Mid-Cenomanian vertebrate faunas of the Western Interior Seaway of North America and their evolutionary, paleobiogeographical, and paleoecological implications

The Western Interior Seaway (WIS) was an epicontinental sea that extended north–south through the middle of North America during the last half of the Cretaceous, linking the Tethys Sea to the south with the boreal paleo-Arctic Ocean to the north. Aptian–Maastrichtian sedimentary deposits from the seaway crop out in the Western Interior Basin of Canada and the United States. Vertebrate remains are commonly well-preserved, and in certain stratigraphic units, abundant. The classic vertebrate fauna of the WIS, from the Upper Cretaceous Niobrara Chalk, has been studied for 150 years, but relatively little has been known of the seaway's earlier faunas until recent years. Studies of mid-Cenomanian vertebrate faunas, particularly from lag deposits (bonebeds and calcarenites) in Canada and the United States have significantly altered the previous picture of the overall biodiversity and biogeography of the WIS during the ‘mid-Cretaceous.’ These mid-Cenomanian faunas include diverse fish assemblages of chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes) as well as various forms of tetrapods, including marine turtles, plesiosaurs, and hesperornithiform and ichthyornithiform birds.Our compilation of faunas indicates that at least 70 vertebrate taxa are recorded from five mid-Cenomanian WIS ‘localities’ situated in Alberta, Saskatchewan, the Black Hills region of South Dakota and Wyoming, Kansas, and Colorado. The faunas include 41 chondrichthyans, 21 osteichthyans, 10 non-avian tetrapods, and five avian taxa. Our study, combined with previous faunal studies, suggests that the combination of the following eight chondrichthyan species characterizes mid-Cenomanian time, which has been referred to as the ‘Woodbinian Age’ in the marine realm of North America: Ptychodus decurrens, P. occidentalis, Squalicorax curvatus, Cretodus semiplicatus, Carcharias amonensis, C. saskatchewanensis, Eostriatolamia tenuiplicatus, and Cretomanta canadensis. This is the geologic time when non-ptychodontid hybodonts became rare faunal components in most areas, and when a number of other fish lineages, particularly teleosts, emerged and diversified. Many of these persisted into the early Campanian, or the ‘Niobrara Age’ that followed the Woodbinian. Therefore, mid-Cenomanian time in the WIS can also be characterized by the genesis of the ‘Niobraran fauna,’ a significant radiation and diversification of fish taxa.The five mid-Cenomanian WIS ‘localities’ highlighted in this study are separated by as much as 18° latitude, a north–south distance of more than 2300 km. These faunas demonstrate some clinal trends, such as more diverse faunas of benthic chondrichthyans mixed with dolichosaurid lizards to the south, and a more diverse fauna of birds to the north. However, the faunas overall show strong taxonomic homogeneity, particularly among many chondrichthyans that were cosmopolitan, indicating that passage throughout the seaway as well as to both the Boreal and Tethys oceans was utilized. These faunas also demonstrate high osteichthyan diversity in the northern waters of the WIS, which were once thought to have supported only a depauperate, cold-adapted fauna. Because of the surprisingly great diversity in vertebrate taxa, ecosystem structure and dynamics of the WIS during the mid-Cenomanian were unquestionably complex.