A Cosmopolitan Osteichthyan Fauna from in and around the Debris Cones of Harvester Ant Nests: Fairpoint Member of the Fox Hills Formation (Maastrichtian), Meade County, South Dakota

Sediments recovered from in and around the debris cones of harvester ant nests in the Upper Cretaceous (Maastrichtian) Fairport Member of the Fox Hills Formation in Meade County, South Dakota contain an osteichthyan assemblage indicative of transitional to marine shoreface deposits. The fauna consists of: Lepisosteus sp., Paralbula casei, Cylindracanthus cf. C. ornatus, Enchodus gladiolus, cf. Hadrodus sp., Siluriformes indet., and teleost indet. Taxa recognized from the Fairpoint Member are relatively common in the upper Cretaceous of North America where osteichthyan faunas characteristically consist of very limited species diversity compared to global, time-contemporaneous osteichthyan occurrences. Taphonomy and poor preservability due in part to the small size of many osteichthyans from the upper Cretaceous of North America may account for some of this observed diversity disparity. However, the central question of whether this disparity in diversity is real or is the result of differential preservation and taphonomic bias, remains unresolved. Fairpoint osteichthyans are members of families that survive the K-T boundary extinction event. Enchodonts, phyllodonts and pcynodonts are well-represented until the Eocene-Oligocene transition where cooling global temperatures result in wide spread marine extinctions. Modern representatives of semionotiforms and acipenserifoms can be found in boreal fresh and brackish water environments where morphology appears to have changed little since the late Mesozoic. The post-Cretaceous success and survivorship of these taxa may be related to their ability to tolerate a broad range of changing environmental conditions and salinities in a shrinking Western Interior Seaway.