Rhinocephalus planiceps was a hake relative with a long, low head and rows of fine recurved teeth. The jaw construction is well suited to grasping small fishes and other soft-bodied prey, matching the broadly predatory ecology of modern hakes. That feeding interpretation is supported by the teeth, although the depth range and precise hunting behaviour of the Eocene species are not directly known.

Three-dimensional London Clay skulls show an elongate merlucciid-like cranium without a strongly ossified eye ring. The species adds a gadiform predator to a fauna otherwise dominated by many unrelated acanthomorph lineages.