Gymnosarda prisca was a tuna-like scombrid known from the caudal skeleton rather than the skull. Its identification rests on the structure of the hypural plates that supported the tail fin. This makes the species unusual among London Clay fishes, many of which are diagnosed from three-dimensional crania.

The preserved tail region indicates a powerful caudal apparatus consistent with an active pelagic swimmer. A fast, predatory lifestyle is a reasonable inference from its scombrid relationships, but jaw structure and diet are not directly known for this species. The fossil is important because it records an early branch of the bonito–tuna radiation through one of the body regions most closely tied to propulsion.