On the spines of the Stethacanthidae (Chondrichthyes), with a description of a new genus from the...

ON THE SPINES OF THE STETHACANTHIDAE (CHONDRICHTHYES),

WITH A DESCRIPTION OF A NEW GENUS FROM

THE MISSISSIPPIAN BEAR GULCH LIMESTONE

by

RICHARD LUND *

ABSTRACT

Several specimens of an elasmobranch from the upper Mississippian Bear Gulch limestone of Mon- tana are described. The first dorsal spine and fin closely resemble Stethacanthus erectus and to a lesser extent Stethacanthus altonensis. The new genus and species Orestiacanthusfergusi is proposed for the specimens, and it is suggested that recent attempts to synonymize the species of Stethacanthus on the basis of spines are unjustified. The specimens also share the enlarged cranial denticles, general form of the palatoquadrate, cladodont teeth and pectoral fin with S. aitonensis. They differ from S. altonensis in proportions of jaws, numbers of tooth families, numbers of prearticular basals and axial radials, morphology of the second dorsal fin, and in the presence of generali- zed squamation in some specimens (but not the female). The pelvic fins differ greatly in having a prominent metapterygium.

RI~SUM]~

Plusieurs sp6cimens d'un Elasmobranche du calcaire mississipien sup~rieur de Bear Gulch (Montana) sont d~crits. Son 6pine dorsale ant~rieure ressemble &roitement/~ ceUe de Stethacanthus erectus et, dans une moindre mesure, /~ celle de Stethacanthus altonensis. Le nora de Orestiacanthusfergusi nov. gen., nov. sp. est propos6 pour ces sp6cimens, et il est sug- g~r6 que les r6cents essais de synonymic des esp~ces de Stethacanthus ~t partir des 6pines ne sont pas justifi6s. Ces sp6cimens partagent 6galement avec S. altonensis des denticules craniens agrandis, la forme g6n6rale du palatocarr6, les dents cladodontes et le type de nageoire pectorale. Ils different de S. altonensis par les proportions des mAchoires, le nombre des families dentaires, le nombre des rayons basaux et axiaux pr6articulaires, la morphologie de la seconde nageoire dorsale, et la pr6sence d'une squamation g6n~ralis6e chez quelques sp6cimens (reals pas chez la femelle). La nageoire pelvienne diff6re profond6ment par la pr6sence d'un m6tapt6rygium pro~minent.

KEY-WORDS : STETHACANTHUS SPINES, ORESTIACANTHUS FERGUSI, CLADODONTIDA (STETHACANTHIDAE), LOWER CARBONIFEROUS (MONTANA).

MOTS-CL]~S : ]~PINES DE STETHACANTHUS, ORESTIACANTHUS FERGUSI, CLADODONTIDA (STETHACANTHIDAE), CAR- BONIFI~RE INFI~RIEUR(MONTANA).

* Biology Department, Adelphi University, Garden City, L.I., New York 11530, U.S.A.

Geobios, n ° 17, fasc. 3 p. 281-295, 9 fig., 2 tabl. Lyon, juin 1984

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INTRODUCTION

Broadly triangular unornamented spines of Cheste- rian age assigned to Stethacanthus altonensis by New- berry (1889) were first identified as first dorsal fin spines of a chondrichthyan by Lund (1974), and several other nominal species were synonymized with this species in that paper. The synonymy was based upon three assumptions : 1) Zangerl, (personal commun. 1973 ; 1981) was correct in his assertion that FMNH PF 2207, a Pennsylvanian specimen, was a female and the same species ; 2) as there were only two, superfi- cially similar specimens of a large shark in the Bear Gulch limestone, the two could be considered the same species with MV 2830 a male and CM 23654 a female ;and 3) that from the variations seen in assumptions 1 and 2 that all or most large Stethacan- thus spines could be considered a single species, the considerably smaller spines named by Eastman (1903) and Hussakoff (1913 ; 1918) excepted. Zangerl (1981) has since synonymized all known Stethacanthus spines into a single species, S. altonensis.

Further collections in the Bear Gulch limestone show that none of the above assumptions were justi- fiable ; that it is not possible to determine the sex of FMNH PF 2207 ; that the two Bear Gulch specimens available to me at that time are, in fact different spe-

cies and CM 23654 is too immature to accurately determine sex ; and that therefore variations in the nominal species of spines are real taxonomic features, albeit of unknown taxonomic level. Zangerl's 1981 synonymy, given without any attempt at explanation, is without merit. There are several complex problems raised by the Bear Gulch material, which will be addressed in a series of papers. The three most basic issues, however, involve the nature of species criteria among Paleozoic Chondrichthyes, the dimensions of the Stethacanthidae, and the relationships of the Ste- thacanthidae to other chondrichtyhyans.

Preparation of specimens is done by needles. Dra- wings are either from photographic bases or made with a drawing tube attachment to an Aus Jena microscope. Photographs are taken with Kodak Tech- nical Pan 2415 film (ASA 25), developed for seven minutes in 1 : 15 Ethol thin emulsion compensating developer.

Abbreviations : American Museum of Natural His- tory (AMNH), New York;Carnegie Museum of Natural History (CM), Pittsburgh, Penna. ; Illinois State Museum (ISM), Springfield, Ill. ; University of Montana Geologic Museum (MV), Missoula, Mont. ; United States National Museum (USNM), Washing- ton, D.C.

SPINES OF THE STETHACANTHIDAE

The genus Physonemus McCoY, 1848 with the type species P. arcuatus McCoY, 1848 was erected for tapering, ornamented, curved spines with denticula- tions projecting along the concave face. The line of skin insertion indicated that the concavity of the spines faced forward. Subsequent species and genera with similar characteristics were reviewed recently by Baird (1957) and Chorn & Frailey (1978). A spine of the Physonemus group was tentatively identified as near Xystracanthus and associated with a cochliodont by Lund (1977a).

St. John & Worthen (1875, 1883) named as species of Physonemus a series of unornamented roughly triangular to hook shaped spines with very thin walled bases, and Davis (1883) added two unornamented spine species to this genus. Newberry (1889) separated the first described of the unornamented spines, P. altonensis as the type of the genus Stethacanthus, and

erected several other species for related large spines (Newberry, 1897). Eastman (1902) and Hussakoff (1913 ; 1918) named several species of small unornamented spines of very similar shape as further mem- bers of the genus Stethacanthus (tabl. 1).

Difficulties in identification of spines on a typologi- cal level arise from several factors. The thin walled hollow bases of the spines cause crushing asymmetries that led Newberry (1889) to hypothesize that the spines were paired structures rather than median. Growth of the spine, from distal tip to proximal region, as in modern shark spines, may leave the proximal region unfinished in immature spines, This may be the case for the spines named S. compressus, P. parvulus and P. chesterensis, and also seems to be the case in P. hamus- piscatorius. Further, the magni- tude of variation in size and shape is unknown, the size of the individual at inception of spine formation is unknown, and potential sex dimorphism is

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Fig. 1 - Spines of the Stethacanthidae, drawn from the original illustrations of the type material (A-D, H-O), to approximately the same scale. See Table 1 for figure references. Epines des Stethaeanthidae, redessin6es d'apr~s les figures originales du mat6riel-type (A-D, H-O), approximativement ~ la m~me 6chelle. Voir tableau 1 pour les r6f~rences des figures originales. A, Physonemus attenuatus DAvis, 1883 ; B, Physonemus proclivus ST. JOHN & WORTHEN, 1875 ; C, Physonemus hamatus DAVIS, 1883 ; D, Physonemus fal- catus ST. JOHN & WORTHEN, 1883 ; E, Stethacanthus altonensis, spine and fin, MV 2830, Bear Gulch limestone, (Lund, 1974). Epine et nageoir¢, Calcaire de Bear Gulch ; F, Orestiacanthusfergusi nov. gen., nov. sp., spine and fin, this paper, l~pine et nageoire (cet article) ; G, Physonemusfalcatus, spine and fin, CM 35463, Bear Gulch limestone. #.pine et nageoire, calcaire de Bear Gulch ; H, Stethacanthus erectus EASTMAN, 1902 ; I, S. humilis HUSSAKOFF, 1913 ; J, S. exilis HUSSAKOFF, 1913 ; K, Orestiacan- thus fergusi nov. gen., nov. sp. ; L, S. depressus (ST. JOHN & WORTHEN, 1875) ; M, S. productus NEWBERRY, 1897 ; N, S. compressus NEWBERRY, 1897, drawing reversed. Dessin invers~ ; O, S. altonensis, (ST. JOHN & WORTHEN, 1875) and O x, distal cross section. Section transversale de la partie distale.

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unknown. Most basically, the spines convey no morphological information about the animals that bore them and therefore no systematic or evolutionary information. There are however, other anatomical features associated with the spine-bearing taxa of the Bear Gulch limestone that contribute greatly to these questions.

The spines of stethacanthids fall into several clearly definable shape categories, irrespective of size, each shape referable to the earliest nominal species. In most cases different sizes have been assigned different names, and as there are no demonstrable intermedia- tes these names should be conserved in the interest of taxonomic stability. The shape categories are :

S. altonensis (fig. 1, 0). Large robust vertical spine apical angle 37.5 ° , posterior margin slopes slightly, forward from base. Prominent anterior shoulder with notch below and anteriorly projecting base. Ventral margin almost straight, with a slight posteroventral notch - also includes the smaller, less robust S. erectus (fig. 1, H ; fig. 2, A).

S. exilis (fig. 2, B). Small robust spine, posterior margin slopes slightly forward from base, anterior margin extends forward to prominent shoulder. Shoulder notched below but base does not extend far forward. Anteroventral margin convex, posteroventral margin notched.

S. humilis (fig. 2, D). Small narrow spine set well back on base, vertical posterior margin. Shoulder does not project forward to leave notch below. Ven- tral margin straight. Also includes the larger ~ S. productus )~ of Eastman, 1902 (see USNM 4879).

S. depressus (fig. 1, L). Moderately narrow spine with apical angle of 24 °, projecting strongly postero- dorsally and not differentiated from base. Posterior margin short, also slopes strongly back. Shoulder moderate, with notch below. Ventral margin concave downward posteriorly. This category also includes the considerably larger S. productus of Newberry, 1897, but not Eastman 1902 ; CM 23654 ; Lund, 1974 is also assigned here.

Physonemusfalcatus (fig. 1, D, G). Spine narrow, projecting anterodorsally to anteriorly, with posterior and topographically dorsal margin concave in cross section. Base with shoulder not notched below, ventral margin straight with posteroventral notch. Also included, P. proclivus, P. hamatus. Type, ISM 8720.

P. attenuatus (fig. 1, A). Spine narrow, projects forward far beyond small base, topographically dorsal margin convex in section. Base lacks significant

anterior projection, shoulder or posteroventral notch. Also possibly includes P. hamus-piscatorius EAST- MAN, 1903.

A fin is known to be associated with the spine in three instances (fig. 1, E-G) is clearly lacking in Physonemus attenuatus and may be missing in CM 23654.

In S. altonensis (MV 2830) the spine is broadly triangular, with a vertically oriented apex, a cartilage core and a separate posteroventral fibrocartilage articular element. The fin articulates with the spine along its posterior edge and with an articular element. The fin is triangular, with a calcified core faced bilaterally with many long calcified tubes radiating from the articular region to the posterodorsal margin. The fin is surmounted with a belt of enlarged denticles (Lund, 1974 ; Zangerl, 1981).

The spine of <t Physonemus )~ faicatus curves forward to an angle of 14.5 ° from the horizontal (MV 5385). A separate articular element is present posteroventrally, but does not articulate with the fin. The fin is in the form of a rod projecting forward above the head, resting in a groove on the (topographically) dorsal surface of the spine. It consists of long calcified tubules around a central core, well consolidated by calcification and tipped by slightly enlarged denticles. The size of the spines show no correlation with growth except that they are present only on sexually mature males, while females lack any traces of first dorsal fin structures. Males also have a dorsal cranial covering of enlarged denticles. The spine of << Physo- nemus ~> attenuatus is quite different in appearance from others of the group in the long distal forward extent and the small base lacking a '~houlder (CM 35473). An articular element is evident in small indivi- duals, but there are no traces of a fin and the articular element is incorporated into the base with spine growth. Males have a dorsal cranial covering of enlarged denticles. No known females.

The small Bear Gulch spines are 15-16 mm high, narrow and sharp, with posterior margin sloping slightly forward. The basal plates are 9 mm in length, with shoulder neither notched nor projecting anteriorly, and base not projecting far beyond the shoulder. The bases are convex anteroventrally, notched posteroventrally with a distinct articular calcification below the notch. The fin is a tall, 15 ° acute triangle of many superficial hollow tubes radiating from the articular apex, with substantial internal and basal (articular) calcification types to form a solid unit. The fin is capped by 6 rows of denticles on each side of spine

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with 7 or more denticles per row. The denticle bases are thick, polygonal and tightly fitted. The anterior row of denticles are short-cusped and the remainder very long cusped, with all cusps directed ventrally. Shorter cusps may have angular cross sections while the remainder are circular in cross section.

These spines most closely resemble S. erectus EAST- MAN, 1902 in apical shape and relationship to the base,

but have basal characteristics of both S. exilis and S. hum#is. A clear assignment of the Bear Gulch small spines to any of the prior names is not possible.

Subclass ELASMOBRANCHII,

Family STETHACANTHIDAE, L U N D , 1974

Orestiacanthus nov. gen.

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Fig. 2 - - A. Stethacanthus erectus, holotype, after Eastman,1902. Scale is 1.5 cm. B. S. exilis, A M N H 5076, holotypc, scale is 5 mm. C. Orestlacanthusfergust, nov. gen., nov. sp. MV 6152, spine and fin of holotype. Scale is 5 mm. Epine et nageoire de l 'holotype. Scale is 5 ram. D, S. humllis, holotype, A M N H 5077. Scale is 5 ram.

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TYPE SPECIES : Orestiacanthusfergusi, n o v . sp . .

DIAGNOSIS:small stethacanthids with first dorsal spine and narrow, well calcified fin, second dorsal fin lacking spine, supported by well spaced two jointed radials. Pectoral fin of 4 articular basals supporting radials, 8 radials on principal basal plate, 1 axial basal with 2 radials, 5 basals with 1 radial each and at least 24 basals following. Pelvic girdles with long metapterygium, leading edge of fin articulated with girdle but majority of radials supported on metapterygium. Caudal fin is low angle heterocercal. Ventral lateral lines present between pectoral fins.

ETYMOLOGY ." named for Orestes St. John, the earliest describer of many spines of the stethacanthid elasmobranchs.

Orestiacanthus fergusi nov. gen., nov. sp. (figs. 2C, 3-8, table 2)

TYPE SPECIMEN : M V 6152, male.

REFERRED SPECIMENS : M V 7695 , C M 35460 , 37525 ,

41054.

DIAGNOSIS : Same as species, only genus.

CAT SPL SPH HL LJL MXH R-DIO R-D20 D2B PVF

MV 6152 9.3 16.5 24.3 24.1 -

MV 7695 - - 25.5 - 64.15 20.0 -

CM 35460 9.0 15.3 -

CM 41054 ii 15.8 25.5 24.4 33.1 34.8 93.7 18.8 -

CM 37525 . . . . . . . . 64.8

Tabl. 2 -- Measurements of Orestiacanthus fergusL Mensurations de Orestiacanthus fergusi. Abbrevia t ions:CAT, catalogue number ;D2B, length of second dorsal fin b a s e ; H L , head l eng th ;LJL , lower jaw length ; MXHT, maximum body height ; PVF, pelvic fin origin to caudal fin fork ; R-D10, rostrum to origin of first dorsal insertion ; R-D20, rostrum to origin of second dorsal insertion ; SPH, spine height ; SPL, spine length.

Abr6viations : CAT, num6ro de catalogue ; D2B, longueur de la seconde nageoire dorsale /~ sa base ; HL, longueur de la t~te ; LJL, longueur de la mandibule ; MXHT, hauteur maximale du corps ; PVF, distance de l'origine de la nageoire pelvienne la fourche de la caudale ; R-D10, distance du rostre A l'origine de l'insertion de la premiere nageoire dorsale ; R-D20, distance du rostre/t l'origine de l'insertion de la seconde nageoire dorsale ; SPH, hauteur de l'6pine ;SPL, longueur de l'~pine.

HORIZON AND LOCALITY " Upper Chesterian (Namu- rian E2b), Bear Gulch member of the Heath Forma- tion, south of Becket, Fergus County, Montana.

ETYMOLOGY : named for Fergus County, Montana.

REMARKS : the spine of Orestiacanthusfergusi clearly closely resembles the ichthyodurulites S. exilis, S. erectus and, to a lesser extent, S. humilis from the Kinderhookian Waverly Shale nodules of Knob Lick, Kentucky (fig. 2, 3). No other morphologic features are associated with the Knob Lick spines, nor is it pro- bable that associated material from the type locality or horizon will become available. Systematically significant anatomical differences between the bearers of these spines are therefore unavailable. Anatomical differences between the Chesterian Stethacanthus

altonensis (Lund, 1974, Zangerl, 1981) and Orestia- canthus fergusi however, justify genus-level separa- tion at the very least. That this, or any other systematic conclusions, cannot be established on the basis of the Knob Lick material renders the Knob Lick names of dubious systematic value.

SQUAMATION : a general squamation is evident, and is particularly well displayed on CM 41054 (fig. 4). There are additional areas of specialized denticles on the dorsal surface of the head, spine, pectoral and pelvic fins. Further, the lateral line canals are framed by specialized dermal denticles in the form of fine half rings.

The general body squamation consists of well spaced unicuspid, sharply pointed denticles that flare broadly basally. The denticles seem to be distributed in rows.

288

Zones of enlarged denticles are found along the ven- trolateral margins of the posterior abdomen. Squa- marion outlines the four external gill openings.

Lateral concentrations of large denticles with thick polygonal bases are found between the anterior end of the ethmoid and the midorbital region of the dorsal surface of the head. Similar denticles cover the dorsal

region of the first dorsal fin ; apexes of these cusps point downward.

The posterior portions of the axes of the pectoral fins of MV 7695 are outlined by fine denticles with fused bases. Pelvics of the male, MV 6152, are den- sely coated with elongate denticles.

Fig. 3 - - Orestlacanthusfergusl nov . g e n . , nov . sp . , C M 41054 . Scale in m m .

One female, lacking head and anterior trunk, is assigned to this species on the basis of axial, dorsal fin and pelvic characters (CM 37524). It lacks any trace of squamation aside from lateral line canals. This is not a preservationai defect.

The trunk lateral line canal gives off fine dorsal and ventral segmental branches diminishing in height posteriorly to the second dorsal fin. There is also a paired ventral lateral line along the abdomen from the scapulocoracoid to approximately the posterior end of the pectoral axis. Cranial lateral line canals are poorly preserved. The supraorbital canals seem to be associated with the enlarged denticles as in male chondren- chelyids (Lund, 1982). The infraorbital canal des- cends ventrally from the postorbital process.

CRANIUM : the braincases are well calcified but crus- hed and display few details (fig. 5, 6). The percent of postorbital braincase to its total length are between 41-46070, with a very short preethmoid region and no obvious rostrum. The otic region is rounded postero- dorsally. Supraorbital shelves are present but are not as broad as in Stethacanthusaltonensis (Lurid, 1974). Ventrally, there are no subocular shelves ; the eth- mold and anterior orbital regions are narrow.

The palatoquadrate has a prominent anterior eth- mold process, circular orbital margin and a single articular facet on the pterygoid process (fig. 5). The height of the pterygoid region is 7607o of its length, descending gradually to a short but prominent quadrate shelf. The quadrate condyle is level with the occlusa l margin .

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Fig. 4 - - Orestiaeanthusfergusi nov. gen., nov. sp., MV 6152, holotype, precaudal skeleton, scale in mm. Abbreviations : H, hyomandibular ; M, pectoral metapterygium ; S, scapulocoracoid, V, vertebrae.

Squelette pr6caudal. Abr6viations : H, hyomandibulaire ; M, m6tapt6rygium pectoral ; S, scapulocoracoide ; V, vert~bres.

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F i g . 5 - - Orestlacanthusfergusl, nov. gen., nov. sp., MV 6152, holotype, palatoquadrate and mandible, scale in mm. Palatocarr6 et mandibule.

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Anterior to the quadrate condyle, there is a slight dorsal excavation of the occlusal margin, followed by a slightly curved tooth bearing region. There are grooves for approximately 12-13 tooth families.

Meckel's cartilage is slender with a slight coronoid process. There are grooves for 12 tooth families, occupying 74% of the jaw. The hyomandibular is robust.

Teeth are cladodont, appear to have 5 cusps, with the central cusp most robust and vertically fluted. The tooth bases are not displayed. Numerous styptobasis -

like denticles (Zangerl & Case, 1976) and less common stemmatodus- like forms are found along the bran- chial arches.

PAIRED FINS AND GIRDLES " the scapular process is tall, uniformly slender and bears an anterodorsal and a posterior process dorsally (fig. 4). The coracoid process is directed ventrally, as in Stethacanthus altonensis (Lund, 1974) and Cobelodus (Zangerl & Case, 1976) but there is no evidence for a separate subcora- cold element.

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Fig. 6 - - Orestiacanthusfergusi, nov. gen., nov. sp., MV 7695A. A, neurocranium in ventral view. B, Neural arches 13-19, contralateral elements dashed. C, Pectoral fin. Abbreviations : B, basiapophysis ; I, infraorbital sensory canal ; N, nasal capsule ; S, supraorbital sensory canal ; SC, scapulocoracoid ; V, ventral sensory canal. Scale in mm.

A, neurocr[tne en vue ventrale ; B, arcs neuraux 13-19, 616ments contralat6raux en pointill6s ; C, nageoire pectorale. Abr6viations : B, basiapophyse ; I, canal sensoriel infraorbitaire ; N, capsule nasale ; S, canal sensoriel supra-orbitaire ; SC, sca- pulocoracoIde ; V, canal sensoriel ventral.

2 9 1 - -

There 4 basals articulating with the scapulocoracoid anterior to the glenoid fossa in CM 41054, followed by a pentagonal basal plate articulating with 8 radials. The radials are relatively short, suggesting considera- ble support of the fin web by other than endoskeletal elements. The axis of the pectoral fin contains 12 visible basals, and is further delineated posteriorly by fringing denticles (CM 7695 ; fig. 6).

The paired pelvic girdle elements are platelike, are not fused at the midline and probably lay approximately horizontally in the ventral body wall with their anteromesial ends in close proximity. A long, stra- plike metapterygium articulates with the posterome- sial corner of each girdle element. The major plane of the metapterygium is oriented perpendicular to the major plane of the girdle. The first 4-5 radials articulate with the posterolateral part of the girdle, but the remaining radials in the female articulate with the lateral surface of the metapterygium (fig. 7). This geometric arrangement is the same as in the modern Raja erinacea, and has not previously been noted in Paleozoic elasmobranchs.

The pelvic fin and girdle of MV 6152 are the same as those of CM 37525. The clasper is supported by an axis consisting of a basal somewhat smaller than the metapterygium, followed by approximately 10 small elements. The clasper itself is composed of four elements. The first basal of the axis supports five unjointed radials of equal size.

The second dorsal fin skeleton consists of 20 well spaced two jointed elements (fig. 8). The posterior four radials and distal basals are supported upon a single small triangular proximal basal plate. There is no known anal plate or anal fin.

A X I A L SKELETON : the vertebral numbers of Orestia- canthus can only be estimated by matching the anterior parts of CM 35460 and MV 7695 to the posterior part of MV 6152 and CM 37525. There are 22-24 segments between the occiput and the anterior end of the second dorsal fin, 20 vertebral segments under the dorsal fin, and approximately 6-9 segments to the beginning of the caudal skeleton, or approximately 48-53 precaudal segments. There are no haemal arches visible anterior to the supports for i

Fig. 7 - - Orestiaeanthusfergusi, nov. gen., nov. sp. Pelvic fins ; Nageoires pelviennes. A, MV 6152, holotype, male. B, CM 37525, female. Scale in mmo

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the ventral lobe of the caudal fin, possibly a preserva- tion deficit of about 6-9 segments. There are at least 29 vertebral segments in the low angle heterocercal tail.

The first nine cervical neural arches are paired, square, and lack prominent spinous processes. Suc- ceeding neural arches are paired, taller and narrower, with thin spinous processes. The anterior 30 vertebrae have visible dorsal root foramina ; ventral roots of spinal nerves may have exited between arches. Verte- brae posterior to the middle of the dorsal fin become

thinner and more sharply inclined from the vertical. The caudal neural arches are paired, thin and closely overlapping, with the last few elements squarish. The ventral lobe of the caudal fin is supported on about eight large but unspecialized haemal spines. The eight radials are unjointed. Succeeding haemals decrease in size to the last few diminutive, squarish elements.

There are stout triangular anterior trunk basa- pophyses. There are no traces of dorsal or ventral intercalary elements nor notochordal calcifications.

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Fig. 8 - - Orestiacanthusfergusi nov. gen., nov. sp., CM 41054, second dorsal fin ; Seconde nageoire dorsale. Scale in m m .

DISCUSSION

Orestiacanthus fergusi is an elasmobranch, revea- ling four external gill openings defined by squamation. It shares a common structure of the first dorsal spine with several nominal taxa of varying sizes assigned to the genera Stethacanthus and Physonemus. The shapes of both fin and spine differ only in degree from that of Stethacanthus altonensis (ST. JOHN &

WORTHEN) (Newberry, 1889 ; Eastman, 1902 ; Hus- sakoff, 1913 ; L u n d , 1974 ; Zangerl, 1981). Most recently Zangerl (1981) has attempted to synonymize all Stethacanthus spines into S. altonensis. Compari- son of the known morphology of S. altonensis (MV 2830) with that of O. fergusi gives some insight into the systematic problems of Stethacanthus.

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Squamation is general in O. fergusi with specialized lateral anterodorsal cranial, first dorsal fin, pelvic and lateral line denticles. General and lateral line squamation are unknown, but not necessarily absent in S. altonensis (Lund, 1974 ; Zangerl, 1981). Enlar- ged cranial denticles cover most of the dorsum of the head in S. aitonensis. First dorsal fin denticles are vir- tually the same in the two taxa ; pectoral and pelvic denticles are unknown but probably absent in S. altonensis. Supraorbital crests are very wide in S. altonensis, moderate in O. fergusi. The palatoquadrate of S.

altonensis has a conspicuous posterior orbital line rising to the high pterygoid process, contrasting with the almost circular outline of O. fergusi. The pterygoid area is considerably higher posteriorly in S. altonensis (height as a percent of length - 76%0), and a very narrow posterior quadrate shelf. The toothed portion of Meckel's cartilage is only 57% of its length ; a coronoid process is conspicuous, and there are grooves for only 6-8 tooth families in S. altonensis.

Fig. 9 - - Stethacanthus altonensls, MV 2830, pelvic fin ; nageoire pelvienne. Male. Scale is 1 cm.

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Pectoral girdles are similarly shaped in the two taxa, as is the general structure of the fin. S. altonensis has ten prearticular basals, while O. fergusi has four elements. The metapterygium supports about 8 radials and a long axis in each, with MV 2830 having radials the length of the axis (omitted in Zangerl, 1981, fig. 81). Pelvic girdles are radically different in the two forms. The pelvic plate of S. altonensis (fig. 9) lacks a pronounced metapterygium and most of the 21 one- jointed radials are supported on the plate itself. The long metapterygium supports 60% of the radials of O. fergusi. Although the clasper of S. altonensis is poorly known, there is no indication of the elongate first basal element of O. fergusi. The second dorsal fin of S. altonensis consists of over 20 one-jointed, high, tightly packed radials with a small anterior basal plate (omitted in Zangerl, 1981, fig. 81), in contrast to the well spaced, two jointed condition in O. fergusi. The claim of Zangerl (1981, pg. 73) that the axial radials of MV 2830 are neurapophyses is not possible. No known Paleozoic chondrichthyan has neurapophyses separate from the arches and round in cross section, and no known vertebrate has neurapophyses articulating with distal pectoral appendages. The pre- servation of Field Museum of Natural History P2207 does not lend itself to a determination of the condition of either pectoral axis structure nor of sex (Zan- gerl, 1981, fig. 81A). The proportions of the spines differ between the Bear Gulch and Field Museum specimens, however. This could be accounted for by dif ferences in age, sex (Lund, 1974), size, environment, geologic age (Namurian and Westphalian) or even of species. There is no current solution to this problem.

Stethacanthus altonensis and Orestiacanthus fergusi share several unique characters. These are the structures of the first dorsal fin and spine and the presence of areas of enlarged denticles on the dorsal aspect of the head and first dorsal fin. They differ most markedly from each other in pelvic fin structure. Stethacanthus has a platelike girdle lacking a long metapterygium, a character shared with << symmoriids >> (Zangerl, 1981). Orestiacanthus, by contrast, has an essentially Euselachian metapterygium (Smith, 1937) upon a paired pelvic girdle resembling the simple triangular plates of non-euselachian Chondrichthyes (Lund, 1977b, 1982 ; Dean, 1909).

Stethacanthus and Orestiacanthus share the form of the teeth, (cladodont) pectoral fin and high pterygoid process of the palatoquadrate with several other forms, the Symmoriidae (Zangerl, 1981), <~ Clado- dus >> neilseni (Traquair, 1888), and others referred to the order Cladodontida (Obruchev, 1967, Moy- Thomas & Miles, 1971). The general anatomy of the Cladodontida is known elsewhere only among the Symmoriidae, which lack a first dorsal fin (Zangerl & Case, 1976 ; Zangerl, 1981) and have a stout postorbital arcade, specialized characters not shared by Ste- thacanthus or Orestiacanthus.

At the present level of knowledge of Paleozoic Chondrichthyes the Stethacanthidae and Symmorii- dae seem to constitute two suprageneric groups within the order Cladodontida. There is clearly no justifica- tion for any synonymy of spines of the Stethacanthi- dae at this time in view of the broad morphological differences between Stethacanthus and Orestiacan- thus.

Acknowledgements

I am grateful to Wendy Lund for the illustrations and to William G. Melton Jr., for his co-leadership of the field crews. The ranchers of the Grassrange-Forest Grove area have extended us access to their land and their hospitality.

This work has been funded by National Science Foundation grants BMS 75-02720, DEB 77-02548 and DEB 79-19492 and by Carnegie Museum of Natural History.

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Manusc r i t d6f in i t i f regu le 10.01.1984

On the spines of the Stethacanthidae (Chondrichthyes), with a description of a new genus from the...

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