Defining the Jurassic–cretaceous Boundary in the Panboreal Superrealm: A Re-View of Correlative Levels Traced by Ammonites and Bivalves

The Jurassic–Cretaceous (J/K) boundary currently lacks a ratified Global Boundary Stratotype Section and Point (GSSP). However, three intervals between the “traditional” base of the Upper Tithonian and the base of the Middle Berriasian have been identified as candidates for the future GSSP level (Grabowski et al., 2026). Due to the absence of globally traceable events with the exception of palaeomagnetic reversals a set of levels suitable for subglobal correlation is required. Here, we consider such levels within the Panboreal Super-realm. In this extensive region, the Volgian and Ryazanian stages are traditionally employed for the boundary interval instead of the Tithonian and Berriasian (Rogov et al., 2024). Among the various fossil groups, ammonites (the preeminent 'geological clock' for the Jurassic) and certain bivalves are of paramount importance. The following succession of ammonite bio-events, principally the first appearance datums (FADs) of key genera with broad geographic ranges, can be recognized within the studied interval (from older to younger): 1) FAD of Epilaugeites; 2) FAD of Praechetaites ex gr. exoticus and Laugeites mesezhnikowi; 3) FAD of Swinnertonia; 4) FAD of Subcraspedites; 5) FAD of Craspedites and the last appearance datum (LAD) of Laugeites, an event that marks the base of the Upper Volgian; 6) FAD of Craspedites (Trautscholdiceras); 7) FAD of Volgidiscus and Chetaites; 8) FAD of Praetollia and LAD of Volgidiscus, defining the Volgian–Ryazanian boundary; 9) FAD of Hectoroceras. Bivalves of the families Buchiidae and Inoceramidae (genera Buchia, Anopaea, “Inoceramus”, and “Parainoceramya”) were characterized by high evolutionary rates during this interval. The following bivalve-based events have been recognized: 1) FAD of Buchia terebratuloides near the Middle–Upper Volgian boundary; 2) FAD of Anopaea, first in NW Europe and the Russian Platform, and later in Siberia; 3) FAD of Buchia unschensis, which may also exhibit minor diachroneity. At approximately the same level, the first “Parainoceramya” golberti appears in Eastern Siberia and, possibly, in NE Europe, while “Inoceramus” subplanus appears in Eastern and Western Siberia; 4) LAD of Anopaea in the Volgian-Ryazanian transition of the Russian Platform and Siberia; 5) FAD of Buchia okensis and B. volgensis, situated slightly above the base of the Hectoroceras kochi ammonite zone. The successions of bioevents described above provide a framework for the more or less precise identification of the J/K boundary, independent of the final decision regarding the GSSP level. This study was supported by the Russian Science Foundation (project no. 25-17-00210).