AN EOCENE SPECIES OF HIODON FROM MONTANA, ITS PHYLOGENETIC-RELATIONSHIPS, AND THE EVOLUTION OF THE POSTCRANIAL SKELETON IN THE HIODONTIDAE (TELEOSTEI)

Hiodon is a Recent genus of fishes endemic to the fresh waters of Nort h America; it is represented by two species: Hiodon tergisus Lesueur a nd H. alosoides (Rafinesque). Two specimens of osteoglossomorph collec ted from the Eocene-Oligocene Kishenehn Formation of northwestern Mont ana represent a new fossil species that is more closely related to the Recent Hiodon than it is to the Eocene dagger Eohiodon. The close rel ationship of this new species to Hiodon is supported by derived charac ters of the nasal, the opercular series, the jaws, the basihyal tooth plate, the infraorbitals, and the caudal skeleton. Comparative study o f the postcranial skeleton of Recent and fossil hiodontids illustrates several stages in the evolution of the Hiodontidae: 1) origin of seve n-rayed pelvic fins by the late Early Cretaceous (evidenced by dagger Yanbiania); 2) development of an anal fin larger than the dorsal fin a nd supported by 16 or more anal pterygiophores at least as early as th e early Late Cretaceous (evidenced by dagger Plesiolycoptera); 3) evol ution of a falcate dorsal fin and a sexually dimorphic anal fin at lea st as early as the Eocene (evidenced by dagger Eohiodon); and 4) incre ase in number of vertebrae accompanied by changes in the caudal skelet on as seen in the new fossil species and in Recent species of Hiodon.