Siler, CameronWilbert, Greg2021-05-252021-05-252021-05-14https://hdl.handle.net/11244/329849The Ogallala Formation is an accumulation of fluvial and lacustrine sediments deposited in the high plains of North America during the middle Miocene to early Pliocene. Most famous for its highly diverse mammalian fauna, the Ogallala Formation also represents an important record for aquatic taxa as well, including in Oklahoma Atractosteus, ?Esox, Ictalurus, Micropterus, ?Aplodinotus, Alligator, turtles, mollusks, freshwater sponges, and diatoms. Here, I describe a new species of buffalo fish (genus Ictiobus) based on a fossil from the Late Miocene (Clarendonian) of western Oklahoma. This specimen was found in a diatomite derived from a brackish lake (due to underlying Permian salt beds), discovered in approximate articulation, upside down, and lacking the caudal half of the postcranial skeleton. The skull preserves roughly 28 elements, along with remnants of the dorsal spines, fin rays, and multiple cycloid scales covering portions of the skeleton. The opercular series on each side lay open, with the dorsal skull bones (frontal, parietal, and supraoccipital) remaining in full articulation. The dentary, premaxilla, and other rostral skull bones are deflected laterally and preserved next to elements of the skull roof. Generally, the bones are well-preserved except where a dense root mass plane has intersected the skeleton and top of the skull. The genus Ictiobus is represented by five extant species: I. bubalus, I. cyprinellus, I. labiosus, I. meridionalis, and I. niger, ranging from Central America to Canada, as well as one Pliocene species, I. aguilerai from Hidalgo, Mexico. The new Oklahoma species can be differentiated from the living and fossil species based on its serrated cleithrum, the large shelf on the anterior end of the gnathic ramus, and the sharp angular interior margin of the preopercle. The addition of this new species adds valuable data for the early evolution of the genus, including potentially important implications for the biogeography of living Ictiobus. I. phyllisae n. sp. is the oldest known species of Ictiobus; it is most closely related to fossil I. aguilerai; the two form a clade with extant I. bubalus. Given the Clarendonian age (about 12 Ma) of I. phyllisae n. sp., initial cladogenesis within Ictiobus began well back in the Miocene.IctiobusCypriniformesCatostomidaeMiocene