Everything, everywhere, all at once? Trends in Diplodocoidea diversity during the Jurassic/Cretaceous transition

Estimating diversity patterns from the fossil record is essential for understanding the factors that shape biodiversity. The Jurassic/Cretaceous transition 143.1 million years (Ma) ago is a particular interval of pronounced ecological turnover accompanied by a global sharp diversity decline, disproportionately affecting larger and more specialised terrestrial taxa such as Sauropoda. Sauropods are a diverse group of large herbivorous dinosaurs, considered a good model for studying long-term biodiversity trends because of their prominent ecological roles and abundant fossil record. In this study, we analyse paleodiversity patterns for Diplodocoidea as a whole and for each of its subclades. Diplodocoidea is a subgroup of neosauropods that includes three main subclades: Dicraeosauridae, Diplodocidae, and Rebbachisauridae. The Flagellicaudata (= Dicraeosauridae + Diplodocidae) were notably affected by the Jurassic/Cretaceous faunal turnover, whereas Rebbachisauridae originated and persisted into the Early to mid-Cretaceous. Our key findings are: (1) high diversity during the Late Jurassic; (2) a sharp decline in diversity just before or during the Jurassic/Cretaceous transition; (3) a diversity peak in the Early Cretaceous Aptian stage; (4) overall higher Diplodocoidea diversity in the Early Cretaceous compared to the Late Jurassic; and (5) distinct geographic distributions, with Flagellicaudata restricted to high latitudes, while Rebbachisauridae occupied the full latitudinal range of Diplodocoidea. Our results suggest that the observed paleodiversity patterns were likely influenced by abiotic factors, and that Flagellicaudata and Rebbachisauridae may have been subject to distinct climatic constraints, potentially reflecting different ecological strategies. Additionally, this study underscores the impact of sampling biases on diversity estimates and challenges previous assumptions about sauropod diversity patterns, opening new directions for future research.