The Late Triassic—Early Jurassic Elliot Formation of South Africa is one of the most important geological formations worldwide for understanding the early evolution of sauropodomorph dinosaurs. However, many of the taxa currently recognized as valid within its lower strata remain either poorly understood, vaguely diagnosed, or both. The recent discovery of an articulated partial skeleton of a single individual of the enigmatic lower Elliot genus Eucnemesaurus provides an important opportunity to expand our understanding of the anatomy and phylogeny of this poorly known taxon. A comprehensive investigation of the morphological relationships of this new specimen identified key features, pertaining primarily to the femoral shaft and distal tibia, which distinguish it from the only other previously named species of Eucnemesaurus—E. fortis. A new species, E. entaxonis, is erected within which to accommodate it. A cladistic analysis confirms the monophyly of Eucnemesaurus, as well as its continued inclusion within the low-diversity ‘Riojasauridae.’ Nonetheless, this result highlights continued uncertainties regarding the constituency of the Riojasaurus hypodigm. The relatively robust pedal architecture of E. entaxonis suggests an unexpectedly early experiment in a slower, subgraviportal form of locomotion within Late Triassic basal Massopoda, whereas the intriguing mosaic of plesiomorphic and derived characters evident in E. entaxonis raises questions regarding the hypothesized population dynamics of the basal-most sauropodomorph taxa of the lower Elliot Formation. This latter concern has particular bearing on newly observed inconsistencies in the prevailing hypodigms of other lower Elliot basal sauropodomorph taxa such as Melanorosaurus.
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