Palaeobiogeographical and palaeobiological aspects of mid- and Late Cretaceous ammonite evolution and bio-events in the Russian Pacific

Both faunal endemicity and provincialism in the mid- and Late Cretaceous of the Pacific realm complicate the recognition of stage boundaries in that area. Correlations with other areas (e.g., Europe, United States Western Interior, Pacific coast of North and South America) therefore have to rely solely on event stratigraphy; not only extinctions, but all biotic events must be considered. During the Cretaceous, Pacific faunas were characterised not so much by the presence of typical ‘Boreal’ assemblages, but rather by the absence or extreme paucity of other widely distributed biota. Clearly, faunal similarities depended more on regional facies development than on their spatial distribution. The regional diversity curve reflects all global mass extinctions, faunal turnovers and radiations. A detailed analysis of ammonite evolution, based on mid- and Late Cretaceous sections from the Russian Pacific coast, plus a comparison with other provinces across the globe, fails to support a trend in decreasing ammonite diversity since the late Albian. On the contrary, ammonites demonstrated a high adaptive ability after each extinction event and recovered from each with new radiations.