Lightning and the thunder: insular dwarfism inferred from long bone histology of the titanosaurian Atsinganosaurus velauciensis

Titanosaurian sauropods include the largest land animals that ever walked on Earth. However, some of them evolved into dwarfed species, linked to their insular habitats. Here, we report on the long bone histology of several mature individuals of the small-sized titanosaur Atsinganosaurus velauciensis from the Upper Cretaceous of Velaux – La Bastide Neuve (Provence, South-Eastern France). The completely remodelled H bone tissue type in all specimens characterizes mature and fully grown individuals. Together with the extensive bone remodelling, the samples range from HOS (Histological Ontogenetic Stages) 14 and from RS (Remodeling Stages) 13 to 14. Considering the reduced size of the sampled femur and humeri, the remodelling process would have begun early in the ontogeny of this titanosaur compared to non-titanosaurian sauropods, at a rate that surpassed the apposition rate. Thus, size reduction of A. velauciensis has to be taken into account to explain the advanced state of its long bone histology. Insular dwarfism is a consistent hypothesis for this combination of features and has been proposed for a series of other titanosaurs from the European archipelago (e.g. the Romanian Magyarosaurus dacus and the Spanish Lirainosaurus astibiae) that show comparable long bone histology and inferred body size.