Pentanogmius is an extinct genus of sail-finned ray-finned fish that lived during the Late Cretaceous in what is now Europe and the United States.

The American species inhabited large areas of the Western Interior Seaway, with fossil finds indicating a range from Texas and Alabama in the south to Manitoba, Canada, in the north.

Over the years a lot of material has been referred to P. evolutus including several well preserved skeletons, making it both the most abundant and best understood members of its family.

[5] In 2009 Joseph Fritsch (unrelated to Antonin) and Kris Howe collected another specimen of Pentanogmius, put on display at the Perot Museum of Nature and Science.

While P. evolutus has a dorsal fin that gradually grows shorter the more posterior it is located, the one of P. fritschi is greatly elongated in the anterior third of the body, reaching a height of up to 75 cm.

This gives P. fritschi a hook-shaped sail, superficially similar to those seen in modern-day Marlin, Swordfish and the extinct Pachyrhizodus.

[6] Unlike Pachyrhizodus and billfish however, species of Pentanogmius possess small and weak pectoral fins located in a much more dorsal position on the body at around the same level as the orbits.

[6] The exact internal relationships between members of Pentanogmius are difficult to determine due to the fragmentary nature of the European species and the limited skeletal material of P. crieleyi.

[7] Abisaadichthys Eusebichthys Protobrama Eoplethodus Paranogmius Bananogmius Luxilites Syntegmodus Niobrara Martinichthys Pseudothryptodus Thryptodus Plethodus Pseudanogmius Moorevillia Pentanogmius Zanclites Bachea Enischnorhynchus Tselfatia Dixonanogmius The rocks, which P. fritschi was preserved in, suggest an off-shore environment from the Late Campanian to Early Turonian, a time at which the Western Interior Seaway covered a broad stretch of the modern United States.

The area covered by water and its depth during this part of the Cretaceous suggests that P. fritschi was most likely an open ocean animal with some adaptations for an active fast-swimming pelagic lifestyle (fusiform body and luniform caudal fin).