A TURTLE FROM THE TANIS KPG MASS-DEATH ASSEMBLAGE: FURTHER EVIDENCE FOR CIRCUM-RIPARIAN DISRUPTION BY A MASSIVE CHICXULUB IMPACT-TRIGGERED SURGE

A cascade of ecological collapse following the Cretaceous-Paleogene (KPg) Chicxulub impact triggered one of the most profound mass-extinctions in Earth’s history. Comparatively little is known, however, about the first mortalities that immediately followed the impact, or the nuances of how individual animals experienced this event. Tanis, an exceptionally well-preserved mass-death assemblage (Thanatocoenosis) at the KPg boundary within the Hell Creek Formation, U.S.A., and which was emplaced by a massive surge in the first minutes/hours of the Paleogene, preserves the first clear paleontological record of mortalities caused by the immediate effects of the Chicxulub impact. Here we report a fossil turtle (c.f. Baenidae) found within this rapidly deposited sediment package, constituting the first articulated chelonian discovered within a KPg boundary event deposit. The extensively preserved tubercular skin covering large portions of the limbs and tail, and completeness and articulation of the osseous elements, provide a valuable opportunity to better understand aspects of the skeletal and soft tissue anatomy of Baenidae from the Hell Creek Formation. The cause of death is implied by a fossil tree limb/branch that impaled the anterior opening of the shell, which also further reveals the turbulent and high-energy nature of the surge event. The stratigraphic location of the fossil in the upper of two units that comprise the surge deposit broadly constrains it to the latter stages of emplacement of the sediment package. Impact ejecta lenses bracket the turtle immediately above and below, and help to further refine the micro-temporal context of this remarkable fossil. This unique and rare glimpse into a likely impact-caused mortality sheds new light on the first effects felt by terrestrial vertebrates and aids in our understanding of the hardships faced by life in the first moments after the Chicxulub impact. The turtle and coeval carcasses of fishes and other vertebrates in the mass-death assemblage provide compelling evidence of the ecologically damaging effects that would have been exacerbated in circum-riparian and lacustrine areas that experienced impact-triggered surges.