THE UPPER CRETACEOUS FOSSIL RECORD OF THE WESTERN INTERIOR BASIN OF NORTH AMERICA IS A MEGALAGERSTÄTTE

A Megalagerstätte represents a body of rock of wide geographic and temporal extent that provides exceptional preservation of both quantity and quality of fossil organisms; it may include multiple individual Lagerstätten. The Western Interior basin extends from northern Mexico to Canada, and, during the Campanian to Maastrichtian, it preserves an exceptional fossil record notably preserved in strata deposited in the seaway and on the coastal plains of Laramidia along the western margin. The nonmarine vertebrate faunas of Laramidia occur from northern Mexico to Alaska and include units with exceptional preservation (e.g., Dinosaur Park Formation in Alberta). Although they are best known for their dinosaur faunas, they also include important assemblages of mammals and non-dinosaurian lower vertebrates. There is no compelling evidence that there was any discrete biogeographic separation of the Campanian or Maastrichtian dinosaur-dominated vertebrate assemblages from north to south between Texas and Alberta in the Western Interior basin. Coal-bearing and other strata also yield important floras of megafossil plants and pollen (e.g., Lagerstätten of Ellesmere Island). At peak flooding, the Western Interior Seaway extended from Arctic Canada and Alaska south to the Gulf of Mexico, with probable intermittent connections to the Hudson Bay region. Strata deposited in the seaway yield outstanding invertebrate and vertebrate faunas (e.g., Niobrara Chalk of Kansas). The invertebrate faunas demonstrate boreal and Tethyan provinciality. The Western Interior basin also includes significant ichnofaunas. The marine invertebrate ichnofaunas of the seaway are diverse and exceptionally preserved. Vertebrate ichnofaunas include the largest sample known of marine and nonmarine dentalites, abundant preservation of marine consumulites and nonmarine and marine coprolites and diverse tetrapod footprint assemblages. The Western Interior Megalagerstätte is a unique preservational setting that resulted from an interplay of causal factors including tectonics, eustasy and climate. Study of the region has been facilitated by the climate, human history and economic geology of the region.