Lessemsauridae is a clade of early sauropodiform dinosaurs that lived in the Triassic and Jurassic of Argentina, South Africa and possibly Lesotho. A phylogenetic analysis performed by Apaldetti and colleagues in 2018 recovered a new clade of sauropodiforms uniting Lessemsaurus, Antetonitrus, and Ingentia which they named Lessemsauridae. It is a node-based taxon, defined as all descendants of the most recent common ancestor of Lessemsaurus sauropoides and Antetonitrus ingenipes.[1] Depending on the definition of Sauropoda, Lessemsauridae is either one of the most basal sauropod taxa, or a sister taxon of Sauropoda. An additional member of the clade was named later in 2018, Ledumahadi.[2] A 2021 study by Pol and colleagues also assigned the genera Kholumolumo and Meroktenos to the group.[3]
Lessemsaurids were quadrupeds. Unlike later sauropods, which had columnar limbs, the forelimbs of lessemsaurids were flexed.[2] Sauropodomorph trackways have been found with widely-spaced front feet, indicating they were made by a trackmaker with flexed forelimbs as in lessemsaurids.[4]
Lessemsaurids could reach large sizes, with Lessemsaurus estimated to have reached a mass of 7 tonnes[1] and the later Ledumahadi estimated to have reached a mass of 12 tonnes.[2]
They had highly pneumatic cervical and dorsal vertebrae, very antero-posteriorly short but tall cervical vertebrae, robust cervicals, a very expanded distal scapula blade, and upright arms.[1][2]
Lessemsaurids first appeared during the middle Norian age of the Triassic. A specimen of an unnamed lessemsaurid from the Elliot Formation of South Africa probably dates to approximately 218 million years ago.[5] Lessemsaurus is a member of the La Esquina Local Fauna,[3] which probably dates to at least 213 million years ago.[6] Antetonitrus and Ledumahadi are found in the Upper Elliot Formation, which dates to the Hettangian and Sinemurian ages of the Jurassic.[7] Lessemsaurids are known from South Africa and Argentina.