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.
The description of S. gracilis highlights its unique features: its dental formula, snout shape, absence of symphyseal fusion, smaller olfactory bulb size, and larger visual cortex compared to other members of the Nothacanthinae family.