Freshwater stingrays from the Fossil Butte Member of the late early Eocene Green River Formation of Wyoming are reviewed, and a new genus and species of fossil stingray is described. †Asterotrygon maloneyi, n.gen., n.sp. is remarkably well preserved and is known from articulated skeletons of juveniles and adults, both males and females. It is distinguished from all Recent and fossil stingrays, including †Heliobatis radians from the same formation, by the unique presence of a dorsal fin covered with dermal denticles directly anterior to the caudal stings. Other characters that in combination distinguish the new fossil genus from all other stingrays include: retention of separate, individual vertebrae extending to the tail extremity instead of a cartilaginous rod posterior to caudal stings; dorsal surface of disc and tail covered by numerous, closely packed, minute denticles; tail relatively stout at base; and relative proportions of disc and tail. †Asterotrygon, n.gen. shares with certain stingray genera postorbital processes of neurocranium separated from a supraorbital process by a small notch in the supraorbital shelf, presence of both dorsal and ventral tail-folds posterior to caudal stings (and internally supported by rudimentary radial elements), and hyomandibulae separated from lower jaws by a gap that originally contained the hyomandibular-Meckelian ligament. A calcified angular cartilage between the hyomandibula and Meckel's cartilage is tentatively identified in †Asterotrygon, n.gen. as well. †Asterotrygon, n.gen. is unquestionably a stingray, presenting many myliobatiform synapomorphies including caudal stings on the dorsal aspect of tail, lack of jugal arches in neurocranium, a thoracolumbar synarcual cartilage posterior to scapulocoracoid, absence of thoracic ribs, and laterally expanded, shelflike postorbital processes. †Asterotrygon, n.gen. and †Heliobatis primitively retain a narrow and slightly arched puboischiadic girdle and primitively lack calcified rostral elements in adults.
A phylogenetic analysis of 23 stingray genera, two outgroups, and 44 informative morphological characters resulted in 35 equally most parsimonious trees. The strict consensus reveals the following hierarchical structure: Hexatrygon (†Asterotrygon, n.gen., Plesiobatis, Urolophidae (Urotrygonidae (†Heliobatis (Potamotrygonidae (amphi-American Himantura, Pteroplatytrygon, Himantura, Taeniura, Dasyatis (Gymnuridae Myliobatidae)))))). Our resulting tree has nodes in common with previous phylogenetic analyses of stingrays (e.g., Hexatrygon is the most basal stingray genus; gymnurids and myliobatids [pelagic stingrays] are well-supported sister-groups), but includes novel components, such as a clade that includes all dasyatid genera (as a polytomy) and the component Gymnuridae Myliobatidae. “Dasyatidae” is not monophyletic in any of the minimum-length trees obtained; Urolophidae (Urolophus and Trygonoptera) and Urotrygonidae (Urobatis and Urotrygon) are both monophyletic, but are not sister-groups. †Asterotrygon, n.gen. forms a clade with urolophids in 21 of the 35 equally most parsimonious trees. Successive approximations wei