A juvenile specimen of Neuquensaurus australis (Dinosauria: Titanosauria) from the Upper Cretaceous of Argentina: implications for sauropod ontogeny and growth dynamics

Abstract

Sauropod dinosaurs are one of the most common components of the Cretaceous continentalfauna of South America. However, juvenile specimens are poorly documented to date. Here, wedescribe an incomplete juvenile titanosaur from the Upper Cretaceous of Argentina. The preservedmaterial belongs to a single individual and is sufficiently diagnostic to classify it as a juvenileNeuquensaurus australis. The specimen represents an individual approximately 44–63% of adultindividual size. Anatomical comparison with adult specimens reveals that fusion between differentstructures (neural arches and their centra, scapula and coracoid, and sacral centra) started lateduring the ontogeny (at least in ontogenetic stages more advanced than inferred for the juvenileindividual here studied). Invasion of pneumatic diverticula within the bones also appears to bedeveloped in a more advanced ontogenetic stage. Overlapping elements between the juvenile andadult skeletons indicate allometric changes for Neuquensaurus australis throughout ontogeny,which contrast with previous hypotheses about isometric growth in titanosaurs. Histologicaldata reveals a rapid growth rate in the juvenile individual. However, the presence of at least onecyclical growth mark in the cortex challenges previous hypotheses regarding growth dynamics insauropod dinosaurs, which postulate that interrupted growth only occurred at advanced ontogeneticstages.