Embryonic dinosaur remains are extremely rare1,2 and with the exception of the specimens described here, are so fragmentary that they have yielded little or no information useful for palaeobiological interpretation. We report here the first well-documented association of dinosaur eggs containing embryos, juveniles and adults from the fossil record. The material, attributable to a new hypsilophodontid, Orodromeus makelai, n. gen., n. sp., and the hadrosaurid Maiasaura peeblesorum, comes from the Upper Cretaceous Two Medicine Formation of western Montana3. Osteo-logical and histological studies of these two animals indicate structural and morphological adaptations for their respective growth patterns and neonate behaviour.