Abstract

Sauropod dinosaurs exhibit an ontogenetic size variation exceeding that for other vertebrates, but very young individuals are required to clarify mechanisms underlying this pattern. A new specimen of Rapetosaurus krausei sheds light on the life of one of the littlest, largest vertebrates. Bones record rapid growth rates as well as the first hatchling lines in a non-avian dinosaur, indicating that Rapetosaurus was only ~2.5 kg at hatching, and obtained a mass of ~40 kg and was ~ 35 cm tall at the hip when it died. Cortical remodeling, limb isometry, and calcified hypertrophic metaphyseal cartilages indicate an active, precocial growth strategy for this fledgling. Characteristically thin cartilage may also signal a temporary reduction in bone elongation in response to starvation within the drought-stressed Cretaceous ecosystem in which Rapetosaurus perished.