Ecological Controls on the Campanian Distribution of Hesperornis (Aves: Hesperornithiformes) in the Western Interior Seaway

The epicontinental Western Interior Seaway (WIS) of Late Cretaceous North America provided a unique marine habitat for cephalopods, fish, marine reptiles, and the foot-propelled diving seabird Hesperornis. While several predator-prey relationships among Hesperornis or other hesperornithiforms and other WIS animals have been hypothesized based on gut contents, bite marks, and coprolites/colonites, ecological relationships have not been quantitatively tested. Paleontological species distribution modeling (SDM) studies have focused on extinct non-marine taxa and marine invertebrates, with only two marine vertebrate studies of extant taxa. Here, two SDM methods were used to test the influence of vertebrate faunas, sedimentary rock type, paleogeography, and outcrop area on Hesperornis occurrences: generalized linear models (GLMs) and occupancy models (an extension of standard GLMs). Results of both model types indicate a decreased probability of Hesperornis presence at elasmosaur-occupied sites and an increased probability of Hesperornis presence with higher paleolatitudes. The negative association between elasmosaurs and Hesperornis in all highest-ranked models, coupled with evidence of similar diets, may indicate food resource competition, but the geographic distributions of both taxa may instead reflect different habitat preferences. The positive impact of paleolatitude may reflect seasonal migrations resulting in large breeding congregations at higher paleolatitudes (based on probable immature specimens), preference for specific environmental conditions or feeding areas (e.g., along oceanographic fronts or upwelling zones), or migration mortality. While Spearman’s correlation test did not identify a western, middle, or eastern WIS preference for Hesperornis, the inclusion of this regional covariate with paleolatitude in the highest-ranked occupancy models may indicate an interaction effect between these two covariates that favored higher Hesperornis abundance in South Dakota and Manitoba along probable mixing zones or fronts. Taxa hypothesized by previous authors to have had ecological interactions with hesperornithiforms were not supported by the models as major factors in the Hesperornis distribution. These include Tylosaurus and polycotylid plesiosaurs, hypothesized feeders on Hesperornis, Enchodus, a teleost consumed by the smaller hesperornithiform Fumicollis, and Squalicorax, an opportunistic shark that consumed hesperornithiforms. This study represents the first paleogeographic model for Hesperornis occurrences and the first GLM and occupancy modeling applications to extinct marine vertebrates. These models examine previous hypotheses and provide novel interpretations for hesperornithiform ecology.

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