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Abstract
The Petalodontiformes are Upper Paleozoic marine euchondrocephalan chondrichthyans known primarily from isolated teeth. Few dentitions have been reported, among them that of the Permian Janassa bituminosa and "Janassa" korni and the Serpukhovian, Mississippian Belantsea montana, Obruchevodus griffithi, Netsepoye hawesi and Siksika ottae. A dentition has been reconstructed for the Pennsylvanian Petalodus ohioensis. New information on dentitions and postcranial morphology is now introduced for petalodont remains from the Bear Gulch Limestone of Montana (USA). These include N. hawesi, Petalorhynchus beargulchensis, several specimens bearing teeth of the Fissodus form, and a new genus and species, Fissodopsis robustus. Analysis of all material reveals dignathic heterodonty as prevalent among the Upper Mississippian petalodonts, with distinct regionalization resulting in large anterior and small lateral teeth. Fissodus-style teeth are found as the lower median tooth position of most heterodont dentitions, with a Ctenoptychius-like upper anterior tooth or teeth, accompanied by lateral upper and lower teeth of Janassa-like form. As heterodonty seems to have been the plesiomorphic condition not only in these petalodonts but for the Euchondrocephali as well, a model is proposed to explain the derived homodont conditions found in Petalodus (Hansen, 1985), Polyrhizodus (Lund, 1983), Belantsea (Lund, 1989), and J. bituminosa (Jaekel, 1899). The principally dentition-based cladistic analysis retains the previously erected families Petalodontidae, Belantseidae, and Janassidae, rejects the assignment of "J." korni to Janassa, and suggests a heterodont clade for the remaining taxa. Two new families are designated on the basis of these data, the Petalorhynchidae and the Obruchevodidae.