Late Cretaceous endemic shallow-marine gastropod genera of the northeast Pacific: biodiversity and faunal changes

Endemic genera of shallow-marine gastropods in the Cretaceous Northeast Pacific Subprovince (NEP), extending from Alaska to northern Baja California Sur, Mexico, are tabulated and discussed in detail for the first time. None are known in Lower Cretaceous or Cenomanian strata, but 43 genera, nearly two-thirds of which are neogastropods, are recognized in Upper Cretaceous strata. Their first appearance was at the beginning of the Turonian, which coincided with the warmest time of the Cretaceous and one of its highest sea-level stands. Fourteen new subtropical endemic genera appeared then, and 10 (71%) were neogastropods. Tethyan-influenced thermophilic mollusks (nerineid, acteonellid, neritid, and cypraeoidean gastropods, as well as rudistid bivalves) were present. A turnover at the Turonian/Coniacian boundary occurred when cooler waters migrated southward, resulting in the subtropical endemics being abruptly and nearly completely replaced by 10 warm-temperate new endemic neogastropods, which commonly had long-living lineages persisting through the Campanian or early Maastrichtian. Eight (80%) of these new genera were neogastropods. During these cooler water times in the Coniacian and nearly all of the Santonian, there was an absence (“gap”) of Tethyan-influenced and other thermophile mollusks. New endemic genera, were added during an origination event in the early Campanian and also during another one in the late Campanian. Fewer and fewer new neogastropod genera appeared during each of these origination events, whereas new endemic genera of non-neogastropods increased. Rudistids and thermophilic mollusks also returned in the Campanian. Most of the post-Turonian endemic genera went extinct at a turnover at the end of the early Maastrichtian (=“Middle Maastrichtian Event”), rather than at the K/Pg boundary mass-extinction event. Two new endemic genera, both neogastropods, originated just before the end of the Maastrichtian. They and three other Cretaceous endemic gastropod genera occur also in the NEP Cenozoic record, thereby imparting a somewhat transitional aspect between these two faunas. The tectonically transported fauna of the middle Albian Alisitos Formation in northern Baja California, Mexico is confirmed to have lived in the tropical-water Caribbean Biotic Province of the Tethyan Realm.