Contributions to the Skeletal Anatomy of Freshwater Stingrays (Chondrichthyes, Myliobatiformes): 1
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Zoosyst. Evol. 88 (2) 2012, 145–158 / DOI 10.1002/zoos.201200013 Contributions to the skeletal anatomy of freshwater stingrays (Chondrichthyes, Myliobatiformes): 1. Morphology of male Potamotrygon motoro from South America Rica Stepanek*,1 and Jrgen Kriwet University of Vienna, Department of Paleontology, Geozentrum (UZA II), Althanstr. 14, 1090 Vienna, Austria Abstract Received 8 August 2011 The skeletal anatomy of most if not all freshwater stingrays still is insufficiently known Accepted 17 January 2012 due to the lack of detailed morphological studies. Here we describe the morphology of Published 28 September 2012 an adult male specimen of Potamotrygon motoro to form the basis for further studies into the morphology of freshwater stingrays and to identify potential skeletal features for analyzing their evolutionary history. Potamotrygon is a member of Myliobatiformes and forms together with Heliotrygon, Paratrygon and Plesiotrygon the Potamotrygoni- dae. Potamotrygonids are exceptional because they are the only South American ba- toids, which are obligate freshwater rays. The knowledge about their skeletal anatomy Key Words still is very insufficient despite numerous studies of freshwater stingrays. These studies, however, mostly consider only external features (e.g., colouration patterns) or selected Batomorphii skeletal structures. To gain a better understanding of evolutionary traits within sting- Potamotrygonidae rays, detailed anatomical analyses are urgently needed. Here, we present the first de- Taxonomy tailed anatomical account of a male Potamotrygon motoro specimen, which forms the Skeletal morphology basis of prospective anatomical studies of potamotrygonids. Introduction with the radiation of mammals. Living elasmobranchs are thus the result of a long evolutionary history. Neoselachians include all living sharks, rays, and Some of the most astonishing and unprecedented ex- skates, and their fossil relatives. Their monophyly is pressions of neoselachian evolution are the adaptation well established and beyond any dispute although the to deep-sea environments with all the required physio- interrelationships of several clades within Neoselachii logical changes (Kriwet & Klug 2009; Klug & Kriwet remain controversial. Major lineages include the Galeo- 2010), the development of filterfeeding and durophagy, morphii, Squalomorphii and Batomorphii, most of and freshwater adaptations (e.g., Compagno 1990; Car- which are key predators in modern marine environ- valho et al. 2004). The fossil record of freshwater ments. Generally perfectly adapted to the environment adapted elasmobranchian clades extends back into the they are living in, several groups developed special Palaeozoic and by the Early Cretaceous, several hybo- adaptations. Fossil evidence suggests that neoselachians dontiform and neoselachian lineages seemingly were have been primarily marine predatory from their ear- fully adapted to freshwater conditions. Early in the liest beginnings and throughout their long evolutionary Cenozoic, various stingrays (e.g., y Asterotrygon, career although freshwater adaption occurred several y Heliobatis) invaded freshwater habitats like rivers and times independently. Major neoselachian radiations are lakes (Carvalho et al. 2004), which probably is in con- recognized in the Early Jurassic, at the end of the Early junction with the most recent neoselachian radiation Cretaceous, in the middle of the Late Cretaceous, and event. in the Cenozoic (Kriwet & Klug 2004; Kriwet & Ben- Nevertheless, despite the development of diverse ton 2004; Kriwet et al. 2009). The most recent elasmo- neoselachian faunas within the Meso- and Cenozoic, branch radiation in the Cenozoic supposedly coincided the palaeoenvironmental specificity of the taxa has gen- * Corresponding author, e-mail: [email protected] # 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 146 Stepanek, R. & Kriwet, J.: Morphology of Potamotrygon motoro (Chondrichthyes, Myliobatiformes) from South America erally been ignored. One of the key questions is what Pectoral fin length, measured from the anterior top of the first ra- environmental changes precipitated these profound dial, which is normally the longest, to the posterior longest radial. The adaptations? The reasons for freshwater adaptations and longest radial doesn’t necessarily have to be the last radial. The width of the fin is measured from mesopterygium to the edge of the fin. the underlying processes are not yet understood. Inter- Pelvic girdle, measured from the anterior top of the prepelvic pro- estingly, some bony fishes and dolphins also entered cess to the posterior end of the mixopterygium, or clasper. the proto-Amazonas as well as stingrays of the family Neurocranium length, measurements are from the anterior end of Potamotrygonidae (e.g. Lovejoy et al., 2010; M. Brito, the nasal capsules to the articulation with the first synarcual. The pers. comm.). All groups nevertheless have a very pat- width is measured from the distal point of the right preorbital process chy fossil record. The oldest known of these very dif- to the left preorbital process. The depth is measured at the posterior ferent groups remains from the Miocene indicate that end of the fronto-parietal fontanel. Length of cervicothoracic synarcual, measured between the anterior they probably adapted to freshwater environments about articulation to the neurocranium and the posterior articulation with the same time. the pelvic girdle. The depth is measured at the highest point including At present, four living potamotrygonid genera are the crest. well-established: Heliotrygon (Carvalho & Lovejoy, Tail length, two distances were measured to establish the complete 2011), Plesiotrygon (Rosa et al., 1987), Potamotrygon length of the tail because of a sharp bend near the articulation of the Garman, 1877, and Paratrygon Dumeril, 1865. sting. Therefore, both anterior and posterior tail proportions related to Although this study is based on a single male speci- the insertion of the spine were measured separately and subsequently combined. In this case, the tail measurements start at the anterior ar- men, it represents the most comprehensive morphologi- ticulation to the pelvic girdle to the posterior end of the tail and com- cal account of any freshwater stingray from South prise the second synarcual and the following vertebrae. America to date and provides important skeletal infor- Abbreviations. aa, angular cartilage a; ab, angular cartilage b; ac, an- mation of freshwater stingrays in general. Miyake torbital cartilage; adf, anterodorsal foramen; ba, branchial arches; (1988) only considered the systematics of the genus Uro- baco, basibranchial capula; bar, scapulacoracoid bar; bpy, basiptery- trygon and Lovejoy (1996) and Carvalho et al. (2004) gium; bhy, basihyal; bridge, bridge projection; cfo, crest foramen; focused on the phylogenetic interrelationship of stingrays chy, ceratohyal; cl, clasper (mixopterygium); cr, crest; cute, curled providing only selected morphological information. terminal; doma, dorsal marginal; epb, epiphysial bar; fpf, fronto-par- Moreover, different and partly misinterpreted morpholo- ietal fontanelle; fsy, first synarcual; hy, hyomandibular; 1st hypo, first hypobranchiale; ilp, iliac process; ins, intermediate segments; isp, gical information of Potamotrygon motoro exist in the ischia process; last, lateral stay; lppp, lateral prepelvic process; ma, literature. mandibular arch (Meckel´s cartilage); map, mandibular process; Thus, the intentions of this paper are to (1) present mono, monospondylous vertebrae; mpa, metapterygial axis; mplate, the first detailed anatomical description of Potamotry- medial plate; mppp, medial prepelvic process; mspy, mesopterygium; gon motoro from the Amazon Basin and (2) review and mpy, metapterygium; na, nasal capsules; nc, neurocranium; ob, orbi- comment the interrelationships and origin of freshwater tal; oc, otic capsule; pcf, pectoral fin; pcg, pectoral girdle; pef, pelvic stingrays. This paper is the first part in a series of mor- fin; peg, pelvic girdle; pf, precerebral fontanelle; poc, preorbital pro- cess; pog, postorbital groove; pop, postorbital process; ppy, proptery- phological studies of freshwater stingrays. gium; pqu, palatoquadrate (upper jaw); ra, radialia; snf, spinal nerve foramina; soc, supraorbital crest; ssy, second synarcual (vertebrae); sp, scapular process; st, sting; veme, ventral marginal. All scale bars Material and methods (if not otherwise stated) equal 1.0 cm. The adult specimen of Potamotrygon motoro that forms the basis of this study was part of a breeding program at the Aquazoo and Lb- Morphology of Potamotrygon motoro becke Museum in Dsseldorf, Germany and was donated to the Mu- seum of Natural History Berlin, Germany for this study after skeletal Morphological characters were described by different preparation. For this, the specimen was mechanically prepared and as authors previously but mostly with the focus on a parti- much soft tissue as possible was removed in a first step. In a next cular organ system or selected skeletal components. step, larder beetles were used for cleaning the skeleton from all re- maining soft tissue. Finally, the skeleton was cleaned using ethyl alco- The morphological description presented in this study hol and bleached with hydrogen peroxide. is based on a single, adult male specimen (Fig. 1). The The morphological terminology follows that of Lovejoy (1996) and total length of the specimen is 350 mm and its total