Santaisaurus yuani (Koh 1940) doesn’t have much presence out there. It’s a small crushed fossil from the Early Triassic of China. Li (1989) nested it with Paliguana at the base of the lepidosauriformes. The large reptile tree agrees with this nesting.
Lucas (2002) considered Santaisaurus a possible eolacertilian and notes it is now represented by three incomplete skeletons. Lucas lists traits that agree with a procolophonid assignment, but notes tooth differences.
Palaeocritti nests Santaisaurus tentatively within the Procolophonidae. However, Santiasaurus has small, subpleurodont teeth, not the the acrodont teth of procolophonids. In this way it presages the pleurodont teeth of sphenodontia, which are derived and closely related through Gephyrosaurus. Fairly large limbs like these (Fig. 1) are also found in the related Palaegama and Saurosternon, two likely arboreal reptiles.
There’s a larger issue
As reported earlier, the traditional Procolophonoidea appears to have an upside-down tree with Owenetta primitive and Procolophon derived, just the opposite of that found in the large reptile tree where Procolophon nests with the primitive Diadectes while the essentially unrelated Owenetta nests with derived lepidosauriformes, like Santaisaurus and Paliguana. Several non-procolophonids nest inbetween.
Koh T-P 1940. Santaisaurus yuani gen. et sp. nov., ein neues Reptil aus der unteren Trias von China. Bulletin of the Geological Society of China 20(1):73-92.
Li J-L1989. A New Genus of Procolophonidae from Lower Triassic of Shaanxi, China. Vertebrata PalAsiatica 27(4):248-267.
Lucas S 2002. Chinese Fossil Vertebrates. Columbia University Press.