S. gracilis had a dental formula of  and had a relatively short snout, with rounded frontal bone as compared to other nothactines. This species lacked symphyseal fusion and this species of primate had comparatively reduced olfactory bulbs and a more expanded visual cortex.
S. gracilis exhibited a dental formula of , a short snout with a rounded frontal bone, lack of symphyseal fusion, reduced olfactory bulbs, and an expanded visual cortex in comparison to other nothactine primates.