Reconstructing trophic palaeoecology & niche partitioning in Cretaceous coastal plain ecosystems using biogeochemical proxies

Background/Question/Methods

The Late Cretaceous of North America (100.5-66 Ma) preserves non-analogue ecological communities and greenhouse environmental conditions. Prior studies have suggested that the composition and diversity of taxa recovered here are strongly tied to their distance from palaeoshoreline and their palaeolatitude, with many species appearing to have restricted biogeographic ranges. These putative differences in palaeocommunities over sub-million year timeframes and over relatively small biogeographic areas suggest that large-bodied (i.e. rhinoceros- to elephant-sized) dinosaurs may have been particularly sensitive to palaeoenvironmental change, in contrast to the much wider latitudinal and altitudinal ranges occupied by many living large-bodied mammals and reptiles. How these co-occurring taxa partitioned their niches can be difficult to determine from examinations of fossils alone. To reconstruct community structure in this system we measured a combination of strontium isotope compositions (proxy for habitat use and movement) and elemental ratio data (tracking biopurification as a trophic/dietary indicator) from the bioapatite tissues of a range of co-occurring extinct species (Nspecies=18), intensively-sampled from a spatiotemporally-constrained bonebed deposit in southern Alberta. Using these data, we reconstructed community structure and trophic ecology, and tested if species in this system were partitioning their niches via detectable differences in habitat use, movement patterns, and dietary specialization.

Results/Conclusions

We find evidence for herbivorous dinosaurs (hadrosaurids, ankylosaurs, ceratopsids) partitioning their niches via differences in habitat use and diet. Hadrosaurids preserved non-overlapping and considerably broader 87Sr/86Sr ranges when compared to ankylosaurs and ceratopids, suggesting distinct habitat occupation (potentially inland vs. coastal). 87Sr/86Sr values also indicated potential feeding-height differences, with hadrosaurids feeding on deep-rooted tall plants (87Sr/86Sr values reflecting bedrock) and ankylosaurs and ceratopsids feeding on low-lying shallow-rooted plants (87Sr/86Sr reflecting multiple input sources). Sr/Ca and Ba/Ca ratios are higher in ceratopsids compared to ankylosaurs and hadrosaurids, potentially indicative of roots and other subterranean plant resources comprising a partial dietary component. Low Sr/Ca, Ba/Ca, and/or Pb/Ca ratios support carnivory in tyrannosaurids, dromaeosaurids, crocodylians, and varanoids. Borioteiioid lizards and multituberculate mammals preserve Sr/Ca, Ba/Ca, and Pb/Ca ratios suggestive of insectivory, though with distinct 87Sr/86Sr ranges indicating differing habitat specializations. Enigmatic theropod dinosaurs such as troodontids and Richardoestesia, which have a wide range of previously hypothesized diets, are recovered here as omnivores and piscivores, respectively, based on preserved Sr/Ca and Pb/Ca ratios relative to other sampled taxa. Going forward, these biogeochemical proxies will facilitate tracking of temporal patterns of community dynamics and ecological responses to climatic changes in this ancient greenhouse system.