Acestrus ornatus was a long-snouted predatory teleost from the early Eocene London Clay sea. Its cranial construction includes an elongate, flattened rostrum and a well-developed eye region, giving it a strongly specialised head unlike that of a generalized coastal fish. The rostrum and streamlined billfish-like organization indicate an active open-water animal, although the exact shape of the complete body is not known from London Clay material.

The feeding behaviour of Acestrus cannot be observed directly, but its long rostrum and predatory affinities make pursuit of fishes and other mobile prey a reasonable interpretation. London Clay specimens are known principally from three-dimensionally preserved cranial and rostral material, so most of what can be said confidently about the animal comes from the head rather than the postcranial skeleton.