THE IMPACT OF SUCCESSIVE DEVONIAN EXTINCTIONS ON CRINOID AND VERTEBRATE TROPHIC INTERACTIONS AND DIVERSITY

Predator-prey interactions define both modern ecosystems and major marine revolutions. Yet, few pervasive predatory relationships have been identified in the fossil record. In addition, little is known about how extinctions affect survivors at different trophic level than victims. In modern ecology, food webs are teased apart through experimental manipulations such as the selective removal of consumers. The two largest extinctions of the Devonian period, the Frasnian-Famennian Kellwasser event (374 million years ago) and end-Devonian Hangenberg event (359 million years ago), were similarly selective in their effects. The loss of reefs at the Frasnian-Famennian boundary provided most of the conditions necessary for a post-extinction radiation of camerate crinoids, including removal of competitors and widespread initiation of new carbonate ramps. However, crinoid diversity remained relatively stable until the devastation of their vertebrate predators during the Hangenberg extinction, 15 million years later. Post-Hangenberg crinoids show the classic signs of predatory release, as the extinction brought an abrupt end to the Mid-Paleozoic Marine Revolution. A Mississippian “Age of Crinoids” was thus established through the combined action of two separate extinction events. However, this age was short lived due to the indirect effects of those same events. As surviving vertebrates radiated in the midst of abundant prey, they evolved new dentitions better suited to the Devonian-era defenses, or “legacy adaptations,” of camerate crinoids. As predicted from introductions of new predators into small ecosystems, this caused a precipitous decline in camerate diversity during the later Mississippian (345-318 million years ago), changing the global crinoid evolutionary fauna. Thus, loss of vertebrate predators during the Hangenberg extinction is linked to both the success and failure of their co-evolved crinoid prey. This study shows that mass extinctions can have unexpected and informative knock-on effects, and that well-known trophic phenomena (e.g. Lotka-Volterra cycles) can operate on macroevolutionary scales.