Grammonus argutus is an otolith-based bythitid, or brotula relative, from the London Clay. Living bythitids are generally secretive bottom-associated fishes, many inhabiting cavities, reefs or deeper marine settings. A demersal lifestyle is therefore plausible for the Eocene animal, but the assignment of the fossil otoliths to the modern genus Grammonus is itself cautious and the complete body is unknown.

The sagittal otolith is elongate with a broad sulcus and fairly symmetrical dorsal and ventral profiles. These ear stones document the presence of bythitid-grade fishes in the Ypresian sea even though skeletal remains attributable to the species have not been recognised.