Diandongosaurus acutidentatus Shang, Wu & Li, 2011 is restudied on the basis of a new specimen. The new specimen provides further information on the palate of the skull, the shoulder girdle, the gastralia and hind limbs, and permits revision of the diagnosis. Newly added or modified diagnostic characters include the presence of a few small vomerine teeth, the presence of an ectopterygoid, lateral elements of gastralia distally swollen and curving dorsally, boomerang-shaped interclavicle and clavicle with a distinct anterolateral process. Slight variations in the vertebral counts, girdle morphology, and phalangeal formulae are attributed to the intraspecific variations. A revised phylogenetic analysis demonstrates that Diandongosaurus is closely related to the Nothosauroidea and that certain Chinese taxa (e.g. Keichousaurus) previously affiliated with the Pachypleurosauridae are more closely related to the Nothosauroidea. The effect of inclusion or exclusion of fragmentary taxa in the phylogenetic analysis is observed in the branching pattern of the cladogram, but it has little effect on the morphological characterization of the major clades.

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