Sr-O-C isotope signatures reveal herbivore niche-partitioning in a Cretaceous ecosystem

Stable and radiogenic isotopes represent powerful tools for reconstructing ecological and environmental patterns in ancient ecosystems. The Cretaceous of North America preserves a diverse record of fossil vertebrates well-suited to analysis using these proxies, contained within many well-sampled and stratigraphically well-characterized intervals. Multiple hypotheses have been offered to explain the diverse assemblages of megaherbivores that co-occurred in the relatively restricted available landmass here, including various forms of niche-partitioning related to habitat preference, dietary specialization and feeding height stratification. Here we analyse the Sr-87/Sr-86, delta C-13 and delta O-18 of bioapatite samples obtained from a range of herbivores, faunivores and endemic taxa, from a spatiotemporally-constrained and intensively-sampled site in the upper Oldman Formation, to test if megaherbivores partitioned their niches based on spatial patterns of occupation and resource-use. We also compare measured strontium values to regional Sr-87/Sr-86 data to assess biogeographical range sizes, habitat breadth and migration potential. We find that hadrosaurs had broad ranges, whereas ankylosaurs and ceratopsids were more spatially restricted. The Sr-87/Sr-86 ranges of hadrosaurs are much wider and do not overlap with those of other ornithischians, potentially related to dietary differences driven by a combination of feeding height-stratification and habitat breadth differences. Ankylosaurs and ceratopsids overlapped extensively in Sr-87/Sr-86, delta C-13 and delta O-18, indicating overlap in the same habitats and intake of similar resources, and suggesting more complex spatiotemporal variation in resource-use patterns, fine-scale dietary differences, and/or sufficient resource-availability to reduce the degree of competition given this theoretical niche overlap. Additional analyses integrating ecomorphological proxies may elucidate these patterns further.