Belonostomus (from belos, 'dart' and stóma 'mouth') is a genus of prehistoric ray-finned fish that was described by Louis Agassiz in 1844. It is a member of the order Aspidorhynchiformes, a group of fish known for their distinctive elongated rostrums. Fossils of Belonostomus have been found worldwide in marine deposits, although some species are known from freshwater habitats. The oldest known species are from the Upper Jurassic of Germany and England, with the youngest known species from the late Maastrichtian. One specimen has been recovered from the late Paleocene of North Dakota, which would suggest that this genus was the only aspidorhynchean to survive into the Cenozoic, although it is possible this fossil may have been reworked from earlier formations. It likely consumed plankton or other small fish, though one specimen from the Late Jurassic of Germany was found with the rhynchocephalian Homoeosaurus as stomach contents.

Taxonomy

Known Belonostomus species are: Indeterminate remains are known from worldwide, including the Gulf Coast of the United States (Alabama, Arkansas, and Texas), Australia, Belgium, Lebanon, and Russia. The former species B. acutus Agassiz, 1844 (syn: B. tenellus Agassiz ex Egerton 1841 ) and B. anningiae Agassiz, 1843 from the early-mid Jurassic of England are now thought to be species of the unrelated saurichthyiform Saurorhynchus. The former species B. carinatus Mawson and Woodward, 1907 from the Hauterivian of Brazil is now thought to be a stem-gar in the family Obaichthyidae. The former species B. microcephalus Winkler, 1862 from the Tithonian of Germany is now thought to be a junior synonym of Aspidorhynchus acustirostris, while B. flexuosus Philips, 1871 is one of A. crassus. The former species B. sweeti Etheridge & Woodward, 1892 is now placed in Richmondichthys. The former species B. pygmaeus Winkler, 1874 from the Tithonian of Germany is thought to be an immature specimen of one of the other Solnhofen Belonostomus species.