REGIONAL AND ENVIRONMENTAL TESTS OF THE ESCALATION HYPOTHESIS DURING THE JURASSIC: A TETHYAN ORIGIN TO ESCALATION?

A central question in paleobiology is whether physical or biological processes were the dominant drivers of Phanerozoic macroevolutionary trends. Increasingly important is understanding the regional and environmental variation in these macroevolutionary trends. Here we track regional ecological patterns during the Jurassic in the midst of the Mesozoic Marine Revolution to test if these trends are consistent with the escalation hypothesis. We use stage-level species occurrence data of bivalves, gastropods, brachiopods, echinoderms, arthropods, corals, and sponges from the Paleobiology Database. The escalation hypothesis asserts that taxonomic groups that are able to adapt to intense predation and grazing/bioturbation should have proliferated, whereas groups that were unable to adapt should have been marginalized or driven to extinction. The escalation hypothesis predicts that the proportions of infaunal, mobile, and cementing occurrences would increase through the Jurassic, whereas the proportions of sessile and free-lying epifauna would decrease.

We tested these hypotheses in five regions (Europe, Middle East, New Zealand, North America, and South America) during the Jurassic and in five depositional environments within Europe. Ecological trends consistent with escalation are rare or lacking in most regions, and most trends consistent with escalation occur in Europe (western Tethys Ocean and adjacent epicontinental seas). Brachiopods and bivalves primarily drive these trends, with increases in shallow infauna, cementing epifauna, and facultative mobile occurrences and decreases in pedunculate, free-lying, and sessile epifauna. Within Europe, trends consistent with escalation occur in nearshore environments, as previously predicted, but also in middle to outer shelf environments, where escalation has been thought not to occur. When regional ecological trends are aggregated, trends in Europe are largely responsible for global ecological patterns. These results suggest that escalation was limited to the western Tethys during the Jurassic, and did not occur elsewhere until later. Our results highlight the need to dissect global patterns regionally and environmentally to understand macroevolutionary dynamics, particularly the origin and spread of ecological revolutions.