Stable isotope record of Triceratops from a mass accumulation (Lance Formation, Wyoming, USA) provides insights into Triceratops behaviour and ecology

Our understanding of Late Cretaceous dinosaur ecosystems from North America has considerably improved through stable isotope analyses on fossil bones and teeth. Oxygen and carbon stable isotopic compositions of structurally-bound carbonate in these fossil apatites are commonly used to infer variations of ingested water and food sources, which are in turn related to environmental and climatic conditions. Incremental isotopic records potentially provide insights into seasonality and migratory behaviour. So far, these reconstructions are based on vertebrate remains from spatiotemporally diverse datasets. Here, we present oxygen (818O) and carbon (813C) isotopic records from a large, spatially and temporally well-constrained, Triceratops bonebed from the Upper Maastrichtian Lance Formation (eastern Wyoming. USA). These isotopic compositions allow to elucidate the palaeoecology of these large herbivores and their ecosystem in detail, as well as their habitat use, diet and possible migration. The 818O signature from incrementally sampled Triceratops teeth reveal relatively low intra-tooth variation (average 1.3 %o), comparable to contemporaneous dinosaur species as well as modern herbivo-rous mammals. Average 813C values (-5.4 %o) are somewhat higher than for modern C3 plant grazers, and hint towards complex interactions during carbon uptake by non-avian herbivorous dinosaurs. Calculated 818O of drinking water (-14.8 %o) combined with the local sedimentology of fine-grained siliciclastic deposits with high total organic and low carbonate contents strongly suggest a freshwater environment. Additionally, the combined average 818O and 813C Triceratops isotope signatures indicate a living environment intermediate between inland forests and coastal floodplains, expanding on earlier theories of ornithischian niche partitioning. Our robust dataset provides meaningful tests of habitat and palaeoecological hypotheses for Triceratops, and highlights the application of spatiotemporally well-constrained fossil remains.