An exquisitely preserved ictalurid catfish from the early Oligocene Brule Formation, South Dakota, U.S.A.

Well-preserved catfish remains from the early Oligocene Poleslide Member of the Brule Formation, South Dakota, represent a new, extinct species of Ictalurus (family Ictaluridae). The type series includes numerous nearly complete articulated skeletons with exceptional bone preservation. †Ictalurus geraldsmithi, sp. nov. exhibits a combination of apomorphic, cranial-muscle attachment sites, some of which characterize all living Ictaluridae and others that more specifically characterize the genus Ictalurus. Within the genus Ictalurus, two subgenera are formally established here. The first is the nominotypical Ictalurus (Ictalurus), informally known as the ‘channel catfish group,’ with at least eight extant and five extinct species including the new species described here. The second subgenus is Ictalurus (Istlarius), informally known as the ‘blue catfish group,’ with at least three extant and two extinct species. Relative to its small body size (approximate standard length is 16–20 cm), †I. geraldsmithi, is unique among the species of Ictalurus in showing an accelerated development of hypertrophied jaw adductor and epaxial muscle attachment sites on the skull roof. Also, the new species uniquely differs from all congeners by its low total vertebral count of 37–38 including the anteriormost five united in the Weberian complex. The 33.3–30.8 Ma age of the Poleslide Member of the Brule Formation establishes a minimum early Oligocene age for the distinctive musculo-skeletal synapomorphies that diagnose the genus Ictalurus. The few older, late Eocene fossils placed in Ictalurus are incompletely preserved and uninformative on many key cranial musculo-skeletal details.