A Bird's Eye View: Hesperornithiforms as Environmental Indicators in the Late Cretaceous Western Interior Seaway

Abstract

Seabirds are considered indicators of ecological hotspots in modern ecosystems because their biogeographic distribution is correlated with physical, chemical, and biological oceanographic factors. Pursuit diving seabirds – those that actively pursue prey underwater – are generally more limited in distribution and more closely tied to oceanographic factors, as diving ability is often gained at the expense of flight capabilities. Today, wing propelled pursuit diving seabirds are restricted to colder waters; foot propelled divers have a broader latitudinal distribution, but do not forage far from shore in marine environments. In comparison, Late Cretaceous Northern Hemisphere seas were host to foot propelled pursuit diving seabirds called hesperornithiforms. Their Western Interior Seaway fossil record, in particular, reflects the broad latitudinal distribution of modern foot propelled divers, but with occurrences of offshore foraging more typical of modern wing propelled divers. The presence of hesperornithiforms in such a wide variety of environments (Boreal, Tethyan, coastal, nearshore, and offshore) suggests different ecological and oceanographic conditions affected biogeographic patterns of Late Cretaceous divers. The purpose of this study is to explore Late Cretaceous hesperornithiform paleobiogeography in light of what is known about how biotic and environmental factors affect modern pursuit diving seabirds to better understand WIS paleoecology.The Late Cretaceous was characterized by greenhouse climate, high sea levels, and epicontinental seas that provided marine paleoenvironments with no modern analogs. Two notable ecological differences between the Late Cretaceous and today are the presence of marine reptiles (and absence of marine mammals) as apex predators and the lack of acanthomorph fish diversity. Spatio-temporal overlap among hesperornithiforms, marine reptiles, and large predatory fishes implies different competition and predator-prey relationships among marine carnivores than today. Differences in swimming capabilities between marine reptiles and marine mammals may partially account for the presence of hesperornithiforms in warm, offshore waters. Additionally, environmental factors relating to the unique paleoceanography of the WIS would have also influenced hesperornithiform populations. Ample shoreline, abundant shallow water habitats, and high primary production made epicontinental seas hotspots for pursuit diving seabirds, despite warmer temperatures. Together, ecosystem structure and the unique oceanographic factors characterizing epicontinental seas both contribute to differences in seabird biogeography between the Late Cretaceous and today.