Marinavis longirostris was a large early Eocene seabird known from parts of a long, narrow, toothless bill with a strongly down-curved terminal hook. The bill is strikingly frigatebird-like, but the available cranial material does not securely place the species within a living bird order; Marinavidae is therefore best treated as an uncertain early marine-bird lineage. No unequivocally associated postcranial skeleton is known, so its wing proportions, flight adaptations, swimming ability and terrestrial locomotion cannot yet be reconstructed confidently.

The hooked bill suggests a surface-feeding marine predator or opportunist capable of seizing slippery prey. Fish, cephalopods and other soft-bodied marine animals are plausible foods by comparison with large hook-billed seabirds, although direct dietary evidence is absent. The species is evolutionarily important because it documents a specialised large seabird early in the Eocene radiation of modern-type waterbirds.