Protarpon oblongus was an elopid relative known from the posterior part of the neurocranium. The surviving skull roof is elongate and carries paired ridges that diverge toward the rear. Its placement among tarpon- and ladyfish-like elopiforms suggests an active marine fish and a broadly predatory feeding ecology, but the jaws and complete body are not known well enough to reconstruct prey capture directly.

The species is therefore anatomically informative but ecologically incomplete. Its fossil shows how useful even the posterior braincase can be in separating early elopiform lineages, while also setting a clear limit on what should be inferred about the living animal.