Mussaurus (meaning "mouse lizard") is a genus of herbivorous sauropodomorph dinosaur that lived in southern Argentina during the Sinemurian stage of the Early Jurassic.

Numerous specimens of varying age found in a single locality suggest that Mussaurus is one of the earliest dinosaurs to have lived in a gregarious lifestyle.

With its possible origin from the Triassic, this complex social behaviour may have given rise to the sauropods' early success as the largest herbivores on land.

Therefore, a cladistic analysis of basal sauropodomorphs performed by Otero and Pol (2013) to test the phylogenetic relationships of Mussaurus, included information only from adult specimens.

[7] As of 2021, Mussaurus represents the earliest unequivocal evidence of complex social behaviour in dinosaurs, with over 100 eggs and skeletal specimens of 80 individuals ranging from embryos to adults found in the same locality.

Changes in the relative proportions of its body during growth (ontogeny) may have caused its centre of mass to move backwards towards its pelvis, resulting in the animal adopting a two-legged (bipedal) stance later in life.