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) MARCELO R. DE CARVALHO Research Associate, Department of Ichthyology, Division of Vertebrate Zoology, American Museum of Natural History Departamento de Biologia Universidade de SaÄo Paulo Av. dos Bandeirante 3900, RibeiraÄo Preto SP, Brazil, 14040-901 ([email protected]) JOHN G. MAISEY Division of Paleontology, American Museum of Natural History ([email protected]) LANCE GRANDE Department of Geology, Field Museum of Natural History Roosevelt Road at Lake Shore Drive, Chicago, IL, 60605-2496 (grande@®eldmuseum.org) BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 284, 136 pp., 53 ®gures, 7 tables Issued June 9, 2004 Copyright q American Museum of Natural History 2004 ISSN 0003-0090 Frontispiece. Artistic rendering of the new genus and species of stingray described in the present paper in its natural environment. This stingray taxon occurred in Fossil Lake, an extinct tropical to subtropical freshwater intermontane lake that formed as a consequence of the orogeny of the Rocky mountains (®g. 1). Sediments from Fossil Lake are exposed as the Fossil Butte Member of the Green River Formation in present- day Wyoming (some 52 million years before present). The adult female is giving birth through viviparity; this scene is inspired by specimen AMNH 11557, in which an adult female stingray is preserved alongside two small stingrays that are inferred to be her aborted late-term fetuses (®g. 3; see text for details). Based on an original painting by David W. Miller and modi®ed from Maisey (1996: pl. 42). CONTENTS Abstract ....................................................................... 4 Introduction .................................................................... 5 Materials and Methods .......................................................... 7 Measurements and Terminology ................................................ 8 Comparative Stingray Material ................................................. 9 Abbreviations ............................................................... 11 Geological Setting ............................................................. 12 Systematic Paleontology ........................................................ 17 ²Asterotrygon, new genus ....................................................... 17 ²Asterotrygon maloneyi, new species ............................................. 24 Anatomical Description ......................................................... 31 Neurocranium ............................................................... 36 Jaws, Hyomandibulae and Visceral Arches ...................................... 44 Synarcual and Vertebral Skeleton .............................................. 50 Appendicular Skeleton ....................................................... 52 Dermal Skeleton ............................................................. 59 Dermal Denticles ............................................................ 59 Caudal Stings ............................................................... 61 Teeth ....................................................................... 63 ²Heliobatis Marsh, 1877 ........................................................ 65 ²Heliobatis radians Marsh, 1877 ................................................ 72 Stingray Characters of ²Asterotrygon ............................................. 72 Phylogenetic Relationships of the Green River Stingrays ........................... 73 Phylogenetic Procedures and Coding ........................................... 73 Character Descriptions ....................................................... 77 Lateral-line Canals ........................................................... 77 Skeleton .................................................................... 78 Mandibular Muscle Plate ..................................................... 93 Hypobranchial Muscle Plate .................................................. 93 Miscellaneous ............................................................... 95 Results of Phylogenetic Analysis .............................................. 97 Discussion and Comparison to Previous Stingray Phylogenies .................... 102 Systematic Issues within Myliobatiformes ..................................... 105 Classi®cation of Myliobatiform Genera ........................................ 108 Biogeographical Implications ................................................... 108 Origin of the Green River Stingrays .......................................... 109 Origin of the South American Freshwater Stingrays ............................. 112 Conclusions .................................................................. 119 Acknowledgments ............................................................ 122 References ................................................................... 123 Appendix 1. Comparative Batoid (Nonstingray) Material Examined ................. 133 Appendix 2. Experimental Phylogenetic Analysis ................................. 134 Index of Stingray Genera and Species ........................................... 136 4 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 284 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 de- scribed. ²Asterotrygon 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 ²Heliobatis radians from the same formation, by the unique presence of a dorsal ®n 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 pro- portions of disc and tail. ²Asterotrygon, 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 calci®ed angular cartilage between the hyomandibula and Meckel's cartilage is tentatively identi®ed in ²Asterotrygon, n.gen. as well. ²Asterotrygon, 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 scapulocor- acoid, absence of thoracic ribs, and laterally expanded, shel¯ike postorbital processes. ²Aster- otrygon, n.gen. and ²Heliobatis primitively retain a narrow and slightly arched puboischiadic girdle and primitively lack calci®ed rostral elements in adults. A phylogenetic analysis of 23 stingray genera, two outgroups, and 44 informative morpho- logical characters resulted in 35 equally most parsimonious trees. The strict consensus reveals the following hierarchical structure: Hexatrygon 1 (²Asterotrygon, n.gen., Plesiobatis, Uro- lophidae 1 (Urotrygonidae 1 (²Heliobatis 1 (Potamotrygonidae 1 (amphi-American Himan- tura, Pteroplatytrygon, Himantura, Taeniura, Dasyatis 1 (Gymnuridae 1 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 1 Myliobatidae. ``Das- yatidae'' is not monophyletic in any of the minimum-length trees obtained; Urolophidae (Uro- lophus and Trygonoptera) and Urotrygonidae (Urobatis and Urotrygon) are both monophy- letic, but are not sister-groups. ²Asterotrygon, 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 1 Myliobatidae. The resulting stingray phylogeny is at odds with previous phylogenies mostly regarding the af®nities 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 con¯ict, 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 con®g- uration, ceratobranchial fusion patterns, and other characters discussed in our study. Three additional synapomorphies