Studies on New Zealand Elasmobranchii. Part I. Two Further Specimens of Arhynchobatis asperrimus Waite, (Batoidei) with an Account of the Skeleton and a Discussion on the Systematic Position of the Species
[Read before a meeting of the Wellington Branch, October 14, 1953: received by the Editor, October 15, 1953.]
Arhynchobatis is a monospecific genus of skates based on A. asperrimus, a moderately deep-water species, endemic to New Zealand. It is a typical member of the Rajidae, differing from other genera in the absence of a second dorsal, and the presence of a complete caudal fin. It has been included recently in the Platyrhinidae, but the endoskeleton is definitely that of the g. Raja.
Waite (1909) established Arhynchobatis as a new genus to contain A. asperrimus, a new species based on a single female specimen 640 mm. in total length and trawled from 66 to 94 fathoms in the Bay of Plenty during the operations of the New Zealand Government Trawling Expedition of 1907. From the literature there has been no other record of the species, but Dr. G. A. Archey has advised me that a dried specimen was found in the Auckland area. This specimen, however, no longer appears to be available. The study material on which the present paper is based comprises two female specimens of A. asperrimus, comparable in size to Waite's specimen, trawled from 50 to 60 fathoms, south of Castlepoint, in September, 1953, by the steam trawlers “Maimai” and “Thomas Currell”. The first of these specimens taken (i. e., the “Maimai” specimen) formed the basis for the following systematic description, and later was dissected on one side so that an account of the skeleton could be prepared. The second specimen taken (i.e., the “Thomas Currell” specimen) was checked against the systematic description of the first, and the variations are included in the description given here.
In his original account, Waite apparently recognised his new genus as belonging to the Rajidae, as he refers to no other family, and considered his specimen as having affinities with Psammobatis in the absence of “a cartilaginous rostral and the character of the nasal valves,” but differing in having a distinct caudal and only one dorsal.
Garman (1913) included Waite's account in his monograph on the Plagiostomia, and places the genus in the family Discobatidae, where it is allied with Discobatus, Platyrhinoidis and Zanobatus. All of these genera are uniform in possessing a rostral cartilage and two dorsals, thus differing from Waite's account of Arhynchobatis where it is stated there is no rostral cartilage and only one dorsal fin.
Whitley (1940) includes Arhynchobatis in the Rajidae, and believes that it is allied with the Australian Pavoraja nitida, though he has not been able to examine any specimens of the former.
Fowler (1941), following Garman, includes Arhynchobatis in the Platy-rhinidae, a family which also includes Zanobatus and Platyrhina (= Discobatus). In a recent review, Richardson and Garrick (1953) having no material available for examination, followed the opinion of Garman and Fowler.
Garman (1913) gives an account of the significant morphology of Platyrhina (= Discobatus) sinensis. His figures (Pl. 66) of an entire specimen 25 ¼ inches in length, is that of an animal generally similar to Trygonorrhina or other of she fiddler rays, in other words having the entire disc longer than broad but rather sharply rounded anteriorly, and with the greatest diameter at two-thirds the distance from the snout to the posterior end of the pectoral disc; the tail at its origin is wider than the interspiracular width, and heavy throughout its length. In contrast, Arhynchobatis has the length of the entire disc subequal to its width, with the greater diameter midway between the snout and the hind margin of the pectoral disc. The tail is narrow at its origin, less than the interspiracular width, and slender throughout its length. These characters, which delineate the general facies of Arhynchobatis, are definitely those of the typical Rajidae and not of the Platyrhinidae (though Arhynchobatis has more rounded lateral angles to the disc than is usual in rajids). The only external diagnostic character which resembles that of the Platyrhinidae is the completeness of the caudal fin. But in Platyrhina the caudal is generally heavy and extensive, while in Arhynchobatis it is small and generally slender, despite its completeness. It would appear, then, that from the external characters, Arhynchobatis should be placed in the Rajidae as originally proposed by Waite. Such a proposition is further substantiated by the nature of the skeleton, which will be discussed later. Waite's suggestion that Arhynchobatis appears to be allied to Psammobatis due to the similarity of the nasal valves and the absence of the rostral cartilage is valid at least for the former character, though the generally similar nature of the nasal valves of all the Rajidae precludes these from being very reliable characters. The rostral cartilage (which is absent in Psammobatis, the anterior face of the mesethmoid being concave) is present in Arhynchobatis as a short, shovel-shaped rostrum, as seen by dissection of the “Maimai” specimen. Such a condition has no parallel in the other Rajidae as known to me, though there is sufficient variation in their rostra to allow Arhynchobatis to be included with them in the family.
The absence of a second dorsal fin in Arhynchobatis makes it unique among the Rajidae. For this reason alone, and despite the obvious similarities between Arhynchobatis and other rajids such as Pavoraja nitida as referred to by Whitley (1940), the distinctness of the species at the generic level is valid. This is confirmed by the presence of the complete caudal fin, a condition not occurring in other rajids. The generic diagnosis given by Waite does, however, need revision, in that the presence of the short rostral cartilage, and the nature of the posterior nasal valve, which can barely be described as triangular, should be noted. The amended generic diagnosis follows:
Arhynchobatis Waite, 1909.
Neurocranium prolonged anteriorly into a short, shovel-shaped rostrum. The pectorals continuous in front, but modified to form a small nasal tip. Ventrals distinct from pectorals, and deeply notched. Tail long, depressed, with well-
developed caudal fin, and a fold on each side. A single dorsal fin, near extremity of tail. Each nostril with two valves, the anterior tube-like, the posterior sub-quadrangular, the latter pair separated by a medial emargination.
Arhynchobatis asperrimus Waite, 1909.
(Text-fig. 1, Figs. A–H, Text-fig. 2.)
Study Material: Female 750 mm. T.L., trawled from 50–60 fathoms south of Castlepoint (East coast of North Island), 4th September, 1953; female 695 mm. T.L., trawled from the same area 10th September, 1953. (Both specimens in the Dominion Museum, the latter No. 1397.)
Description: Body rajoid, greatly depressed, disc rhomboidal with curved lateral and posterior angles, and an acutely pointed, slightly protruding tip breaking the obtuse anterior angle. Pelvics distinct and deeply notched at their origin and insertion. Skin smooth, glabrous below, everywhere thorny above. The entire dorsal surface of the disc, and the dorsal and lateral aspects of the tail, including the dorsal and caudal fins, densely covered with small denticles, each consisting of a diamond-shaped basal plate bearing an erect, elongate, retrorsely-directed. filamentous spine, so that these surfaces appear delicately hirsute. In the interorbital and interspiracular areas there are a number of larger spines, each consisting of an irregularly-shaped basal plate bearing an erect, coarsely-ridged, conical spine directed posteriorly so that its tip is above or beyond the posterior third of the basal plate. A patch of similar spines is on the humeral region, in the form of a cross, laterally extending along the pectoral girdle, and anteroposteriorly along the vertebral column. Anteriorly this median array of spines terminates midway or less than half the distance between the pectoral girdle and the eyes, while its posterior prolongation is about half the length of its anterior. The lateral extensions along the pectoral girdle are slightly wider than the greatest interspiracular width. The peduncle is strongly armed with large spines which extend for a short distance anteriorly on to the disc, nearly or to the level of the pelvic origin. There is a middorsal row of about thirty large, laterally-compressed tubercles with elongate, sub-rectangular bases bearing bluntly conical, posteriorly-directed spines, each of which is strongly ridged. This row terminates just, anterior to the dorsal fin. Laterally there are additional rows of spines similar to those on the head and humeral regions. On the anterior quarter of the peduncle, and extending on to the disc, about two irregular rows are present on each side, but posteriorly there is only one row which terminates at the level of the dorsal fin base Smaller spines are irregularly interspersed between the middorsal and lateral rows of spines, and a few of these extend on to the caudal fin. In addition the whole dorsal and lateral surface of the peduncle, and the dorsal and caudal fin, are covered with the bristle-like denticles mentioned earlier.
Length of the disc (including the pelvics) slightly more than its width and 1.8 in the total length. Greatest width of disc at the level of the pectoral girdle and 1.9 in the total length. Greatest depth of body at the pectoral symphysis, 10·0 in total length. Tail long, slender and depressed, its length about equal to the width of the disc and 1·9 in the total length, its width at its origin 1·1 in the interspiracular width. Distally it carries a single small dorsal and a complete caudal fin, and laterally there is a small cutaneous fold along most of its length.
Text-Fig. 1.—A. asperrimus, female, 750 mm. total length. Figs. A and B, dorsal and ventral views (dotted lines indicate underlying skeletal elements); Fig. C, portion of upper tooth-plate, showing median and first lateral rows of teeth on both sides; Fig. D, lateral view of distal end of peduncle, showing dorsal and caudal fins; Figs. E and F, dorsal and lateral views of portion of peduncle; Figs. G and H, dorsal and lateral views of dermal denticles from left shoulder region. N.B.: Figs. A, B and D show only the major spination, the smaller dermal denticles not indicated.
Text-Fig. 2. —Ventral view mouth and nostrils, A. asperrimus, female, 750 mm. total length. ANV, anterior nasal valve; N, nostril; PNV, posterior nasal valve; VEL, maxillary velum.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
Length of snout 1·6 to 1·7 in the head. Interorbital 2·7 to 2·9 in the snout. Eyes completely lateral, their length 1·3 to 1·4 in the interorbital. The upper margin to each iris produced downwards as a frilled pupillary flap. Spiracles subovate, posterolateral to the eyes, their length 1·0 to 1·2 in the length of the eye. Length of head, 5·5 to 5·8 in the total length. Gill slits small, sinuous, the fifth slit smallest. Distance between first and second slits subequal to that between second and third, and third and fourth, and slightly greater than that between fourth and fifth. Width between the first slits 1·3 to 1·4 in the head and 1·4 times that between the 5th slits. Distance from anterior edge of disc to nostrils 2·0 to 2·2 in the head. Internarial width 2·8 to 3·0 in the head. Nostrils ovoid, and almost transverse, laterally bounded by the anterior nasal valves which are expanded into a rim with a blunt anterior prolongation and a fimbriated posterolateral margin. Posterior nasal valves supporting a fleshy flap which separates the nostrils and is secured medially by a frenulum. This flap is deeply and smoothly emarginated over the middle third. The free posterolateral margins of the flap are subquadrangular, irregularly fimbriated posteriorly, and have each a short blunt lateral projection which extends just behind the nostril. Mouth wide and slightly curved, its width 3·0 in the head. Laterally it is bounded by deep nasoral grooves, anteriorly reaching the nostrils and posteriorly curving smoothly medially to be confluent when the mouth is opened. Teeth forming a wide pavement 15–1–14(15)/15–1–15, with six to seven rows of teeth functional. The base of a fully formed tooth is subcircular in outline. The teeth in the posterior rows each with
a short, blunt cusp directed posteriorly. Teeth in the anterior rows with the cusps eroded.
A prominent maxillary velum is pendant from the roof of the mouth just poster or to the upper tooth-plate. Medially it is deeply notched to form two lateral portions, each of which is shallowly notched about half way along its border, which is finely fimbriated.
Dorsal fin small and originating on the distal fifth of the tail. Its height 1.7 to 2.0 in its basis and 1.6 to 2·0 times the depth of the tail at the same level. Anterior margin convex posterior margin straight or concave, and the posterior angle smoothly but sharply curved and extending well posterior to the insertion of the fin. Origin of the caudal well behind the dorsal, the interspace being equal to the dorsal basis. Caudal elongate, ovate in outline, complete, its height 2·2 in its length, and the latter about twice the interorbital width. Epiural lobe about twice the height of the hypural. The lateral peduncular keels originate at the level of the posterior edge of the pelvic fin and extend ventrolaterally along each side of the tail, terminating just posterior to the origin of the caudal fin. Their width is about one-quarter the width of the tail at the origin of the caudal fin.
Pectorals completely fused into the body, the propterygia extending around and in front of the head where they almost meet to form the tip of the disc. The anterior margin of the disc obtusely angled with the exception of the tip of the snout, which is acute, and breaks the outline. The free lateral margin of each pectoral almost straight anteriorly, but smoothly curved laterally and posteriorly. Posterior insertion deeply notched Length of pectoral 2·0 to 2.2 in the total length Origin of pelvics slightly more than one-third of total length from anterior edge of disc, and well anterior to insertion of pectorals. Anterior portion of each pelvic free, and extended laterally as a broad, flat appendage not reaching the pectoral margin. Posterior portion of the pelvic extending the disc, though separated from the pectoral by a deep notch. Lateral and posterior margins smoothly curved, and the posterior insertion strongly notched. Length of the complete pelvic 3·0 in the length of the pectoral. Cloacal aperture at the level of the posterior insertion of the pectoral Posterolateral to it are two small abdominal pores.
Colour above, dusky brown, below cream.
The Endoskeleton of Arhynchobatis asperrimus. (Text-fig. 3.)
The neurocranium is strongly depressed, constricted to a narrow waist, and has prominent lateral nasal and otic capsules as is usual in most batoids. The mesethmoid cartilage produced anteriorly as a short shovel-shaped rostrum (R), open above by the anterior fontanelle (rostra of a similar form occur in some sharks—e.g., Squalus). The rostrum is extended by a very slender, median, laterally compressed, accessory rostral cartilage (AR), which loosely articulates with the tip of the rostrum proximally and distally is expanded into a horizontal thin flat plate supporting the tip of the disc. The articulation of this cartilage and its slenderness, suggests that it is a chondrified ligament (viz., the median longitudinal septum) rather than a true rostral cartilage. Median rostral cartilages occur in the Pristidae, where they are very elongate, broad, and bear lateral teeth; in the Rhinobatidae where they are similarly elongate and laterally exten-
Text-Fig. 3.—Ventral view endoskeleton A. asperrimus, female, 750 mm. total length. AC, antorbital supporting cartilage; ANV, anterior nasal valve cartilage; AR. accessory rostral cartilage; BF, branchial filament; BP, basipterygium; C, coracoid; CB, ceratobranchial, CH, ceratohyal; DPH, dorsal pseudohyoid; EP, epibranchial; HB, hypobranchial; HC, hyoid copula; HM, hyomandibula; M, Meckel's cartilage; MBC, median hypo-basibranchial cartilage; MSP, mesopterygium; MTP, metapterygium; PB, pelvic bar; PNV, posterior nasal valve cartilages; PP, propterygium; PQ, palato-quadrate; PR, preaxial radial; PRE, lateral prepelvic process; R. rostrum; RA, radialia; SC, scapula; SP, spiracular cartilage; VC, vertebral column; VF, fused vertebral tube; VP, lateral vertebral plate; VPH, ventral pseudohyoid; VS scapulary process of vertebral column.
sive; in the Platyrhinidae where they are short and blunt with laterally articulated accessory projections; in some of the Torpedinidae where they may be long or short, stout or slender; and in some of the Rajidae. In the latter family the rostrum may be long and tapered as in Raja erinacea (Garman, Pl. 68, Fig. 1); just protruding from the neurocranium as a flat convex plate as in Sympterygia acuta (Garman, Pl. 68, Figs. 3, 4); or absent entirely, the mesethmoid having a concave anterior face as in Psammobatis mira (Garman, Pl. 69, Figs. 1, 2). The condition of the rostrum in Arhynchobatis resembles somewhat that of Platyrhina sinensis (Garman, Pl. 66, Fig. 2), though in the latter species the rostrum is longer and blunter than in Arhynchobatis, and there are also accessory lateral rostral cartilages which do not occur in Arhynchobatis. Although none of the Rajidae as known to me have a rostrum corresponding to that of Arhynchobatis, the variation in the rostral region of the members of this family, as described above, would allow Arhynchobatis to be included with them.
Antorbital supporting cartilages (AC) are present in Arhynchobatis as short stout rods, one on each side, arranged transversely and articulating proximally on the lateral face of the nasal capsule, and distally on the propterygial cartilage (PP). Similar cartilages occur in all batoids, but in the Pristidae they are flat and elongate and directed posteriorly; in the Rhinobatidae and Platyrhmidae they are plate-like or similar to those of the Pristidae; in the Torpedinidae they are laterally extended and divaricated; and in the Dasyatidae, Myliobatidae and Mobulidae they are small, tapered and directed posteriorly. Only in the Rajidae is there a close approximation to the condition in Arhynchobatis.
The cartilages of the nasal valves (ANV, PNV) in Arhynchobatis are very similar to their homologues in the Rajidae (Garman, Pl. 68, Figs. 1, 2, 4). The lateral border of the anterior valve is supported by a small strap-like, hook-shaped cartilage. The posterior valve is supported anterolaterally by a small pointer cartilage articulating on the anterior edge of the foramen of the nasal capsule, and posteriorly by a large cartilage expanded ventrally into a sub-rectangular plate which extends the posterolateral corners of the nasal flap, and dorsally into a similar plate which lies between the nasal capsule and the palato-quadrate. Homologues of these cartilages occur in all other batoids, and vary greatly in their shape and divarication, but with the exception of the Rajidae they do not closely resemble those of Arhynchobatis. This especially applies to Platyrhina sinensis, as can be seen from Garman's illustration (German, Pl. 66. Fig. 2).
Because the jaw suspension of all batoids is hyostylic, the jaws and the hyomandibula show relatively little variation. Those of Arhynchobatis are no exception; both the palatoquadrate (PQ) and Meckel's cartilage (M) being wide, rather slender cartilages with flexible median symphyses. The hyomandibula (HM) is elongate and articulates proximally on the lateral face of the otic capsule. Distally it supports the distal end of Meckel's cartilage by means of a heavy ligamentous articulation, while Meckel's cartilage in turn supports the palatoquadrate by means of a ball and socket joint as in Raja clavata (Daniel, p. 63). There is no articulation between the palatoquadrate and the neurocranium.
A lenticular spiracular cartilage (SP) supporting the spiracular valve is present, articulating proximally on the posterior wall of the orbit, and distally
on the hyomandibula. Similar cartilages occur in all rays, and are not diagnostic, as they may be in sharks.
The elements of the hyoid arch in Arhynchobatis comprise the hyomandibula already described, a dorsal and ventral pseudohyoid bearing hyal filaments, a minute ceratohyal and a complete hyoid copula. The dorsal and ventral pseudohyoids (being the segmented ceratohyal of Garman) are two slender rod-like cartilages (DPH and VPH), equivalent in function to epibranchial, and ceratobranchial cartilages, and forming an arch anterior to the first branchial arch. They support the anterior wall of the first gill slit. The validity of the homologous cartilages as pseudohyoids and not as a segmented ceratohyal in Rhynchobatus, Torpedo, Pristis and Raja is discussed by de Beer (1932), and earlier by Krivetski (1917), whose conclusions apply equally well to Arhynchobatis.
The dorsal pseudohyoid articulates proximally midway along the anterior edge of the first epibranchial cartilage, as is usual in many batoids, though in some of the Torpedinidae the articulation is on the proximal portion of the hyomandibula (Garrick, unpublished MS on Typhlonarke), and in the Dasyatidae it may be on the neurocranium and posterior to the hyomandibula (Daniel, p. 64). From Garman's figure of Pristis microdon (Pl. 64, Fig. 2) it would appear that in that species it articulates with the distal end of the hyomandibula and also with the first epibranchial cartilage. The ventral pseudohyoid articulates with the ceratohyal, as is to be expected. Both of the pseudohyoids bear hyal filaments, there being six on the dorsal element and seven on the ventral. Each filament (BF) is a long slender rod expanded distally into a flat irregular plate. This type of filament is very usual in the batoids, but uncommon in the sharks.
The ceratohyal (CH) is a small flat cartilage, pointed medially. It articulates laterally with the ventral pseudohyoid and posteriorly with the anteromedian edge of the first ceratobranchial cartilage. It abutts against but does not directly articulate with the lateral extremity of the hyoid copula. It is present in all batoids as a similar small cartilage, though in the Torpedinidae it is relatively much larger (Garman, Pl. 67, Fig. 3), and in Rhynchobatus it still bears hyal filaments (de Beer, 1932, Pl. 21, Fig. 15).
The hyoid copula (HC) is a slender strap of cartilage, smoothly arched anteriorly and expanded laterally into two pointed processes, an anterior and a posterior. The posterior process articulates distally with the median tip of the first ceratobranchial cartilage. Complete hyoid copulae are present in the Pristidae, where they are almost transverse and expanded into large, bluntly pointed plates laterally; in the Rhinobatidae where they are similar to those of the Pristidae or else squarely arched anteriorly and lacking distinct lateral processes; in the Platyrhinidae (Platyrhina sinensis as in Garman, Pl. 66) where they are sinuous and also lack lateral processes; and in the Rajidae where they closely resemble that of Arhynchobatis.
The branchial arches of Arhynchobatis are almost identical with those of the Rajidae. The pharyngobranchial cartilages are slender, pointed and directed posteromedially. The first three are discrete, the fifth and sixth fused distally, as occurs in the majority of batoids. The epibranchial (EB) and ceratobranchial cartilages (CB) are complete, the first four being flat, strap-like cartilages bearing on their posterior margins branchial filaments identical to those on the pseudohyoids. The epibranchials bear six filaments and the ceratobranchials
seven. The most dorsal filament on each epibranchial, and the most ventral filament on each ceratobranchial, is flattened and fused distally to the adjacent filament of the same arch.
There are no filaments on the fifth arch. The fifth epibranchial cartilage (EB5) is in the form of a stouter plate than those preceding, and distally articulates near to but not at the extremity of the fifth ceratobranchial cartilage (CB5). The latter cartilage is a heavy rod articulating proximally on the plate-shaped hypo-basibranchial cartilage and distally on the anteromedial aspect of the lateral portion of the pectoral girdle. Such a relationship between the fifth arch and the pectoral girdle is found in all batoids.
The remaining ventral elements of the branchial arches, that is the hypo-branchial and basibranchial cartilages, are represented by two cartilages only. Medially there is a large subcircular cartilaginous plate (MBC) with two elongate, pointed, anterolateral processes, and a single blunt median posterior process. Articulating laterally on this plate are the fifth ceratobranchial cartilages previously mentioned, while immediately anterior to them, on each side, there is a small irregular flat cartilage (HB3–4) representing hypobranchial elements. The latter cartilage has two short, blunt lateral processes which articulate with the third and fourth ceratobranchials, and a small pointed anteromedian process. The lateral articulations suggest that this cartilage represents at least the third and fourth hypobranchial elements, while its median articulation with the large subcircular plate suggests that the latter is in part a basibranchial cartilage (though Garman labels the homologous cartilage in the Rajidae as the fifth hypobranchial only, Pl. 68). However, as the fifth ceratobranchial articulates directly on this plate, it may well combine hypobranchial elements, not only posteriorly but also anteriorly in the form of the two anterolateral processes.
Similar ventral branchial elements occur only in some of the Rajidae—e.g., Sympterygia acuta as illustrated by Garman (Pl. 68, Fig. 4). Raja erinacea has the median plate divided into anterior and posterior portions (Garman, Pl. 68 Fig. 1), while in Psammobatis mira there are two separate hypobranchial elements, on each side (Garman, Pl. 69, Fig. 2).
The vertebral column of Arhynchobatis is like that of all other batoids in that anteriorly it is fused into a continuous tube (VT), the fusion extending well posterior to the pectoral girdle and involving almost forty vertebrae (The latter condition is characteristic of the more advanced rays.) In the branchial region, the ventrolateral margins of the column are produced dorsolaterally into flat plates (VP) (the post-branchial or lateral stays of Garman) which serve as muscle attachment regions, and are best developed in the Pristidae, and show decreased development in the Rhinobatidae, Rajidae and Dasyatidae respectively (Garman, Pl. 55). At the level of the pectoral girdle, the fused neural arches abruptly extend laterally as a stout cartilaginous bar (VS) which firmly articulates with the dorsomedial extremity of the scapula. Such extensions are best developed in the Rajidae, though they are present to a lesser degree in all batoids except the Torpedinidae which have the scapulae either completely fused (Torpedo) or else joined by a median dorsal cartilage which is distinct from the vertebral column (Typhlonarke). Posteriorly the vertebral column (VC) becomes compressed and bears prominent transverse processes.
The pectoral girdle comprises a wide, almost transverse, coracoid bar (C), greatly expanded laterally into a short anterior and an elongate posterior process which serve for the articulation of the basalia. Dorsally the girdle continues without interruption as the scapulae (SC) which pass medially as stout transverse bars to articulate with the distal end of the processes from the fused neural arches.
The propterygium (PP) is a stout, tapered, segmented cartilage, articulating proximally on the anterior lateral process of the pectoral girdle, and smoothly curving anteriorly around and in front of the head to reach the tip of the snout and the extremity of the accessory rostral cartilage. Half-way along its length, it is supported on its medial face by the antorbital supporting cartilage. Laterally it bears numerous segmented radialia (RA).
Similarly elongate propterygia, reaching to or approximating to the mid-line at the tip of the snout, occur in the Platyrhinidae, most of the Rajidae, the Dasyatidae and the Myliobatidae.
The metapterygium (MTP) is a similar elongate cartilage articulating proximally on the posterior lateral process of the pectoral girdle, and extending posteriorly, flanking the abdominal cavity and curving medially. It also bears numerous lateral radialia.
The mesopterygium (MSP) is represented by two elements, of which the anterior is the larger. The validity of both as mesopterygial elements is evidenced by their being bound within a common connective tissue sheath. The anterior larger element articulates anteriorly with the posterolateral extremity of the propterygium, and medially with the lateral face of the pectoral girdle. Laterally it bears six radialia, the bases of which are fused to it and do not articulate as do those on the propterygium and metapterygium. The posterior mesopterygial element articulates medially on the lateral face of the pectoral girdle and bears three radialia similar to those on the anterior element.
Divided mesopterygia occur in several batoid families, particularly the Dasyatidae and Myliobatidae (Garman, Pls. 71, 73). In Garman's illustrations of the Rajidae (Pls. 68, 69) several genera including Psammobatis are shown to have mesopterygia like those of Arhynchobatis, though only the large anterior element is labelled as such. It is obvious, however, that the smaller element is equally of mesopterygial origin, when the nature of the connective tissue sheaths binding the cartilages is examined, and the multiple origin of radialia from each cartilage is considered.
The pelvic girdle of Arhynchobatis is typically that of a rajid, in that it comprises a stout, almost transverse pelvic bar (PB) bearing prominent lateral pre-pelvic and iliac processes. The prepelvic processes (PRE) arise from the anterolateral edge of the pelvic bar, and are elongate, pointed and directed anteriorly. The iliac processes arise posterolaterally on the pelvic bar from the dorsal surface, and are stout, bluntly pointed, curved anteriorly and directed dorsally. Articulating on a socket on the lateral aspect of the pelvic bar is the heavy preaxial radial (PR) which, together with three successive posterior radialia of smaller size, supports the anterior appendage of the pelvic fin. The basipterygium (BP) is a stout, tapered, segmented cartilage, articulating proximally on the posterolateral aspect of the pelvic bar and directed posteriorly. Laterally it bears numerous radialia which support the posterior part of the pelvic fin.
Transverse pelvic bars occur in all of the batoid families except the Dasyatidae, Myliobatidae and Mobulidae where they are arched forward and bear a median prepelvic process. However, in the Rhinobatidae the lateral prepelvic processes are very stout and plate-like (Rhinobatus productus in Daniel p. 80, Fig. 86B); in the Platyrhinidae they are barely produced (Garman, Pl. 66, Fig. 2); and in the Torpedinidae they may be elongate as in Torpedo californicus and arising from a posteriorly arched pelvic bar (Garman, Pl. 53, Fig. 2), or expanded into flat plates distally as in Typhlonarke (Garrick, unpubl. MS). Amongst the Rajidae the condition of the pelvic bar and associated processes most closely approximates that of Arhynchobatis (see Garman, Pl. 53, Fig. 1).
From the foregoing description of the skeleton of Arhynchobatis asperrimus it can be seen that the affinities of this species are with the Rajidae. The structure of the rostrum conforms to the type present in the Rajidae, although it does not agree completely with that of any of the rajid genera known to me; and although it agrees with the type of rostrum occurring in Platyrhina sinensis, that in itself is by no means enough evidence to retain the species in the Platyrhinidae. The median accessory rostral cartilage appears to be a novelty for the Rajidae, but if it is a chondrified ligament, as it appears to be, then its presence offers no obstacle to placing Arhynchobatis in this family. The antorbital supporting cartilages and the cartilages of the nasal valves agree closely with those of the Rajidae, the latter in particular offering valuable evidence on affinities considering the amount of variation shown by these cartilages in the various families. The jaws, hyomandibula and spiracular cartilage are not of great diagnostic value at the family level, but the remainder of the hyoid arch, the pseudohyoids, ceratohyal and hyoid copula are certainly indicative of the Rajidae, as are the branchial arches and in particular the ventral branchial elements. The vertebral column, though showing the characteristic features of all batoids in being fused anteriorly and bearing processes for the articulation of the pectoral girdle, is modified like those of other members of the Rajidae in that the fusion extends well behind the level of the pectoral girdle, and the pectoral processes are dorsolateral bars which are laterally very extensive. The latter affect the pectoral girdle, making it in turn typically rajid. The basalia and radialia are diagnostic only in that they separate Arhynchobatis from such highly specialised families as the Pristidae, Torpedinidae, Mobulidae, etc., but they do not separate it from the more closely related families with anterior terminal or quasi-terminal pectoral fins.
The pelvic girdle similarly is at once distinct from those of families with forward arched bars and median prepelvic processes (Dasyatidae, Myliobatidae and Mobulidae). But even in the remaining families with transverse pelvic bars, there are none apart from the Rajidae with similarly elongate, pointed lateral prepelvic processes such as are present in Arhynchobatis.
The skeletal distinctions between Arhynchobatis and the g. Raja are, with the exception of that of the median accessory rostral cartilage, almost entirely quantitative (i.e., differences in relative lengths and proportions of cartilages), or else where there are minor qualitative distinctions, homologues of these may be found
in other genera. [E.g., the large median hypo-basibranchial plate of Raja erinacea is shown by Garman (Pl. 68, Fig. 1) to be divided into an anterior and posterior portion, whereas in Arhynchobatis and Psammobatis mira (Garman, Pl. 69, Fig. 2) it is complete.]
It would appear then that Arhynchobatis may be regarded as a typical rajid, with only very minor skeletal distinctions separating it from the genotype of the family, and from the other genera.
The distinctions between Arhynchobatis and the Platyrhinidae (as exemplified by Platyrhina sinensis) are much more marked. The rostra are certainly similar, though Arhynchobatis has a median accessory rostral cartilage, and Platyrhina has lateral, divaricated accessory rostrals. The plate-like antorbital supporting cartilages and the divarications of the nasal valve cartilages of Platyrhina contrast strongly with the short, rod-like antorbitals and the nasal valve cartilages as described previously of Arhynchobatis. Similarly, the sinuous hyoid copula, the subrectangular median hypo-basibranchial cartilage, and the divided hypobranchials of Platyrhina differ markedly from the smoothly arched hyoid copula with prominent lateral processes, the subcircular median hypobasibranchial cartilage, and the fused hypobranchials of Arhynchobatis. But the most striking difference between the two skeletons is in the pelvic girdle. In Platyrhina the pelvic bar is very wide, slightly arched forward medially, and lacks distinct lateral prepelvic processes. In Arhynchobatis the pelvic bar is relatively much narrower, is almost transverse, and bears the characteristic rajid lateral prepelvic processes, which extend well forward as elongate pointed cartilages.
Such differences are obviously too fundamental to allow Arhynchobatis to be included in the Platyrhinidae, especially as the similarities that do occur are no more than one would expect between two members of such a morphologically and hence systematically compact and specialised group as the Batoidei.
The general facies of Arhynchobatis asperrimus are essentially those of the Rajidae, the disc shape, slender tail and other proportions supporting this view.
A short, shovel-shaped rostral cartilage is present, not reaching to the tip of the disc but continued by a slender, median, accessary rostral cartilage the latter probably representing a chondrified ligament.
The endoskeletal features are similar to those of the Rajidae, particularly the antorbital supporting cartilages, the nasal valve cartilages, the ventral branchial elements and the pelvic girdle.
The general facies and the endoskeleton exclude A. asperrimus from the Platyrhinidae, and indicate its affinities within the Rajidae.
The lack of a second dorsal fin, and the completeness of the caudal fin. are unique characters in the Rajidae, and as such merit the distinction of Arhynchobatis at the generic level.
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