INFAUNAL RESPONSE THROUGH MASS-EXTINCTION EPISODES: PREDICTIVE MODELS AND OBSERVED PATTERNS

Three hypothesized, predictive end-member infaunal ecosystem behavioral responses composed of positive, negative, and no feedback are compared to existing, empirical infaunal records of the big five Phanerozoic mass extinctions—latest Ordovician, Late Devonian, end-Permian, end-Triassic, and end-Cretaceous—and evaluated against new ichnofaunal data from the Late Devonian (Frasnian-Famennian, F-F) mass extinction recorded within the intracratonic Pilot basin of western Utah. All of the big five events are characterized by negative responses within the infaunal realm, measured by such parameters as ichnodiversity, ichnofabric index, tiering, maximum burrow depth, and burrow size; however, the magnitude of the negative response varies in proportion apparently to the severity and signature of the extinction, determined independently by such proxies as epifaunal biodiversity loss, sedimentation and facies patterns, and geochemistry. Furthermore, responses of both short-term positive and no feedback are superimposed on the overall negative infaunal trends, indicative of other such complicating influences as feeding strategy, oxygen tolerance, and facies preference of the specific behavior within the ecosystem. Of the big five crisis intervals, the end-Permian infaunal record has received the most detailed study. Consistent with epifaunal patterns, the marine infauna records a major, two-part extinction phase followed by protracted survival and recovery intervals, which are consistent with the persistent and severe anoxic conditions evidenced independently by other proxies. Although more high-resolution data are needed, it appears that infaunal ecosystem response across the end-Cretaceous mass extinction may have displayed a pattern of more abrupt collapse and more rapid recovery; this pattern seems significantly different from those observed in the other four of the big five events and may be characteristic of infaunal response to large extraterrestrial impacts. We suggest that detailed, high-resolution records of infaunal response through mass-extinctions can provide a new tool for evaluating and assessing the overall nature and mechanism(s) of extinction, including the potential recognition of crises driven by such geologically catastrophic processes as impact.