patagonicus specimens (in addition to the 11 individuals discovered decades ago 15, 17). Our recent expeditions to the type locality have yielded 69 new M. Abbreviations: mc: medullary cavity so: secondary osteons. Gonzalez) represent the size of the different specimens representing the various ontogenetic stages of M. Arrowheads indicate the position of lines of arrested growth. patagonicus ( l–o), cortical bone histology of four different sized specimens of M. patagonicus (anterodorsal process of dentary) ( e), 3D reconstruction of neonate 21 MACN-PV 4111 ( f) 3D reconstruction of skull anatomy of neonate MACN-PV 4111 ( g), associated skeletons of one-year old juveniles MPM-PV 1813 ( h), 3D reconstruction of juvenile MPM-PV 1813/4 ( i), associated skeletons of two adult individuals MPM-PV 1868 and MPM-PV 1869 ( j), skull of adult individual MPM-PV 1868 ( k), Skeletal reconstruction of the different growth stages of M. ( a) nest with eggs MPM-PV 1887 ( b), isolated egg MPM-PV 1875 ( c) 3D reconstruction of embryo within egg MPM-PV 1879 ( d), right dentary of embryonic remains MPM-PV 1879 showing autapomorphic traits of M. Specimens of Mussaurus patagonicus collected from the Laguna Colorada Formation. Later the anatomy of juvenile skulls was described 16, and more recently five incomplete adult specimens have been described and identified as M. 2) associated with two partially preserved eggs. The sauropodomorph Mussaurus patagonicus was originally described 15 from here based on several well-preserved post-hatchling specimens (Fig. 1) that provides new information about the social behaviour of early sauropodomorph dinosaurs. The Laguna Colorada Formation 14 of southern Patagonia (Santa Cruz Province, Argentina) contains an exceptional fossil locality (Fig. Behaviour has not been regarded as playing a role in the early success of these dinosaurs due to the scarcity of relevant information. Proposed reasons for their early success 3, 9 include their ability to opportunistically adapt to niches left empty after the extinction of other herbivores 1 or to out-perform their competition with superior high-browsing herbivory, large body size, and rapid growth rates 4, 9, 10, 11, 12, 13. The predominance of early sauropodomorph dinosaurs in terrestrial ecosystems extended for almost 40 million years 7 (ca. 200 Ma), which left them as the only large herbivores in terrestrial ecosystems of the Early Jurassic 4, 6. Sauropodomorphs were subsequently unaffected by the Triassic–Jurassic extinction event (ca. By the end of the Triassic, sauropodomorphs had replaced other herbivores (therapsids and other archosaurs) and were the most abundant tetrapods in many terrestrial ecosystems 1, 4, 6. Soon after dinosaurs originated, early sauropodomorphs (forerunners of the gigantic quadrupedal sauropods) underwent a remarkable adaptive radiation landmarked by the acquisition of herbivory 1, 2, 3, 4, large body sizes 5, and high taxonomic diversity and specimen abundance 2, 4, 6. The presence of sociality in different sauropodomorph lineages suggests a possible Triassic origin of this behaviour, which may have influenced their early success as large terrestrial herbivores. These findings provide the earliest evidence of complex social behaviour in Dinosauria, predating previous records by at least 40 My. Our new discoveries indicate the presence of social cohesion throughout life and age-segregation within a herd structure, in addition to colonial nesting behaviour. Most specimens were found in a restricted area and stratigraphic interval, with some articulated skeletons grouped in clusters of individuals of approximately the same age. Here we report an exceptional fossil occurrence from Patagonia that includes over 100 eggs and skeletal specimens of 80 individuals of the early sauropodomorph Mussaurus patagonicus, ranging from embryos to fully-grown adults, with an Early Jurassic age as determined by high-precision U–Pb zircon geochronology. For instance, knowledge on their behaviour is limited, although herding in sauropodomorphs has been well documented in derived sauropods from the Late Jurassic and Cretaceous.
Sauropodomorph dinosaurs dominated the herbivorous niches during the first 40 million years of dinosaur history (Late Triassic–Early Jurassic), yet palaeobiological factors that influenced their evolutionary success are not fully understood.