It is more difficult to determine the climate of southeastern Australia when the dinosaur fossil beds were laid down , towards the end of the Early Cretaceous: these deposits contain evidence of permafrost, ice wedges, and hummocky ground formed by the movement of subterranean ice, which suggests mean annual temperatures ranged between and ; oxygen isotope studies of these deposits give a mean annual temperature of to . However the diversity of fossil vegetation and the large size of some of fossil trees exceed what is found in such cold environments today, and no-one has explained how such vegetation could have survived in the cold temperatures suggested by the physical indicators – for comparison Fairbanks, Alaska presently has a mean annual temperature of . An annual migration from and to southeastern Australia would have been very difficult for fairly small dinosaurs in such as "Leaellynasaura", a herbivore about to long, because seaways to the north blocked the passage to warmer latitudes. Bone samples from "Leaellynasaura" and "Timimus", an ornithomimid about long and high at the hip, suggested these two dinosaurs had different ways of surviving the cold, dark winters: the "Timimus" sample had lines of arrested growth (LAGs for short; similar to growth rings), and it may have hibernated; but the "Leaellynasaura" sample showed no signs of LAGs, so it may have remained active throughout the winter. A 2011 study focusing on hypsilophodont and theropod bones also concluded that these dinosaurs did not hibernate through the winter, but stayed active.