Zoosyst. Evol. 88 (2) 2012, 145–158 / DOI 10.1002/zoos.201200013

Contributions to the skeletal anatomy of freshwater stingrays (, ): 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- rays, detailed anatomical analyses are urgently needed. Here, we present the first de- 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 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 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 , 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 like rivers and times independently. Major neoselachian radiations are lakes (Carvalho et al. 2004), which probably is in con- recognized in the Early , 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 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 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 Compagno (1999). For this study, the specimen was documented with disc width is 170 mm. Dorsal fins are absent. a digital camera. Small skeletal elements were studied and analysed using a magnifier with a twelve-fold magnification (Eschenbach). Measurements were taken on dried cartilages with digital callipers Calcified cartilages with a measuring range from 0–150 mm and a resolution of 0.01 mm (Fig. 1a). The accuracy is: þ/–– 0.0 mm (< 100 mm) and 0.03 mm The skeleton mainly consists of cartilage. Densely cal- (> 100 mm), respectively, and the repeatability is 0.01 mm. These cified tissue occurs in the double cones of vertebral measurements are important for establishing confidential meristic data centra (= alveolar calcification sensu Moss 1967). Most for comparisons with other stingray specimens in prospective studies. skeletal elements are covered by tesserae of tiny hexa- Meristic accounts are provided in Table 1 and measurements used gonal calcium-phosphate crystals (= tesserate cartilage). herein are: Total length, measured from the anterior tips of pectoral fins to These tesserae are functionally important in stiffening posterior end of the tail. the axial skeleton or parts of the cranium (e.g. jaws as Width of disc, measured between extreme outer corners of disc. adaptation to durophagy) when arranged in several

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Figure 1. Dorsal view Potamotrygon motoro; a. Measurement points; b. Main elements. Abbreviations are explained in the meth- ods section.

# 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim museum-zoosyst.evol.wiley-vch.de 148 Stepanek, R. & Kriwet, J.: Morphology of Potamotrygon motoro (Chondrichthyes, Myliobatiformes) from South America

Table 1. Measurements of the studied male specimen of Potamytrogon motoro in mm. Abbreviations are: le – left; ri – right. structure length width depth neurocranium 52 29 17 first synarcuum 27.5 – 12 tail 230 –– pectoral fin ri 190/le 188 ri 58/le 58 – pelvic girdle þ clasper 117 –– layers (Dean & Summers 2006). The multiplication of concave protrusion (Fig. 2). The apertures are consider- tesserate layers in the jaws of Potamotrygon is analo- ably wider than long and inclined antero-ventrally, so gous to cortical thickening (Summers 2000). These tes- that the apertures face downward. In pelagic stingrays, serae are perichondrial in origin. such as Dasyatis violacea, the nasal capsules and the rest of the neurocranium form an angle while in non- pelagic stingrays the nasal capsules and the rest of the Neurocranium neurocranium are in the same plane (Miyake 1988). In The shape of the neurocranium is related to functional the study presented here, the examined Potamotrygon aspects of the jaws and locomotion. It encloses the motoro specimen displays the latter condition. Both, brain and the olfactory, auditory, and visual organs. In Miyake (1988) and Lovejoy (1996) show in their inves- dorsal view, the neurocranium is rectangular broadening tigations the ventro-lateral expansion of the nasal cap- anteriorly because of the large nasal capsules. In lateral sules, but this character seems difficult to quantify. view, the neurocranium is box-like with the base being The fontanel is divided into an anterior and a poster- horizontal. ior portion. The anterior precerebral fontanel and the Stingrays lack a rostral cartilage. According to posterior fronto-parietal fontanel are incompletely se- Miyake (1988) only the freshwater stingrays Potamotry- parated by a median constriction, which represents the gon and Plesiotrygon as well as some urolophids pos- reduced epiphyseal bar (Fig. 2a). An incomplete or sess what is called a “functional rostrum” (Fig. 1). The strongly reduced epiphyseal bar is common in many ex- anterior ends of the pectoral fins form this rostrum, tant and extinct stingrays (Carvalho et al. 2004). which articulate via the antorbital cartilages with the The precerebral fontanel is wider than the fronto-par- nasal capsules. This structure is assumed to be homolo- ietal fontanel, which narrows posteriorly (Fig. 2a). The gous to the rostrum of and skates by length of both fontanels measures 45 mm. The w- Miyake (1988). shaped form of the anterior margin of the nasal cap- The two nasal capsules, which enclose the olfactory sules continues into the anterior margin of the precer- organs, are located at the anterior end of the cranium ebral fontanel. In the examined specimen the roof of and display a w-shaped anterior margin with a medially the brain, the tectum orbitale, is indeterminable.

Figure 2. Neurocranium and branchial archs; a. Dorsal view; b. Ventral view. Abbreviations are explained in the methods section.

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The orbital region is only slightly longer than the oti- and mandibular arches are present in stingrays. Either co-occipital region. The space between both orbitals is the hyomandibula attaches directly to the mandible as relatively narrow and shorter than the full length of the in Hexatrygon and Plesiobatis or there is a stout liga- neurocranium as in other stingrays. ment, the “hyomandibulo-Meckelian tendon” of McEa- The supraorbital shelf is well developed and its chran et al. (1996) connecting the hyomandibular di- length more or less equals the distance between the rectly to the mandible as in Paratrygon.In preorbital and postorbital process. The anterior region Potamotrygon, the hyomandibula also connects to of the orbit is completely enclosed by the preorbital Meckel’s cartilage by a strong ligament, in which small process, which does not extend over the nasal capsules. calcifications, the angular cartilages may occur. The In other stingrays such as rogersi and Urolo- number of angular cartilages within the ligament of Po- phus cruciatus, the preorbital process extends over the tamotrygon varies from one (as also found in Plesiotry- nasal capsules (e.g. Lovejoy, 1996). gon) to two (Carvalho et al. 2004). Additionally, the ar- The well-developed postorbital process (sensu su- rangement and size of the angular cartilages may vary praorbital process in Carvalho at al. 2004) marks the as seen in, for instance, Potamotrygon boesemani,in posterior margin of the orbit. It is separated from the which these cartilages are arranged parallel and have supraorbital shelf by a notch, extends laterally and is the same size (Rosa et al. 2008) conversely to the con- dorso-ventrally flattened. This process is divided in an dition displayed by the specimen of Potamotrygon stud- anterior and a posterior portion by the postorbital ied here (Fig. 2b). groove (Fig. 2a). According to Lovejoy (1996), this In the examined specimen of Potamotrygon motoro groove provides the passage for the intra-orbital lateral in this study, two angular cartilages are present on both line canal and is characteristic for potamotrygonids but sides of the jaws, which differ in size and form also some other stingrays such as cruciatus. (Fig. 3a). The anterior angular cartilage (angular a) In other stingrays, the canal passes through a dorso- measures 9.0 mm in length and is considerably longer ventral positioned opening into the process. The evolu- and arched than the posterior one. It directly connects tionary significance of this character momentarily re- to the mandibular and hyomandibular. The second pos- mains dubious pending further studies. In skates and terior angular cartilage (angular b) is very small meas- several non-myliobatiforms, the postorbital process is uring 3.0 mm and is drop-shaped. Thus it is only a poorly developed or absent as in electric rays (Miyake third the length of the anterior one. It is located near 1988: fig. 59). the articulation of the hyomandibula with angular a. The suborbital shelf is a horizontal plate ventral to These calcifications serve for reinforcing the articula- the orbit. In the examined Potamotrygon motoro it is tion (Garman 1913) but also enlarge the distance be- missing. The sclerotic ring enclosing the eyeballs is tween jaws and the neurocranium considerably conver- oval. sely to the direct articulation mode. These modifications The aperture for the iris is large and elongated and a in jaw articulation are related to and improve the function medial foramen for the optic nerve is present. of the jaws during feeding. It appears that the connective The otic capsules housing the inner ears are situated ligament is important for functional aspects of opening posterior to the postorbital process and are broadly and closure of the jaws. rounded. The hyomandibulae attach latero-ventrally to The presence, number and size of angular cartilages the posterior region of the otic capsule. are ambiguous and difficult to evaluate for use in phy- The occipital region includes the posterior end of the logenetic analyses. In amphi-American Himantura spe- neurocranium. Here, the first cervical centrum of the cies, i.e., these elements vary strongly in size and num- vertebral column articulates with the occiput. On each ber conversely to the condition found in Potamotrygon side there is an occipital condyle that articulates with species (Lovejoy 1996). Angular cartilages are absent the first basiventral. The otico-occipital region occupies in Paratrygon, which represents the plesiomorphic con- only 1/3 of the total length of the neurocranium. dition within Potamotrygonidae. Consequently, the pre- sence of angular cartilages cannot be regarded a syna- pomorphy of Potamotrygonidae as previously suggested Mandibular arch and hyomandibulae but is a synapomorphy of Potamotrygon þ Plesiotry- gon.InPlesiotrygon a single robust, elongated angular The elements of the mandibular arch comprising the cartilage is present and no variation in their number is palatoquadrate and Meckel’s cartilage and the hyoman- discernable. In Paratrygon the connection between hyo- dibulae are part of the viscerocranium. The hyomandi- mandibula and Meckel’s cartilage is considerably short- bula of Potamotrygon are elongated and represent the er and only a very small cartilage might be developed, upper portion of the hyoid arch. It forms the hyostylic the homology of which remains ambiguous (Lovejoy type of articulation of the jaws with the neurocranium. 1996). The ventral element of the hyoid arch, the ceratohyale In other stingrays such as Myliobatis, Aetobatus, Rhi- or pseudohyoid, also is elongated. noptera and , a cartilage element close to the According to Nishida (1990) and Lovejoy (1996) two anterior tip of the hyomandibular was observed by main types of connection between the hyomandibulae Lovejoy (1996). This element, however, seemingly has

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Figure 3. a. Ventral view of the hyomandibular and palatoquadratum; b. Ventral view of the branchial archs; c. Dorsolateral view of the branchial arch, the first synacuum and the pectoral girdle; d. Posterolater view of the pectoral girdle; e. Dorsal view of the pectoral fin with all radials; f. Dorsal view of the pelvic girdle with the articulated and the clasper; g. Ventrolateral view of the basipterygium and the mixopterygia. Abbreviations are explained in the methods section.

museum-zoosyst.evol.wiley-vch.de # 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Zoosyst. Evol. 88 (2) 2012, 145–158 151 not the same function as the angular cartilages in the stingrays and might be segmented, unsegmented or ligament of Potamotrygon, Himantura and Plesiotrygon even absent. In the examined specimen, the basihyal is and their development and homologies remain uncer- unsegmented and the elements are completely fused tain. (Fig. 3b). Its anterior margin is slightly convex, whereas The wing-shaped mandibular process in the exam- the posterior edge is almost straight. This condition ined specimen of Potamotrygon motoro, which is close seems to be extremely variable and therefore, its phylo- to the attachment between mandibular arch and angular genetic inference must be reviewed of a larger sample a is well developed and overlap both jaws (Fig. 3a). Ac- size. cording to Lovejoy (1996), it is also well developed in However, the basihyal in Lovejoy’s (1996: fig. 7) in- other potamotrygonids, Dasyatis, Taeniura, and Himan- vestigation of Potamotrygon motoro is shown to be seg- tura but less developed in Urolophus, , and Uro- mented similar to the condition found in Dasyatis, trygon. This variation in form and size makes it difficult Himantura, Taeniura, Urobatis, and the remaining to code this character in phylogenetic hypothesis. potamotrygonid taxa. The basihyal of Plesiobatis, Hexa- The symphysis of the mandibles is not fused in Pota- trygon, Urolophus, Gymnura, and Aetoplatea is unseg- motrygon but meets along a vertical suture (Fig. 3a). In mented (Lovejoy 1996). Due to that extreme variation, other myliobatiforms, such as Rhinoptera and Dasyatis, more data on the morphology of the basihyal, which is the mandibles (as well as the hyomandibulae) are en- potentially important for phylogenetic reconstructions, is tirely fused (Summers 2000: Fig. 2). necessary. Teeth of Potamotrygon motoro are unicuspid, small The first hypobranchial is elongate and the anterior and arranged in multiple rows along the jaws. In some end is forked. The proximal end articulates with the ba- derived myliobatiforms such as Myliobatis, Aetomy- sihyal, whereas the distal edge articulates with the first laeus, and Rhinoptera teeth are broadened resulting ceratobranchials. from the fusion of individual teeth and are arranged The three anterior ceratobranchials meet each other densely forming characteristic grinding dentitions. medially. The first ceratobranchial articulates with the second one but only the third ceratobranchial articulates with the medial plate. Branchial skeleton According to Lovejoy (1996), the pseudohyal is fused to the first ceratobranchial, which represents an The branchial skeleton, which also is part of the viscer- apomorphy of stingrays. The pseudohyal is not pre- ocranium, comprises five arches (Fig. 3b). The ventral served in the studied specimen, which certainly repre- branchial skeleton consists of an enlarged central med- sents an artefact. In other stingrays such as Urolophus, ial plate, which resulted from the fusion of the basi- the pseudohyal contacts the first ceratobranchial but is branchial copula and the basibranchial components not fused to it (compare Lovejoy 1996). In other taxa, (Miyake & McEachran 1991; Carvalho et al. 2004), a the first two ceratobranchials are fused to the pseu- short and transversely directed basihyal, a pair of short dohyal. and anteriorly directed hypobranchials, and five pairs In the examined Potamotrygon motoro specimen the of ceratobranchials. first three ceratobranchials are fused (Fig. 3b). How- In the examined specimen, a single small bridge pro- ever, Lovejoy (1996) indicates four fused ceratobran- jects ventrally from the medial plate (Fig. 3b). Accord- chials in Potamotrygon motoro. It appears that these fu- ing to Lovejoy (1996), this bridge forms a shelter for sion patterns are complex and variable within genera the aorta and afferent branchial vessels. Such projec- and species (compare Lovejoy 1996: fig. 7). The fourth tions also are present in Plesiobatis, Hexatrygon, Uro- ceratobranchial completely articulates with the medial batis, Urotrygon, Urolophus, and Gymnura (Lovejoy plate. 1996), but are absent in some potamotrygonids and The ceratobranchials articulate dorsally with the epi- some other stingrays such as Dasyatis and pelagic My- branchials and through small pharyngobranchials with liobatis species. the occipital region of the neurocranium (first branchial The anterior end of the medial plate is perforated by arch) and the anterior region of the first synarcual (sec- a large foramen between the first and second cerato- ond to fourth branchial arches). Only the fifth epibran- branchials. At the posterior end of the medial plate is a chials articulate directly with the scapulocoracoid. sting-shaped projection, which represents the basibran- It appears that the proximal first gill ray at the cera- chial copula. It forms the ventral cover of the pericar- tobranchial is wider than the following ones and wing- dial cavity. shaped. The last ray of the epibranchial, where it ar- The five pairs of ceratobranchials originate at the ticulates to the synarcual, resembles the first gill ray of central medial plate. According to Miyake & McEa- the ceratobranchial. The number of rays of the ventral chran (1991), the second to fifth hypobranchials and ceratobranchials and dorsal epibranchials varies. The basibranchials are coadunate to the central medial plate first ceratobranchial bears ca. nine rays and there are in stingrays. five rays on the last ceratobranchial, respectively. The Anteriorly, the hypobranchials are laterally attached number of rays (excluding the wing-shaped ray) varies to the basihyal. The basihyal is variably developed in between nine and 11 at the epibranchials.

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Vertebral column Pectoral girdle and scapulocoracoid

Vertebral centra are spool-like calcified elements origi- The pectoral girdle (Fig. 3d) is situated directly poster- nating directly posterior to the neurocranium and ex- ior to the branchial arches and consists of different tending to the distal end of the caudal fin. The posterior fused elements, mainly the coracoid and the scapular margin of the occipital region is formed by a hemicen- process. The coracoid itself consists of two pairs of trum, which forms the contact to the vertebral column. fused elements, which are ventrally positioned and form Within stingrays the vertebral column also comprises a bar (Fig. 3d). Laterally, it fuses with the scapular pro- two synarcuals, the cervicothoracic and the thoracolum- cess and curves dorsally. In the examined Potamotrygon bar synarcual, respectively. In the examined specimen motoro, the scapulocoracoid displays several foramina. the scapulocoracoid separates both synarcuals (Fig. 1). Dorsally, there is a large antero-dorsal foramen and a The transition from mono- to diplospondylous ver- similar large antero-ventral foramen below. While the tebrae is at the level of the pelvic girdle (Fig. 3d). Di- postero-dorsal foramen is absent, the postero-ventral plospondylous vertebrae extend to about the middle of foramen is a small opening. It is situated postero-later- the tail sting. The vertebral centra of the remaining tail ally, where the coracoid is connected to the scapular are fused forming an unsegmented rod. process. Neural arches are laterally flattened and spatulate as Most stingray taxa have four scapulocoracoid forami- it is characteristic for stingrays (Carvalho et al. 2004). na, the antero-dorsal, antero-ventral, postero-dorsal and Thoracic ribs are lacking, which is a synapomorphy of the postero-ventral ones (Lovejoy 1996). According to myliobatiforms. The missing pleural (ventral) ribs re- Nishida (1990) the postero-dorsal foramen is absent in present an artefact. Urolophus, Urobatis, and Urotrygon, whereas Lovejoy (1996) observed small postero-dorsal foramina in Uro- trygon rogersi. Cervicothoracic Synarcual In most stingrays, the foramen of the scapular pro- The first synarcual articulates anteriorly with the neu- cess is present. According to Miyake (1988) it is absent rocranium and posteriorly with the intersynarcual ver- in Plesiobatis, Urolophus, Potamotrygon, Paratrygon, tebrae close to the pectoral girdle (Fig. 3c). Its length and Plesiotrygon. This interpretation is supported by in the studied specimen measures 27.5 mm and the this study for Potamotrygon. maximum height in the middle part is 11.8 mm (includ- Dorsal to the scapular processes, the suprascapulae ing the crest). The width is 7.7 mm. The cervicothor- articulate on both sides and are fused dorsally to the acic synarcual forms an elongated rigid tube consisting first synarcual. Because of these dorsal and ventral ar- of several fused vertebrae. The lateral surfaces of the ticulations, the pectoral girdle is flattened and ring- synarcual are perforated by spinal nerve foramina along shaped. The fifth epibranchial and ceratobranchial of its complete length. Ventral to these foramina, short the branchial arches articulate directly with the antero- prolongations extend laterally from the synarcual, medial aspect of the scapulacoracoid. Both condyles which are not pierced by any foramina. are adjacent. The scapular process supports the pectoral The lateral stay is situated dorsally to the spinal fin and has three different condyles for its articulation. nerve foramina in the examined specimen (Fig. 3c), Anteriorly, there is a procondyle for the propterygium, which confers with the observations of Lovejoy (1996) intermediate the mesocondyle for the mesoptergydium for Potamotrygon. This condition also occurs in Plesio- and posterior the metacondyle for the metapterygium. trygon but not in Paratrygon. In all other stingrays the lateral stay (if present) is situated ventrally to the fora- Pectoral fin mina (Lovejoy 1996). The degree of the lateral extension of the lateral stay The pectoral fin (Fig. 3e) is supported by the pectoral varies among stingrays and rays in general (Claeson girdle and consists of three cartilaginous ele- 2008). It is thin in Urotrygon and Pacific coast Uroba- ments. The first anterior element is the propterygium, tis specimens (Lovejoy 1996). In the specimen studied followed by the mesopterygium and posteriorly by the here, the first synarcual is only slightly higher than metapterygium. All three elements are very different in wide, whereas it is wider than high in Paratrygon size. The propterygium of the examined specimen is (Lovejoy 1996). segmented anteriorly, elongated, flattened and rod- Dorsally, a medial crest projects over the whole shaped with a length of 69 mm. It curves anteriorly length with a height of 7.6 mm. Anteriorly, the crest where it articulates via the antorbital cartilage with the does not connect to the neurocranium. Posteriorly, it at- nasal capsule. The first segmentation of the proptery- taches to the dorsal region of the pectoral girdle, where gium occurs directly at the level of the nasal capsule the suprascapular connects. Here, a foramen penetrates (Figs 1b, 3e) at the contact with the antorbital cartilage. the basis of the crest. The synarcual articulates poster- This condition is also present in Urolophus, Urotry- iorly with the scapulocoraracoid with two large facets gon, Urobatis, Plesiotrygon, Paratrygon, Taeniura, and of its lateral projections. These projections pass ante- amphi-American Himantura (Lovejoy 1996). In other riorly into the lateral stay. taxa, such as Dasyatis, Indo-West Pacific Himantura,

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Gymnura, and Myliobatis the segmentation occurs ante- The sting-shaped prepelvic process of the investi- rior to the nasal capsules. gated specimen is very distinct. In most of the stingrays The propterygium articulates with the scapulocora- this process is moderately developed or reduced (Love- coid and contacts the mesopterygium in Plesiotrygon joy 1996: fig. 11). In Paratrygon, Plesiotrygon, and the conversely to the condition found in all other stingrays. pelagic stingrays Rhinoptera and Mobula this process Consequently, this character is supposed to support the also is elongated (Lovejoy 1996). monophyly of Potamotrygon þ Plesiotrygon (Carvalho Ventrally, a wing-shaped process, the ischial process, et al. 2004). projects proximal from each side. Another process, the The mesopterygium is considerable smaller than the iliac process, projects dorsally to the ischial process. It two other elements in the studied specimen. It is unseg- appears that there is only a single condyle, the basal mented, flat, and rhomboidal and measures 9.0 mm condyle, on the lateral aspect, which articulates with (Fig. 3e). It articulates laterally with the scapulacora- the pelvic basipterygium. The first radial articulates coid but seemingly not with the posterior end of the anteriorly to the anterior aspect of the basipterygium. propterygium or with the metapterygium. In Plesiotry- The skeleton of the clasper is formed by the basipteri- gon, the mesopterygium also does not contact the prop- gal axis and attaches posteriorly to the basipterygium terygium, whereas the mesopterygium articulates with of the pelvic fin (Fig. 3g). It is segmented and includes the posterior aspect of the propterygium in other sting- several curled cartilages. The length of the basipteri- rays. The mesopterygium can be segmented, such as in gyium is 18 mm. It is almost straight, elongated and Gymnura and Myliobatis, or be absent or fused with dorso-ventrally flattened. The basipterygium articulates the scapulacoracoid, such as Aetobatus, Rhinoptera, and via several intermediate segments with the dorsal mar- Mobula. ginal and the ventral marginal of the clasper. The dorsal The metapterygium is shorter than the propterygium, marginal measures 35 mm and articulates to the curled but also elongated, flattened and rod-shaped. It is terminal. The ventral marginal measures 15 mm. The 52 mm long and is distinctly curved anteriorly. pelvic fin radials are morphologically similar to those of The number of radials varies in stingrays and it has the pectoral fin. They are segmented and very slender. to be established if this is connected to the size of the individual or if this represents a taxonomic signal. Dis- tally to the basals, the radials broaden. The radials are Thoracolumbar synarcual and vertebrae elongated thin cartilages that support the ceratotrichia, which support the fin web. The last segments of the The presence of a second or thoracolumbar synarcual is radials are bifurcated. The bifurcation of the first four a synapomorphy of stingrays. It consists of several radials is less distinct with only the most distal parts fused vertebrae and is positioned posterior to the pelvic being branched. girdle (Fig. 3d). The corresponding vertebral centra The segmentation of the radials varies from nine seg- have a pair of basidorsals and a pair of basiventrales. ments anteriorly to 20 segments medially, 12 segments Two arch-like structures are attached to the basidorsals posteriorly. In the examined stingray, 96 radials are pre- forming the neural arch. Thoracic ribs are absent. served in the right pectoral fin, whereas 98 radials from The vertebrae anterior to the second synarcual are the left one indicating that the number of radials is not monospondylous consisting of a single centrum and a constant in a single specimen. The extremely fragile single pair of basiventrals and basidorsals. The verte- ceratotrichia are not preserved in the examined speci- brae posterior to the pelvic girdle are diplospondylous men. extending to about the middle portion of the sting. The vertebral centra of the remaining tail are fused forming an unsegmented rod (Fig. 1b) conversely to the condi- tion seen in Hexatrygon, Plesiobatis, Urolophus, Uro- Pelvic girdle and puboischiadic bar batis, and Urotrygon. In these taxa, diplospondylous The pelvic girdle (Fig. 3f) supports the pelvic fins lat- vertebrae extend to the tip of the tail. erally in stingrays and is simple. The puboischiadic bar The spine of the specimen in this study displays the is transversely flattened, slightly arched anteriorly and characteristic morphology for potamotrygonids in that bears some characteristic processes. According to Love- it is acute and laterally barbed. It measures 44 mm in joy (1996), the puboischiadic bar is strongly arched in length with 2/3 of its length are laterally flattened. Aetobatus, Rhinoptera, and Mobula, but less so in all other stingrays. One elongated process, the medial pre- pelvic process, extends anteriorly from the mid-portion Comparison of the pelvic girdle (Fig. 3f). Laterally, the rather short prepelvic processes extend anteriorly on each side. Ad- Potamotrygon motoro was examined and described only ditional processes extend posteriorly from the end of sporadically or incompletely in previous studies. The the U-shaped arch. In Heliotrygon, this process extends new results will be compared and discussed with pub- to the anterior one-third of the metapterygial length lished results (Lovejoy 1996; da Silva & Carvalho (Carvalho & Lovejoy, 2011) 2011; Carvalho & Lovejoy 2011) in the following chap-

# 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim museum-zoosyst.evol.wiley-vch.de 154 Stepanek, R. & Kriwet, J.: Morphology of Potamotrygon motoro (Chondrichthyes, Myliobatiformes) from South America ter to provide a basis for further comparisons and dis- The postorbital process is well developed in stingrays cussions as more studies on different taxa are available. as in the examined Potamotrygon motoro and Urotry- Miyakes (1988) investigation of Urotrygon was used gon rogersi (Lovejoy 1996). A lateral groove also is de- for some distinct elements. veloped as shown in Potamotrygon boesemani (Rosa et al. 2008, fig. 8) and Urolophus cruciatus (Lovejoy 1996, fig. 5C). In other skates or non-stingray batoids, Rostral cartilage the postorbital process is poorly developed or absent as in electric rays (Miyake 1988, fig. 59). The developing In stingrays, the rostral cartilage is not developed and of the lateral groove appears to create a new passage of the anterior edge of the disc is rounded. According to the infra-orbital lateral line canal. In other stingrays as Miyake (1988), nevertheless, a snout support is present in potamotrygonids, the canal passes through a foramen in the freshwater stingrays Potamotrygon and Plesiotry- in the postorbital process as shown in Urolophus cru- gon as well as in some species of urolophids. It is ciatus. The significance of this character remains un- formed by the anterior end of the pectoral fin and the clear. Rosa et al. (2008) examined the close related Po- articulation via the antorbital cartilage with the nasal tamotrygon boesemani and their investigation of the capsules. Miyake (1988) considers this cartilage to be neurocranium bears high resemblance to the examined homologous with the rostral appendix of guitarfishes Potamotrygon motoro (Fig. 2a) in this study. The preor- and skates. bital process doesn’t extent over the nasal capsules, a postorbital groove is present and the epiphysial bar is Neurocranium represented as a reduced remnant of a bridge that sepa- rates the cranial roof. According to Rosa et al. (2008) In pelagic stingrays, such as Dasyatis violacea, the na- the postorbital process laterally extends and is flattened sal capsules and the rest of the neurocranium form an just as the examined P. motoro in this study. angle while in non-pelagic stingrays the nasal capsules and the rest of the neurocranium are in the same plane (Miyake 1988). In the study presented here, the exam- Mandibular arch and hyomandibular ined Potamotrygon motoro specimen displays the same character. Both, Miyake and Lovejoy show in their in- The relative position, shape and size of the hyomandi- vestigations the ventro-lateral expansion of the nasal bular cartilage is be related to the morphological and capsules, but it seems to be a character that is difficult functional features. Major varieties exist in the connec- to quantify. In the specimen examined here, this feature tion between the mandibular arch and the hyomandibu- also is present. lar element. According to Lovejoy (1996), the connec- It appears that the epiphysial bar is only represented tion between hyomandibular and jaw elements in as a reduced remnant of a bridge in the examined Pota- stingrays can be direct or indirect via several skeletal motrygon motoro, which separates the cranial roof into components. The direct connection is present in Hexa- the anterior and the posterior fontanelle (Fig. 2a). One trygon and Plesiobatis for instance. In the examined reason could be that during the cleaning process, the freshwater stingray Potamotrygon motoro and in other Larder beetles also destroyed much of the delicate car- taxa, such as Paratrygon aireba, an indirect connection tilage. Another reason could be that the cartilage was via angular-a and angular-b is present (Fig. 3a), which so thin that it was destroyed during mechanically pre- is absent in Heliotrygon (Carvalho & Lovejoy 2011). paration. Although the epiphysial bar is variably devel- The Potamotrygon boesemani (Rosa et al. 2008, oped in different species, it always divides the neuro- fig. 8B) shows also both angular-a and -b. The angulars cranium into the anterior precerabral fontanelle and the are parallel and have nearly the same length conversely posterior fronto-parietal fontanelle. Some other sting- to the condition found in the P. motoro in this study rays, i.e., Urotrygon daviesi, Urolophus cruciatus and (Fig. 3a). However, previous investigations (e.g., Love- other freshwater stingrays have an incomplete epiphy- joy 1996, fig. 6) exemplify that both character states sial bar (Miyake 1988). Nevertheless, there are two fon- may occur in several taxa. Consequently, this character tanella developed. is not unambiguous to use it for phylogenetic analyses. The space between the orbitals various in several The difference within the several components connect- species, but in stingrays the interorbital region is rela- ing the hyomandibular and the mandibular arch is the tively narrow (Lovejoy 1996) as it also is in the studied only feature that can be employed and compared. In specimen here. amphi-American Himantura species, these elements The neurocranial processes vary in extension within vary strongly in sizes and number conversely to the the stingrays. The preorbital process might extend over condition found in Potamotrygon species (Lovejoy the nasal capsules as in Urotrygon rogersi and Urolo- 1996). According to Lovejoy (1996) and the examined phus cruciatus (Fig. 2a and Lovejoy 1996, fig. 5C). Potamotrygon motoro in this study, the two different an- However, in the examined Potamotrygon motoro it is gular cartilages are characteristic for this . For ex- restricted to the posterior region of the nasal capsules ample, in Plesiotrygon only one robust elongate angu- as shown in Figure 2a. lar-a is present and the second angular-b is absent. In

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Paratrygon the connection is considerably shorter and mented as in Dasyatis, Himantura, Taeniura, Urobatis only a very small cartilage is developed, the homology (Lovejoy 1996, fig. 7A), and all other potamotrygonids of which is unclear (Lovejoy 1996). It appears that (e.g. Carvalho & Lovejoy, 2011). Unsegmented ba- these cartilages are functionally important. Although sihyals are present in Plesiobatis (Lovejoy 1996, the size and number of the cartilages vary within the fig. 7B), Hexatrygon, Urolophus, Gymnura (Lovejoy species, it seems that they have a similar functional 1996, fig. 7E), and Aetoplatea. Due to that extreme var- role. This hypothesis is supported by the fact that the iation within the segmentation it has to be more speci- connections are more than only a simple ligamentous fic, which character state is developed in which taxon connection. and additional material is needs to be analysed. In other stingrays such as Myliobatis, Aetobatus, Rhi- The pseudohyal is not preserved in the specimen ex- noptera and Mobula, a cartilage element close to the amined herein. This could be the result of the employed anterior tip of the hyomandibular was observed by preparation method, because this element is very fra- Lovejoy (1996). Nevertheless, it appears that these car- gile. According to Lovejoy (1996), the pseudohyal is tilages do not have the same functional role as the car- fused to the first ceratobranchial, which is an apomor- tilages in Potamotrygon, Himantura and Plesiotrygon. phy for stingrays. However, there are species that lack In the examined specimen of Potamotrygon motoro, this fusion and other taxa, in which the degree of the the wing-shaped mandibular process is well developed connection between these elements varies. For example, (Fig. 3a). According to Lovejoy (1996), it is also well in Urolophus there is only a connection between pseu- developed in other potamotrygonids, Dasyatis, Tae- dohyal and first ceratobranchial. Another possibility is niura, and Himantura but less developed in Urolophus, the fusion between the first two ceratobranchials and Urobatis, and Urotrygon. This variation in form and the pseudohyal. In the examined Potamotrygon motoro size makes it difficult to code this character in phyloge- the first three ceratobranchials are fused (Fig. 3b). netic hypothesis. However, Lovejoy’s Potamotrygon motoro displays four Summers (2000) examined myliobatid stingrays, such fused ceratobranchials. It appears that these fusion pat- as Rhinoptera bonasus, Rhinobatos lentiginosus, Dasya- terns are complex and variable within genera and spe- tis sabina, and Dasyatis sayi. According to this study cies (Lovejoy 1996, fig. 7) the jaws of the examined specimens are all very robust and larger than those of other stingrays. The mandibu- lar symphysis and the hyomandibular symphysis are en- Scapulocoracoid tirely fused (Summers 2000, Fig. 2), controversially to Most stingray taxa have four scapulocoracoid foramina: the condition seen in Potamotrygon motoro of this the anterodorsal, anteroventral, posterodorsal and the study, which clearly displays the fused suture. posteroventral foramina (Lovejoy 1996, fig. 9). Accord- In Myliobatis, Aetomylaeus, and Rhinoptera flattened ing to Nishida (1990) the posterodorsal foramen is ab- teeth bands forming a grinding dentition as in all exam- sent in Urolophus, Urobatis, and Urotrygon. However, ined specimen by Summers (2000, fig. 7). In all other Lovejoy (1996) observed small posterodorsal foramina stingrays, including the examined Potamotrygon mo- in some stingray such as e.g., Urotrygon rogersi. In the toro, teeth stripes are shown as in Figure 3a. The ar- examined specimen, no posterdorsal foramen is present. rangement of the teeth is mainly related to the feeding Hence, it appears that this character varies within this mechanism. species and a broad species analysis is required. In most stingrays, the foramen of the scapular pro- cess is present. According to Miyake (1988) it is absent Branchial arches in Plesiobatis, Urolophus, Potamotrygon, Paratrygon, and Plesiotrygon. There are neither a foramina nor a Bridges over the aorta and other vessels projects from fossa. This interpretation is supported by this analysis. the medial plate. These projections are present in the examined Potamotrygon motoro specimen (Fig. 3b), and in Plesiobatis, Hexatrygon, Urobatis, Urotrygon, Urolo- Pectoral fin phus, and Gymnura (Lovejoy 1996). However, these projections are not present in all potamotrygonids and The pectoral propterygium is segmented anteriorly. In stingrays, e.g. dasyatids and pelagic myliobatids. There- the investigated Potamotrygon motoro the segmentation fore, this character is not useful for phylogentic ana- occurs directly at the level of the nasal capsules as lyses. shown in Figure 3E. This condition is also present in The basihyal is variably developed in the different amphi-American Himantura, Heliotrygon, Paratrygon, stingray taxa. It could be segmented, unsegmented or Plesiotrygon, Urolophus, Urotrygon, Urobatis and Tae- absent. Additionally, the degree of segmentation varies niura (Lovejoy, 1996; Carvalho & Lovejoy, 2011). In extremely. In the here-examined Potamotrygon motoro, other taxa, such as Indo-West Pacific Himantura, Da- the basihyal is not segmented or strongly fused Fig. 3b). syatis, Gymnura, and Myliobatis the segmentation oc- However, in Lovejoy´s (1996, fig. 7F) investigation of curs anterior to the nasal capsules (Lovejoy 1996, Potamotrygon motoro the basihyal is shown to be seg- fig. 10B). Another possibility is the segmentation pos-

# 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim museum-zoosyst.evol.wiley-vch.de 156 Stepanek, R. & Kriwet, J.: Morphology of Potamotrygon motoro (Chondrichthyes, Myliobatiformes) from South America terior to the nasal capsules in other outgroups (Lovejoy much variation within the taxa requiring more material 1996). to be studied before the significance of this character In most stingrays the mesopterygium articulates with can be established. the posterior aspect of the propterygium. According to In the Potamotrygon motoro specimen of this study, Lovejoy (1996, figs 10D and E), this form of articula- the first synarcual is only slightly higher than wide. tion is absent only in Potamotrygon and Plesiotrygon. Conversely, the first synarcual is wider than higher in The mesopterygium can be segmented, such as in Gym- Paratrygon (Lovejoy 1996). The presence of the second nura and Myliobatis, or be absent or fused with the sca- synarcual is an apomorphic character for stingrays. pulacoracoid, such as Aetobatus, Rhinoptera, and Mo- Diplospondylous vertebrae start at about the level of bula. In the examined P. motoro the mesopterygium is the pelvic girdle and continue to the tip of the tail as in not segmented (Fig. 3e). The number of radials varies Hexatrygon, Plesiobatis, Urolophus, Urobatis, and Uro- in the different stingray taxa and it has to be estab- trygon (Lovejoy 1996). In these taxa, the cartilaginous lished if this is connected to the size of the individual rod is absent, which is present in other stingray taxa, or if there are some other reasons for this. such as the investigated P .motoro.

Pelvic girdle Conclusions The most conspicuous character in the investigated Po- Most species of Potamotrygon have been defined by tamotrygon motoro specimen is the sting-shaped prepel- their coloration patterns. However, an extreme intraspe- vic process (Fig. 3f). In most stingrays this process only cific coloration variation occurs in many species de- is minute or small developed (Lovejoy 1996, fig. 11). pending on several factors including conditions However, in Heliotrygon, Paratrygon, Plesiotrygon, Po- and others. Consequently, the current number of de- tamotrygon, and the pelagic stingrays such as Rhinop- scribed species might be exaggerated. Additionally, the tera and Mobula it also is characteristicly elongated coloration pattern does not provide any information for (Lovejoy 1996). addressing evolutionary or phylogenetic issues. Anato- The puboishchiadic bar is arched anteriorly in the ex- mical characters are more important in this respect. amined specimen as in other stingray taxa (Lovejoy However, anatomical information for potamotrygonids 1996). According to Lovejoy (1996), it is extremely still is sparse. All available anatomical information of arched in Aetobatus, Rhinoptera, and Mobula, but less Potamotrygon and closely related taxa comes from few so in all other stingrays. studies, which either focused on extinct stingrays (e.g. Carvalho et al. 2004) or analysed the interrelationships Vertebrae of stingrays in general (e.g. Lovejoy 1996). Recently, da Silva & Carvalho (2011) presented a detailed mor- The first synarcual displays the most conspicuous dif- phological account of Potamotrygon tatianae. So far, ferences within this species with regard to the lateral the inter- and intraspecific as well as sexual and onto- stay and the base of the lateral stay. The base of the genetic variation of the skeletal system of potamotrygo- lateral stay is pierced by foramina. According to Love- nids have not been established. joy (1996) the lateral stay contacts the synarcual dor- The description presented here is the most detailed sally to the foramina and is not pierced as the specimen morphological account of a male specimen of Potamo- examined here (Fig. 3c). This character is observed in trygon motoro to date. It provides additional informa- the examined P. motoro and by Lovejoy (1996) in his tion to those presented in the studies of Lovejoy (1996) examined P. motoro, P. constellate, Urobatis jamaicen- and Carvalho et al. (2004). Nevertheless, more speci- sis and Plesiobatis iwamae (Lovejoy 1996, fig. 8) In all mens including females and individuals of different on- other taxa, the lateral stay joins the synarcual ventrally togenetic stages are needed to examine the variation of to the foramina and is pierced as in Urobatis halleri, skeletal characters and to identify features, which are Paratrygon aireba, and other taxa (Lovejoy 1996). important for phylogenetic inferences. Consequently, The lateral stay might be reduced or absent but it is this study is considered a first step to elucidate the present in most stingrays. The degree of the extension morphological anatomy of freshwater stingrays in gen- of the lateral stay also varies. It is delicate in Urotrygon eral and to assemble reliable morphological characters and Pacific coast Urobatis specimens (Lovejoy 1996). for inferring relationships and evolutionary aspects of The degree of the lateral projection also varies among this highly interesting group. So far, the monophyly of batoids in general. According to Claeson (2008), in Potamotrygonidae including Heliotrygon, Paratrygon, Raja inornata the lateral stay is present midway the syn- Plesiotrygon and Potamotrygon is largely based on phy- arcual and curves dorsolateral similar than in the ex- siological (urea excretion in urine) and soft-part fea- amined Potamotrygon motoro in this study. P.motoro tures (reduced rectal gland) (Carvalho et al. 2004). The features only a medial crest. However, Raja inornata only skeletal character supporting their monophyly is additional features synarcual spines, which extend to the very distinct median prepelvic process (Carvalho the medial crest (Claeson 2008, Fig. 1). There is too et al. 2004: fig. 16B). The function of this prepelvic

museum-zoosyst.evol.wiley-vch.de # 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Zoosyst. Evol. 88 (2) 2012, 145–158 157 process remains unclear momentarily. The absence of cies of Neotropical freshwater stingrays from the Amaton basin such an elongated prepelvic process in extinct fresh- (Chondrichthyes: Potamotrygonidae). – Zootaxa 2776: 13–48. water stingrays such as y Asterotrygon and y Heliobatis Claeson, K. M. 2008. Variation of the synarcual in the California Ray, Raja inornata (: Rajidae). – Acta Geologica from the Eocene Green River Formation (U.S.A.) (Car- Poloniaca 58: 121–126. valho et al. 2004) indicates that this feature is not re- Compagno, L. J. V., 1990. Relationship of the megamouth shark, lated to the adaptation to freshwater conditions. We Megachasma pelagios (Lamniformes: Megachasmidae), with com- also were not able to identify any other skeletal feature ments on its feeding habits. In Pratt, H. L., Gruber, S. H. & Ta- that might be related to a freshwater life-style. Thus, it niuchi, T. (eds). Elasmobranchs as living resources: Advances in is currently not possible to identify freshwater adapted the biology, ecology, systematics, and the status of . batoids by unique dental or skeletal characters and sedi- NOAA Technical Report NMFS 90: pp. 357–379. Compagno, L. J. V. 1999. Endoskeleton. In Hamlett, W. C. (ed.). mentological information still is necessary for interpret- Shark, skates, and rays. The biology of elasmobranch fishes. The ing the occurrence of fossil freshwater stingrays. John Hopkins University, Baltimore: pp. 69–92. Resolving dental features in freshwater stingrays is Da Silva, J. P. & Carvalho, M. R. de 2011. A new species of Neotro- urgently needed to describe dental morphologies in re- pical freshwater stingray of the genus Potamotrygon Garman, cent taxa and identify fossil freshwater stingrays. So 1877 from the Ro Madre de Dos, Peru (Chondrichthyes: Pota- far, the only known fossil remains are isolated bucklers, motrygonidae). – Papis Avulsos de Zoologia 51: 139–154. tubercles, spines, and oral teeth from the Miocene of Dean, M. N. & Summers A. P. 2006. Mineralized cartilage in the ske- leton of chondrichthyan fishes. – Zoology 190: 164–168. South America (Frailey 1986; Deynat & Brito 1994; Deynat, P. P. & Brito, P. M. 1994. Rvision des tubercules cutans de Lundberg 1997, 1998; Brito & Deynat 2004). The only raies (Chondrichthyes, ) du Bassin du Paraa, tertiaire known fossil record of dasyatid freshwater stingrays d’Amrique du Sud. – Annales de Palontologie 80: 237–251. constits of caudal spines and oral teeth of Dasyatis afri- Feibel, C. S. 1994. Freshwater stingrays from the Plio-Pleistocene of cana from the Plio-Pleistocene of Ethiopia and Kenya the Turkana Basin, Kenya and Ethiopia. – Lethaia 26: 359–366. (Feibel 1994). Thus, the incomplete fossil currently Frailey, C. D. 1986. Late Miocene and Holocene mammals, exclusive cannot contribute to inferring any origination dates and of the Notoungulata, of the Ro Acre region, western Amazonia. – Contributions in Science 374: 1–46. inference of the timing of their origins and adaptation Garman, S. 1913. The Plagiostomia (Sharks and Rays). Memoirs of to freshwater conditions rests on the interpretation of the Museum of Comparative Zoology, Harvard College, Cam- their systematic position within Myliobatiformes and bridge Massachusetts 36: 1–528. the identification of their sister group. For this more Klug, S. & Kriwet, J. 2010. Timing of deep-sea adaptation in dogfish and detailed morphological data is necessary, which sharks: insights from a supertree of extinct and extant taxa. – Zo- presently is not available. ologica Scripta 39: 331–342. Kriwet, J. & Benton, M. 2004. Neoselachian (Chondrichthyes, Elas- mobranchii) diversity across the Cretaceous- boundary. – Palaeogeography, Palaeoclimatology, Palaeoecology, 214: 181– Acknowledgements 194. Kriwet, J. & Klug, S. 2004. Late Jurassic selachians (Chondrichthyes, We would like to thank the Aquazoo and Lbbecke Museum Dssel- Elasmobranchii) from southern Germany: Re-evaluation on taxon- dorf (especially Silke Stoll) for the donation of the studied specimen omy and diversity. – Zitteliana, A44: 67–95. of Potamotrygon motoro, and for the helpful information. P. Bartsch Kriwet, J. & Klug, S. 2009. Fossil record and origin of squaliform (Museum fr Naturkunde, Berlin, Germany) is acknowledged for sharks (Chondrichthyes, Neoselachii). In Gallucci, V., McFarlane, making the specimen available for this study. Special thanks go to the G. & Bargmann, G. (eds). Biology and Management of dogfish members of the former palaeoichthyological working-group at the sharks. 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