COMPETITION AND NICHE PARTITIONING IN THE STRUCTURING OF MEGAHERBIVOROUS DINOSAUR COMMUNITIES IN LARAMIDIA: EVIDENCE, CHALLENGES, AND OPPORTUNITIES

Megaherbivorous dinosaur diversity in Laramidia was high, peaking in the late Campanian. This rich diversity has, in part, led some to argue for the existence of a superabundant dietary resource base, while others have argued that such resources were instead limiting and that megaherbivore coexistence was facilitated by dietary niche partitioning. Evidence for the latter hypothesis has come by way of the confluence of cranial morphometrics, jaw mechanics, feeding height reconstruction, dental microwear analysis, and, most recently, enamel isotope analysis. The evidence so far suggests differential resource use between ankylosaurid and nodosaurid ankylosaurs, centrosaurine and chasmosaurine ceratopsids, and hadrosaurine and lambeosaurine hadrosaurids.

Although this model of Laramidian dinosaur ecology has not received substantial pushback, it nevertheless suffers from possible inadequacies. For example, ongoing discovery of new species in Alberta, Montana, and Utah challenges the premise that only a single representative of each ceratopsid subfamily coexisted. Similarly, recent reinterpretation of the Panoplosaurus mirus holotype skull strongly supports its taxonomic separation from the contemporaneous nodosaurid Edmontonia rugosidens, a view which has not found unanimous acceptance. These challenges necessitate revision of the model via appeal to common vs. rare or transient taxa, or to resource partitioning at finer taxonomic scales.

Resource limitation and the competitive exclusion that follows from it have been invoked to account for how the apparent provincialism exhibited by late Campanian dinosaur communities was maintained. However, the presence of certain closely related taxa (e.g., Gryposaurus monumentensis/Gryposaurus notabilis, “Chasmosaurus” russelli/Utahceratops gettyi) in southerly and northerly regions of Laramidia is at odds with this hypothesis. Similarly, the accumulating evidence for frequent dinosaur migration between Asia and Laramidia throughout the Late Cretaceous may indicate reduced competition within the intermediary Beringia, despite presumably reduced primary productivity. These apparent tensions reveal intriguing avenues for research.