In addition, virtual osteohistology allowed us to visualize the seasonal fluctuations of osteocyte lacunar density and volume, which are poorly expressed in the physical 2D thin sections 18 (Fig. The 3D nature of the synchrotron data enables optimal projection of the bone deposition pattern across multiple cross-sectional planes and resolved the exact relationship between seasonality and cyclical bone apposition in superb detail 20.
To corroborate the annual nature of the LAGs using virtual high-resolution osteohistology 17, 18, three-dimensional (3D) volumes were produced with propagation phase-contrast synchrotron radiation micro-computed tomography 19 on beamline BM05 of the European Synchrotron Radiation Facility, France. We prepared dermal bone slices of six acipenseriform specimens as microscopic slides and subjected these to osteohistological assessment, during which lines of arrested growth (LAGs) were easily recognized (Fig. To trace appositional growth and pinpoint the season in which bone apposition terminated, we first assessed the preservation of bone growth patterns across the studied specimens.
Within the thanatocoenotic accumulation, abundant acipenseriforms-sturgeons and paddlefishes-became oriented along the seiche flow directions and buried alive with numerous impact spherules in their gills 5 (Fig. 1a–c) and plant matter in the suspended load while impact spherules rained down from the sky 5. As the seiche proceeded upstream, it advected bones, teeth, bivalves, ammonites, benthic foraminifera (Extended Data Fig. Tens of minutes after the impact, the seiche agitated large volumes of water and soil in the estuary of the Tanis river 5. The majority of macrofossils encountered at the Tanis locality represent direct casualties of the K–Pg bolide impact that were buried within the impact-induced seiche deposit 5. The Tanis event deposit in North Dakota (USA) is an exceptional seiche deposit preserving a rich thanatocoenosis (that is, a mass death assemblage) of latest Cretaceous biota at the top of the Hell Creek Formation. Whether seasonal timing of the onset of these marked changes affected the selectivity of the K–Pg extinction could not yet be established owing to the lack of suitable records. Although direct effects of the impact devastated a vast geographical area, the global mass extinction probably unfolded during its aftermath, which involved rapid climatic deterioration estimated to have lasted up to several thousands of years 9, 10, 11. Direct consequences of the impact, including impact glass fallout, large-scale forest fires and tsunamis, are geologically documented more than 3,500 km from the Chicxulub impact crater 5, 6, 7, 8. Among archosaurs, for example, all pterosaurs and non-avian dinosaurs succumbed in the K–Pg mass extinction, while crocodilians and birds survived into the Palaeogene period 3, 4. The Cretaceous-Palaeogene (K–Pg) mass extinction event affected biodiversity with high but poorly understood taxonomic selectivity. We postulate that the timing of the Chicxulub impact in boreal spring and austral autumn was a major influence on selective biotic survival across the Cretaceous–Palaeogene boundary. Annual life cycles, including seasonal timing and duration of reproduction, feeding, hibernation and aestivation, vary strongly across latest Cretaceous biotic clades.
Osteohistology together with stable isotope records of exceptionally preserved perichondral and dermal bones in acipenseriform fishes from the Tanis impact-induced seiche deposits 5 reveal annual cyclicity across the final years of the Cretaceous period. Here, by studying fishes that died on the day the Mesozoic era ended, we demonstrate that the impact that caused the Cretaceous–Palaeogene mass extinction took place during boreal spring. The timing of the impact and its aftermath have been studied mainly on millennial timescales, leaving the season of the impact unconstrained.
This event caused the highly selective extinction that eliminated about 76% of species 3, 4, including all non-avian dinosaurs, pterosaurs, ammonites, rudists and most marine reptiles. The Cretaceous–Palaeogene mass extinction around 66 million years ago was triggered by the Chicxulub asteroid impact on the present-day Yucatán Peninsula 1, 2.