Rotundorhynchus brittanicus was a long-snouted marine teleost known from its robust rostrum. The bill is deeper and more rounded in section than those of many other London Clay rostral fishes, and its internal canal system shows that it was a vascularised extension of the skull. Its exact family relationships remain outside a modern billfish family, so ecology should be inferred cautiously.

An active predatory lifestyle is plausible for a fish carrying such a reinforced rostrum, but whether the bill was used to strike, slash, probe or serve another function cannot be demonstrated from the fossil alone. The species is especially useful for showing that superficially similar Eocene bills were built in substantially different ways.