THE JURASSIC AND ITS PLACE IN THE HISTORY OF ESCALATION

The hypothesis of escalation asserts that enemies — competitors, predators, and disease agents — drive much of adaptive evolution and ecological expansion. The Jurassic and Cretaceous periods of the later Mesozoic era had been seen as times of substantial worldwide escalation both on land and in marine environments. New analyses of the times of origin of competition- and predation-related innovations (cementation in bivalves, powered flight in vertebrates, novel sculptural types in gastropods, and many others) indicate that these are concentrated in the Late Triassic and at various times in the Cretaceous. Relatively few innovations occur during the Jurassic, and these are concentrated mainly early and late in the period. Geographically and tectonically favorable conditions during the Jurassic appear instead to be associated with worldwide diversification and with the spread of escalation-related gains made during the Triassic. These radiations and expansions may have been further abetted by the modest extinctions of the Jurassic, particularly during the early part of the period. The Jurassic was therefore chiefly a time of consolidation rather than of innovation.