A New Petalodont Chondrichthyan from the Bear Gulch Limestone of Montana, USA, with Reassessment of Netsepoye hawesi and Comments on the Morphology of Holomorphic Petalodonts

A new holomorphic petalodont from the Bear Gulch Limestone, Obruchevodus griffithi, is described and features of the related Netsepoye hawesi are reinterpreted. Comparison of these taxa with the holomorphic petalodonts Janassa bituminosa, Belantsea montana, and Siksika ottae provide insight into petalodont anatomical form and variation. All holomorphic material supports holostyly and nested, subcranial branchial arches. Teeth occur in families with linguo-labial replacement but alignment of most families relative to the jaw ramus results in a staggered tooth alignment between adjacent tooth families. Symphysial teeth are the exception to this pattern. Tooth retention is clearly indicated only in the homodont Janassa bituminosa. In contrast there is no evidence of tooth retention in examined petalodonts with a heterodont dentition. There is variation in styles of squamation among all forms; sexual dimorphism in scale development and distribution are suggested as well. Despite depressiform and compressiform conditions there is commonality in possession of a lobular, anteriorly extended and high aspect ratio pectoral fin, large pelvic fins, and absence of fin spines. Pectoral fins provide the primary means of locomotion. Variation in the ventral aspect of the pelvic girdle anatomy exists between that interpreted for Janassa and that documented by Obruchevodus and Netsepoye. Male claspers are long and highly flexible. Laterally compressed body forms display two dorsal fins (first being largest) that are variable in their vertical expanse along the length of the fin. Cranial anatomy shows some correspondence to general features of the Holocephali. Meckel's cartilage is dorso-ventrally deep, as in extant chimaeroids, with contralaterals in symphysial fusion. Mandibular labial cartilages, when present, may be sexually dimorphic.