Promegalops sheppeyensis was a tarpon relative represented by an elongate neurocranium from the London Clay. Tarpons are active predatory fishes with large mouths, and the long, low cranial construction of Promegalops is compatible with a similar fish-eating or generalist carnivorous role. That ecology is inferred from its relationships because the known material does not preserve a complete feeding apparatus.

The species contributes to evidence that elopiform fishes were diverse in the early Eocene sea. Its skull is unusually flat and carries paired raised ridges along the roof, while the orbital region lacks a strongly ossified sclerotic ring.