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On the Relationships of the Petalodontiformes (Chondrichthyes)
Article in Paleontological Journal · December 2014 DOI: 10.1134/S0031030114090081
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On the Relationships of the Petalodontiformes (Chondrichthyes)1 R. Lunda, b, c, E. D. Grogana, b, c, and M. Fatha aBiology Department, Saint Joseph’s University, 5600 City Avenue, Philadelphia PA 19131 bCarnegie Museum of Natural History 4400 Forbes Avenue, Pittsburgh, PA 15213 cAcademy of Natural Sciences in Philadelphia 1900 Benjamin Franklin Parkway, Philadelphia, PA 19103 e mail: [email protected] Received February 21, 2012
Abstract—The Petalodontiformes are Upper Paleozoic marine euchondrocephalan chondrichthyans known primarily from isolated teeth. Few dentitions have been reported, among them that of the Permian Janassa bituminosa and “Janassa” korni and the Serpukhovian, Mississippian Belantsea montana, Obruchevodus grif fithi, Netsepoye hawesi and Siksika ottae. A dentition has been reconstructed for the Pennsylvanian Petalodus ohioensis. New information on dentitions and postcranial morphology is now introduced for petalodont remains from the Bear Gulch Limestone of Montana (USA). These include N. hawesi, Petalorhynchus bear gulchensis, several specimens bearing teeth of the Fissodus form, and a new genus and species, Fissodopsis robustus. Analysis of all material reveals dignathic heterodonty as prevalent among the Upper Mississippian petalodonts, with distinct regionalization resulting in large anterior and small lateral teeth. Fissodus style teeth are found as the lower median tooth position of most heterodont dentitions, with a Ctenoptychius like upper anterior tooth or teeth, accompanied by lateral upper and lower teeth of Janassa like form. As hetero donty seems to have been the plesiomorphic condition not only in these petalodonts but for the Euchondro cephali as well, a model is proposed to explain the derived homodont conditions found in Petalodus (Hansen, 1985), Polyrhizodus (Lund, 1983), Belantsea (Lund, 1989), and J. bituminosa (Jaekel, 1899). The principally dentition based cladistic analysis retains the previously erected families Petalodontidae, Belantseidae, and Jan assidae, rejects the assignment of ”J.” korni to Janassa, and suggests a heterodont clade for the remaining taxa. Two new families are designated on the basis of these data, the Petalorhynchidae and the Obruchevodidae.
Keywords: Petalodontiformes, petalodont dentitions, heterodonty, Fissodus, Janassa, Petalorhynchus, Missis sippian
DOI: 10.1134/S0031030114090081
INTRODUCTION1 MATERIALS AND METHODS A petalodont tooth has been distinguished from Imaging and Measurements similar teeth as consisting of a distinct labially situated Specimens were digitally scanned at original size, crown with a Iingually extended heel, and a root that in RGB color, and with resolution up to 6400 ppi using originates below the lingual edge of the heel (e.g. an Epson Perfection V500 color flatbed scanner. Newberry and Worthen, 1866; Lund, 1989), resulting Images were corrected for color casts, levels (white in a sigmoid tooth profile. Teeth with these properties and black points), brightness and contrast with Corel were originally included in a Family Petalodontidae by Paint Shop Pro X2, rendered into black and white and Newberry and Worthen (1866), but the systematic reduced to the resolution required for publication. level of this group has gradually been elevated to an Drawings were prepared with Adobe Illustrator CS4, Order (Patterson, 1965) to accommodate the accumu from drawings rendered with a microscope drawing lated increase in primarily tooth based taxa. Whole tube, as well as prepared from overlays of scanned fos and partial dentitions are rare, and specimens that dis sil images. Original drawings based upon studies made play additional cranial or postcranial information are for Lund (1989) were also digitized into scanned rarer still. However, re examination of previously images. Anatomical measurements were made using described taxa and the recovery of new specimens the guide and measurement tools in Adobe Illustrator. from the Serpukhovian (Upper Mississippian) of Montana, USA, now make it possible to attempt a character based phylogenetic analysis of the known Terminology petalodont dentitions. In the following pages, formal taxonomic names 1 The article is published in the original. are italicized, but names used to describe particular
1015 1016 LUND et al.
(a) (b) (c) (d) (e) (f)
(g) (h) (i) (j) (k)
Fig. 1. Teeth of the Fissodus form. All teeth drawn to same scale. (a) Netsepoye hawesi, lingual view, CM 46092; (b, c) “Fissodus”, CM 62709 A and B; (d) “Ctenoptychius” (Janassa) korni, from Brandt, 1996, Fig. 4b; (e) “Ctenoptychius” (Janassa) korni, from Brandt, 1996, Fig. 4c; (f) “Ctenoptychius” (Janassa) korni, from Brandt, 1996, Fig. 6b; (g) Siksika ottae, MV 6199, from Lund, 1989, Fig. 12; (h, i) Fissodus bifidus, from St. John and Worthen, 1875, Pl. XIII, Figs. 1, 2, in labial (h) and lingual (i) views; (j) Fissodopsis robustus sp. nov., CM 62710; (k) “Fissodus”, CM 48824. tooth forms that are not, or may not be restricted to a in Peripristis semicircularis (Newberry and Worthen, single taxon are not italicized. 1866) or Siksika ottae (Lund, 1989). Heterodonty is prevalent among the known petal A Janassa like tooth displays a low crown, which odont dentitions, but is not present in all the taxa. As varies from a horizontal to a slightly peaked profile several of the known specimens with dentitions are that is smooth to denticulate, the absence of a basin, small and some are probably juveniles or subadults, and an extended heel approximating a perpendicular neither ontogenetic addition of tooth positions nor to the crown, with several strong ridges, and a promi ontogenetic heterodonty can be excluded. In addition, nent root. the denticulate condition described for some teeth A Fissodus like tooth displays a high acuminate may simply reflect the unworn state of unerupted or and somewhat trenchant bifid crown, with the crown recently erupted teeth, but since this cannot be deter edge smooth to slightly cuspidate, no basin, a conspic mined on the basis of available material and the char uous heel with few to several weak to strong ridges, and acter has been repeatedly used in older literature, the a long narrow root. character state has been included in our analysis. Three basic tooth forms are variably displayed in the several petalodont dentitions, Petalodus like, Phylogenetic Analyses Ctenoptychius like, and Janassa like (Plate 7). A Fis Cladistic analyses were performed with the pro sodus like form is also present as an apparent lower gram TNT (Goloboff et al., 2008) upon two matrixes median tooth in the dentitions of several different taxa (matrix Chon58). The first matrix contains 122 char (Fig. 1). There is no evidence to support any of these acters and 61 taxa and includes a theoretical zero tooth forms as the plesiomorphous petalodont condi based outgroup as well as placoderm, acanthodian and tion for an entire dentition. osteichthyan outgroups. The second matrix is com A Petalodus like tooth displays a crown that varies prised of the available morphological and dentition from high to low and acuminate to low acuminate, characters of the Petalodontiformes, and contains with the crown edge not trenchant and smooth to 34 characters and 15 taxa including a zero based out lightly denticulate, with a slight basin, a minor heel, group as well as Gregorius rexi (Lund and Grogan, and few, low basal ridges. 2004a) and two Bear Gulch Limestone species related A Ctenoptychius like tooth displays a high crown to Chomatodus. that is acuminate to sharply acuminate and trenchant, Institutional abbreviations. CM, Carnegie with the crown edge denticulate to cuspidate, and a Museum of Natural History, Pittsburgh, Pennsylva heel with a conspicuous basin and few, strong basal nia, USA; FMNH, Field Museum of Natural History, ridges. The crown may be proximo distally straight, as Chicago, Illinois, USA; MV, University of Montana in Belantsea montana, or proximo distally curved, as Geological Museum, Missoula, Montana, USA.
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SYSTEMATIC PALEONTOLOGY to the median sagittal plane) of the lateral teeth is CLASS CHONDRICHTHYES small (1.5 mm wide) and has a slightly peaked crown HUXLEY, 1880 and a rectangular heel. The distal tooth is wider than its neighbor (1.9 mm) is and has a horizontal slightly SUBCLASS EUCHONDROCEPHALI LUND ET GROGAN 1997B denticulate crown and a perpendicularly oriented rectangular heel that bears at least three lingual ridges. Order Petalodontiformes Patterson, 1965 The presumed lower median tooth is of the Fissodus Fissodopsis gen. nov. form (2.5 mm wide), less than one half the width of the Fissodopsis robustus sp. nov. median upper, with a narrow heel. There are at least Plate 7, Fig. 7; Plate 8, Fig. 1 four accompanying small lateral teeth; the most prox imal has a trenchant acuminate denticulate crown and E t y m o l o g y. The genus name is derived from the an extended heel while the more distal teeth all have Fissodus shaped lower median tooth, with recogni extended ridged heels. Crown shapes are uncertain. tion that the Fissodus shaped tooth is found in many This gives a possible upper jaw tooth position number different petalodont dentitions as well as this one. of 11 and a lower jaw tooth position number of seven Type species. Fissodopsis robustus, new species. teeth. Multiple replacement teeth are evident but no D i a g n o s i s. Moderate sized petalodont with tooth retention is indicated. narrow compressiform neurocranium; basis cranii dis Material. Holotype only. tinctly wider than dorsal aspect, well mineralized hyoid complex and elements of a nested branchial Comparison. Fissodopsis robustus compares to arch skeleton. Dentition of regionally specialized other petalodonts as described below: Obruchevodus anterior and lateral teeth; median upper double the grifflthi, CM 48833, holotype (Grogan et al., this vol width of the bifid median lower, three anterior lower ume). The mouth of O. griffithi is terminal, and the teeth, five small upper lateral tooth families and prob ethmorostrum is anteroventrally extended. The upper able three lower families. dentition has a large acuminate median tooth of Ctenoptychius form, with five lateral teeth in the C o m p o s i t i o n. This genus is monotypic. upper and four lateral teeth in the lower ramus. One Etymology. The species name after robustus, sharp cusp of a lower anterior tooth of equal dimen connoting the robust median upper tooth and the sions to the upper can be seen in occlusion with the robust mineralization of elements of the pharyngeal median upper tooth, but further details are not clearly endoskeleton. displayed. The anterior teeth are flanked by four or five H o l o t y p e. CM 62710, head, hyoid and bran lateral teeth per ramus, each of which is accompanied chial arch elements and dentition. Sole specimen. by replacement teeth. There is an abrupt size, angular, P r o v e n a n c e. Serpukhovian Upper Mississip and shape change from large transversely oriented pian Bear Gulch Lens, Bear Gulch Member of the anterior teeth to the smaller and relatively uniform Heath Formation, Big Snowy Group, from south of sized lateral teeth; these have three to four lingual Becket, Montana, USA. ridges but lack extended heels. The proximal crowns of D e s c r i p t i o n (Fig. 1j; Fig. 3). The post eth the lateral teeth are rounded in profile, transitioning to moid neurocranium of F. robustus is preserved in very low rounded profiles distally. The labial surfaces dorso ventral view and is strongly laterally com of the teeth lack basal ridges, and the edges of the pressed. The anterior part of the ethmoid region is crowns are lightly denticulated with up to approxi crushed and distorted, and the relationships of the mately 12 cusplets. O. griffithi displays dignathic heter teeth to the neurocranium suggest that the ethmoid odonty. and mouth were extended anteroventrally as in Belant Netsepoye hawesi, CM 46092, holotype (Lund, sea and Obruchevodus. The short otic region is very 1989) (Fig. 2). Reexamination of the type specimen narrow and bears a sharp sagittal crest, with the basi has clarified significant aspects of its anatomy (Grogan trabecular processes extending ventrally rather than et al., this volume) and dentition (Fig. 2). N. hawesi splaying out ventrolaterally. A long suborbital shelf is exhibits dignathic heterodonty and a division of the present ventrally. The orbital and posterior ethmoid teeth in each jaw into clearly separated and morpho regions widen from the narrow otic. Large, well min logically distinct anterior and lateral tooth regions. eralized and firmly articulated basihyal, hypohyal, and Anterior teeth are transverse to the sagittal plane and ceratohyal cartilages, and several branchial cartilages, laterals are subparallel to the sagittal plane. The mouth have been displaced away from the neurocranium; the of N. hawesi appears to be terminal. There are three branchial cartilages bear a crust of fine denticles. large anterior upper teeth, two of which flank the The length of the head from occiput to the root of median, all of Ctenoptychius form, with acuminate, the anterior median tooth is 2.6 cm. Teeth are of denticulate, and trenchant crowns and extended lin uncertain relationship/orientation to the neurocra gual basins. The median upper tooth is positioned in nium. There is one large, broad median spatulate pre mosaic with and posterior to the flanking teeth in the sumed upper tooth, with a wide (6 mm) crown taper type specimen. Roots of the anterior teeth are very ing to a narrow blunt root. The most proximal (closest long and rather narrow. There is an abrupt shift to the
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Plate 7
123
6
45
7
8
Explanation of Plate 7 Figs. 1–8. Teeth of some Petalodontiformes: (1) Petalodus ohioensis; (2) Belantsea montana, MV 7698, holotype, median and right anterior upper; (3) Netsepoye hawesi, CM 46092, holotype, median and left anterior upper; (4) Obruchevodus griffithi, CM 48833, holotype, right lateral lower; (5) Janassa clarki, MV 7697, holotype; (6) Netsepoye hawesi, CM 46092, holotype, lat eral upper teeth; (7) Fissodopsis robustus, holotype, CM 62710, posterior lateral presumed lower teeth; (8) “Janassa” sp., FMNH PF 8424. Dentition as preserved, anterior trenchant tooth at upper right. Scales in mm. six visible lateral teeth on each ramus. These are small bear multiple lingual ridges, and the roots are elon and rectangular in occlusal (oral) view. The most prox gate, posteriorly slanted, and have bulbous distal ends. imal crowns are low, rounded, and denticulate in labial There is a large bifid lower median tooth, with a view, while the crowns of successive teeth transition to long multi ridged heel and a narrow extended root. a completely planar janassiform posterior tooth. The The replacement for this tooth lies far posterior to the heels are extended perpendicular to the crown and remaining dentition and has the same orientation.
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BTP
SC OR (b) G OT
FM B H
(a) (c)
Fig. 2. Fissodopsis robustus sp. nov., holotype CM 62710. (a) head in dorsal view with teeth and hyoid and branchial elements; (b) upper dentition as preserved; (c) lower dentition as preserved. Abbreviations: (B) branchial elements, (BTP) basitrabecular process, (FM) foramen magnum, (G) glossohyal (H) hyoid arch, (OR) orbit, (OT) otic bulla, (SC) sagittal crest. Anterior is up, scale bars are 1 cm.
There is a sharp change in both size and orientation to almost to the dorsal midline. Each articulate with one the two small laterals. The first lateral tooth is or two large pectoral cartilages but mineralization is obscured, and the posterior lateral tooth is 2/3 the minimal and further details cannot be resolved. A belt length of the three combined occluding upper lateral of scales extends transversely behind the shoulder gir teeth. This tooth is janassiform, with an extended heel dle. A large scale is present in the midline of the head bearing 3–4 low ridges and a low horizontal corru antorbitally, two pairs of large scales are located near gated crown that bears few cusplets. While it is possible the otic midline, and smaller, single pointed scales are that a more distal tooth existed, it is not evident. distributed rather sparsely across the remainder of the Petalorhynchus beargulchensis Lund (1989), head. Small scales demarcate the supraorbital lateral CM 78542 (Fig. 4, Plate 8, Fig. 2). The type specimen line canals. The head is 13.8 mm long from occiput to of Petalorhynchus beargulchensis Lund, 1989 the tip of the anteriormost tooth; the otic and orbital (CM 41032) is a poorly formed and possibly fetal indi regions are, respectively, 2 and 5 mm long. The head is vidual with little endoskeletal mineralization and few 10 mm at the posterior margin of the head, its widest teeth, conforming to the generic definition in details point, 8.5 mm wide at the anterior margin of orbit, and of tooth shape. A second specimen, CM 78542, has 3.2 mm wide interorbitally. The dorsoventral preserva subsequently been found, the head of which is pre tion of head and dentition indicates a terminal to sub served in dorso ventral view. The labial surfaces of the terminal mouth in an ethmoid region that is not upper median teeth each bear a vertical ridge that cul anteroventrally extended. The dentition occupies the minates in a button like basal swelling. This is consid anterior 3.3 mm of head length. The lower jaw articu ered a diagnostic character of the genus Petalorhyn lates antorbitally, 1 mm anterior to the orbital margin. chus (Davis, 1883). The sharply peaked teeth of the The upper dentition contains a large acuminate tren specimen make it possible to attribute it to Petalorhyn chant median tooth conforming to those of other chus beargulchensis Lund, 1989. The postcranial members of the genus, flanked by a minimum of three aspect of the specimen is globular and defined princi gradationally smaller tooth families to either side of pally by pigments and coarse monocuspid scales. the median tooth. All families contain a replacement Traces of long pectoral and possible pelvic fin radials tooth. The lower dentition consists of a bifid tooth of are visible as pigmented impressions. There are no Fissodus form flanked by three gradationally smaller indications of median fins. The shoulder girdles lie more distal tooth positions. The three anterior lower against the posterior margin of the head and extend teeth have strongly ridged heels, but heels are linguo
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(a) (b) (c)
Fig. 3. Netsepoye hawesi, CM 46095 holotype, dentition reconstructed. Anterior is up, uncertain borders of teeth marked with dashed lines. (a) upper dentition with lateral teeth in labial view; (b) lateral upper teeth in oral view; (c) lower dentition. Scale bars in mm. labially narrow in other teeth. A replacement median “Fissodus”, CM 62709 (Figs. 1b, 1c). Several teeth tooth lacking a root is placed distant to the functional are present in a coprolite along with most of a palaeo one; other replacement teeth and probable developing niscoid actinopterygian (Cyranorhis: Lund and Pop teeth are also present. There is neither evidence nor lin, 1997). There are only two fully resolvable teeth, suggestion of retention of any of the teeth as has been and both are bifid. There are shattered parts of two described for P. psittacinus by Davis (1883). The num other teeth, one of which is larger than is either of the bers of tooth families, however, do agree with previous bifid teeth. descriptions of P. psittacinus. Siksika ottae: In light of the position of a bifid tooth as a median lower tooth wherever it is observed in a dentition, and the presence of a median tooth in both The Fissodus—Janassa Problems upper and lower jaws, the dental reconstruction of Fissodus: The specimen labeled as the type of Fisso Siksika is reversed accordingly (Lund, 1989; Fath, dus bifidus St. John et Worthen, 1875 is an isolated 2012). The upper dentition contains a small median tooth listed as Illinois State Museum, Worthen collec acuminate and slightly denticulate tooth plus about tion, number 70814 from the Chester Limestone of three distal teeth per ramus. The lower dentition con Illinois (Leary and Turner, 1996). The outlines of the sists of a large strongly denticulate median bifid tooth original figures of St. John and Worthen (1875) are (Fig. 1g) plus about three lateral teeth per ramus. given in Figs. 1h and 1i There is a sharp change from median to lateral, the lat The median lower tooth of Netsepoye hawesi is a eral teeth having deep lingual basins; crowns of the lat Fissodus shaped tooth (Fig. 1a). In addition to that eral teeth are strongly anteroposteriorly curved and tooth of N. hawesi and Fissodopsis robustus, two other peaked except the posteriormost crown. S. ottae shows Bear Gulch petalodont specimens also bear different dignathic heterodonty, with resemblances to Peripristis Fissodus shaped teeth. in the proximodistal curvature of most of the teeth. “Fissodus”, CM 48824 (Fig. 1k). The upper denti Janassa bituminosa specimens vary in numbers of tion shows about four teeth per ramus, the upper tooth families. Hancock and Howse (1870) give a median tooth is 2 mm wide, and reveals no abrupt size median plus three paired teeth for a total of seven change in the upper dentition. The lower jaw has a occluding teeth per jaw. Jaekel (1899, Pl. XV) illus total of three teeth, a large median bifid tooth of trates a median lower tooth position plus two paired 3.2 mm width, and a paired lateral tooth of less than 1 and a small third tooth position. Jaekel (1899, Pl. XIV) mm in width. The mouth is terminal and the lower illustrates one median upper tooth position plus one jaws are transversely oriented relative to the long axis paired and a small second position. The dentition is of the head. The width of the lower jaw is 8.4 mm and homodont, with shape shift in the most distal family. there is no space for additional teeth on the jaw. Eye The mouth is subterminal in J. bituminosa. The most pigments indicate small dorsally facing eyes. There are distal teeth are denticulate. preserved remnants of the shoulder girdle and parts of Cf. Ctenoptychius (“Janassa”) korni: The presumed a pectoral fin. upper median tooth (Brandt, 1996, Fig. 3) has two
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M D
O D
S
(a) (b)
TB R (c) (d)
Fig. 4. Petalorhynchus beargulchensis, CM 78542. (a) dorsal aspect of head in ventral view; (b) ventral aspect of head in dorsal view; (c) palatal dentition, restored; (d) mandibular dentition, restored. Dashed lines indicate elements whose precise edges can not be determined. Abbreviations: (D) enlarged denticle groups, (M) mandibular articulation, (O) orbit, (R) replacement median lower tooth, (S) shoulder girdle, (TB) tooth buds. Scales in mm. stout primary and two smaller secondary cusps, and a assigned genus Ctenoptychius but there is no justifica small oblique heel with a few ridges. The proximate tion for assigning them to the genus Janassa. lateral tooth has stout primary and smaller secondary The mouth of Belantsea montana is terminal, the cusps, and the most distal tooth has low well rounded ethmorostrum is anteroventrally extended, jaws are costate cusps and a small heavily ridged oblique heel. short and deep, and anterior teeth are transverse to the The bifid lower median tooth (Brandt, 1996, Figs. 4 sagittal plane. There are three lower jaw teeth, none and 6; Figs. 1D, 1E, 1F) has small secondary cusps, are bifid, and the most lateral is diminutive. Teeth are and is flanked to either side by three partially overlap all trenchant and acuminate, and of the Ctenoptych ping small lateral teeth with low thin crowns and small ius form. heavily ridged heels. There are a total of seven teeth The type species of Petalodus, P. hastingsae (Owen, per ramus. Tooth forms are distinctly different in each 1845) is represented only by isolated teeth. Petalodus jaw; as illustrated this fish has dignathic heterodonty. ohioensis has been reconstructed as bearing a median There are, however, problems interpreting the collec tooth plus four distal teeth per ramus for a total of tive remains as those of a single taxon. The teeth in approximately nine teeth per jaw (Hansen, 1985). Pet Brandt, 1996, Fig. 3 are profoundly different from alodus ohioensis is homodont. The type species of those in Figs. 4 and 6 (l.c.). Further, the lower jaw Polyrhizodus, P. magnus (McCoy, 1848) is represented teeth of Fig. 4 (l.c.) differ strongly in cusp shapes and only by isolated teeth, but a unique specimen of heel lengths from those in Fig. 6 (l.c.). There is little Polyrhizodus digitatus bears five tooth families on a justification for retaining these fish in the originally ramus (Lund, 1983). Polyrhizodus digitatus is homo
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Plate 8
H
N
G
B
T
1
T
O
2
PALEONTOLOGICAL JOURNAL Vol. 48 No. 9 2014 ON THE RELATIONSHIPS OF THE PETALODONTIFORMES (CHONDRICHTHYES) 1023 dont with slight size and shape changes along the jaw; conditions for the Euchondrocephali as well as for the its teeth are of the Petalodus form. Petalodontiformes. Of the homodont Petalodonti formes Belantsea, Polyrhizodus and, presumably, Peta lodus, lack teeth of the lateral morphologies (i.e. Jan DISCUSSION assa like teeth), while Janassa lacks teeth of anterior Many of the petalodont dentitions discussed here (proximal) morphologies. Regional specializations as are dignathic heterodont dentitions, and this poses seen in the reduction or elimination of either anterior significant problems in identifying taxa on the basis of or lateral regions of petalodont dentitions is the most individual teeth as well as in understanding the evolu plausible hypothesis for the secondarily homodont tionary pathways of the Order Petalodontiformes. Yet Petalodus, Belantsea, or Janassa conditions. some resolution to the evolutionary diversification and There is considerable morphological similarity relationships of the petalodonts may be derived from between the crowns of the anterior teeth of Tanao studies on the developmental control of tooth forma dus/Climaxodus wisei and those of Antliodus, Petalo tion and morphogenesis and, specifically, on the ori dus, and Polyrhizodus, but the bases of the crowns and gins of heterodonty (Jernvall and Thesleff, 2000). roots of the teeth of T. wisei do not conform to the def Development of a single tooth involves a cascade of initions of petalodont teeth. Similarly, the posterior epithelial mesenchymal interactions and intra and tooth crowns of T. wisei resemble the tooth crowns of intercellular signaling that results in the final erupted Janassa (Hancock and Atthey, 1870; Woodward, product. The development of a homodont dentition 1919). The arrangement, however, of the T. wisei den involves reiteration across the dental lamina of the tition and its proximo distal increase in size is oppo same signaling cascade that produces a single tooth site to that of any known petalodont dentition. There (a tooth module), while slight downstream modifica is a phylogenetic relationship between all these taxa, tions may produce minor differences in the final teeth but the data speak to a more diverse evolutionary radi (Jernvall and Thesleff, 2000; Thesleff, 2003). Hetero ation that includes both the Petalodontiformes and donty involves region wide modifications to the pat other members of the Euchondrocephali. The only tern of successive iterations of the basic tooth module parsimonious hypothesis for the origin and diversifica (Jernvall and Thesleff, 2000). This regionalization tion of the Petalodontiformes within this larger radia results from regulatory gene changes upstream in the tion of the Euchondrocephali involves regional coro signaling cascade. As applied to the petalodont data, nal specializations from a stock that also produced the this model of tooth formation and morphogenesis non petalodont Tanaodus and Chomatodus clades. would explain the division of the petalodont hetero The Chomatodus clade displays a further distinction dont condition into distinct anterior versus lateral from other Euchondrocephali in the reverse size gradi dental fields and in the distinctive patterning of upper ent, small proximal teeth to large distal teeth. Tanao versus lower jaw dentition. Any downstream modifica dus wisei is unique among this group in the orienta tions of the regulatory signaling within each region tions and directions of growth of its tooth families. would generate the sort of variations seen in these Teeth of the general shape of Fissodus bifidus occur fields. across several taxa of petalodonts from the Bear Gulch Yet, heterodonty and regionalization within the Limestone and are consistent as the lower median dentition are not restricted to the Petalodontiformes. positions of dignathic heterodont dentitions. Varia These conditions are found throughout the Euchon tions in the form of the bifid tooth across these taxa drocephali, in the differentiated symphysial and man appear distinctive. The Fissodus shaped tooth in Net dibular arch dentitions of the Gregoriidae (Lund and sepoye hawesi is significantly wider relative to its height Grogan, 2004b), Helodus (Grogan and Lund, 1999), than the illustration of the type specimen of F. bifidus the Eugeniodontiformes (Zangerl, 1981), Harpacan St. John et Worthen, 1875 (Figs. 1h, 1i), but it does thus (Lund and Grogan, 2004a), in Tanaodus/Climax have an extended, ridged heel of the same proportions odus (Woodward, 1919), as well as the anterior versus as that of F. bifidus. The bifid tooth of CM 62709, not posterior dentitions of the Chomatodus Lisgodus associated with a dentition, is essentially identical to group and the Holocephali (Lund and Grogan, that of N. hawesi. Crowns of the teeth of Fissodopsis 1997A). Regionalized specializations are particularly robustus, CM 62710, are not fully visible, but the large evident in the abrupt shifts in sizes, morphologies, and median tooth is Fissodus shaped, and there is an tooth position angles in Netsepoye, Obruchevodus, and abrupt size change from the median teeth to small lat Fissodopsis. Thus, heterodonty and regionalization eral teeth with extended, ridged heels. Further, there is within the dentition appear to be the plesiomorphous a narrow waist between the crown and the heel in the
Explanation of Plate 8 Figs. 1 and 2. (1) Fissodopsis robustus, holotype, CM 62710 B; also see Text Fig. 3. Anterior is down. (2) Petalorhynchus bear gulchensis, CM 78542 B; also see Text Fig. 4, anterior is up. Abbreviations: (B) branchial arch element, (G) glossohyal, (H) cer atohyoid elements, (N) neurocranium, (O) orbit, (T) teeth. Scale bars equal 1 cm.
PALEONTOLOGICAL JOURNAL Vol. 48 No. 9 2014 1024 LUND et al. large teeth of F. robustus, which is a unique distin Fissodopsis. Thus, the specimen is distant from J. bitu guishing character. The bifid replacement tooth of minosa. We find the teeth and dentition of “J.” korni Petalorhynchus beargulchensis, CM 78542, and the most closely resemble that of Petalorhynchus bear lateral teeth are also of similar shapes, as in “Ctenopty gulchensis. chius” korni, but there is a size gradation rather than The lateral teeth of Netsepoye, Obruchevodus, an abrupt shift from median to lateral teeth in both FMNH PF 8424, and Fissodopsis are very similar in jaws. None of these has the same tooth numbers or shape but distinct in details. The most proximal crown shapes as N. hawesi. None of these Fissodus style is somewhat acuminate, and may be denticulate or teeth has the same shape or proportions as the type lightly cuspidate, the succeeding distal crown is den specimen of Fissodus bifidus. ticulate but lower and less acuminate, culminating in There is a bifid lower median tooth in Siksika ottae, the most distal having a denticulate, horizontal, non but it is strongly denticulate and lacks a prominent lin acuminate profile. Thus, lateral tooth crown profiles gual heel, and is thus distinct from Fissodus. The of these fish change gradually or stepwise distally. Net accompanying teeth are trenchant, with prominent sepoye and Fissodopsis have wide, long, squared heels lingual basins. Neither shapes nor numbers of the approximately perpendicular to the crowns, and are teeth in the dentition of S. ottae are close to those of virtual miniatures of a Janassa style tooth. The heels any other specimen or taxon discussed here. of the lateral teeth of Obruchevodus are at an obtuse Pennsylvanian bifid teeth with extended heels were angle to the crowns but are also squared and strongly referred to Fissodus (F. [Cholodus] inaequalis, F. den ridged. tatus,) by Eastman (1903) who remarked that the cleft The distalmost families of teeth of all these petal crown was the principal character distinguishing odonts under study often appear very small relative to Pennsylvanian Fissodus from Janassa. Permian teeth the more proximal teeth, particularly in the smallest with extremely extended heels have generally been holomorphic specimens. Comparison to larger speci referred to Janassa (e.g. J. unguiculata Branson, 1916; mens, particularly noticeable in the several specimens “J”, korni Brandt, 1996). Janassa lacks any tooth with of Janassa bituminosa, suggests that these diminutive a bifid crown. teeth may have become proportionately larger as the In summary, the type specimen (tooth) named Fis animal grew and as jaw growth could accommodate sodus bifidus St. John et Worthen, 1875 is distinct from more distal families. Thus, the number of tooth fami that of any other bifid tooth discussed here; bifid lower lies reported per upper or lower jaw may not be diag symphysial teeth occur across several taxa from the nostic for a species, but would vary with the size or age Mississippian through the lower Permian that bear dis of the preserved specimen. parate lateral dentitions; and there seems to be a trend through this stratigraphic range toward longer ridged lingual heels. The name Fissodus bifidus cannot be Cladistic Characters Beyond Teeth and Dentition definitively associated with any other known tooth or It can now be stated that the known petalodont cra dentition at any geological time except from the type nia are holostylic (Grogan et al., this volume). The locality, but the general bifid tooth shape can be asso angles of the palatobasal arcade (basitrabecular pro ciated with the lower median tooth position of several cess) reflect the geometry of the neurocrania, and thus distinctly different petalodont taxa. may affect the position of preservation of the head. If All Fissodus like teeth preserved with their no other factors affect cranial preservation, a widely replacements were positioned almost horizontally laterally splayed arcade or a dorsoventrally flattened with reference to the head and the lateral teeth of the skull would cause the neurocranium to come to rest in jaws, forming a beak like structure in combination a dorsoventrally preserved position, as seen in Siksika, with the opposing anterior upper teeth. This arrange Petalorhynchus, and Janassa. The laterally com ment is unique among the Petalodontiformes. pressed neurocranium or ventrally projecting arcade Janassa bituminosa is represented by holomorphic of a compressiform fish will facilitate the head coming material, by dentitions, and by isolated teeth. How to rest on its side. This is seen in Belantsea and ever, there has been a tendency through time for Obruchevodus, with Fissodopsis displaying ethmoid researchers to identify any petalodont tooth with an crushing as a result of coming to rest in dorso ventral extended and multi ridged heel as a Janassa tooth position. With an arcade projecting at around 45° to (“Janassa” korni, “Janassa” FMNH PF 8424, for the sagittal plane, the neurocranium of a fusiform fish examples). The data presented herein demonstrate could come to rest at either orientation, or at other that these assignments fail when compared to either angles (e.g. undescribed Bear Gulch euchondroceph J. bituminosa or to other heterodont dentitions. Thus, alan, CM 62705, codenamed Chom2). the Field Museum specimen (Plate 7, Fig. 8) has Labial cartilages are rarely well preserved in the fos ridged heels on its lateral teeth but there is an anterior, sil record. However, in Belantsea montana, a chain of worn, trenchant tooth present, and the shapes of the at least three well mineralized labials are lateral to the crowns and heels of the lateral teeth vary in a manner mandibular articulation and do not bear any special quite similar to those of Obruchevodus, Netsepoye, and ized denticles. The well preserved type specimen of
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B. montana is female, lacking pelvic claspers. The type 1899; Lund, 1989), but these skeletal hypotheses can specimen of Netsepoye hawesi, a male with pelvic be neither substantiated nor falsified at present. claspers, displays at least three narrow labial cartilages Important details of pectoral fin structure are now that support at least five sharp denticles. The type known from Janassa, Belantsea, Netsepoye, and specimen of Obruchevodus griffithi is a male, with pel Obruchevodus, although pelvic details are scant. Pec vic claspers, and bears two broad labial cartilages, one toral girdles are placed closely behind the heads, of which supports two large bifid denticles. The speci restricting the branchial chamber to a subotic position men of Janassa bituminosa illustrated by Jaekel (1899) (note the ventral ends of the coracoid processes of displays one wide labial cartilage, and shows no spe J. bituminosa in Jaekel (1899, Pl. XV, Fig. 1). These cialized denticles, lacks pelvic claspers, and thus is fishes all have very large, broad, long, preaxially devel either female or juvenile (Grogan et al., this volume; oped fins of low aspect ratio that extend at least to see below). These data suggest that large labial denti midorbital level or further anterior. Fish with this fin cles may be a secondary sexual character in petal type are dependent upon oscillatory and/or undula odonts (Grogan et al., this volume). tory movements of the fins for slow propulsion in rela The plesiomorphous mandibular arch condition tively calm habitats (Combes and Daniel, 2001; for known gnathostomes is for the palatoquadrate to Thorsen and Westneat, 2004). Within this categoriza articulate anteriorly upon the ethmoid part of the neu tion, however, structural details of the fins differ in rocranium behind the ventral edge of the nasal cap ways that have strong implications for fine tuning of sule, but not to meet in a median symphysis. Parasym their swimming. These details particularly concern the physial cartilages plesiomorphously lie anterior and anterior, or leading edges, of the fins and thus the lateral to the anterior ends of the palatoquadrates, in amount of potential thrust and control that could have iniopterygians for instance (Grogan and Lund, 2009). been generated during swimming and maneuvering The ventral unit of the mandibular arch, Meckel’s car (Thorsen and Westneat, 2004). These differences tilage, plesiomorphously joins proximally with one or clearly have ecological and systematic significance a pair of symphysial cartilages that lay in the midline. as well. These pre mandibular arch elements bear tooth Belantsea has a long axis composed of a small num whorls or teeth. Tooth whorls in chondrichthyans are ber of blocky basals and a few substantial anterior basal linguo labially incrementally increasing dental ele cartilages, followed by about 24 axials. There is exten ments borne anterior to the mandibular arch, and they sive involvement of unmineralized supports in the dis can be distinctly different from the mandibular denti tal fin. Fins project ventrolateral and extend forward to tion. This distinctiveness can be evidenced by gross the level of the jaw articulation and backward almost to differences in shape, size, histology, and growth his the origin of the pelvic fin. This evidences a pectoral tory from mandibular arch dentitions. Tooth whorls stroke probably capable of both passively and actively may be mounted upon distinct parasymphysial and passing undulatory waves (Combes and Daniel, 2001). symphysial cartilages or, in the holostylic condition, Pectoral and pelvic fins were the primary propulsive presumably upon complexes of cartilage anlagen fused organs. to the neurocranium. Whorls and their cartilages may Netsepoye has 26 posterior axial cartilages; the project considerably anterior to the mandibular sym anterior basals extend to the level of the jaw articula physis (Hitchcock, 1887; Lund and Grogan, 2004a, tion and support seven or more radials, and the fin Grogan and Lund, 2009). itself reaches further anterior. The fin also reaches There is a paucity of data on the fine structure of backward almost to the origin of the pelvic fin. There the anterior end of the head in Paleozoic Chondrich are four blocky basals below the scapular articulation. thyes in general and for the petalodonts in particular. The structure of this fin suggests an undulatory pecto At present it is known that the upper dentitions of the ral stroke as the primary propulsive mode of the fish. known Petalodontiformes meet in the midline and Obruchevodus has 32 posterior axials, and about large anterior and median upper and lower teeth or 7 basals below the scapulocoracoid. A narrow first and presumed lower teeth are known from the few taxa large blocky second basal support anteriorly inclined with dentitions. It is further known that in the hetero radials. The first nine radials are stout and closely fit dont Petalodontiformes, the anterior and posterior together. The posterior axials support thin serial radi tooth regions display differentiated patterns of orien als typical of undulatory fin motions. The fin itself tation, size, and morphology. For the purposes of the reaches from level of the jaw articulation almost to the current cladistic analysis, the weight of the evidence origin of the pelvic fin. The endoskeleton evidences a suggests that the anterior dental positions of the peta stiffly braced leading fin edge coupled with a flexible lodonts should be coded as symphysially and parasym posterior fin. The structure of this fin evidences a pos physially derived dental elements. It can be hypothe sible combination of oscillatory and undulatory pecto sized either that the supporting skeletal elements are ral strokes as the propulsive modes of the fish (Rosen derived from ancestral symphysial and parasymphysial berger, 2001). cartilages having later fused to the jaws, or that the Janassa bituminosa has an anteriorly extended pec jaws are fused at the symphysis (Hansen, 1978; Jaekel, toral fin with a long multi segmented anterior axis that
PALEONTOLOGICAL JOURNAL Vol. 48 No. 9 2014 1026 LUND et al. extends, lateral to the head, to the level of the jaw The phylogenetic tree (Fig. 5a) based on the avail articulation. Other internal structures are not preserved. able holomorphic data emphasizes the early and broad The anterior support and extension of this fin is compa diversification of the euchondrocephalan Chondrich rable in structure and function to that of skates and rays thyes. A basal cluster of taxa shares more plesiomor (Macesic and Kajiura 2010), but the pelvic fins are dis phous suspensoriums and postcranial morphologies. tant from the pectoral fins of Janassa. Pectoral undula The Chomatodus group is placed as the sister group of tory propulsion aided by axial undulation was probably the Petalodontiformes, and this clade is located the primary swimming mode of Janassa. among other euchondrocephalan taxa with more Reassessment of the pelvic girdle of Janassa bitumi derived suspensoriums and dentitions. nosa by comparison to those of skates and rays is reve A separate cladistic matrix was constructed latory (Grogan et al., this volume). The purported iliac employing available information for supposed petal process of J. bituminosa (Jaekel, 1899) is directly com odonts with whole or partial dentitions. The matrix parable to the horizontally oriented lateral pelvic pro used a theoretical outgroup and Gregorius rexi to cesses of Raja eglanteria and Narcine brasiliensis polarize the character states (Fig. 5b). The Chomato (Macesic and Kajiura, 2010), and the purported mix dus group is placed basal to Megactenopetalus. Mega opterygium or “coarse finger” is in the precise position ctenopetalus (Pristodontidae) is situated as the sister of the enlarged first basal cartilages of these modern group to the Petalodontiformes (node 9). Siksika is fish. The illustrated specimen of Janassa bituminosa placed as a basal petalodontiform taxon, and Belantsea lacks pelvic claspers and is therefore reinterpreted as is placed as the sister taxon to all more crown ward either a juvenile or a female individual. taxa. The homodont taxa Polyrhizodus digitatus and Data on petalodont dorsal fins is sparse and thus Petalodus ohioensis cluster into one group, maintain provide insufficient information for phylogenetic ing their long accepted association. Janassa is placed analysis. There are no dorsal fin spines in any known as the sister to a heterodont clade composed of petalodont, but they occur in non petalodont [C. korni + Petalorhynchus] + [Netsepoye + euchondrocephalans. Hansen (1978) illustrates a dor {Obruchevodus + Fissodopsis}]. sal fin spine associated with the dentition of Mega ctenopetalus. A small first dorsal fin spine is present in CHOM2. CONCLUSIONS The pectoral fins of members of the outgroups Chom2 (CM 62705), Chom3 (CM 46185) and Nessie These are tentative and uncertain phylogenetic (MV 6182) all have very long multi element pectoral relationships, because they are based on very incom axes (>15 axials) and all have the postaxial fin devel plete information. Nevertheless, several assertions can oped rather than the preaxial fin. be advanced about the systematic significance of these results. The Petalodontiformes are a morphologically and dentally diverse Chondrichthyan order united in Phylogenetic Analysis possession of the petalodont tooth characters as origi The Class Chondrichthyes as currently understood nally described by Newberry and Worthen (1866). The (Fig. 5, node 3), comprises a basal taxon containing Petalodontidae, Janassidae, and Belantseidae are pre Puccapampella (Maisey, 2001) and the Iniopterygii viously recognized and apparently sustainable families (Grogan and Lund, 2009), and above it, node 4 which on the basis of current data. There is an additional subtends the branches leading to two morphologically clade, with Janassa as its sister taxon, that subtends distinct groups. Node 4 has been called the Euchon two groups, all of which are united in possession of drichthyes (Grogan et al., 2012). Node 5 marks the Fissodus shaped lower median teeth and large, ridged Elasmobranchii. Node 6 was originally proposed as heels on lateral teeth. One branch contains Petalo the Paraselachii by Lund (1977) but subsequent stud rhynchus + “Ctenoptychius” korni, which possess rela ies demonstrated that the included members pre tively few tooth positions (3–4) of gradational size and sented as a paraphyletic assemblage. Subsequently the morphologic relationships. This branch is recognized term Euchondrocephali was proposed for node 6 on as the new Family Petalorhynchidae with Petalorhyn the basis of additional taxa and morphological evi chus as its typical genus. The other branch contains dence (Lund and Grogan, 1997B). The topology of Netsepoye hawesi, Obruchevodus griffithi, and Fissod this cladogram has remained stable throughout the opsis robustus, which display higher numbers of tooth addition of considerable new information. The data positions that are regionally specialized into larger matrix for the present cladogram is essentially the anterior and smaller lateral teeth. This branch is rec same as that given in Grogan et al. (2012). ognized as the new family Obruchevodidae, with
Fig. 5. Phylogenetic relationships of the Petalodontiformes. (a) Matrix Chon58_2_3, using 122 characters and 61 taxa, that yielded 1 retained tree with Length of 1065, traditional search, no pruning or regrafting; (b) Principally petalodont dental matrix using 34 characters and 15 taxa, that yielded 1 retained tree with Length 145, traditional search, no pruning or regrafting.
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Outgroup Onychodus 1 Kalops Hadronector Dicksonosteus 2 Ptomacanthus Acanthodes Puccapampella Rainerichthys Papilionichthys Promyxele Iniopteryx Cervifurca Sibyrhynchus Iniopterygian1 Iniopterygian2 Iniopterygian6 3 Iniopterygian7 Iniopterygian9 Iniopterygian8 Delphyodontos Doliodus Squalus Nodes 5 Egertonodus Hamiltonichthys 1 Osteichthyes Thrinacoselache 2 Acanthodii Placodermi Diplodoselache 3 Chondrichthyes Squatinactis Tristychius 4 Euchondrichthyes Stethacanthus altonensis 5 Elasmobranchii 4 Damocles 6 Euchondrocephali Falcatus Srianta dawsoni 7 Paraselachii 7 Bealbonn 8 Chomatodus group S. srianta 9 Petalodontiformes S. iarlis Gregorius 10 Belantseidae 6 Debeerius 11 Petalodontidae Heteropetalus 12 Janassidae Helodus Orodus 13 Petalorhynchidae Omithoprion 14 Obruchevodidae Caseodus 8 Chom3 Chom2 Belantsea 9 Netsepoye Obruchevodus Harpacanthus Harpagofututor Traquairius nudus Menaspis T. agkistrocephalus T. spinosus Squaloraja Acanthorhina (a) Callorhinchus cf Erismacanthus Physonemus Echinochimaera snyderi E. meltoni Outgroup Gregorius Chom3 Chom2 Megactenopetalus Siksika 9 10 Belantsea 11 Polyrhizodus Petalodus 12 Janassa bituminosa 13 “Ctenoptychius” korni Petalorhynchus beargulchensis Netsepoye (b) 14 Fissodopsis Obruchevodus
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Obruchevodus designated as its typical genus. The Grogan, E.D., Lund, R., and Greenfest Allen, E., The ori position of the poorly known Siksika ottae is basal. gin and relationships of early chondrichthyansin Biology of Sharks and Their Relatives, Carrier, J.C., Musick, J.A., and Heithaus, M.R., Eds., Boca Raton: CRC Press, 2012, ACKNOWLEDGMENTS chap. 1, pp. 3–29. Grogan, E.D., Lund, R., and Path, M., A new petalodont We thank the ranch families of central Montana for chondrichthyan from the Bear Gulch Limestone of Mon their support and friendship across these many years; tana, USA, with reassessment of Netsepoye hawesi and com this project could not have been accomplished without ments on the morphology of holomorphic petalodonts, them. Thanks are also due to the many volunteers who This volume. have given of their time and efforts to move mountains Hancock, A. and Atthey, T., On the generic identity of Cli of rock over the past 43 years. Finally, we thank our maxodus and Janassa, two fossil fishes related to the rays, Russian colleagues, especially Alexander Ivanov and Trans. Nat. Hist. Soc. Northumberland, Durham and New Oleg Lebedev, for organizing the II International castle upon Tyne, 1870, vol. 3, pp. 330–339. Obruchev Symposium and this special volume of Hancock, A. and Howse, R., On Janassa bituminosa Schlo papers and for their hospitality and scholarly theim, from the marl slate of Midderidge, Durham, Ann. exchanges. We thank the reviewers for their contribu Mag. Nat. Hist., 1870, vol. 4, pp. 47–62. tions to the quality of this paper. Hansen, M.C., A presumed lower dentition and a spine of a Permian petalodontiform chondrichthyan, Megactenopeta lus kaibabanus, J. Paleontol., 1978, vol. 52, pp. 55–60. REFERENCES Hansen, M.C., Systematic relationships of petalodontiform Brandt, S., Janassa korni (Weigelt)—Neubeschreibung chondrichthyans, Ninth Int. Congr. Strat. Geol. 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