buried in the lower Flood layers (Upper Triassic series) were mostly small. Although there was an average size increase upward into younger Flood layers, it was not universal or statistically strong. There were many small dinosaurs buried throughout the entire spectrum of dinosaur-bearing rocks. The largest sauropodomorphs were buried in the Upper Jurassic series, about midway through the Flood record of dinosaurs. Whereas, ornithopods showed an increase in size from earliest to latest Flood deposits, attaining greatest size in the Upper Cretaceous series. Theropods generally showed an increase in size from the earliest deposits to the later Flood deposits, but had a small peak in size in the Middle Jurassic deposits also. The sizes of dinosaurs buried at different times in the Flood seem to have been influenced by several factors, including habitat elevation, mobility, and reaction to danger or intelligence. The larger brain size and mobility of the coelurosaur theropods may explain their prevalence in the Cretaceous system. Higher mobility of theropods in general may explain their greater than expected proportion of footprints also.