Rare occurrences of dinosaurian embryos are punctuated by even rarer preservation of their development. Here we report on dental development in multiple embryos of the Early Jurassic Lufengosaurus from China, and compare these to patterns in a hatchling and adults. Histology and CT data show that dental formation and development occurred early in ontogeny, with several cycles of tooth development without root resorption occurring within a common crypt prior to hatching. This differs from the condition in hatchling and adult teeth of Lufengosaurus, and is reminiscent of the complex dentitions of some adult sauropods, suggesting that their derived dental systems likely evolved through paedomorphosis. Ontogenetic changes in successive generations of embryonic teeth of Lufengosaurus suggest that the pencil-like teeth in many sauropods also evolved via paedomorphosis, providing a mechanism for the convergent evolution of small, structurally simple teeth in giant diplodocoids and titanosaurids. Therefore, such developmental perturbations, more commonly associated with small vertebrates, were likely also essential events in sauropod evolution., Dinosaurs had some of the most complex dentitions known. Here, Reisz et al. characterize dental development across embryonic, hatchling and adult Lufengosaurus, an Early Jurassic sauropodomorph dinosaur, and suggest that derived sauropod dentition evolved by paedomorphosis (juvenilization).