A Santonian–Campanian Sphenoceramus assemblage from Arctic Canada – expression of a circum‐Arctic taphonomic event?

Fossils of the giant, iconic genus Sphenoceramus have been documented from the Cenomanian to early Maastrichtian Kanguk Formation of the Canadian Arctic Sverdrup Basin since the 1960s and assigned to no less than seven species in the literature. They are accompanied by the small oxytomid bivalve Hypoxytoma at several localities, but no additional taxa have been reported from the same horizons. Fieldwork on western Axel Heiberg Island has produced a remarkable, much richer fossil assemblage composed of abundant Sphenoceramus, infaunal bivalves (Astarte, Pholadomya, Mactromya), gastropods (potential Naticopsidae and Aporrhaidae), driftwood and an enigmatic, foliate phosphatic fossil. Zircon U-Pb dating of an overlying volcanic ash layer indicates a stratigraphic age close to the Santonian–Campanian boundary for this assemblage. Curiously, outside this Sphenoceramus interval, which comprises up to five closely spaced sideritic horizons, only a handful of isolated ammonites or bivalves are known from the entire Kanguk Formation. The association of Sphenoceramus and Hypoxytoma occurs in several parts of the Arctic and beyond, including West and East Greenland, the East European Platform and the Polar Urals, and extends as far south as Kazakhstan. In several of these areas, as well as in the Sverdrup Basin, this association forms one of very few notable macrofossil assemblages within the Upper Cretaceous succession. This, together with the extraordinary size of Sphenoceramus, suggests the occurrence of a spatially extensive but stratigraphically restricted ecologic and taphonomic window, which promoted the establishment of the Sphenoceramus association together with its subsequent preservation in otherwise nearly barren strata.