Protargentinolithus extenuatus was an argentinid-grade fish known from London Clay sagittal otoliths. Argentines are generally small to medium marine fishes, many living in deeper shelf waters and feeding on zooplankton and small nekton. A comparable mid-water or near-bottom feeding ecology is plausible for this Eocene taxon, but its body and jaws are not preserved in the London Clay record.

Its otoliths are relatively large and robust, with a short rostrum and compact anterior sulcus. The species is important because later revision showed that the older generic assignment did not accurately reflect its relationships, illustrating how otolith taxonomy can change as comparative collections improve.