Xiphiorhynchus priscus was a large long-snouted predatory teleost and the best-known member of an extinct Eocene billfish-like family. Both upper and lower jaws were extended into a long rostral apparatus, producing a very different skull from the single-bill arrangement of living swordfish. Fine teeth along the elongated jaws indicate that the rostrum remained part of the feeding surface rather than becoming completely toothless.

The long head and predatory dentition suggest an active marine fish pursuing smaller nekton. Exactly how the rostrum was used is unknown, so comparisons with modern billfish striking behaviour remain speculative. London Clay skull and rostral remains show that highly specialised long-jawed predators were already established in early Eocene pelagic ecosystems.