Acanthomorph teleosts (spiny-rayed fishes) account for approximately a third of extant vertebrate species. They appeared during the Late Cretaceous and have been a major component of aquatic biodiversity since the early Cenozoic. They occupy today most trophic levels and ecological niches in aquatic environments, however very little is known about those that were adopted by the earliest representatives of the group. Here, we report on an exceptional glimpse into the ecological diversity of early spiny-rayed fishes provided by the unusual preservation of a newly discovered specimen of the freshwater acanthomorph Spinocaudichthys from the Upper Cretaceous of Morocco. A combination of major-to-trace elemental mapping methods reveals that the gross morphology of the specimen's intestine has been remarkably preserved owing to the rapid mineralization of iron hydroxides around it. Differing with the typically short and straight intestinal tract of carnivorous teleosts, the intestine in Spinocaudichthys is long and highly convoluted, indicating a probable herbivorous diet. Acanthomorphs would therefore have conquered various ecological niches in their early evolutionary history, prior to their subsequent phylogenetic diversification in both marine and freshwater environments that followed the K-Pg extinction event.