The Arctic hosts an extraordinary wealth of terrestrial fossil biotas of Late Cretaceous age representing a diverse and highly productive near-polar ecosystem that has no modern analogue． Compared to the rest of the Late Cretaceous Maastrichtian plant diversity was at its lowest and the temperature regime the coolest，yet the semi-open forests supported a rich dinosaur fauna made up of a wide range of body sizes and feeding strategies． The combination of mild winter temperatures and continuous darkness lasting several months imposed severe constraints on primary productivity． Plant survival strategies involved almost universal winter loss of foliage， which in turn limited food supply for non-migratory overwintering herbivorous animals． A combination of leaf form and tree ring studies has been used to quantify year round variations in temperature and determine the tim- ing of spring bud-break and autumnal leaf fall． While Maastrichtian winter temperatures were cold enough (down to － 10℃ for brief intervals) for frequent frosts and snowfall，summer temperatures were cool but highly variable and at ～ 83°N along the north Alaskan coast frequently fell below + 10℃ ． Theropod egg shell frag- ments at ～ 76°N in the Maastrichtian of Northeastern Ｒussia may indicate that dinosaur reproduction took place in the Arctic ecosystem，as distinct from taking place at lower latitude breeding grounds reached by migration． This raises the question of nest management and specifically the maintenance of incubation temperatures，and the duration of incubation． Of critical importance to year-round residency is the timing of hatching and juvenile care before winter darkness set in，temperatures fell to near freezing and food resources became limited．