Contributions to the skeletal anatomy of freshwater ...homepage.univie.ac.at/rica.stepanek/PDF/Stepanek kriwet 2012-P.motoro.pdf · Contributions to the skeletal anatomy of freshwater

Contributions to the skeletal anatomy of freshwater stingrays(Chondrichthyes, Myliobatiformes):1. Morphology of male Potamotrygon motoro from South America

Rica Stepanek*, 1 and J�rgen Kriwet

University of Vienna, Department of Paleontology, Geozentrum (UZA II), Althanstr. 14, 1090 Vienna, Austria

Introduction

Neoselachians include all living sharks, rays, andskates, and their fossil relatives. Their monophyly iswell established and beyond any dispute although theinterrelationships of several clades within Neoselachiiremain controversial. Major lineages include the Galeo-morphii, Squalomorphii and Batomorphii, most ofwhich are key predators in modern marine environ-ments. Generally perfectly adapted to the environmentthey are living in, several groups developed specialadaptations. Fossil evidence suggests that neoselachianshave been primarily marine predatory from their ear-liest beginnings and throughout their long evolutionarycareer although freshwater adaption occurred severaltimes independently. Major neoselachian radiations arerecognized in the Early Jurassic, at the end of the EarlyCretaceous, in the middle of the Late Cretaceous, andin the Cenozoic (Kriwet & Klug 2004; Kriwet & Ben-ton 2004; Kriwet et al. 2009). The most recent elasmo-branch radiation in the Cenozoic supposedly coincided

with the radiation of mammals. Living elasmobranchsare thus the result of a long evolutionary history.

Some of the most astonishing and unprecedented ex-pressions of neoselachian evolution are the adaptationto deep-sea environments with all the required physio-logical changes (Kriwet & Klug 2009; Klug & Kriwet2010), the development of filterfeeding and durophagy,and freshwater adaptations (e.g., Compagno 1990; Car-valho et al. 2004). The fossil record of freshwateradapted elasmobranchian clades extends back into thePalaeozoic and by the Early Cretaceous, several hybo-dontiform and neoselachian lineages seemingly werefully adapted to freshwater conditions. Early in theCenozoic, various stingrays (e.g., y Asterotrygon,y Heliobatis) invaded freshwater habitats like rivers andlakes (Carvalho et al. 2004), which probably is in con-junction with the most recent neoselachian radiationevent.

Nevertheless, despite the development of diverseneoselachian faunas within the Meso- and Cenozoic,the palaeoenvironmental specificity of the taxa has gen-

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

Received 8 August 2011Accepted 17 January 2012Published 28 September 2012

Key Words

BatomorphiiPotamotrygonidaeTaxonomySkeletal morphology

Abstract

The skeletal anatomy of most if not all freshwater stingrays still is insufficiently knowndue to the lack of detailed morphological studies. Here we describe the morphology ofan adult male specimen of Potamotrygon motoro to form the basis for further studiesinto the morphology of freshwater stingrays and to identify potential skeletal featuresfor analyzing their evolutionary history. Potamotrygon is a member of Myliobatiformesand 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 anatomystill is very insufficient despite numerous studies of freshwater stingrays. These studies,however, mostly consider only external features (e.g., colouration patterns) or selectedskeletal 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 thebasis of prospective anatomical studies of potamotrygonids.

* Corresponding author, e-mail: [email protected]

erally been ignored. One of the key questions is whatenvironmental changes precipitated these profoundadaptations? The reasons for freshwater adaptations andthe underlying processes are not yet understood. Inter-estingly, some bony fishes and dolphins also enteredthe proto-Amazonas as well as stingrays of the familyPotamotrygonidae (e.g. Lovejoy et al., 2010; M. Brito,pers. comm.). All groups nevertheless have a very pat-chy fossil record. The oldest known of these very dif-ferent groups remains from the Miocene indicate thatthey probably adapted to freshwater environments aboutthe same time.

At present, four living potamotrygonid genera arewell-established: Heliotrygon (Carvalho & Lovejoy,2011), Plesiotrygon (Rosa et al., 1987), PotamotrygonGarman, 1877, and Paratrygon Dumeril, 1865.

Although this study is based on a single male speci-men, it represents the most comprehensive morphologi-cal account of any freshwater stingray from SouthAmerica to date and provides important skeletal infor-mation of freshwater stingrays in general. Miyake(1988) only considered the systematics of the genus Uro-trygon and Lovejoy (1996) and Carvalho et al. (2004)focused on the phylogenetic interrelationship of stingraysproviding only selected morphological information.Moreover, different and partly misinterpreted morpholo-gical information of Potamotrygon motoro exist in theliterature.

Thus, the intentions of this paper are to (1) presentthe first detailed anatomical description of Potamotry-gon motoro from the Amazon Basin and (2) review andcomment the interrelationships and origin of freshwaterstingrays. This paper is the first part in a series of mor-phological studies of freshwater stingrays.

Material and methods

The adult specimen of Potamotrygon motoro that forms the basis ofthis study was part of a breeding program at the Aquazoo and L�b-becke Museum in D�sseldorf, Germany and was donated to the Mu-seum of Natural History Berlin, Germany for this study after skeletalpreparation. For this, the specimen was mechanically prepared and asmuch soft tissue as possible was removed in a first step. In a nextstep, larder beetles were used for cleaning the skeleton from all re-maining soft tissue. Finally, the skeleton was cleaned using ethyl alco-hol and bleached with hydrogen peroxide.

The morphological terminology follows that of Lovejoy (1996) andCompagno (1999). For this study, the specimen was documented witha digital camera. Small skeletal elements were studied and analysedusing a magnifier with a twelve-fold magnification (Eschenbach).Measurements were taken on dried cartilages with digital calliperswith 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(> 100 mm), respectively, and the repeatability is 0.01 mm. Thesemeasurements are important for establishing confidential meristic datafor comparisons with other stingray specimens in prospective studies.Meristic accounts are provided in Table 1 and measurements usedherein are:

Total length, measured from the anterior tips of pectoral fins toposterior end of the tail.

Width of disc, measured between extreme outer corners of disc.

Pectoral fin length, measured from the anterior top of the first ra-dial, which is normally the longest, to the posterior longest radial. Thelongest radial doesn’t necessarily have to be the last radial. The widthof the fin is measured from mesopterygium to the edge of the fin.

Pelvic girdle, measured from the anterior top of the prepelvic pro-cess to the posterior end of the mixopterygium, or clasper.

Neurocranium length, measurements are from the anterior end ofthe nasal capsules to the articulation with the first synarcual. Thewidth is measured from the distal point of the right preorbital processto the left preorbital process. The depth is measured at the posteriorend of the fronto-parietal fontanel.

Length of cervicothoracic synarcual, measured between the anteriorarticulation to the neurocranium and the posterior articulation withthe pelvic girdle. The depth is measured at the highest point includingthe crest.

Tail length, two distances were measured to establish the completelength of the tail because of a sharp bend near the articulation of thesting. Therefore, both anterior and posterior tail proportions related tothe insertion of the spine were measured separately and subsequentlycombined. In this case, the tail measurements start at the anterior ar-ticulation to the pelvic girdle to the posterior end of the tail and com-prise the second synarcual and the following vertebrae.

Abbreviations. aa, angular cartilage a; ab, angular cartilage b; ac, an-torbital cartilage; adf, anterodorsal foramen; ba, branchial arches;baco, basibranchial capula; bar, scapulacoracoid bar; bpy, basiptery-gium; bhy, basihyal; bridge, bridge projection; cfo, crest foramen;chy, ceratohyal; cl, clasper (mixopterygium); cr, crest; cute, curledterminal; doma, dorsal marginal; epb, epiphysial bar; fpf, fronto-par-ietal fontanelle; fsy, first synarcual; hy, hyomandibular; 1st hypo, firsthypobranchiale; ilp, iliac process; ins, intermediate segments; isp,ischia process; last, lateral stay; lppp, lateral prepelvic process; ma,mandibular arch (Meckels cartilage); map, mandibular process;mono, monospondylous vertebrae; mpa, metapterygial axis; mplate,medial plate; mppp, medial prepelvic process; mspy, mesopterygium;mpy, metapterygium; na, nasal capsules; nc, neurocranium; ob, orbi-tal; oc, otic capsule; pcf, pectoral fin; pcg, pectoral girdle; pef, pelvicfin; peg, pelvic girdle; pf, precerebral fontanelle; poc, preorbital pro-cess; pog, postorbital groove; pop, postorbital process; ppy, proptery-gium; pqu, palatoquadrate (upper jaw); ra, radialia; snf, spinal nerveforamina; soc, supraorbital crest; ssy, second synarcual (vertebrae);sp, scapular process; st, sting; veme, ventral marginal. All scale bars(if not otherwise stated) equal 1.0 cm.

Morphology of Potamotrygon motoro

Morphological characters were described by differentauthors previously but mostly with the focus on a parti-cular organ system or selected skeletal components.The morphological description presented in this studyis based on a single, adult male specimen (Fig. 1). Thetotal length of the specimen is 350 mm and its totaldisc width is 170 mm. Dorsal fins are absent.

Calcified cartilages

The skeleton mainly consists of cartilage. Densely cal-cified tissue occurs in the double cones of vertebralcentra (= alveolar calcification sensu Moss 1967). Mostskeletal elements are covered by tesserae of tiny hexa-gonal calcium-phosphate crystals (= tesserate cartilage).These tesserae are functionally important in stiffeningthe axial skeleton or parts of the cranium (e.g. jaws asadaptation to durophagy) when arranged in several

Stepanek, R. & Kriwet, J.: Morphology of Potamotrygon motoro (Chondrichthyes, Myliobatiformes) from South America146

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

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layers (Dean & Summers 2006). The multiplication oftesserate layers in the jaws of Potamotrygon is analo-gous to cortical thickening (Summers 2000). These tes-serae are perichondrial in origin.

Neurocranium

The shape of the neurocranium is related to functionalaspects of the jaws and locomotion. It encloses thebrain and the olfactory, auditory, and visual organs. Indorsal view, the neurocranium is rectangular broadeninganteriorly because of the large nasal capsules. In lateralview, the neurocranium is box-like with the base beinghorizontal.

Stingrays lack a rostral cartilage. According toMiyake (1988) only the freshwater stingrays Potamotry-gon and Plesiotrygon as well as some urolophids pos-sess what is called a “functional rostrum” (Fig. 1). Theanterior ends of the pectoral fins form this rostrum,which articulate via the antorbital cartilages with thenasal capsules. This structure is assumed to be homolo-gous to the rostrum of guitarfishes and skates byMiyake (1988).

The two nasal capsules, which enclose the olfactoryorgans, are located at the anterior end of the craniumand display a w-shaped anterior margin with a medially

concave protrusion (Fig. 2). The apertures are consider-ably wider than long and inclined antero-ventrally, sothat the apertures face downward. In pelagic stingrays,such as Dasyatis violacea, the nasal capsules and therest of the neurocranium form an angle while in non-pelagic stingrays the nasal capsules and the rest of theneurocranium are in the same plane (Miyake 1988). Inthe study presented here, the examined Potamotrygonmotoro specimen displays the latter condition. Both,Miyake (1988) and Lovejoy (1996) show in their inves-tigations the ventro-lateral expansion of the nasal cap-sules, but this character seems difficult to quantify.

The fontanel is divided into an anterior and a poster-ior portion. The anterior precerebral fontanel and theposterior fronto-parietal fontanel are incompletely se-parated by a median constriction, which represents thereduced epiphyseal bar (Fig. 2a). An incomplete orstrongly reduced epiphyseal bar is common in many ex-tant and extinct stingrays (Carvalho et al. 2004).

The precerebral fontanel is wider than the fronto-par-ietal fontanel, which narrows posteriorly (Fig. 2a). Thelength of both fontanels measures 45 mm. The w-shaped form of the anterior margin of the nasal cap-sules continues into the anterior margin of the precer-ebral fontanel. In the examined specimen the roof ofthe brain, the tectum orbitale, is indeterminable.


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 – –

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


The orbital region is only slightly longer than the oti-co-occipital region. The space between both orbitals isrelatively narrow and shorter than the full length of theneurocranium as in other stingrays.

The supraorbital shelf is well developed and itslength more or less equals the distance between thepreorbital and postorbital process. The anterior regionof the orbit is completely enclosed by the preorbitalprocess, which does not extend over the nasal capsules.In other stingrays such as Urotrygon rogersi and Urolo-phus cruciatus, the preorbital process extends over thenasal capsules (e.g. Lovejoy, 1996).

The well-developed postorbital process (sensu su-praorbital process in Carvalho at al. 2004) marks theposterior margin of the orbit. It is separated from thesupraorbital shelf by a notch, extends laterally and isdorso-ventrally flattened. This process is divided in ananterior and a posterior portion by the postorbitalgroove (Fig. 2a). According to Lovejoy (1996), thisgroove provides the passage for the intra-orbital lateralline canal and is characteristic for potamotrygonids butalso some other stingrays such as Urolophus cruciatus.In other stingrays, the canal passes through a dorso-ventral positioned opening into the process. The evolu-tionary significance of this character momentarily re-mains dubious pending further studies. In skates andseveral non-myliobatiforms, the postorbital process ispoorly developed or absent as in electric rays (Miyake1988: fig. 59).

The suborbital shelf is a horizontal plate ventral tothe orbit. In the examined Potamotrygon motoro it ismissing. The sclerotic ring enclosing the eyeballs isoval.

The aperture for the iris is large and elongated and amedial foramen for the optic nerve is present.

The otic capsules housing the inner ears are situatedposterior to the postorbital process and are broadlyrounded. The hyomandibulae attach latero-ventrally tothe posterior region of the otic capsule.

The occipital region includes the posterior end of theneurocranium. Here, the first cervical centrum of thevertebral column articulates with the occiput. On eachside there is an occipital condyle that articulates withthe first basiventral. The otico-occipital region occupiesonly 1/3 of the total length of the neurocranium.

Mandibular arch and hyomandibulae

The elements of the mandibular arch comprising thepalatoquadrate and Meckel’s cartilage and the hyoman-dibulae are part of the viscerocranium. The hyomandi-bula of Potamotrygon are elongated and represent theupper portion of the hyoid arch. It forms the hyostylictype of articulation of the jaws with the neurocranium.The ventral element of the hyoid arch, the ceratohyaleor pseudohyoid, also is elongated.

According to Nishida (1990) and Lovejoy (1996) twomain types of connection between the hyomandibulae

and mandibular arches are present in stingrays. Eitherthe hyomandibula attaches directly to the mandible asin Hexatrygon and Plesiobatis or there is a stout liga-ment, the “hyomandibulo-Meckelian tendon” of McEa-chran et al. (1996) connecting the hyomandibular di-rectly to the mandible as in Paratrygon. InPotamotrygon, the hyomandibula also connects toMeckel’s cartilage by a strong ligament, in which smallcalcifications, the angular cartilages may occur. Thenumber of angular cartilages within the ligament of Po-tamotrygon varies from one (as also found in Plesiotry-gon) to two (Carvalho et al. 2004). Additionally, the ar-rangement and size of the angular cartilages may varyas seen in, for instance, Potamotrygon boesemani, inwhich these cartilages are arranged parallel and havethe same size (Rosa et al. 2008) conversely to the con-dition displayed by the specimen of Potamotrygon stud-ied here (Fig. 2b).

In the examined specimen of Potamotrygon motoroin this study, two angular cartilages are present on bothsides of the jaws, which differ in size and form(Fig. 3a). The anterior angular cartilage (angular a)measures 9.0 mm in length and is considerably longerand arched than the posterior one. It directly connectsto the mandibular and hyomandibular. The second pos-terior angular cartilage (angular b) is very small meas-uring 3.0 mm and is drop-shaped. Thus it is only athird the length of the anterior one. It is located nearthe articulation of the hyomandibula with angular a.

These calcifications serve for reinforcing the articula-tion (Garman 1913) but also enlarge the distance be-tween jaws and the neurocranium considerably conver-sely to the direct articulation mode. These modificationsin jaw articulation are related to and improve the functionof the jaws during feeding. It appears that the connectiveligament is important for functional aspects of openingand closure of the jaws.

The presence, number and size of angular cartilagesare ambiguous and difficult to evaluate for use in phy-logenetic analyses. In amphi-American Himantura spe-cies, i.e., these elements vary strongly in size and num-ber conversely to the condition found in Potamotrygonspecies (Lovejoy 1996). Angular cartilages are absentin Paratrygon, which represents the plesiomorphic con-dition within Potamotrygonidae. Consequently, the pre-sence of angular cartilages cannot be regarded a syna-pomorphy of Potamotrygonidae as previously suggestedbut is a synapomorphy of Potamotrygon þ Plesiotry-gon. In Plesiotrygon a single robust, elongated angularcartilage is present and no variation in their number isdiscernable. In Paratrygon the connection between hyo-mandibula and Meckel’s cartilage is considerably short-er and only a very small cartilage might be developed,the homology of which remains ambiguous (Lovejoy1996).

In other stingrays such as Myliobatis, Aetobatus, Rhi-noptera and Mobula, a cartilage element close to theanterior tip of the hyomandibular was observed byLovejoy (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 viewof the branchial arch, the first synacuum and the pectoral girdle; d. Posterolater view of the pectoral girdle; e. Dorsal view of thepectoral fin with all radials; f. Dorsal view of the pelvic girdle with the articulated pelvic fin and the clasper; g. Ventrolateral viewof the basipterygium and the mixopterygia. Abbreviations are explained in the methods section.


not the same function as the angular cartilages in theligament of Potamotrygon, Himantura and Plesiotrygonand their development and homologies remain uncer-tain.

The wing-shaped mandibular process in the exam-ined specimen of Potamotrygon motoro, which is closeto the attachment between mandibular arch and angulara is well developed and overlap both jaws (Fig. 3a). Ac-cording to Lovejoy (1996), it is also well developed inother potamotrygonids, Dasyatis, Taeniura, and Himan-tura but less developed in Urolophus, Urobatis, and Uro-trygon. This variation in form and size makes it difficultto code this character in phylogenetic hypothesis.

The symphysis of the mandibles is not fused in Pota-motrygon but meets along a vertical suture (Fig. 3a). Inother myliobatiforms, such as Rhinoptera and Dasyatis,the mandibles (as well as the hyomandibulae) are en-tirely fused (Summers 2000: Fig. 2).

Teeth of Potamotrygon motoro are unicuspid, smalland arranged in multiple rows along the jaws. In somederived myliobatiforms such as Myliobatis, Aetomy-laeus, and Rhinoptera teeth are broadened resultingfrom the fusion of individual teeth and are arrangeddensely forming characteristic grinding dentitions.

Branchial skeleton

The branchial skeleton, which also is part of the viscer-ocranium, comprises five arches (Fig. 3b). The ventralbranchial skeleton consists of an enlarged central med-ial plate, which resulted from the fusion of the basi-branchial copula and the basibranchial components(Miyake & McEachran 1991; Carvalho et al. 2004), ashort and transversely directed basihyal, a pair of shortand anteriorly directed hypobranchials, and five pairsof ceratobranchials.

In the examined specimen, a single small bridge pro-jects ventrally from the medial plate (Fig. 3b). Accord-ing to Lovejoy (1996), this bridge forms a shelter forthe aorta and afferent branchial vessels. Such projec-tions also are present in Plesiobatis, Hexatrygon, Uro-batis, Urotrygon, Urolophus, and Gymnura (Lovejoy1996), but are absent in some potamotrygonids andsome other stingrays such as Dasyatis and pelagic My-liobatis species.

The anterior end of the medial plate is perforated bya large foramen between the first and second cerato-branchials. At the posterior end of the medial plate is asting-shaped projection, which represents the basibran-chial copula. It forms the ventral cover of the pericar-dial cavity.

The five pairs of ceratobranchials originate at thecentral medial plate. According to Miyake & McEa-chran (1991), the second to fifth hypobranchials andbasibranchials are coadunate to the central medial platein stingrays.

Anteriorly, the hypobranchials are laterally attachedto the basihyal. The basihyal is variably developed in

stingrays and might be segmented, unsegmented oreven absent. In the examined specimen, the basihyal isunsegmented and the elements are completely fused(Fig. 3b). Its anterior margin is slightly convex, whereasthe posterior edge is almost straight. This conditionseems to be extremely variable and therefore, its phylo-genetic inference must be reviewed of a larger samplesize.

However, the basihyal in Lovejoy’s (1996: fig. 7) in-vestigation of Potamotrygon motoro is shown to be seg-mented similar to the condition found in Dasyatis,Himantura, Taeniura, Urobatis, and the remainingpotamotrygonid taxa. The basihyal of Plesiobatis, Hexa-trygon, Urolophus, Gymnura, and Aetoplatea is unseg-mented (Lovejoy 1996). Due to that extreme variation,more data on the morphology of the basihyal, which ispotentially important for phylogenetic reconstructions, isnecessary.

The first hypobranchial is elongate and the anteriorend is forked. The proximal end articulates with the ba-sihyal, whereas the distal edge articulates with the firstceratobranchials.

The three anterior ceratobranchials meet each othermedially. The first ceratobranchial articulates with thesecond one but only the third ceratobranchial articulateswith the medial plate.

According to Lovejoy (1996), the pseudohyal isfused to the first ceratobranchial, which represents anapomorphy of stingrays. The pseudohyal is not pre-served in the studied specimen, which certainly repre-sents an artefact. In other stingrays such as Urolophus,the pseudohyal contacts the first ceratobranchial but isnot fused to it (compare Lovejoy 1996). In other taxa,the first two ceratobranchials are fused to the pseu-dohyal.

In the examined Potamotrygon motoro specimen thefirst three ceratobranchials are fused (Fig. 3b). How-ever, Lovejoy (1996) indicates four fused ceratobran-chials in Potamotrygon motoro. It appears that these fu-sion patterns are complex and variable within generaand species (compare Lovejoy 1996: fig. 7). The fourthceratobranchial completely articulates with the medialplate.

The ceratobranchials articulate dorsally with the epi-branchials and through small pharyngobranchials withthe occipital region of the neurocranium (first branchialarch) and the anterior region of the first synarcual (sec-ond to fourth branchial arches). Only the fifth epibran-chials articulate directly with the scapulocoracoid.

It appears that the proximal first gill ray at the cera-tobranchial is wider than the following ones and wing-shaped. The last ray of the epibranchial, where it ar-ticulates to the synarcual, resembles the first gill ray ofthe ceratobranchial. The number of rays of the ventralceratobranchials and dorsal epibranchials varies. Thefirst ceratobranchial bears ca. nine rays and there arefive rays on the last ceratobranchial, respectively. Thenumber of rays (excluding the wing-shaped ray) variesbetween nine and 11 at the epibranchials.

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Vertebral column

Vertebral centra are spool-like calcified elements origi-nating directly posterior to the neurocranium and ex-tending to the distal end of the caudal fin. The posteriormargin of the occipital region is formed by a hemicen-trum, which forms the contact to the vertebral column.Within stingrays the vertebral column also comprisestwo synarcuals, the cervicothoracic and the thoracolum-bar synarcual, respectively. In the examined specimenthe scapulocoracoid separates both synarcuals (Fig. 1).

The transition from mono- to diplospondylous ver-tebrae is at the level of the pelvic girdle (Fig. 3d). Di-plospondylous vertebrae extend to about the middle ofthe tail sting. The vertebral centra of the remaining tailare fused forming an unsegmented rod.

Neural arches are laterally flattened and spatulate asit is characteristic for stingrays (Carvalho et al. 2004).Thoracic ribs are lacking, which is a synapomorphy ofmyliobatiforms. The missing pleural (ventral) ribs re-present an artefact.

Cervicothoracic Synarcual

The first synarcual articulates anteriorly with the neu-rocranium and posteriorly with the intersynarcual ver-tebrae close to the pectoral girdle (Fig. 3c). Its lengthin the studied specimen measures 27.5 mm and themaximum height in the middle part is 11.8 mm (includ-ing the crest). The width is 7.7 mm. The cervicothor-acic synarcual forms an elongated rigid tube consistingof several fused vertebrae. The lateral surfaces of thesynarcual are perforated by spinal nerve foramina alongits complete length. Ventral to these foramina, shortprolongations extend laterally from the synarcual,which are not pierced by any foramina.

The lateral stay is situated dorsally to the spinalnerve foramina in the examined specimen (Fig. 3c),which confers with the observations of Lovejoy (1996)for Potamotrygon. This condition also occurs in Plesio-trygon but not in Paratrygon. In all other stingrays thelateral stay (if present) is situated ventrally to the fora-mina (Lovejoy 1996).

The degree of the lateral extension of the lateral stayvaries among stingrays and rays in general (Claeson2008). It is thin in Urotrygon and Pacific coast Uroba-tis specimens (Lovejoy 1996). In the specimen studiedhere, the first synarcual is only slightly higher thanwide, whereas it is wider than high in Paratrygon(Lovejoy 1996).

Dorsally, a medial crest projects over the wholelength with a height of 7.6 mm. Anteriorly, the crestdoes not connect to the neurocranium. Posteriorly, it at-taches to the dorsal region of the pectoral girdle, wherethe suprascapular connects. Here, a foramen penetratesthe basis of the crest. The synarcual articulates poster-iorly with the scapulocoraracoid with two large facetsof its lateral projections. These projections pass ante-riorly into the lateral stay.

Pectoral girdle and scapulocoracoid

The pectoral girdle (Fig. 3d) is situated directly poster-ior to the branchial arches and consists of differentfused elements, mainly the coracoid and the scapularprocess. The coracoid itself consists of two pairs offused elements, which are ventrally positioned and forma bar (Fig. 3d). Laterally, it fuses with the scapular pro-cess and curves dorsally. In the examined Potamotrygonmotoro, the scapulocoracoid displays several foramina.Dorsally, there is a large antero-dorsal foramen and asimilar large antero-ventral foramen below. While thepostero-dorsal foramen is absent, the postero-ventralforamen is a small opening. It is situated postero-later-ally, where the coracoid is connected to the scapularprocess.

Most stingray taxa have four scapulocoracoid forami-na, the antero-dorsal, antero-ventral, postero-dorsal andthe postero-ventral ones (Lovejoy 1996). According toNishida (1990) the postero-dorsal foramen is absent inUrolophus, Urobatis, and Urotrygon, whereas Lovejoy(1996) observed small postero-dorsal foramina in Uro-trygon rogersi.

In most stingrays, the foramen of the scapular pro-cess is present. According to Miyake (1988) it is absentin Plesiobatis, Urolophus, Potamotrygon, Paratrygon,and Plesiotrygon. This interpretation is supported bythis study for Potamotrygon.

Dorsal to the scapular processes, the suprascapulaearticulate on both sides and are fused dorsally to thefirst synarcual. Because of these dorsal and ventral ar-ticulations, the pectoral girdle is flattened and ring-shaped. The fifth epibranchial and ceratobranchial ofthe branchial arches articulate directly with the antero-medial aspect of the scapulacoracoid. Both condylesare adjacent. The scapular process supports the pectoralfin and has three different condyles for its articulation.Anteriorly, there is a procondyle for the propterygium,intermediate the mesocondyle for the mesoptergydiumand posterior the metacondyle for the metapterygium.

Pectoral fin

The pectoral fin (Fig. 3e) is supported by the pectoralgirdle and consists of three basal cartilaginous ele-ments. The first anterior element is the propterygium,followed by the mesopterygium and posteriorly by themetapterygium. All three elements are very different insize. The propterygium of the examined specimen issegmented anteriorly, elongated, flattened and rod-shaped with a length of 69 mm. It curves anteriorlywhere it articulates via the antorbital cartilage with thenasal capsule. The first segmentation of the proptery-gium occurs directly at the level of the nasal capsule(Figs 1b, 3e) at the contact with the antorbital cartilage.

This condition is also present in Urolophus, Urotry-gon, Urobatis, Plesiotrygon, Paratrygon, Taeniura, andamphi-American Himantura (Lovejoy 1996). In othertaxa, such as Dasyatis, Indo-West Pacific Himantura,



Gymnura, and Myliobatis the segmentation occurs ante-rior to the nasal capsules.

The propterygium articulates with the scapulocora-coid and contacts the mesopterygium in Plesiotrygonconversely to the condition found in all other stingrays.Consequently, this character is supposed to support themonophyly of Potamotrygon þ Plesiotrygon (Carvalhoet al. 2004).

The mesopterygium is considerable smaller than thetwo other elements in the studied specimen. It is unseg-mented, flat, and rhomboidal and measures 9.0 mm(Fig. 3e). It articulates laterally with the scapulacora-coid but seemingly not with the posterior end of thepropterygium or with the metapterygium. In Plesiotry-gon, the mesopterygium also does not contact the prop-terygium, whereas the mesopterygium articulates withthe posterior aspect of the propterygium in other sting-rays. The mesopterygium can be segmented, such as inGymnura and Myliobatis, or be absent or fused withthe scapulacoracoid, such as Aetobatus, Rhinoptera, andMobula.

The metapterygium is shorter than the propterygium,but also elongated, flattened and rod-shaped. It is52 mm long and is distinctly curved anteriorly.

The number of radials varies in stingrays and it hasto be established if this is connected to the size of theindividual or if this represents a taxonomic signal. Dis-tally to the basals, the radials broaden. The radials areelongated thin cartilages that support the ceratotrichia,which support the fin web. The last segments of theradials are bifurcated. The bifurcation of the first fourradials is less distinct with only the most distal partsbeing branched.

The segmentation of the radials varies from nine seg-ments anteriorly to 20 segments medially, 12 segmentsposteriorly. In the examined stingray, 96 radials are pre-served in the right pectoral fin, whereas 98 radials fromthe left one indicating that the number of radials is notconstant in a single specimen. The extremely fragileceratotrichia are not preserved in the examined speci-men.

Pelvic girdle and puboischiadic bar

The pelvic girdle (Fig. 3f) supports the pelvic fins lat-erally in stingrays and is simple. The puboischiadic baris transversely flattened, slightly arched anteriorly andbears some characteristic processes. According to Love-joy (1996), the puboischiadic bar is strongly arched inAetobatus, Rhinoptera, and Mobula, but less so in allother stingrays. One elongated process, the medial pre-pelvic process, extends anteriorly from the mid-portionof the pelvic girdle (Fig. 3f). Laterally, the rather shortprepelvic processes extend anteriorly on each side. Ad-ditional processes extend posteriorly from the end ofthe U-shaped arch. In Heliotrygon, this process extendsto the anterior one-third of the metapterygial length(Carvalho & Lovejoy, 2011)

The sting-shaped prepelvic process of the investi-gated specimen is very distinct. In most of the stingraysthis process is moderately developed or reduced (Love-joy 1996: fig. 11). In Paratrygon, Plesiotrygon, and thepelagic stingrays Rhinoptera and Mobula this processalso is elongated (Lovejoy 1996).

Ventrally, a wing-shaped process, the ischial process,projects proximal from each side. Another process, theiliac process, projects dorsally to the ischial process. Itappears that there is only a single condyle, the basalcondyle, on the lateral aspect, which articulates withthe pelvic basipterygium. The first radial articulatesanteriorly to the anterior aspect of the basipterygium.

The skeleton of the clasper is formed by the basipteri-gal axis and attaches posteriorly to the basipterygiumof the pelvic fin (Fig. 3g). It is segmented and includesseveral curled cartilages. The length of the basipteri-gyium is 18 mm. It is almost straight, elongated anddorso-ventrally flattened. The basipterygium articulatesvia several intermediate segments with the dorsal mar-ginal and the ventral marginal of the clasper. The dorsalmarginal measures 35 mm and articulates to the curledterminal. The ventral marginal measures 15 mm. Thepelvic fin radials are morphologically similar to those ofthe pectoral fin. They are segmented and very slender.

Thoracolumbar synarcual and vertebrae

The presence of a second or thoracolumbar synarcual isa synapomorphy of stingrays. It consists of severalfused vertebrae and is positioned posterior to the pelvicgirdle (Fig. 3d). The corresponding vertebral centrahave a pair of basidorsals and a pair of basiventrales.Two arch-like structures are attached to the basidorsalsforming the neural arch. Thoracic ribs are absent.

The vertebrae anterior to the second synarcual aremonospondylous consisting of a single centrum and asingle pair of basiventrals and basidorsals. The verte-brae posterior to the pelvic girdle are diplospondylousextending to about the middle portion of the sting. Thevertebral centra of the remaining tail are fused formingan unsegmented rod (Fig. 1b) conversely to the condi-tion seen in Hexatrygon, Plesiobatis, Urolophus, Uro-batis, and Urotrygon. In these taxa, diplospondylousvertebrae extend to the tip of the tail.

The spine of the specimen in this study displays thecharacteristic morphology for potamotrygonids in thatit is acute and laterally barbed. It measures 44 mm inlength with 2/3 of its length are laterally flattened.

Comparison

Potamotrygon motoro was examined and described onlysporadically or incompletely in previous studies. Thenew results will be compared and discussed with pub-lished results (Lovejoy 1996; da Silva & Carvalho2011; Carvalho & Lovejoy 2011) in the following chap-

Zoosyst. Evol. 88 (2) 2012, 145–158 153


ter to provide a basis for further comparisons and dis-cussions as more studies on different taxa are available.Miyakes (1988) investigation of Urotrygon was usedfor some distinct elements.

Rostral cartilage

In stingrays, the rostral cartilage is not developed andthe anterior edge of the disc is rounded. According toMiyake (1988), nevertheless, a snout support is presentin the freshwater stingrays Potamotrygon and Plesiotry-gon as well as in some species of urolophids. It isformed by the anterior end of the pectoral fin and thearticulation via the antorbital cartilage with the nasalcapsules. Miyake (1988) considers this cartilage to behomologous with the rostral appendix of guitarfishesand skates.

Neurocranium

In pelagic stingrays, such as Dasyatis violacea, the na-sal capsules and the rest of the neurocranium form anangle while in non-pelagic stingrays the nasal capsulesand the rest of the neurocranium are in the same plane(Miyake 1988). In the study presented here, the exam-ined Potamotrygon motoro specimen displays the samecharacter. Both, Miyake and Lovejoy show in their in-vestigations the ventro-lateral expansion of the nasalcapsules, but it seems to be a character that is difficultto quantify. In the specimen examined here, this featurealso is present.

It appears that the epiphysial bar is only representedas a reduced remnant of a bridge in the examined Pota-motrygon motoro, which separates the cranial roof intothe anterior and the posterior fontanelle (Fig. 2a). Onereason could be that during the cleaning process, theLarder beetles also destroyed much of the delicate car-tilage. Another reason could be that the cartilage wasso thin that it was destroyed during mechanically pre-paration. Although the epiphysial bar is variably devel-oped in different species, it always divides the neuro-cranium into the anterior precerabral fontanelle and theposterior fronto-parietal fontanelle. Some other sting-rays, i.e., Urotrygon daviesi, Urolophus cruciatus andother freshwater stingrays have an incomplete epiphy-sial bar (Miyake 1988). Nevertheless, there are two fon-tanella developed.

The space between the orbitals various in severalspecies, but in stingrays the interorbital region is rela-tively narrow (Lovejoy 1996) as it also is in the studiedspecimen here.

The neurocranial processes vary in extension withinthe stingrays. The preorbital process might extend overthe nasal capsules as in Urotrygon rogersi and Urolo-phus cruciatus (Fig. 2a and Lovejoy 1996, fig. 5C).However, in the examined Potamotrygon motoro it isrestricted to the posterior region of the nasal capsulesas shown in Figure 2a.

The postorbital process is well developed in stingraysas in the examined Potamotrygon motoro and Urotry-gon rogersi (Lovejoy 1996). A lateral groove also is de-veloped as shown in Potamotrygon boesemani (Rosaet al. 2008, fig. 8) and Urolophus cruciatus (Lovejoy1996, fig. 5C). In other skates or non-stingray batoids,the postorbital process is poorly developed or absent asin electric rays (Miyake 1988, fig. 59). The developingof the lateral groove appears to create a new passage ofthe infra-orbital lateral line canal. In other stingrays asin potamotrygonids, the canal passes through a foramenin the postorbital process as shown in Urolophus cru-ciatus. The significance of this character remains un-clear. Rosa et al. (2008) examined the close related Po-tamotrygon boesemani and their investigation of theneurocranium bears high resemblance to the examinedPotamotrygon motoro (Fig. 2a) in this study. The preor-bital process doesn’t extent over the nasal capsules, apostorbital groove is present and the epiphysial bar isrepresented as a reduced remnant of a bridge that sepa-rates the cranial roof. According to Rosa et al. (2008)the postorbital process laterally extends and is flattenedjust as the examined P. motoro in this study.

Mandibular arch and hyomandibular

The relative position, shape and size of the hyomandi-bular cartilage is be related to the morphological andfunctional features. Major varieties exist in the connec-tion between the mandibular arch and the hyomandibu-lar element. According to Lovejoy (1996), the connec-tion between hyomandibular and jaw elements instingrays can be direct or indirect via several skeletalcomponents. The direct connection is present in Hexa-trygon and Plesiobatis for instance. In the examinedfreshwater stingray Potamotrygon motoro and in othertaxa, such as Paratrygon aireba, an indirect connectionvia angular-a and angular-b is present (Fig. 3a), whichis absent in Heliotrygon (Carvalho & Lovejoy 2011).The Potamotrygon boesemani (Rosa et al. 2008,fig. 8B) shows also both angular-a and -b. The angularsare parallel and have nearly the same length converselyto the condition found in the P. motoro in this study(Fig. 3a). However, previous investigations (e.g., Love-joy 1996, fig. 6) exemplify that both character statesmay occur in several taxa. Consequently, this characteris not unambiguous to use it for phylogenetic analyses.The difference within the several components connect-ing the hyomandibular and the mandibular arch is theonly feature that can be employed and compared. Inamphi-American Himantura species, these elementsvary strongly in sizes and number conversely to thecondition found in Potamotrygon species (Lovejoy1996). According to Lovejoy (1996) and the examinedPotamotrygon motoro in this study, the two different an-gular cartilages are characteristic for this taxon. For ex-ample, in Plesiotrygon only one robust elongate angu-lar-a is present and the second angular-b is absent. In



Paratrygon the connection is considerably shorter andonly a very small cartilage is developed, the homologyof which is unclear (Lovejoy 1996). It appears thatthese cartilages are functionally important. Althoughthe size and number of the cartilages vary within thespecies, it seems that they have a similar functionalrole. This hypothesis is supported by the fact that theconnections are more than only a simple ligamentousconnection.

In other stingrays such as Myliobatis, Aetobatus, Rhi-noptera and Mobula, a cartilage element close to theanterior tip of the hyomandibular was observed byLovejoy (1996). Nevertheless, it appears that these car-tilages do not have the same functional role as the car-tilages in Potamotrygon, Himantura and Plesiotrygon.

In the examined specimen of Potamotrygon motoro,the wing-shaped mandibular process is well developed(Fig. 3a). According to Lovejoy (1996), it is also welldeveloped in other potamotrygonids, Dasyatis, Tae-niura, and Himantura but less developed in Urolophus,Urobatis, and Urotrygon. This variation in form andsize makes it difficult to code this character in phyloge-netic hypothesis.

Summers (2000) examined myliobatid stingrays, suchas Rhinoptera bonasus, Rhinobatos lentiginosus, Dasya-tis sabina, and Dasyatis sayi. According to this studythe jaws of the examined specimens are all very robustand larger than those of other stingrays. The mandibu-lar symphysis and the hyomandibular symphysis are en-tirely fused (Summers 2000, Fig. 2), controversially tothe condition seen in Potamotrygon motoro of thisstudy, which clearly displays the fused suture.

In Myliobatis, Aetomylaeus, and Rhinoptera flattenedteeth bands forming a grinding dentition as in all exam-ined specimen by Summers (2000, fig. 7). In all otherstingrays, including the examined Potamotrygon mo-toro, teeth stripes are shown as in Figure 3a. The ar-rangement of the teeth is mainly related to the feedingmechanism.

Branchial arches

Bridges over the aorta and other vessels projects fromthe medial plate. These projections are present in theexamined Potamotrygon motoro specimen (Fig. 3b), andin Plesiobatis, Hexatrygon, Urobatis, Urotrygon, Urolo-phus, and Gymnura (Lovejoy 1996). However, theseprojections are not present in all potamotrygonids andstingrays, e.g. dasyatids and pelagic myliobatids. There-fore, this character is not useful for phylogentic ana-lyses.

The basihyal is variably developed in the differentstingray taxa. It could be segmented, unsegmented orabsent. Additionally, the degree of segmentation variesextremely. In the here-examined Potamotrygon motoro,the basihyal is not segmented or strongly fused Fig. 3b).However, in Lovejoys (1996, fig. 7F) investigation ofPotamotrygon motoro the basihyal is shown to be seg-

mented as in Dasyatis, Himantura, Taeniura, Urobatis(Lovejoy 1996, fig. 7A), and all other potamotrygonids(e.g. Carvalho & Lovejoy, 2011). Unsegmented ba-sihyals are present in Plesiobatis (Lovejoy 1996,fig. 7B), Hexatrygon, Urolophus, Gymnura (Lovejoy1996, fig. 7E), and Aetoplatea. Due to that extreme var-iation within the segmentation it has to be more speci-fic, which character state is developed in which taxonand additional material is needs to be analysed.

The pseudohyal is not preserved in the specimen ex-amined herein. This could be the result of the employedpreparation method, because this element is very fra-gile. According to Lovejoy (1996), the pseudohyal isfused to the first ceratobranchial, which is an apomor-phy for stingrays. However, there are species that lackthis fusion and other taxa, in which the degree of theconnection between these elements varies. For example,in Urolophus there is only a connection between pseu-dohyal and first ceratobranchial. Another possibility isthe fusion between the first two ceratobranchials andthe pseudohyal. In the examined Potamotrygon motorothe first three ceratobranchials are fused (Fig. 3b).However, Lovejoy’s Potamotrygon motoro displays fourfused ceratobranchials. It appears that these fusion pat-terns are complex and variable within genera and spe-cies (Lovejoy 1996, fig. 7)

Scapulocoracoid

Most stingray taxa have four scapulocoracoid foramina:the anterodorsal, anteroventral, posterodorsal and theposteroventral foramina (Lovejoy 1996, fig. 9). Accord-ing to Nishida (1990) the posterodorsal foramen is ab-sent in Urolophus, Urobatis, and Urotrygon. However,Lovejoy (1996) observed small posterodorsal foraminain some stingray such as e.g., Urotrygon rogersi. In theexamined specimen, no posterdorsal foramen is present.Hence, it appears that this character varies within thisspecies 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 absentin Plesiobatis, Urolophus, Potamotrygon, Paratrygon,and Plesiotrygon. There are neither a foramina nor afossa. This interpretation is supported by this analysis.

Pectoral fin

The pectoral propterygium is segmented anteriorly. Inthe investigated Potamotrygon motoro the segmentationoccurs directly at the level of the nasal capsules asshown in Figure 3E. This condition is also present inamphi-American Himantura, Heliotrygon, Paratrygon,Plesiotrygon, Urolophus, Urotrygon, Urobatis and Tae-niura (Lovejoy, 1996; Carvalho & Lovejoy, 2011). Inother taxa, such as Indo-West Pacific Himantura, Da-syatis, Gymnura, and Myliobatis the segmentation oc-curs anterior to the nasal capsules (Lovejoy 1996,fig. 10B). Another possibility is the segmentation pos-

Zoosyst. Evol. 88 (2) 2012, 145–158 155


terior to the nasal capsules in other outgroups (Lovejoy1996).

In most stingrays the mesopterygium articulates withthe posterior aspect of the propterygium. According toLovejoy (1996, figs 10D and E), this form of articula-tion is absent only in Potamotrygon and Plesiotrygon.The mesopterygium can be segmented, such as in Gym-nura and Myliobatis, or be absent or fused with the sca-pulacoracoid, such as Aetobatus, Rhinoptera, and Mo-bula. In the examined P. motoro the mesopterygium isnot segmented (Fig. 3e). The number of radials variesin the different stingray taxa and it has to be estab-lished if this is connected to the size of the individualor if there are some other reasons for this.

Pelvic girdle

The most conspicuous character in the investigated Po-tamotrygon motoro specimen is the sting-shaped prepel-vic process (Fig. 3f). In most stingrays this process onlyis minute or small developed (Lovejoy 1996, fig. 11).However, in Heliotrygon, Paratrygon, Plesiotrygon, Po-tamotrygon, and the pelagic stingrays such as Rhinop-tera and Mobula it also is characteristicly elongated(Lovejoy 1996).

The puboishchiadic bar is arched anteriorly in the ex-amined specimen as in other stingray taxa (Lovejoy1996). According to Lovejoy (1996), it is extremelyarched in Aetobatus, Rhinoptera, and Mobula, but lessso in all other stingrays.

Vertebrae

The first synarcual displays the most conspicuous dif-ferences within this species with regard to the lateralstay and the base of the lateral stay. The base of thelateral stay is pierced by foramina. According to Love-joy (1996) the lateral stay contacts the synarcual dor-sally to the foramina and is not pierced as the specimenexamined here (Fig. 3c). This character is observed inthe examined P. motoro and by Lovejoy (1996) in hisexamined P. motoro, P. constellate, Urobatis jamaicen-sis and Plesiobatis iwamae (Lovejoy 1996, fig. 8) In allother taxa, the lateral stay joins the synarcual ventrallyto the foramina and is pierced as in Urobatis halleri,Paratrygon aireba, and other taxa (Lovejoy 1996).

The lateral stay might be reduced or absent but it ispresent in most stingrays. The degree of the extensionof the lateral stay also varies. It is delicate in Urotrygonand Pacific coast Urobatis specimens (Lovejoy 1996).The degree of the lateral projection also varies amongbatoids in general. According to Claeson (2008), inRaja inornata the lateral stay is present midway the syn-arcual and curves dorsolateral similar than in the ex-amined Potamotrygon motoro in this study. P. motorofeatures only a medial crest. However, Raja inornataadditional features synarcual spines, which extend tothe medial crest (Claeson 2008, Fig. 1). There is too

much variation within the taxa requiring more materialto be studied before the significance of this charactercan be established.

In the Potamotrygon motoro specimen of this study,the first synarcual is only slightly higher than wide.Conversely, the first synarcual is wider than higher inParatrygon (Lovejoy 1996). The presence of the secondsynarcual is an apomorphic character for stingrays.

Diplospondylous vertebrae start at about the level ofthe pelvic girdle and continue to the tip of the tail as inHexatrygon, Plesiobatis, Urolophus, Urobatis, and Uro-trygon (Lovejoy 1996). In these taxa, the cartilaginousrod is absent, which is present in other stingray taxa,such as the investigated P .motoro.

Conclusions

Most species of Potamotrygon have been defined bytheir coloration patterns. However, an extreme intraspe-cific coloration variation occurs in many species de-pending on several factors including habitat conditionsand others. Consequently, the current number of de-scribed species might be exaggerated. Additionally, thecoloration pattern does not provide any information foraddressing evolutionary or phylogenetic issues. Anato-mical characters are more important in this respect.However, anatomical information for potamotrygonidsstill is sparse. All available anatomical information ofPotamotrygon and closely related taxa comes from fewstudies, which either focused on extinct stingrays (e.g.Carvalho et al. 2004) or analysed the interrelationshipsof stingrays in general (e.g. Lovejoy 1996). Recently,da Silva & Carvalho (2011) presented a detailed mor-phological account of Potamotrygon tatianae. So far,the inter- and intraspecific as well as sexual and onto-genetic variation of the skeletal system of potamotrygo-nids have not been established.

The description presented here is the most detailedmorphological account of a male specimen of Potamo-trygon motoro to date. It provides additional informa-tion to those presented in the studies of Lovejoy (1996)and Carvalho et al. (2004). Nevertheless, more speci-mens including females and individuals of different on-togenetic stages are needed to examine the variation ofskeletal characters and to identify features, which areimportant for phylogenetic inferences. Consequently,this study is considered a first step to elucidate themorphological anatomy of freshwater stingrays in gen-eral and to assemble reliable morphological charactersfor inferring relationships and evolutionary aspects ofthis highly interesting group. So far, the monophyly ofPotamotrygonidae including Heliotrygon, Paratrygon,Plesiotrygon and Potamotrygon is largely based on phy-siological (urea excretion in urine) and soft-part fea-tures (reduced rectal gland) (Carvalho et al. 2004). Theonly skeletal character supporting their monophyly isthe very distinct median prepelvic process (Carvalhoet al. 2004: fig. 16B). The function of this prepelvic



process remains unclear momentarily. The absence ofsuch an elongated prepelvic process in extinct fresh-water stingrays such as y Asterotrygon and y Heliobatisfrom the Eocene Green River Formation (U.S.A.) (Car-valho et al. 2004) indicates that this feature is not re-lated to the adaptation to freshwater conditions. Wealso were not able to identify any other skeletal featurethat might be related to a freshwater life-style. Thus, itis currently not possible to identify freshwater adaptedbatoids by unique dental or skeletal characters and sedi-mentological information still is necessary for interpret-ing the occurrence of fossil freshwater stingrays.

Resolving dental features in freshwater stingrays isurgently needed to describe dental morphologies in re-cent taxa and identify fossil freshwater stingrays. Sofar, the only known fossil remains are isolated bucklers,tubercles, spines, and oral teeth from the Miocene ofSouth America (Frailey 1986; Deynat & Brito 1994;Lundberg 1997, 1998; Brito & Deynat 2004). The onlyknown fossil record of dasyatid freshwater stingraysconstits of caudal spines and oral teeth of Dasyatis afri-cana from the Plio-Pleistocene of Ethiopia and Kenya(Feibel 1994). Thus, the incomplete fossil currentlycannot contribute to inferring any origination dates andinference of the timing of their origins and adaptationto freshwater conditions rests on the interpretation oftheir systematic position within Myliobatiformes andthe identification of their sister group. For this moreand detailed morphological data is necessary, whichpresently is not available.

Acknowledgements

We would like to thank the Aquazoo and L�bbecke Museum D�ssel-dorf (especially Silke Stoll) for the donation of the studied specimenof Potamotrygon motoro, and for the helpful information. P. Bartsch(Museum f�r Naturkunde, Berlin, Germany) is acknowledged formaking the specimen available for this study. Special thanks go to themembers of the former palaeoichthyological working-group at theMuseum f�r Naturkunde, Berlin, Germany, especially Stefanie Klug(University of Bristol, UK) for advice and assistance during this pro-ject. For critical remarks we thank anonymous reviewers and for cor-rections, comparative information and helpful ideas, we thank GetulioRincon (Universidade Paulista-UNIP).

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Contributions to the skeletal anatomy of freshwater ...homepage.univie.ac.at/rica.stepanek/PDF/Stepanek kriwet 2012-P.motoro.pdf · Contributions to the skeletal anatomy of freshwater