SEASONAL CALIBRATION OF THE CHICXULUB IMPACT: A HISTOLOGICAL AND INVERTEBRATE BEHAVIORAL APPROACH (Invited Presentation)

Fine-scale chronology of the Chicxulub impact event permits better understanding of the early stages of ecological decline and biotic response after Earth’s most recent mass-extinction. The long-term post-impact consequences such as climate change and mass extinction are well studied, however the immediate biotic effects and the mode/tempo of extinction patterns remain unclear. The season during which the impact occurred could have played a major role in the form and severity of the immediate biotic response, potentially shaping the early trajectory of extinctions, however, previous studies have failed to fully resolve this seasonal/temporal detail. The present study aimed to determine the boreal season during which the Cretaceous-Paleogene (KPg) impact event occurred by means of histological and histo-isotopic analyses of bone tissue from fish (Acipenseriformes) coeval with a Chicxulub impact-triggered surge event in the Hell Creek Formation (North Dakota, USA). Well-defined growth bands in the osseous tissue recorded a multi-year chronology from the last decades of the Cretaceous, terminating during the favorable growth phase that correlates with Spring/Summer. ẟ¹⁸O and ẟ¹³C data from incremental samples along histological transects reveal a similar regular periodicity that suggests death synchronously occurred during the Spring or Summer, shortly after the fish spawning season. Osteological evidence further reinforces this interpretation through the limited range of growth attained by sub-yearling juvenile fishes at the time of death, and by indications of seasonal insect activity including leaf mining behavior. In addition to ascertaining the time of year for the impact, which can have profound implications for initial biotic response or susceptibility to post-impact rigors, data from this study can also provide a valuable multi-year baseline leading up to the impact event. These multiple lines of evidence provide significant insight to better understanding the response of Earth’s current and future biomes. The fossil record provides the hindsight we need to recognize and interpret the intimate relationship between evolution, extinction, and the eventual recovery of Earth biomes.