Asterotrygon, DE CARVALHO & MAISEY & GRANDE, 2004

DE CARVALHO, MARCELO R., MAISEY, JOHN G. & GRANDE, LANCE, 2004, 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), Bulletin of the American Museum of Natural History 2004 (284), pp. 1-136 : 17-24

† Asterotrygon , new genus

DIAGNOSIS: The presence of a dorsal fin covered by dermal denticles, just anterior to the caudal stings, is autapomorphic for † Asterotrygon , n.gen. The following unique combination of characters further distinguishes † Asterotrygon , n.gen. from both fossil and Recent stingray genera: dorsal surface of disc, snout, and tail, as well as base and sides of tail, covered by closely packed denticulation; denticles minute, with posteriorly pointed hooklike crowns and stellate bases, not forming discrete bucklers (also with distinct series of enlarged spines forming rows over tail and part of dorsal disc surface); individual vertebral centra extending to distal tip of tail, posterior to caudal stings (instead of an unsegmented cartilaginous or notochordal rod extending to distal tip of tail, posterior to caudal stings); disc and tail length almost equal; tail stout at base, tapering distally but not continuing caudally as a slender ‘‘whip’’; disc circular to oval in outline (disc length and width almost equal, except in FMNH 14567).

REMARKS: Fossil stingrays that are morphologically similar to † Asterotrygon , n.gen. and known from more­or­less complete skeletons include the monotypic † Heliobatis Marsh, 1877 , occurring in the same localities and horizon as † Asterotrygon , n.gen., and †‘‘ Dasyatis ’’ muricata and †‘‘ Dasyatis ’’ dezignoi, both from the Eocene (Lutetian) of Monte Bolca, Italy ( Jaekel, 1894). The new genus differs from all three taxa by the presence of a conspicuous dorsal fin (located just anterior to caudal stings, absent in all three taxa), a more rounded and smaller disc (disc trapezoidal or rhomboidal in the Monte Bolca taxa and many specimens of † Heliobatis ), a shorter and much stouter tail at base (in all three taxa tail is slender at base, not tapering greatly, and is whiplike in †‘‘ Dasyatis ’’ muricata), and the presence of heavy denticulation over dorsal surface of disc, dorsal fin, snout, and over almost entire tail region (sparse denticles, if present, occur as enlarged spines in generally a single row over middisc and tail regions in all three taxa). † Asterotrygon , n.gen. further differs from †‘‘ Dasyatis ’’ muricata in having a much shorter tail (less than or equal to disc length in † Asterotrygon , n.gen., much greater than disc length in †‘‘ D.’’ muricata; Jaekel, 1894: 143, fig. 32).

† Asterotrygon , n.gen. can be distinguished from the Recent genera Potamotrygon Garman, 1877 , Paratrygon Duméril, 1852 , and Plesiotrygon Rosa, Castello, and Thorson, 1987 (Potamotrygonidae) by the absence of the median prepelvic process. The combina­

region (axial cartilage dark, impressions of terminal cartilages are indicated by arrowhead). Some sections of pectoral disc are missing. Anterior to top.

tion of external features used to separate the new genus from fossil stingrays also separates it from all Recent myliobatiform genera as well. From potamotrygonids, Dasyatis Rafinesque, 1810 , Pastinachus Rüppell, 1828 , Himantura Müller and Henle, 1837 , Taeniura Müller and Henle, 1837 , Pteroplatytrygon Fowler, 1910 , and Urogymnus Müller and Henle, 1837 (Dasyatidae) , † Asterotrygon , n.gen. is distinguished by the presence of the dorsal fin (absent in all above genera), presence of individual vertebrae extending beyond caudal sting to posterior tip of tail (implying the absence of the cartilaginous notochordal rod at level of caudal stings, present in the above genera), circular to slightly oval disc shape (as opposed to a more trapezoidal or rhomboidal disc in most species, except Urogymnus ), and a conspicuously thicker tail at base that is not whiplike (large specimens of Potamotrygon may have a thick, not whiplike, tail as well). The questionable dasyatid Urolophoides giganteus Lindberg, 1930 ( Urolophoides Lindberg, 1930 is most likely a junior synonym of Dasyatis Rafinesque, 1810 ) has a short, stout tail, but lacks the dorsal fin and intense shagreen, as well as having a strongly rhomboidal disc ( Lindberg and Legeza, 1959; Nishida and Nakaya, 1990). Urogymnus is further distinguished from † Asterotrygon , n.gen. by its lack of stings and proportionally shorter distance between eyes and anterior tip of disc ( Compagno and Roberts, 1984; Last and Stevens, 1994).

† Asterotrygon , n.gen. differs from Plesiobatis Nishida, 1990 (Plesiobatidae) in its moderate snout length and anterior disc contour (snout very long and anterior disc point­ ed in Plesiobatis ), presence of dorsal fin (absent in Plesiobatis ), and greater proximity of eyes to anterior margin of disc (eyes very reduced and located far from snout tip in Plesiobatis ; Last and Stevens, 1994). † Asterotrygon , n.gen. is distinct from urotrygonids ( Urobatis Garman, 1913 and Urotrygon Gill, 1863 ) and urolophids ( Urolophus Müller and Henle, 1837 and Trygonoptera Müller and Henle, 1841 ) by having a dorsal fin covered by denticles (dorsal fin completely absent in the former two genera and present in some species of the latter two genera, but never coated with denticles), intense covering of dermal denticles over dorsal surface (dorsal disc and tail surface generally with sparse denticulation in the former two genera, and mostly naked in the latter two genera), and lack of elongated caudal fin (invariably present in all four genera, with conspicuous dorsal and ventral lobes that are internally supported by radial cartilages).

† Asterotrygon , n.gen. is easily separated from Hexatrygon Heemstra and Smith, 1980 (Hexatrygonidae) by the presence of dorsal fin (absent in Hexatrygon ) and by snout and disc shape (snout extremely elongated, triangular, and somewhat demarked from disc in Hexatrygon ). From butterfly rays ( Gymnuridae , Gymnura Kuhl, 1823 , and Aetoplatea Valenciennes, 1841 ), † Asterotrygon , n.gen. is distinguished by disc shape (much broader than long in gymnurids, but disc width and length are about equal in † Asterotrygon , n.gen.), stout tail that is about equal to disc length (tail slender and short in gymnurids), and covering of dermal denticles (disc mostly naked in gymnurids). Eagle, cownose, and manta rays ( Myliobatidae ), including † Promyliobatis gazolae from Monte Bolca, are easily separated from † Asterotrygon , n.gen. by disc shape (invariably broader than long in these groups), head anterior to and separated from disc (pectoral disc projects anterior to head in † Asterotrygon , n.gen.), less intense dorsal denticulation (usually naked dorsal surface in pelagic stingrays), length of tail (usually long and whiplike in myliobatids, but shorter and stout in † Asterotrygon , n.gen.), naked dorsal fin (dorsal fin covered in denticles in † Asterotrygon , n.gen.), presence of cartilaginous rod extending posteriorly from region of caudal stings instead of individual vertebrae as in † Asterotrygon , n.gen., presence of cephalic extensions (‘‘cephalic fins’’) in Mobula Rafinesque, 1810 and Manta Bancroft, 1828 (absent in † Asterotrygon , n.gen.), and by differences in dentition (teeth numerous, small, and closely packed, with subtriangular cusps in † Asterotrygon , n.gen., as in most nonmyliobatid stingrays; teeth in all myliobatids except manta rays are arranged in broad toothplates). Anatomical features that further distinguish † Asterotrygon , n.gen. from some or all of the above genera are discussed in the skeletal description below.

Many Recent genera assigned to the Dasyatidae and Urolophidae are based on external characters not readily available in fossils (e.g., lack of tail­folds on both upper and lower surfaces of tail in Himantura ; lack of dorsal tail­fold, but tall and long ventral tailfold extending to distal tip of tail in Taeniura , etc.). Skeletal characters unique to most nonmyliobatid genera have not been found, and the skeleton is generally very conservative in both Recent and fossil nonmyliobatid genera. The definition given here to the new genus † Asterotrygon , n.gen. is nevertheless consistent with generic diagnoses of Recent stingrays, allowing for a quick and straightforward identification of all † Asterotrygon , n.gen. specimens examined. No other fossil or Recent stingray genus has a dorsal fin covered with hooklike denticles and the unique combination of individual vertebrae extending posteriorly to distal tip of tail, closely packed denticulation over disc, snout and tail, and stout tail at base.

ETYMOLOGY: The new generic name is de­

TABLE 2

Measurements and Counts Conducted on Specimens of † Asterotrygon maloneyi , n.gen., n.sp. Values are expressed as mm/percentage of disc width, except for total length (TL) and disc length (DL), which are shown in mm. See Measurements and Terminology for abbreviations of parameters. All specimens are female except FMNH PF 12989.

rived from the Greek asteros, meaning ‘‘star’’, and trygon, the Greek word for stingray, in reference to the star­shaped bases of the dermal denticles scattered over dorsal disc and tail regions (see description of denticles below; also fig. 25). Gender feminine.

TYPE­ SPECIES: † Asterotrygon maloneyi , new species.

INCLUDED SPECIES: Presently considered to be monotypic.

† Asterotrygon maloneyi , new species