Eothynnus salmoneus was a carangid, or jack relative, represented by three-dimensional skulls from the London Clay. Its cranial construction includes a large eye ring and an exceptionally high median keel along the skull roof. Those features suggest a visually oriented, active marine fish, broadly comparable in locomotor ecology with modern jacks, although the full body and swimming performance are not preserved.

Carangids are typically predators or opportunistic feeders on smaller fishes and invertebrates, and a carnivorous role is a reasonable inference for Eothynnus. The species is particularly useful for showing how diverse early carangiform skull shapes had become by the Ypresian.