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. 2020 Nov 4;238(4):874–885. doi: 10.1111/joa.13354

A new synapomorphy in the pelvic girdle reinforces a close relationship of Zanobatus and Myliobatiformes (Chondrichthyes: Batoidea)

João Paulo Capretz Batista da Silva 1,✉, Thiago Silva Loboda 2, Ricardo de Souza Rosa 1
PMCID: PMC7930767  PMID: 33150584

Abstract

The rays of the order Myliobatiformes present several diagnostic characters, the most striking one being the presence of a serrated sting on the dorsal region of the tail. Although several morphological hypotheses have been proposed supporting the monophyly and interrelationships of its members, few characters of the appendicular skeleton were employed. In the present study, we analyzed comparatively the pelvic girdle morphology across all the groups of rays to investigate the distribution of the ischial process. To understand its significance, we tested this character of the pelvic girdle as a potential synapomorphy for the Myliobatiformes plus Zanobatus. Accordingly, the phylogenetic position of Zanobatus as a sister taxon to Myliobatiformes is reinforced and its pelvic girdle morphology reinterpreted in relation to previous morphological studies.

Keywords: appendicular Skeleton, elasmobranchs, morphology, phylogeny, stingrays

Ventral view of the pelvic girdle of Potamotrygon motoro exhibiting a highly developed ischial process (red arrowhead).

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1. INTRODUCTION

The rays of the order Myliobatiformes Compagno, 1973, form a monophyletic group sharing several synapomorphies that were explored in previous and recent morphological studies (Carvalho et al., 2004; Claeson et al., 2010; Lovejoy, 1996; Marramà, Carnevale et al., 2019; Marramà et al., 2019a; 2019b; McEachran et al., 1996; Nishida, 1990). Some of the most striking diagnostic features of the group are the presence of a serrated caudal sting, the absence of thoracic ribs, and the presence of a second synarcual (thoracolumbar) (Carvalho et al., 2004; Compagno, 1973, 1977; Lovejoy, 1996). Phylogenetic relationships among the several stingray lineages are not yet fully established and there are several morphological studies that still consider some taxa (e.g., Dasyatis and Himantura) as potentially problematic (Carvalho et al., 2004; Lovejoy, 1996; McEachran & Aschliman, 2004; McEachran et al., 1996; Miyake, 1988; Nishida, 1990; Rosa, 1985; Rosenberger, 2001). Accordingly, the phylogenetic position of some taxa is still debatable, as is the case of the genus Zanobatus Garman 1913, which has been considered as more closely related to Myliobatiformes (Aschliman, Claeson et al., 2012; Aschliman, Nishida et al., 2012; McEachran & Aschliman, 2004; McEachran et al., 1996) or as a member of the order Rhinopristiformes (Naylor et al., 2012). The order Rhinobatiformes was never recovered as a monophyletic group under a cladistic analysis (e.g., Aschliman, Claeson et al., 2012; Aschliman, Nishida et al., 2012; McEachran & Aschliman, 2004; Naylor et al., 2012; Nishida, 1990), and recent molecular and morphological hypotheses (Brito et al., 2019; Last et al., 2016; Marramà et al., 2020; Villalobos‐Segura et al., 2019) failed to recover the original arrangement, and therefore, the monophyly of Rhinopristiformes (sensu Naylor et al., 2012). Consequently, the interrelationships and classification of the batoids are not consensual or yet established. To increase the number of morphological characters that support the monophyly of Myliobatiformes but also in an attempt to elucidate the systematic status of the panray Zanobatus, a new character related to the appendicular skeleton is described based on a comparative analysis of the pelvic girdle across all ordinal groups of rays (Batoidea). This new character involves a process on the pelvic girdle and is discussed here as an additional synapomorphy of the order Myliobatiformes. Its homology is also discussed addressing its putative greater distribution across other groups of Batoidea (e.g., Rajiformes sensu stricto) as suggested by previous morphological studies and also its probable convergent presence in a shark taxon within the order Orectolobiformes.

2. MATERIALS AND METHODS

The pelvic skeleton was studied mainly by direct dissections of ethanol preserved specimens but also by examination of cleared and stained specimens following the methodology of Dingerkus and Uhler (1977). The musculature associated with the pelvic girdle and fins was removed to expose the skeleton in preserved specimens. Terminology of skeletal structures primarily follows Nishida (1990), Carvalho et al. (2004), and Silva (2014). Although the recognized orders of Batoidea were reduced to four with the proposition of Rhinopristiformes (Last et al., 2016; Naylor et al., 2012), for convenience we maintain the orders Rhinobatiformes and Rhiniformes following Compagno (1999) and Weigman (2016). Moreover, we did not follow the new generic arrangement for the species of rays as proposed by Last et al. (2016) so that our taxa employment would be in accordance with the taxon names used in the morphological study of Aschliman, Claeson et al. (2012). We only employ the new genus Styracura (Carvalho et al., 2016) for the Amphi‐American species (e.g., “Himantura” schmardae), considering that it has being consistently shown to be phylogenetically placed well outside the other Himantura species, as the sister to the South American freshwater stingrays (Aschliman, Claeson et al., 2012; Aschliman, Nishida et al., 2012; Lovejoy, 1996; McEachran & Aschliman, 2004; McEachran et al., 1996; Naylor et al., 2012). Accordingly, our aim here was to test a morphological character of the pelvic girdle as a putative synapomorphy for Zanobatus +Myliobatiformes and not to propose changes in the classification of batoids.

The morphological character explored herein, the presence of an ischial process on the pelvic girdle, was submitted to the tests of homology as conceptualized by Patterson (1982), including initially the similarity and conjunction tests. After the proposal of primary homology (Pinna, 1991), the character was incorporated in the morphological matrix of Aschliman, Claeson et al. (2012) dealing exclusively with the interrelationships of Batoidea (Sup 1) and tested for congruence (Patterson, 1982; Pinna, 1991). We included the description of the morphology of the pelvic girdle of Zanobatus separately to facilitate the comparisons with members of Myliobatiformes. A maximum parsimony analysis by tree bisection–reconnection (TBR) was performed under TNT v. 1.5 (Goloboff et al., 2008) with random sequence addition (100 replicates) and retained trees limit (MaxTrees) set to 5000 (following the parameters set by Aschliman, Claeson et al., 2012). We followed the equal weighting of characters as adopted by Aschliman, Claeson et al., (2012) and Aschliman, Nishida et al., (2012) to avoid assumptions of which characters are more relevant to reconstruct the phylogeny, and consequently, down‐weighting homoplastic characters with an impact in the resultant topology. Our aim was to test if a morphological character of the pelvic girdle would be recovered as a putative synapomorphy for Zanobatus +Myliobatiformes testing it with other equally weighted characters. The resulting consensus cladogram was built with the software WINCLADA version 3.02 (Maddison & Maddison, 2011). Material examined was from the following institutions: Academy of Natural Sciences of Drexel University, Philadelphia (ANSP); California Academy of Sciences, San Francisco (CAS); Smithsonian Institution, National Museum of Natural History, Washington DC (USNM); American Museum of Natural History, New York (AMNH); Muséum national d’Histoire naturelle, Paris (MNHN); Universidade do Estado do Rio de Janeiro (UERJ); and Museu de Zoologia da Universidade de São Paulo, São Paulo (MZUSP). The abbreviations TL and DW used throughout the text refer to total length and disc width, respectively; N/C refers to uncatalogued specimens; C&S refers to cleared and stained specimens.

2.1. Materials examined

Pristidae: Anoxypristis cuspidata (C&S): AMNH 3268 (female, 300 mm TL); Pristis microdon: UNSM 81066 (male, 945 mm TL); Pristis pristis: MPEG 2324 (male, 750 mm TL); Pristis zijsron: ANSP 101398 (male, 725 mm TL). Platyrhinidae: Platyrhina sinensis (C&S): AMNH 44055 (male, 140 mm DW). Rhinidae: Rhina ancylostoma: CAS 56636 (male, 465 mm TL). Rhynchobatidae: Rhynchobatus djeddensis: CAS N/C (female, 500 mm TL). Rhinobatidae: Aptychotrema vicentiana: AMNH 98505 (male, 350 mm DW), AMNH 98515 (male, 320 mm DW); Glaucostegus halavi: ANSP 109057 (male, 600 mm TL); Glaucostegus typus: AMNH 98724 (male, 150 mm DW); Rhinobatos horkelii: MZUSP N/C (female, 468 mm TL); MZUSP N/C (female, 350 mm TL); Rhinobatos lentiginosus (C&S): AMNH 8913 (male, 100 mm DW); Rhinobatos percellens: AMNH 55622 (male, 50 mm DW); Trygonorrhina sp.: AMNH 214211 (male, 600 mm DW), AMNH 214213 (male, 600 mm DW), AMNH 214215 (male, 400 mm DW); Zanobatus schoenleinii: MNHN N/C (male, 345 mm DW); Zapteryx brevirostris: MZUSP N/C (female, 549 mm TL). Narcinidae: Narcine brasiliensis: UERJ 1128 (female, 260 mm DW), AMNH 2488 (female, 80 mm DW). Torpedinidae: Tetronarce nobiliana: MZUSP N/C (female, 170 mm DW), UERJ 1249 (female, 230 mm DW); Torpedo fuscomaculata: ANSP 109257 (female, 95 mm DW); Torpedo torpedo (C&S): AMNH 4128 (male, 60 mm DW). Arhynchobatidae: Atlantoraja cyclophora: MZUSP N/C (male, 364 mm DW); Psammobatis bergi: MZUSP N/C (female, 205 mm DW); Psammobatis extenta: MZUSP N/C (female, 152 mm DW); Rioraja agassizi: MZUSP N/C (female, 320 mm DW); Sympterygia acuta: MZUSP N/C (female, 256 mm DW); Sympterygia bonapartei: MZUSP N/C (male, 260 mm DW). Rajidae: Dipturus mennii: UERJ 2104 (female, only pelvic girdle skeleton analyzed); Gurgesiella atlantica: MZUSP N/C (female, 210 mm DW); Fenestraja plutonia: USNM 222168 (male, 110 mm DW); Leucoraja erinacea: USNM 395725 (male, 235 mm DW); Leucoraja lentiginosa: ANSP 193260 (male, 145 mm DW). Anacanthobatidae: Anacanthobatis folirostris: USNM 222428 (female, 295 mm DW); Crurijara rugosa: USNM 222270 (female, 225 mm DW). Potamotrygonidae: Styracura schmardae: USNM 361688 (male, 300 mm DW); Plesiotrygon iwamae: MZUSP N/C (male, 380 mm DW); Paratrygon aiereba: MZUSP N/C (female, 220 mm DW), MZUSP 104648 (female, 150 mm DW); Potamotrygon falkneri: MZUSP 106265 (male, 231 mm DW); Potamotrygon limai: MZUSP 104033 (male, 359 mm DW); Potamotrygon orbignyi: MZUSP 103898 (male, 335 mm DW); Potamotrygon tatianae: MZUSP 107671 (male, 351 mm DW); Potamotrygon humerosa: MZUSP N/C (male, 237 mm DW); Potamotrygon histrix: MZUSP N/C (female, 310 mm DW); Potamotrygon motoro: MZUSP N/C (female, pelvic fin skeleton only); Heliotrygon gomesi: MZUSP 108294 (female, 125 mm DW). Urotrygonidae: Urobatis jamaicensis: ANSP 102744 (female, 168 mm DW); Urotrygon rogersi: CAS 235565 (male, 225 mm). Urolophidae: Trygonoptera sp.: AMNH 214470 (male, 300 mm DW); Urolophus halleri: CAS 17327 (female, 190 mm DW); Urolophus maculatus: CAS 18378 (male, 160 mm DW). Dasyatidae: Dasyatis americana: USNM 039397 (female, 280 mm DW); Dasyatis guttata: MZUSP N/C (female, 330 mm DW); Dasyatis hypostigma: UERJ 832 (female, 300 mm DW), UERJ 998 (female, 260 mm DW); Dasyatis zugei: CAS 232293 (male, 161 mm DW); Himantura gerrardi: CAS 41679 (male, 185 mm DW); Himantura imbricata: CAS 41658 (male, 167 mm DW), ANSP 178832 (male, 173 mm DW); Himantura uarnak: CAS 68150 (male, 450 mm DW); Neotrygon kuhlii: MZUSP N/C (male, 320 mm DW); Pteroplatytrygon violacea: MZUSP N/C (male, 355 mm DW); Taeniura lymma: MZUSP N/C (female, 166 mm DW), AMNH 44079 (female, 310 mm TL). Gymnuridae: Gymnura micrura: MZUSP N/C (female, 150 mm DW), MZUSP N/C (male, 140 mm DW); MZUSP N/C (male, 200 mm DW). Myliobatidae: Myliobatis freminvillei: MZUSP N/C (female, 400 mm DW); Myliobatis californica: CAS 214025 (male, 225 mm DW). Rhinopteridae: Rhinoptera bonasus: UERJ 338 (female, 466 mm DW); Rhinoptera sp.: UERJ 2176 (female, 250 mm DW). Stegostomatidae: Stegostoma fasciatum: AMNH 17205 (C&S) (female, 482 mm TL), CAS 80843 (female, 350 mm TL).

3. RESULTS

3.1. General description of the puboischiadic bar in rays

The puboischiadic bar (pib), commonly known as the pelvic girdle, is a transverse cartilaginous bar that supports the pelvic fins, and in batoids it can be either straight, subtly arched or markedly arched in dorsal view. This bar bears different numbers of condyles (cfr‐condyle for the first enlarged pelvic radial; cbp‐condyle for the basipterygium) for the articulation of the first enlarged radial (fer) and also for the basipterygium (bp). Additionally, obturator foramina (of) in various numbers pierce the lateral regions of the pelvic girdle for the passage of spinal nerves reaching the pelvic fins (Figure 1). Virtually, all the Torpediniformes analyzed, the Rajiformes, almost all Rhinobatiformes (except Zanobatus) and Anoxypristis (Pristiformes) have a straight puboischiadic bar (i.e., without any marked curvature on its anterior or posterior margins). In Zanobatus (Rhinobatiformes), in Rhiniformes (Rhina and Rhynchobatus), in most of the Pristiformes (except Anoxypristis) and in most of the Myliobatiformes (Trygonoptera, Dasyatis americana, D.guttata, D.hypostigma, Styracura schmardae, Neotrygon kuhlii, Pteroplatytrygon, Taeniura, Myliobatis spp., Urolophus spp., Urobatis, Urotrygon, and potamotrygonids), the puboischiadic bar is moderately arched. Only in Dasyatis zugei, Himantura gerrardi, Himantura uarnak, Himantura imbricata, Rhinoptera spp., and Gymnura micrura, the puboisquiadic bar is extremely arched.

Figure 1.

Figure 1

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Ventral view of the pelvic girdle of Potamotrygon motoro: MZUSP N/C, female, pelvic girdle and fin only. Abbreviations: bp, basipterygium; cbp, condyle for basipterygium; cfr, condyle form first enlarged pelvic radial; clp, clasper; fer, first enlarged pelvic radial; ip, iliac process; isp, ischial process; lpp, lateral prepelvic process; mpp, median prepelvic process; of, obturator foramina; ppp, postpelvic process; pvr, pelvic radials. Scale bar =10 mm

3.2. Pristiformes

Pelvic girdle (pib) anteroposteriorly narrow and arched, convex on its anterior margin and concave on its posterior margin. Lateral prepelvic process (lpp) wide, somewhat rounded and projecting anterolaterally from each anterolateral border of the puboisquiadic bar. All members of this order, except Anoxypristis, bear a faint and triangular median prepelvic process (mpp). One to three small and obliquely oriented obturator foramina (of) on each enlarged lateral region of the puboischiadic bar. A rounded condyle for the first enlarged radial (cfr) present at the lateral extremity of the pelvic girdle, just above the iliac process. A second rounded condyle for the basipterygium (cbp), twice the size of the cfr, located lateromedially to the iliac process (ip). Ischial process (isp) absent. Iliac process short and narrow, wider at its base and tapering distally, ending in a rhomboid tip (Figure 2a).

Figure 2.

Figure 2

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Ventral views of the pelvic girdles of (a) Pristis zijsron: ANSP 101398, male, 725 mm TL; (b) Rhina ancylostoma: CAS 56636, male, 465 mm TL; (c) Rhinobatos lentiginosus (C&S): AMNH 8913, male, 100 mm DW; (d) Platyrhina sinensis (C&S): AMNH 44055, male, 140 mm DW). Abbreviations are listed in Figure 1

3.3. Rhinobatiformes and Rhiniformes

Pelvic girdle (pib) laterally wide and corresponding to a transverse straight bar, being slightly convex on its anterior margin and concave on its posterior margin. In Rhina and Rhynchobatus, the pelvic girdle is noticeably arched. Each anterolateral region of the puboischiadic bar provided with an approximately triangular and anterolaterally oriented prepelvic process (lpp). Rhina and Rhynchobatus also bear a short and anteriorly directed median prepelvic triangular process (mpp). Each expanded lateral region of the puboischiadic bar usually bearing two small and obliquely oriented obturator foramina (of). An oval condyle for the first enlarged radial (cfr) at the lateral extremity of the pelvic girdle, just above the iliac process. Condyle for the basipterygium (cbp) also oval and located lateromedially to the iliac process. Postpelvic processes (ppp), corresponding to small and shallow rounded projections, present on the posteromedian margin of the puboischiadic bar, except in Rhina, Rhynchobatus, and Aptychotrema. Ischial process (isp) absent. Iliac process (ip) (obliterated by the pelvic girdle in Figure 2c) long (nearly 1/4 the width of the pelvic girdle) and somewhat rectangular, slightly projected dorsally and narrowing faintly distally (Figure 2b–d).

3.4. Torpediniformes

Pelvic girdle (pib) anteroposteriorly narrow and "U"‐shaped. Its anterior margin slightly concave and posterior margin convex. Lateral prepelvic processes (lpp) varying in extension but usually long (as long as the width of the puboisquiadic bar), wide on their bases and tapering anteriorly into a spatulate tip. Usually three small and obliquely oriented obturator foramina (of) at each lateral extremity of the puboischiadic bar. A round condyle for the first enlarged radial (cfr) at the lateral extremity of the pelvic girdle, just above the iliac process. Condyle for the basipterygium (cbp) rounded and located lateromedially to the iliac process. Two short and nearly triangular postpelvic processes (ppp) present at the posteromedian region of the puboisquiadic bar in Torpedo, Narcine, and Hypnos. Ischial process absent. Iliac process (ip) (obliterated by the pelvic girdle in Figure 3a) short, narrowing slightly from its base toward dorsal region, and slightly curved anteriorly (Figure 3a).

Figure 3.

Figure 3

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Ventral views of the pelvic girdles of (a) Torpedo torpedo (C&S): AMNH 4128, male, 60 mm DW and (b) Dipturus mennii: UERJ 2104, female, pelvic girdle skeleton only. Abbreviations are listed in Figure 1

3.5. Rajiformes

Pelvic girdle (pib) slightly arched, its anterior margin convex and its posterior margin concave. Its anteroposterior extension equivalent in length to the anteroposterior extension of the coracoid bar. Narrow and variably developed (short or long) anterolaterally oriented lateral prepelvic processes (lpp) projecting from each anterolateral region of the pelvic girdle. These processes tapering distally and generally ending in a pointed tip. One to four obliquely oriented obturator foramina (of) present at each enlarged lateral region of the puboisquiadic bar. An elliptical condyle for the first enlarged radial (cfr) at the lateral face of the pelvic girdle, just above the iliac process. Condyle for the basipterygium (cbp) rounded and generally located immediately under the obturator foramina. Ischial process absent. Iliac process (ip) rectangular, somewhat hook‐like (curved anteriorly) and dorsally oriented (Figure 3b).

3.6. Zanobatus

In Zanobatus, the pelvic girdle (pib) is noticeably arched. Each anterolateral region of the puboischiadic bar bears an approximately triangular and anterolaterally oriented lateral prepelvic process (lpp). Median prepelvic process absent. Two small and obliquely oriented obturator foramina (of) located above the condyles for the first enlarged radial (cfr) and for the basipterygium (cbp). Both condyles are close to each other. A nearly triangular and prominent ischial process (isp) project posteromedially from each postero‐inner corner of the pelvic girdle. Iliac process (ip) (obliterated by the pelvic girdle in Figure 4a) somewhat rectangular and narrowing faintly distally, projecting dorsally from each posterolateral margin of the pelvic girdle (Figure 4a).

Figure 4.

Figure 4

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Ventral views of the pelvic girdles of (a) Zanobatus schoenleinii MNHN N/C, male, 345 mm DW) and (b) Plesiotrygon iwamae: MZUSP N/C, male, 380 mm DW. Abbreviations are listed in Figure 1

3.7. Myliobatiformes

Pelvic girdle (pib) noticeably arched, with a convex anterior margin and usually bearing a shallow or long (slightly shorter than the pelvic girdle width) anteromedian prepelvic triangular process (mpp). Posterior margin markedly concave. Lateral prepelvic process (lpp) variably developed but usually prominent and nearly triangular, protruding anterolaterally from each anterolateral region of the pelvic girdle. Two to four small obturator foramina (of) obliquely positioned at each enlarged lateral region of the puboischial bar. A rounded condyle for the first enlarged radial (cfr) at the lateral extremity of the pelvic girdle, above the iliac process. A second rounded condyle for the basipterygium (cbp), with 1/3 the size of the cfr, located lateromedially to the iliac process. A marked and posteromedially oriented triangular ischial process (isp) present at each inner posterior corner of the pelvic girdle. Iliac process (ip) (obliterated by the pelvic girdle in Figure 8c) short (usually less than 1/5 the width of the pelvic girdle) and plate‐like, slightly expanded distally, and protruding posterodorsally from each posterolateral region of the pelvic girdle (Figures 1, 4b, 5, 6, 7, 8).

Figure 8.

Figure 8

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Ventral views of the pelvic girdles of (a) Trygonoptera sp.: AMNH 214470, male, 300 mm DW; (b) Urobatis halleri; CAS 17327, female, 190 mm DW; (c) Urotrygon rogersi: CAS 235565, male, 225 mm DW; (d) Urobatis jamaicensis: ANSP 102744, female, 168 mm DW. Abbreviations are listed in Figure 1

Figure 5.

Figure 5

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Ventral views of the pelvic girdles of (a) Dasyatis americana: USNM 039397, female, 280 mm DW); (b) Dasyatis zugei: CAS 232293, female, 161 mm DW; (c) Dasyatis hypostigma: UERJ 832, female, 300 mm DW; (d) Dasyatis guttata: MZUSP N/C, female, 330 mm DW. Abbreviations are listed in Figure 1

Figure 6.

Figure 6

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Ventral views of the pelvic girdles of (a) Himantura gerrardi: CAS 41679, male, 185 mm DW; (b) Himantura uarnak: CAS 68150, male, 450 mm DW; (c) Himantura imbricata: CAS 41658, male, 167 mm DW; (d) Neotrygon kuhlii: MZUSP N/C, male, 320 mm DW. Abbreviations are listed in Figure 1

Figure 7.

Figure 7

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Ventral views of the pelvic girdles of (a) Pteroplatytrygon violacea: MZUSP N/C, male, 355 mm DW; (b) Styracura schmardae: USNM 361688, male, 300 mm DW; (c) Rhinoptera bonasus: UERJ 338, female, 466 mm DW. Abbreviations are listed in Figure 1

3.8. Orectolobiformes

Pelvic girdle arched and anteroposteriorly enlarged (pib). Its anterior margin slightly convex and posterior margin markedly concave. Lateral prepelvic processes (lpp) absent. A condyle for the first enlarged (cfr) radial and a condyle for the basipterygium (cbp) situated at each posterolateral corner of the pelvic girdle. Each lateral extremity of the pelvic girdle enlarged and bearing three small rounded and obliquely oriented obturator foramina (of‐ obturator foramina are reduced and hard to visualize). Each posterolateral corner of the pelvic girdle bearing a short, triangular, and posteriorly oriented ischial process (isp). Iliac processes (ip) are absent (Figure 9).

Figure 9.

Figure 9

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Ventral views of the pelvic girdles of Stegostoma fasciatum (C&S). (a) CAS 80843, female, 350 mm TL; (b) AMNH 17205, female, 482 mm TL. Abbreviations are listed in Figure 1

3.9. Tests of similarity and conjunction

The two characters of the pelvic girdle were submitted to a test of similarity adopting primarily the topography (position) and structural composition as criteria. These characters clearly differ in position and construction as demonstrated by their descriptions below. Simultaneously, these characters were submitted to a test of conjunction and the postpelvic process and the ischial process of the puboischiadic bar were interpreted as independent characters and not part of the same transformational series (i.e., character states). Consequently, our conjectures of homology (primary homologies) resulted in two distinct characters. The first pelvic character described below correspond to character 69 (presence of postpelvic processes) which is modified from Aschliman, Claeson et al. (2012). The character 90 is a new character proposed herein and is related to the presence of ischial processes in the pelvic girdle. The description of both characters is as follows:

69—Postpelvic processes of the puboischiadic bar: (0) absent; (1) present. (modified).

The puboisquiadic bar may bear on its posteromedian margin a pair of posteriorly oriented, short, and approximately triangular processes. These processes were described as the postpelvic processes (Nishida, 1990) and employed as a character in previous morphological hypotheses involving the interrelationships of batoids. They have been also recognized only in members of Platyrhinidae (including Platyrhina and Platyrhinoidis) and have been consistently recovered as an exclusive synapomorphy of this family (Aschliman, Claeson et al., 2012; McEachran & Aschliman, 2004; McEachran et al., 1996; Nishida, 1990). However, the distribution of the postpelvic process is widespread and also found in other members of the order Rhinobatiformes (Glaucostegus, Rhinobatos, Zapteryx, and Trygonorhina) (Figure 2b–d) and Torpediniformes (Tetronarce, Torpedo, Narcine, Diplobatis, and Hypnos (Figure 3a; Claeson, 2014, supp plate 7)) (1). In the rest of the analyzed elasmobranchs, the posterior margin of the pelvic girdle lacks a process of equivalent form and position (0) (Figures 1, 2a, 3b, 4, 5, 6, 7, 8, 9).

90—Ischial process on the puboischiadic bar: (0) absent; (1) present. (new proposed character)

The ischial process is a prominence that provides additional anchoring point for the depressor musculature of the pelvic fin (Miyake, 1981988). This process is absent in sharks (with the exception of Stegostoma fasciatum) (Figure 9) and in virtually all groups of rays (0), excepting Zanobatus (Figure 4a) and the Myliobatiformes (Figures 1, 4b, 5, 6, 7, 8) (1). In Stegostoma fasciatum (Orectolobiformes), nearly triangular and prominent processes are topographically equivalent to the ischial processes found in the pelvic girdle of Myliobatiformes (Figure 9). These processes are positioned at the posterolateral inner corners of the pelvic girdle. In some Rhinobatiformes and Torpediniformes, the posteromedian margin of the pelvic girdle bears a pair of short and approximately triangular postpelvic processes (Figures 2b–d, 3a). However, considering the topography (central in the Torpediniformes and Rhinobatiformes and lateral in Myliobatiformes), the condition in these groups is distinct and consequently interpreted as non‐homologous (0).

3.9.1. Test of congruence

A matrix with 40 terminal taxa and 90 equally weighted characters based on Aschliman, Claeson et al. (2012) yielded four equally parsimonious trees of 206 steps of length, consistency index (CI) 0.62, and retention index (RI) 0.90. The consensus tree has the same topology as that recovered by Aschliman, Claeson et al. (2012), indicating Torpediniformes as a basal group and a sister taxon to a polytomy including Pristis,Rhynchobatus,Rhina,Rhinobatos,Zapteryx,Trygonorrhina, rajids, and a clade containing the remaining batoids. Both characters (69 and 90) were submitted to a test of congruence. Only the new pelvic character proposed herein (character 90, presence of an ischial process) was recovered as a synapomorphy (secondary homology sensu Pinna, 1991) for Zanobatus+Myliobatiformes. The postpelvic process was recovered as highly homoplastic and with several independent occurrences in Torpedo,Hypnos,Narcine,Rhinobatos,Zapteryx,Trygonorrhina,Platyrhina, and Platyrhinoidis. Consequently, only the ischial process (character 90) passed all the tests of homology. A strict consensus tree with characters 69 and 90 superimposed are presented in Figures 10 and 11, respectively.

Figure 10.

Figure 10

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Maximum parsimony optimization of the modified character 69 (puboischiadic bar with postpelvic processes) superimposed (in blue) on the cladogram of the Batoidea (L = 206, CI=62, RI=90)

Figure 11.

Figure 11

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Maximum parsimony optimization of the new proposed character 90 (presence of ischial processes on the puboischiadic bar) superimposed (in red) on the cladogram of the Batoidea (L = 206, CI=62, RI=90)

4. DISCUSSION

To understand the relevance of the ischial process as a new synapomorphy for the Myliobatiformes, we explored the morphology of the pelvic girdle of 62 species of rays across the six recognized orders (sensu Compagno, 191999) of Batoidea. Additionally, the phylogenetic significance of this character followed three different tests to corroborate it as a secondary homologue (=synapomorphy) grouping Zanobatus and Myliobatiformes: a test of similarity, a test of conjunction, and finally a test of congruence (Patterson, 1982), with the incorporation of this new character in the morphological matrix of Aschliman, Claeson et al. (2012).

In relation to the test of similarity, we compared the ischial process observed in Zanobatus and Myliobatiformes with the postpelvic processes observed in the pelvic girdle of some members of the orders Rhinobatiformes and Torpediniformes. These orders bear faint median postpelvic triangular processes which may serve as additional anchoring points for the pelvic fin depressor, although its significance as a supplementary area for the origin of this muscle (considering that these processes are visibly short) is unclear. This character was employed in the phylogenetic analyses of Nishida (1990), McEachran et al. (1996), McEachran and Aschliman (2004), and Aschliman, Claeson et al. (2012) as a synapomorphy of Platyrhinidae. However, it is widespread across Rhinobatiformes and could be observed in other members of the order, including Glaucostegus,Rhinobatos,Zapteryx, and Trygonorhina. Additionally, median postpelvic processes are also present in Torpediniformes (Tetronarce nobiliana,Torpedo torpedo,Torpedo fuscomaculata,Narcine brasiliensis; Figure 10). In the anatomical work of Claeson (2014), exploring the systematics of Torpediniformes, we could also identify them in Hypnos, but not in Narke and Temera (Claeson, 2014, supp plate 7).

Carvalho (2004) considered the postpelvic processes in Platyrhina as irregularities in the posterior surface of the puboischiadic bar and not as discrete processes as interpreted in other morphological studies (Aschliman, Claeson et al., 2012; McEachran & Aschliman, 2004; McEachran et al., 1996; Nishida, 1990). Additionally, Carvalho (2004) considered this character to be unreliable to phylogenetically diagnose the family Platyrhinidae due to the variable development of these processes in both Platyrhina (Figure 2d) and Platyrhinoidis, as well as in other rhinobatiforms (Rhinobatos,Zapteryx, and Trygonorhina). Likewise, Claeson et al. (2013), did not consider the presence of these processes as a synapomorphy of the Platyrhinidae due to their absence in a fossil representative of the family (†Tethybatis selachoides). Nonetheless, Claeson et al. (2013) coded Zanobatus as possessing postpelvic processes, even though this taxon presents ischial processes, a condition shared with the rays of the order Myliobatiformes (Figure 4a).

The ischial processes on the pelvic girdle protrude posteromedially from each posterolateral inner corner of the puboisquiadic bar and provide additional anchoring points for the depressor pelvic muscles in Zanobatus (Figure 4a) and Myliobatiformes (Figures 1, 4b, 5, 6, 7, 8), with a putative role in the mobility of the pelvic fins. Moreover, in Myliobatiformes the propterygium (or first enlarged radial) depressors, levators, retractors, and protractors originate on the puboischiadic bar, considering that the lateral prepelvic processes are significantly reduced (Macesic & Kajiura, 2010). In addition to that, only in Myliobatiformes the flexor muscles of the clasper originate on the puboischiadic bar, and are also probably associated with the depressor musculature of the pelvic fin, considering that both muscles were described as mixed at their origins (Moreira & Carvalho, 2018). Conversely, in Rajiformes and Torpediniformes, the pelvic girdle has a different morphology (“U” shaped in Torpediniformes and a straight transverse bar in most of the Rhinobatiformes and Rajiformes) from what is observed in Myliobatiformes (arched and usually convex anteriorly and concave posteriorly), and all the musculature associated with the propterygium originates from the lateral prepelvic process of the pelvic girdle (Macesic & Kajiura, 2010). Besides that, the flexor muscles of the clasper in Rhinobatiformes, Rajiformes, and Torpediniformes originate from the proximal half of the basipterygium and not from the puboisquiadic bar as observed in Myliobatiformes (Moreira & Carvalho, 2018). Taking into account the variable morphology of the pelvic girdle across the orders of Batoidea, it is probable that several distinct muscles (associated with both the pelvic fins and clasper), that originate in a necessarily arched pelvic girdle as observed in Zanobatus and Myliobatiformes, may need additional anchoring points for that diverse pelvic musculature. However, we are unsure if the ischial process may provide additional anchoring points for other pelvic or clasper muscles besides the pelvic fin depressor.

Miyake (1988) had already suggested that the ischial process would be restricted to the Myliobatiformes and would be absent only in Hexatrygon. However, we could not analyze this taxon to clarify the presence or absence of the ischial process and consequently we consider its presence as unknown (?). Carvalho (2004) also reported the presence of ischial processes in Zanobatus and made it clear that these processes would not be homologous to the postpelvic processes visualized for some Rhinobatiformes (e.g. Platyhrina). In the remaining batoid taxa analyzed, the posterior margin of the pelvic girdle does not present processes with equivalent position and morphology. Nevertheless, the condyle for basipterygium (cbp) was mistakenly identified as the ischial process for some members of the order Rajiformes and Torpediniformes in previous morphological works such as for Malacoraja (Carvalho et al., 2003), Dipturus (Moreira et al., 2011) (Figure 4b) and Electrolux (Compagno, 1999). Taking into account the considerations above and applying the test of conjunction, the postpelvic processes found in members of Rhinobatiformes and Torpediniformes are clearly an independent character from the ischial processes found in Myliobatiformes and Zanobatus (i.e., they are not part of the same transformational series), representing different conjectures of primary homology (Figure 10).

After its inclusion in a morphological matrix and its submission to a test of congruence, the ischial processes on the pelvic girdle are corroborated here as a synapomorphy for Zanobatus +Myliboatiformes (Figure 11). The phylogenetic position of Zanobatus as a sister taxon of Myliobatiformes is in accordance with recent morphological (Aschliman, Claeson et al., 2012; McEachran & Aschliman, 2004; McEachran et al., 1996; Villalobos‐Segura et al., 2019) and molecular (Aschliman, Nishida et al., 2012; Last et al., 2016) studies, and a greater support for this sister grouping is thus reinforced by the presence of a ischial process on the pelvic girdle.

Among sharks, the ischial process was found only in Stegostoma fasciatum (Orectolobiformes) and represents most likely an independent acquisition in this group considering its phylogenetic position and also that the remaining elasmobranchs, the Hybondontiformes and the Holocephali do not present an equivalent process (Maisey, 1982; Silva, 2014).

Finally, it is noteworthy that characters related to the appendicular skeleton are useful to clarify phylogenetic interrelationships within elasmobranchs (Silva & Carvalho, 2015; Silva et al., 2015, 2018). Besides the presence of an ischial process on the pelvic girdle discussed herein, other characters related to the paired fins and girdles were previously suggested as a putative synapomorphy for Zanobatus +Myliobatiformes (Silva & Carvalho, 2015), as the presence of an exclusive facet for propterygium posterior to the procondyle, which confers a greater support to this anterior pectoral basal. Consequently, we strongly encourage the employment of characters related to the appendicular skeleton as a potential source of data when considering the phylogenetic relationships across elasmobranchs.

AUTHORS’ CONTRIBUTIONS

Conceived and designed the manuscript: J.P.C.B.S.; analyzed the data: J.P.C.B.S., T.S.L., R.S.R.; wrote the paper: J.P.C.B.S., T.S.L., R.S.R.; Funding support: J.P.C.B.S., T.S.L.

ACKNOWLEDGMENTS

Aléssio Datovo and M. Gianeti (MZUSP), U. Gomes and H. Santos (UERJ), B. Séret (MNHN), J. Sparks, R. Arindell, and B. Brown (AMNH), J. Williams, R. Vari, J. Clayton, S. Raredon, K. Murphy, and D. Pitassy (USNM), D. Catania, L. Rocha, J. Fong, M. Hoang (CAS), M. Sabaj Pérez, J. Lundberg, M. Arce, and K. Luckenbill (ANSP) are sincerely thanked for providing access to specimens that were crucial for this paper, as well as for their support. This project was funded by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) through grants to the first (2010/52677‐6 and 2012/22692‐9) and second (2007/56739‐3) authors.

Capretz Batista da Silva J.P., Silva Loboda T., de Souza Rosa R.. A new synapomorphy in the pelvic girdle reinforces a close relationship of Zanobatus and Myliobatiformes (Chondrichthyes: Batoidea). J. Anat. 2021;238:874–885. 10.1111/joa.13354

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