The reign of dinosaurs, dominant on land for about 150 million years, terminated at the close of the Cretaceous. They did not all disappear at precisely the same moment, but from a peak of about 120 genera near the middle of the period, dwindled in numbers down to the end of Cretaceous at which time a large number of genera became extinct (Newell, 1962; Colbert, 1965, esp. Fig. 6, Table 14). The cause (or causes) of their extinction has long been a problem of major concern to students of earth history, yet a satisfactory explanation has not been found. This report considers the possibility that extinction may have resulted primarily from stresses produced by decreasing equability of climate during the later Cretaceous.