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

Sauropod dinosaurs are one of the most common components of the Cretaceous continental fauna of South America. However, juvenile specimens are poorly documented to date. Here, we describe an incomplete juvenile titanosaur from the Upper Cretaceous of Argentina. The preserved material belongs to a single individual and is sufficiently diagnostic to classify it as a juvenile Neuquensaurus australis. The specimen represents an individual approximately 44–63% of adult individual size. Anatomical comparison with adult specimens reveals that fusion between different structures (neural arches and their centra, scapula and coracoid, and sacral centra) started late during the ontogeny (at least in ontogenetic stages more advanced than inferred for the juvenile individual here studied). Invasion of pneumatic diverticula within the bones also appears to be developed in a more advanced ontogenetic stage. Overlapping elements between the juvenile and adult skeletons indicate allometric changes for Neuquensaurus australis throughout ontogeny, which contrast with previous hypotheses about isometric growth in titanosaurs. Histological data reveals a rapid growth rate in the juvenile individual. However, the presence of at least one cyclical growth mark in the cortex challenges previous hypotheses regarding growth dynamics in sauropod dinosaurs, which postulate that interrupted growth only occurred at advanced ontogenetic stages.