FRESHWATER STINGRAYS OF THE GREEN RIVER FORMATION OF WYOMING (EARLY EOCENE), WITH THE DESCRIPTION OF A NEW GENUS AND SPECIES AND AN ANALYSIS OF ITS PHYLOGENETIC RELATIONSHIPS (CHONDRICHTHYES: MYLIOBATIFORMES)

Abstract 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 weighting adds only one additional node in relation to the strict consensus, which unites Pteroplatytrygon, Dasyatis, and Himantura sensu stricto (in a polytomy) with Gymnuridae + Myliobatidae. The resulting stingray phylogeny is at odds with previous phylogenies mostly regarding the affinities of amphi-American Himantura and Taeniura, which do not form a monophyletic group with the South American freshwater stingrays (Potamotrygonidae) in any of the minimum-length trees obtained. Similar to most elasmobranch groups, stingrays display much character conflict, and cladogram topologies are very sensitive to changes in character coding. Due to a high degree of character variation present in certain generic-level terminal taxa, a more fully representative species-level phylogeny is necessary to clarify the systematic importance of tail-fold configuration, ceratobranchial fusion patterns, and other characters discussed in our study. Three additional synapomorphies of stingrays were uncovered by our study, pertaining to the configuration of the basihyal, first pair of hypobranchial cartilages, and to the forward extension of the basibranchial copula. Our phylogenetic results imply the following biogeographic patterns: the relationships of †Asterotrygon, n.gen. demonstrate a strong Indo-west Pacific historical correlation, while †Heliobatis displays an affinity with the Americas; the node containing the greatest diversity of modern stingrays (“Dasyatidae” + (Gymnuridae + Myliobatidae)) evolved only after an American stingray lineage was established sometime earlier than the early Eocene; and potamotrygonids date at least from the late early Eocene, and not the Miocene, as previous studies have implied. The mechanism responsible for the invasion of the potamotrygonid ancestor into South America could indeed have been a marine transgression as advocated by other authors, albeit a much earlier (pre-Miocene) one, during either the Late Cretaceous or the late Paleocene to early Eocene.