Mid-Cretaceous Vertebrate Faunas of the Western Interior Sea of Northern North America

The Western Interior Sea was an epicontinental sea that extended north-south through the middle of North America during the last half of the Cretaceous. Recent studies of Cenomanian vertebrate faunas from lag deposits in the prairie provinces and states of Canada and the United States have significantly altered the previous picture of the overall biodiversity and biogeography of the Western Interior Sea during the ‘mid-Cretaceous.' Geographically widespread mid-Cenomanian lag deposits in the form of bonebeds and calcarenites contain abundant inoceramid mollusk prisms and vertebrate debris, particularly remains of chondrichthyan and osteichthyan fishes. These bioaccumulations also contain bones and teeth of marine reptiles such as turtles, elasmosaurs, pliosaurs and dolichosaurs as well as hesperornithiform and ichthyornithiform birds.

The localities in Manitoba, Saskatchewan, Kansas, and Colorado are separated by as much as 15 degrees latitude (a north-south distance of approximately 1900 km). Their faunas demonstrate some clinal trends, such as a more diverse fauna of benthic chondrichthyans mixed with dolichosaurs to the south, and a more diverse fauna of plesiosaurs and birds to the north. However, the faunas also show strong taxonomic homogeneity, particularly many chondrichthyan fishes that were cosmopolitan, indicating easy passage throughout the sea as well as to both Boreal and Tethys oceans. These faunas add significantly to osteichthyan fish diversity in the northern waters of the Western Interior Sea, which were thought to have supported only a depauperate, cold-adapted fauna.

Some of the bioaccumulations are located far from any reconstructed Cenomanian shorelines, but they appear to represent lag deposits shallow enough to have been affected by storm wave base. Others represent nearshore deposits with occurrences of some terrestrial taxa. The lithological and paleontological data suggest a general trend of transgression, punctuated by a series of fairly dramatic lowstands that created basin-wide, shallow water habitat during the mid-Cretaceous.