Orthopristis rectangulus was a haemulid, or grunt relative, known from London Clay otoliths. Living grunts are mostly coastal or shelf fishes that feed on benthic invertebrates and small fishes, often moving between structured habitats and open sediment. A similar trophic role is plausible for the Eocene species, but its London Clay record consists of ear stones rather than jaws or stomach contents.

The otolith preserves the characteristic haemulid organization of a broad anterior ostium and a more sharply bent posterior cauda. This inner-ear evidence shows that grunt-like percomorphs were already present in the warm early Eocene seas of Britain.