New biogeochemical insights into Mesozoic terrestrial paleoecology and evidence for omnivory in troodontid dinosaurs

Abstract

The Cretaceous paleocommunities of North America preserve a rich record of biodiversity that suggests many species occupied narrow biogeographic ranges in comparison to their ecological equivalents in extant systems. How taxa in these systems partitioned their niches and structured their communities can be difficult to determine from fossils alone, which has led to a variety of hypotheses concerning diets and habitat use. Here, we examine element ratios (Sr/Ca, Ba/Ca) in the enamel of a suite of co-occurring vertebrate taxa sampled from a spatiotemporally constrained interval in the Oldman Formation of Alberta, Canada, to reconstruct trophic structure, and use δ13C, δ18O, and 87Sr/86Sr compositions to test for niche partitioning and habitat use among hadrosaurids, ceratopsids, and ankylosaurs. We also test previously proposed dietary hypotheses of troodontid theropods. In large ornithischians, we find Ba/Ca and Sr/Ca ratios that are consistent with herbivory, with hadrosaurs distinct from ceratopsids and ankylosaurids in their 87Sr/86Sr ranges, a pattern that is indicative of differences in habitat use/breadth, dietary plant sources, and feeding height. The sampled mammals, varanoid lizards, dromaeosaurids, and tyrannosaurids preserve a gradient of lower Sr/Ca and Ba/Ca ratios that is consistent with animal-dominant omnivorous to faunivorous diets. Troodontids, which have been variably hypothesized as either faunivorous, omnivorous, or herbivorous due to their distinct and unusual dentition, preserve Sr/Ca and Ba/Ca ratios that fall between those of the ornithischians and the dromaeosaurids. From these multi-proxy data, we interpret troodontids as mixed-feeding to plant-dominant omnivores. These proxies represent a valuable tool for understanding the trophic and community ecology of Cretaceous ecosystems and hold enormous potential for future research in paleobiology.

Morphological disparity and structural performance of the dromaeosaurid skull informs ecology and evolutionary history

“…We suspect that future work with expanded datasets will find that dromaeosaurid skull shapes reflect a mix of shared evolutionary history and functional adaptations, and that our proposed sequence of cranial evolution will be refined as the dromaeosaurid fossil record is expanded. Additional understanding of the subtleties of dromaeosaurid ecology, for instance through isotope analyses [94] or expansion of feeding trace studies [21], will also aid in calibrating the morphofunctional data to corresponding ecologies. Despite these caveats, this study demonstrates the ability of skull shape and functional metrics to discern non-avialan theropod ecology at lower taxonomic levels and identify variants of carnivorous feeding in the fossil record.…”