GIANT ICHTHYOSAURS PROVIDE INSIGHTS INTO LATE TRIASSIC MARINE ECOSYSTEM STRUCTURE

Tracking secular shifts in marine ecosystem structure over geologic time is important for understanding how life in the oceans responds to global change and how modern ecosystems were assembled. The Triassic Period, between ~252 and 201 million years ago, witnessed several significant changes to marine ecosystems with consequences that are still apparent in today’s oceans. One innovation is the first appearance of large-bodied marine tetrapod predators, culminating in giant forms such as the ichthyosaur Shonisaurus (>11 m total length). Despite a long history of study, the ecological roles of these giant ichthyosaurs, and a clear understanding of when and why they disappeared, remains enigmatic. To elucidate the paleoecology of Shonisaurus and its ecosystems, we initiated a long-term project to examine the vertebrate paleontology of Upper Triassic marine strata of western Nevada. New specimens demonstrate that Shonisaurus popularis possessed robust teeth with carinae, suggesting it was an apex predator capable of taking large prey. This taxon was abundant and widespread during the late Carnian through early Norian, and related giant forms persisted in Nevada and other northern hemisphere sites until the end-Triassic mass extinction (ETE). No known ichthyosaurs of similar size persisted after this event, although multiple other marine tetrapod lineages attained similar sizes later in the Mesozoic and Cenozoic. This pattern suggests that a fundamental change in ecosystem structure occurred at the ETE that caused the permanent disappearance of giant apex predator ichthyosaurs. Because modern marine predator body size is driven by the tradeoffs between metabolic cost, feeding efficiency, and prey availability, we suggest that the collapse of marine food webs during the ETE made it impossible for these giant ichthyosaurian predators to obtain enough prey. When marine food webs recovered sufficiently to support giant predators, other marine reptile groups assumed this ecological role. It is presently unclear whether the failure of ichthyosaurs to attain giant sizes later in the Mesozoic reflects competitive exclusion or limitation by some other constraint.