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UNRAVELING SPECIES CONCEPTS FOR THE HELICOPRIONTOOTH WHORL

LEIF TAPANILA1,2AND JESSE PRUITT2

1Department of Geosciences, 921 S. 8th Ave., Idaho State University, Pocatello, ID 83209-8072, USA; and 2Division of Earth Sciences,921 S. 8th Ave., Idaho Museum of Natural History, Pocatello, ID 83209-8096, USA, ,[email protected].

UNRAVELING SPECIES CONCEPTS FOR THE HELICOPRIONTOOTH WHORL

LEIF TAPANILA1,2AND JESSE PRUITT2

1Department of Geosciences, 921 S. 8th Ave., Idaho State University, Pocatello, ID 83209-8072, USA; and 2Division of Earth Sciences,921 S. 8th Ave., Idaho Museum of Natural History, Pocatello, ID 83209-8096, USA, ,[email protected].

ABSTRACT—The genus Helicoprion (Chondricthyes, Euchondrocephali) is preserved primarily by a continuous spiral rootthat forms the base for more than 130 teeth. Helicoprion is found globally in Lower Permian marine rocks and at least 100specimens exist in public collections worldwide. Ten species of the genus are reviewed in the context of newmorphometric analyses conducted on dozens of specimens. Helicoprion whorls share a common inner spiral geometry thatresults in exponential growth of tooth and root dimensions. Novel growth equations permit calculation of whorl diameter,volution number and tooth count from incomplete specimens. The results of meristic and geometric analyses identifytaxobases that distinguish three emended species concepts. Differentiation of form is evident among specimens only afterthe second volution or roughly the 85th tooth. Helicoprion davisii has widely spaced, stout teeth with tall cutting surfacesand is distinguished from H. bessonowi, which has narrow, closely spaced teeth with short cutting surfaces. Helicoprionergassaminon is an intermediate form, having narrow, closely spaced teeth with tall cutting surfaces. Several largespecimens in the study are too dissimilar to place in the new emended species concepts.

INTRODUCTION

THE SPIRAL tooth whorl of Helicoprion is a marvel inevolutionary novelty that has captivated generations of

paleontologists worldwide. Ten species of Helicoprion areknown from a 20 my interval of Lower Permian (Cisuralian–early Guadalupian) marine rocks deposited along the warm-water epicontinental basins and seaways of Pangaea (Fig. 1). Aninventory of public collections reveals that of the roughly 100specimens of Helicoprion the bulk come from two miningdistricts: one-quarter of specimens are from the type locality inthe Ural mountains in Russia and more than half are from theRocky Mountain foothills in Idaho, U.S.A. (online Supplemen-tal data Table 1). With access to the single largest collection ofHelicoprion at the Idaho Museum of Natural History, this studypresents a new analysis of whorls in an attempt to redefinespecies concepts using modern geometric techniques that haveproven useful in other chondricthyan tooth studies (e.g., Nyberget al., 2006; Whitenack and Gottfried, 2010).

FORM AND PRESERVATION OF THE WHORL

More than 100 enameloid-covered teeth are arranged along acommon root that forms a cartilaginous spiral, with the smallestteeth at the axis of the spiral (adaxial). Teeth gradually increasein size away from the spiral center (abaxial), and may exceed 10cm in length (Fig. 2). With the exception of the hook-shapedjuvenile tooth arch located at the spiral center, all Helicoprion

teeth have a roughly triangular, laterally compressed, cuttingsurface, often preserved with serrations. The upper crown issupported by the middle part of the tooth, which leads to acommon root structure on the innermost part of each volution.Lower projections of individual teeth curve in an abaxialdirection, such that the root of one tooth is shingled below thecrown of the previous, smaller tooth in the series. The lowerprojection of enameloid does not cover the lowermost part of theroot, and this exposed portion is called the shaft. The shaft of theroot rests on top of cartilage that encapsulates the previousvolution of teeth. In a complete whorl, the abaxial part of the

spiral terminates with an extended root that lacks a middle orupper portion of the tooth crown.

Preservation of Helicoprion strongly favors the tooth whorlover the remainder of the cartilaginous body. Fossils of the toothwhorl are frequently incomplete, where only a small arc ormaybe two volutions of teeth are preserved. It is particularlyrare for specimens to have complete teeth, and more frequentthat the teeth are medially truncated or recorded as externalimpressions. The calcified root tends to have greater preserva-tion potential.

The taphonomy of Helicoprion presents great challenges formorphologically based systematic and phylogenetic analyses.Helicoprion whorls are often preserved at the center of aconcretionary nodule (e.g., most Idaho specimens). The part andcounterpart are infrequently retained together, and morecommonly one half is not retained at all. Concretionarypreservation confers additional problems: cartilaginous jawand post-cranial elements that could be preserved extendbeyond the boundary of the concretion and are not retained.Furthermore, in the process of reducing the size of a concretion,overzealous mechanical preparation of the fossil has oftenremoved subtle elements of the cartilaginous jaw. Without post-crania, all characters are limited to the teeth and the supportingwhorl.

Institutional abbreviations.—AMNH, American Museum ofNatural History, New York, U.S.A.; DG, private collection; FortHall, Fort Hall Reservation, Shoshone Bannock Tribal Museum,Idaho, U.S.A.; GSA, Geological Society of America Office,Boulder, Colorado, U.S.A.; IMNH, Idaho Museum of NaturalHistory, Pocatello, U.S.A.; LACM, Los Angeles County Muse-um, USA; PIN, Palaeontological Institute of the RussianAcademy of Sciences, Moscow, Russia; PMO, PaleontologiskMuseum, Oslo, Norway; TsNIGR, Central Research GeologicalMuseum, St.-Petersburg, Russia; UMNH, Natural History Muse-um of Utah, Salt Lake City, U.S.A.; UMUT, University Museum,University of Tokyo, Japan; UNMMPC, W.M. Keck EarthScience and Mineral Engineering Museum, University of Nevada,Reno, U.S.A.; USNM, National Museum of Natural History,U.S.A.; WAMAG, Western Australian Museum, Perth, Australia;

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Journal of Paleontology, 87(6), 2013, p. 965–983

Copyright � 2013, The Paleontological Society

0022-3360/13/0087-965$03.00

DOI: 10.1666/12-156

YIGM, Yichang Institute of Geology and Mineral Resources,China.

REVIEW OF HELICOPRION SPECIES CONCEPTS AND TAXOBASES

The first specimen of Helicoprion, WAMAG 9080, wasdescribed from a small curved series of 15 teeth connected by acommon root and found without provenance in WesternAustralia (Woodward, 1886). Woodward designated a newspecies, Edestus davisii, later reassigned to the newly created

genus Helicoprion in Karpinsky’s landmark publication in 1899.Forty years later Teichert (1940) confirmed Karpinsky’srecombination with two additional West Australian specimensof H. davisii showing a complete spiral form. Karpinsky’s 1899monograph thoroughly described the type species, Helicoprionbessonowi, presenting six specimens from a quarry in the LowerPermian rocks of the Ural Mountains, Russia. SpecimensTsNIGR 1/1865 and 3/1865 from Karpinsky (1899, pls. I, II)preserve more than three volutions of the whorl and the teeth arethoroughly prepared from the matrix.

In 1907 and 1909, Hay presented three specimens of a newgenus and species he called Lissoprion ferrieri that were foundin the economic phosphate deposits on the borderlands of Idahoand Wyoming. The holotype of H. ferrieri consists of a meagerseries of three teeth but one of the additional specimens figuredin 1909 demonstrates at least two volutions of teeth, which ledKarpinsky (1911) to reclassify the Idaho-Wyoming fossils asHelicoprion ferrieri.

The next two species of Helicoprion to be named also comefrom the western part of the United States. Wheeler (1939)described two well-preserved 2.5 to 3 volution specimens from amine in western Nevada (UNMMPC 1001) and glacial morainedeposits in eastern California (UNMMPC 1002). Each specimenwas designated as holotype of a new species and each suffersfrom ill-defined provenance. Based on upslope geology, theglacial rocks containing the holotype of Helicoprion sierrensisare likely from the Goodhue Formation (Harwood, 1992). But itis Wheeler’s second species that remains enigmatic. Wheelerreported that the holotype of Helicoprion nevadensis wascollected by miners excavating the volcanic-sedimentary Ro-chester Trachyte and therefore assigned an Artinksian (EarlyPermian) age to the deposit using index-fossil biostratigraphy.Some decades later, Silberling (1973) re-evaluated the lithologyof the specimen and determined that the rock type wasinconsistent with those found in the Rochester area, thereforecalling into dispute the provenance and age of the specimen.Most recent geochronology of Rochester Trachyte (¼KoipatoFormation) and the Rochester mining area demonstrates an earlyTriassic age for the region (249 Ma; Vetz, 2010). There is noway to test the age of the holotype because it is missing.

An uncataloged specimen figured in Mullerreid’s (1945)description of Helicoprion mexicanus forms a large, 10-tootharc, but the specimen is very poorly preserved and contains

TABLE 1—Measurements and calculations to describe the form of the teeth and whorl of Helicoprion.

Form Category Measurement Description

Whorl Equiangular spiral Alpha angle Angle between spiral and radiusRaup spiral c, d, e Spiral variables

W (calculated) Rate of whorl expansionD (calculated) Distance between generating curve and spiral axis

Polar coordinates r, theta (origin unknown) Relative expansion of the inner spiraltheta (origin known) Absolute volution number

Size Dmax Maximum diameter of whorlTooth count T# Tooth numberTeeth per volution T#, theta (origin known) Insertion angle

Tooth/Root Linear A (¼LM2 to LM5) Volution heightB (¼LM1 to LM3) Tooth widthC Crown heightD Tooth spacingS (¼LM7 to LM5) Shaft height

Angular Apical angle Angle at tooth apexBasal angle Angle of basal projection

Geometric LM1–LM6 Six-landmarkLM1–LM7 Seven-landmark

Linear ratios (LM2 to LM3) / A Upper ratioD/A Tooth spacing ratioS/A Shaft ratio

FIGURE 1—Early Permian paleogeography of Helicoprion specimens.Phosphoria Sea: 1, Idaho, Utah, Wyoming, U.S.A.; Eastern Panthalassia: 2,Alberta–British Columbia, Canada; 3, Nevada–California, U.S.A; 4, Texas,USA; 5, Mexico; Laurussian epeiric sea: 6, Melville Island, Canada; 7,Ellesmere Island, Canada; 8, Spitsbergen, Norway; 9, middle Urals, Russia;Paleotethys: 10, southern Urals, Kazakhstan; 11, Hubei, China; 12, Laos; 13,Japan; Neotethys: 14, Western Australia. More than 50 percent of Helicoprionspecimens come from 1 (diamond), and 25 percent from 9 (square). All otherlocations (circle) have produced five or fewer specimens. Paleogeographicbase map modified after Ron Blakey, NAU Geology (Blakey, 2012).

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FIGURE 2—Helicoprion tooth whorls. 1–5, H. davisii, IMNH 37899, IMNH 30897, Fort Hall, UMNH 5131, and UNMMPC 1002, respectively; 6, H.bessonowi, TsNIGR 1/1865; 7, 8, H. ergassaminon, IMNH 30900 and AMNH 4948, respectively; 9, 10, unknown species, IMNH 14095 and DG5, respectively.Species designations follow emended diagnoses in this paper. Specimens 1–3, 7–10 from Phosphoria Formation, Idaho; 4 from Phosphoria Formation, Wyoming;5 from ?Goodhue Formation, California; 6 from Divya Formation, Russia. All scale bars¼2 cm.

TAPANILA AND PRUITT—HELICOPRION SPECIES CONCEPTS 967

weak stratigraphic control. This holotype is missing (Sour-Tovar et al., 2000) but is included in our study based on a figurein Mullerreid (1945; fig. 1).

Of lesser specimen quality, Obruchev (1953, pl. VI, fig. 2)figured the impression and root of two teeth collected in theUrals of Russia, the caption of which designates a new speciesH. karpinskii. A diagnosis was not provided by Obruchev (1953)as required by the ICZN. The current location of this fossil isunknown (Lebedev, 2009). We consider H. karpinskii to be anomen nudum.

In 1966, Danish ichthyologist Bendix-Almgreen publishedresults from his study of Helicoprion specimens in thecollections of Idaho museums. His monograph is the mostcomprehensive study of the genus since Karpinsky’s originalwork, and it primarily focuses on a reconstructive model for theanimal. He also described a new species, H. ergassaminon, froma single specimen preserving nearly four volutions. Hisdesignation of a new species, as the epithet implies, was basedon breakage and grooves on the teeth that he interpreted aspatterns of biting and wear. Bendix-Almgreen (1966) lists therepository for the holotype (‘‘Idaho 5’’) as the University ofIdaho in Moscow but that specimen is currently missing.

During a geological survey on the Norwegian island of WestSpitsbergen, one specimen of Helicoprion was discovered, PMOA-33961. Siedlecki (1970) reports on teeth and impression oftwo partial volutions of an individual specimen and designated itas a new species, H. svalis.

The tenth and final species of Helicoprion was recentlydiscovered when road construction in Hubei Province, Chinarevealed a very large, complete whorl of more than fourvolutions. Chen et al. (2007) erected H. jingmenense on thebasis of this exceptional specimen (YIGM V 25147).

In reviewing the 127 year history of discovery anddesignation of Helicoprion species, several patterns emerge.At the time of their designation, nine of the 10 species weredescribed using only one specimen (H. bessonowi is theexception), and four of 10 were described with only a partialvolution of between two to 15 teeth (e.g., H. davisii, H. ferrieri,H. mexicanus, and H. karpinskii) in a genus that typicallycontains more than three volutions and .120 teeth. With suchlimited data, many of the species concepts lack sufficientaccounting of variation within and among individual specimens,even though most authors acknowledge that variation iscommon amongst specimens (Karpinsky, 1899; Teichert,1940; Bendix-Almgreen, 1966). Perhaps the most significantverdict on whether Helicoprion species are redundant is theobservation that, since their original description, only H.bessonowi and H. ferrieri have been diagnosed in depositsother than their respective type localities, and for four of the 10species (H. sierrensis, H. karpinskii, H. ergassaminon, and H.svalis), the holotype is the sole example from the 100Helicoprion specimens in the public fossil record. Confoundingthe systematics problem, at least three holotypes are presentlymissing from their designated repository (e.g., H. nevadensis, H.mexicanus, and H. ergassaminon) and one was not formallydesignated or diagnosed (H. karpinskii).

Taxobases applied to Helicoprion morphology have relied onangles and proportions to describe tooth shape, and number ofteeth per volution to characterize the whorl. Often these metricsare treated as species-defining constants, although severalauthors have attempted to account for ontogenetic variationsby measuring a few teeth per volution. No study to date hasprovided continuous measures for an entire whorl.

This study revises morphological taxobases and speciesconcepts of Helicoprion by developing a database of

Helicoprion specimens that includes type specimens of the 10species, and 90 others that have not received as much researchattention. Two dozen specimens, including several completewhorls, were selected for a variety of geometric and meristicanalyses to account for tooth and whorl shape variation withinand among specimens.

MATERIALS AND METHODS

Reconstruction of Helicoprion whorl.—Dozens of hypotheticalreconstructions in the literature have placed the whorl extendingoutward from the upper jaw (Karpinsky, 1899), as paired lowerand upper jaw elements (Eaton, 1962), or as pectoral or caudaldenticles (Hay, 1909). Recent CT scans of Helicoprion specimenIMNH 37899 (Tapanila et al., 2013) demonstrate that a singlewhorl was supported at the midpoint (symphysis) of the lower jawas predicted by earlier workers (Bendix-Almgreen, 1966; Zangerl,1981; Lebedev, 2009). In this reconstruction, a complete whorl isoriented with the newest, largest teeth issuing from thedorsoposterior of the mouth (abaxial), and the roots of the teethadvancing on a curve in the anterior direction (adaxial) (Fig. 3).Abaxial and adaxial terms are preferred over anterior/posteriorbecause the position of teeth rotates continuously during the lifeof the animal, but the position relative to the spiral center remainsthe same.

Whorl parameters.—Form of the whorl is characterized using avariety of methods to test their efficacy in distinguishingspecimens (Table 1). Traditional measures of volution number,whorl diameter and tooth counts were performed for severalspecimens, focusing on the most complete whorls in thecollection. We define tooth number 2 (i.e., T#2) as the first toothafter the juvenile tooth arch, and count subsequent teeth abaxially.Owing to challenges in visualizing the innermost teeth of thewhorl, underestimation of tooth count is the most likely bias, butnot by more than two or three teeth.

Spiral morphology is described in several ways to facilitatecomparison to previous research (Fig. 3). Spiral growth isdescribed by measuring the alpha angle (a) between the tangentof the spiral curve and the radial vector taken at the inner marginof the root. If this angle is constant throughout the spiral, it is anequiangular (logarithmic) spiral.

To facilitate comparison to Raup’s studies (1966, 1967) onspiral form in planispiral shells, we measured three spiralvariables (c, d, and e; Fig. 3.2) to calculate w¼(d/e)2 and D¼(c/d) parameters. Note that Raup’s calculated D differs from thetooth spacing measurement in Figure 3.2.

Polar coordinate measurements relate radius and relativevolution in radians [r, H] following Cortie (1992). The radialdistance is measured from the spiral center to the inner margin ofthe root. This measure is taken over several volutions of thewhorl, but these are reported as relative volutions because theabsolute origination (i.e., H¼0) of the whorl was not visible inmany specimens.

In a few of the most complete specimens, where the juveniletooth arch is visible, we tracked tooth number (T#), absolutevolution number, and maximum whorl diameter. Absolutevolution number is measured as degrees of rotation around thespiral axis, starting from the second tooth apex (T#2¼08), and thevalue is presented in decimal degrees.

The maximum diameter of the whorl is measured from the apexof a tooth (T#) to the apex of the smaller tooth located 1808 fromits position, corresponding to the anteroposterior dimension of thewhorl at the time T# was generated. All measurements of spiralmorphology were conducted on orthogonal digital images usingGIMP or Photoshop software.

Tooth parameters.—Following the same approach as withwhorl form, many different measurements are applied to tooth


shape to establish which of these might serve as useful taxobases

for species concepts (Fig. 3.1). Specimens were measured either

from scaled photographs or directly on specimens using digital

calipers.

Geometric morphometry was conducted on multiple teeth from

a total of 23 Helicoprion specimens. Seven of the ten Helicoprion

type specimens are included in the analysis. Helicoprion

karpinskii is excluded because its type lacks identifiable

FIGURE 3—Terminology for Helicoprion tooth-root system and whorl. 1, tooth number and size increases abaxially; landmarks (LM1 to LM7) define the upper,middle, and lower parts of the tooth as well as the height of shaft; meristic variables capture linear dimensions and angles of the tooth (A, B, C, basal and apicalangle) and tooth spacing (D); 2, the spiral whorl is comprised of enameloid teeth (light gray) capping a continuous root, whose lower exposure is termed the shaft(dark gray); encapsulating cartilage (white) envelops earlier volutions of teeth and forms a base for the succeeding root; Raup’s variables (c, d, e) and the alphaangle describe spiral shape.

TABLE 2—Alpha angle (degrees) of Helicoprion spiral measured at inner root.

SpecimenIMNH36701

IMNH11905

IMNH30898

IMNH30900

IMNH36509

IMNH36510

PIN1769/6

FortHall

GSA30897

TsNIGR3/1865

TsNIGR1/1865

Mean 83.3 83.1 81.8 83.0 82.4 83.6 81.3 81.4 81.5 83.8 81.6S.D. 4.9 2.8 3.1 2.2 3.0 4.0 5.0 4.7 4.4 2.5 5.5Min 74.0 77.6 78.1 79.1 78.5 78.7 75.6 72.8 74.7 80.5 76.0Max 90.0 88.1 85.6 85.7 88.1 88.2 88.7 87.0 85.5 87.1 87.0N 11 11 8 10 9 9 8 10 5 6 3


landmarks, and the only known image of the missing type H.ergassaminon is too poor for landmark analysis. The type H.ferrieri is a very small specimen of three teeth; instead thisspecies is represented by several H. ferrieri from Idaho (e.g.,IMNH 37899 and IMNH 36510) diagnosed by Bendix-Almgreen(1966).

Whorl specimens were digitally photographed orthogonal tothe spiral axis, or in some instances, original figures or plates ofHelicoprion from literature sources were digitally scanned.Images were rotated or mirrored such that the tooth is orientedwith the apex up and adaxial to the right. A subset of all availableteeth from each whorl was selected for landmark analysis basedon clarity of six or seven landmark positions. Landmarks are acombination of Type I (LM1 and 3), Type II (LM2, 4, 5, and 6),and Type III (LM7) points (Fig. 3).

Determining reliable landmark points is compromised by thedegree of exposure of the fossil from matrix and by breakage ofthe fossil. To mitigate shape variation caused by poor landmarkexposure, only teeth with the best level of exposure and toothcompleteness were chosen for landmark analysis, although werecognize that some of the variation in our results is likely fromthis issue.

Landmark coordinates were acquired using tpsDig2 software,and imported in MorphoJ for further analysis. Each series of teethfrom a whorl were treated as one dataset, each tooth with its ownset of group classifiers. A full Procrustes superposition was usedto rotate and scale each set of landmarks prior to generatingcovariate matrices and PCA transformations. Results of PCA areused to identify shape parameters that are significant in defining

species. Statistical analyses were conducted in PAST software(Hammer et al., 2001).

MORPHOMETRY

Spiral geometry.—The spiral shape of the whorl, as measuredby the inner margin of the cartilaginous root, is approximated byan equiangular spiral (i.e., logarithmic spiral). The small degreeof variance does not correlate with volution number or whorldiameter. The mean a angle for 11 specimens ranged from 81–838(S.D.¼2–58) (Table 2).

Raup’s parameters define it as planispiral, with successivevolutions that abut or are slightly open, i.e., W@1/D (Table 3).This character is consistent with spiral shells produced by mostcoiled cephalopods, where abutting volutions afford greater shellstrength and minimize the production of shell material (Raup,1967).

Polar coordinates for the spiral geometry of 11 specimens (Fig.4), independent of origin, show very similar expansion ratesamong whorls. Regression of log transformed polar coordinates(H, ln r) produces a strong linear correlation for each specimen,r2.0.92 (Table 4). One-way ANCOVA test for parallelism ofslopes for all 11 specimens is not rejected (F¼1.42, p¼0.18).Spiral geometry appears to be conservative within the genus andmay be a useful generic taxobase.

FIGURE 4—Polar coordinates (radius, H) of Helicoprion whorls measured at the inner margin of the root; H¼0 radians is estimated from position of juveniletooth arch in all specimens except for IMNH 11095, where the center of the whorl is not preserved. Data are presented in Table 4.

TABLE 3—Raup’s spiral variables for select Helicoprion specimens.

Specimen Ontogeny c (cm) d (cm) e (cm) w D

IMNH 36701 Adult 4.26 11.46 6.98 2.70 0.37IMNH 30900 Adult 2.21 5.14 4.02 1.63 0.43IMNH 30900 Juvenile 0.95 2.21 1.92 1.33 0.43Fort Hall Adult 2.26 5.70 3.36 2.88 0.40Fort Hall Adult 1.90 4.46 2.61 2.91 0.43Fort Hall Juvenile 0.94 1.90 0.98 3.73 0.50TsNIGR 1/1865 Adult 3.78 7.59 5.21 2.12 0.50

TABLE 4—Regression of polar coordinates (radius, H) for spiral whorls ofselect Helicoprion specimens. See Figure 4 for corresponding data points.Variables k and m are provided for the exponential function, radius¼kemH.Radius in mm, H in radians.

Specimen k m r2

IMNH 30897 3.7166 0.1342 0.9877IMNH 36701 4.4501 0.1404 0.9921IMNH 11905 6.4905 0.1273 0.9932Fort Hall 5.1312 0.1264 0.9734IMNH 30900 3.4719 0.1270 0.9680IMNH 30898 2.6553 0.1575 0.9221IMNH 36509 5.0396 0.1269 0.9724IMNH 36510 4.5427 0.1350 0.9921TsNIGR 1/1865 4.9100 0.1203 0.9922TsNIGR 3/1865 2.2586 0.1463 0.9931PIN 1769/6 4.5463 0.1310 0.9796


Tooth and whorl growth.—Linear dimensions of teeth grow

exponentially with tooth number and volution number (Figs. 5, 6;

Table 5). Transformed volution height (ln A) for each specimen

shows a strong linear correlation after T#30 (r2.0.93). Growth

rate of volution height (T#.30) is not equivalent for any

combination of specimens, except among TsNIGR 1/1865, 3/

1865, and IMNH 30900 (test for parallel slope, F¼1.62, p¼0.20).

Transformed tooth width (ln B) shows similar exponential growth

by tooth number for IMNH 30897, IMNH 36701 and YIGM V

25147 (test for parallel slope, F¼2.688, p¼0.07), and for IMNH

348 and IMNH 30900 (F¼1.147, p¼0.29). All other combinations

of specimens in Table 5 yield no statistical similarity.

The exponential growth of tooth and whorl dimensions makesthese size parameters strongly dependent on volution number. Atvolutions less than 1.5, approximately T#,60, specimens showvery similar values for linear metrics A, B, C, D, and maximumdiameter (Figs. 5, 6), but small variations are amplified in latervolutions. With additional increments of volution, specimensdiverge greatly in tooth dimensions, and less so by maximumwhorl diameter. By the second volution (T#60–80), specimenswith high growth rates (e.g., IMNH 30897, IMNH 36701, andYIGM V 25147) have volution height 40 percent greater thanspecimens with lower growth rates (e.g., IMNH 30900, TsNIGR1/1865, and UNMMPC 1002). By contrast, maximum whorldiameter during the second volution is similar among specimens.

FIGURE 5—Meristic analysis of teeth plotted by tooth number. Tooth number 1 is located at the center of the spiral.


For example at volution 2.5, the large-toothed IMNH 36701 has avolution height of 58 mm compared to the 35 mm of TsNIGR 1/1865 whereas their maximum whorl diameters are similar at ~18cm. Both whorls share the same logarithmic spiral as measured by

the inner root margin (Fig. 4) so the disparity between theirvolution heights is accommodated in the whorl by theencapsulating cartilage, i.e., the distance from tooth apex to thebase of the subsequent root. In this instance, more of the volution

FIGURE 6—Measurements by volution of the whorl. 1, volution height (A); 2, tooth number; 3, insertion angle of teeth measured as degrees of volution dividedby tooth number; 4, maximum whorl diameter measured from tooth apex at volution number.


in TsNIGR 1/1865 is occupied by encapsulating cartilage than bythe root and teeth as compared to the same volution in IMNH36701 (Fig. 7).

The total number of volutions recorded by Helicoprion whorlsis therefore significant in accounting for size-based variationamong specimens, and highlights the importance of consideringvolution number when comparing specimens. For example, thefinal whorl diameter of IMNH 37899 is half that of IMNH 30897,with maximum volution heights of 60 mm and 126 mm,respectively. Yet examining these two specimens at equalvolution number shows they have similar volution height andwhorl diameters up to the final volution (3.25) of IMNH 37899.The additional 0.65-volution produced by IMNH 30897, to a totalof 3.9 volutions, resulted in a near doubling of its size. It is verylikely that, had IMNH 37899 produced a similar total of 3.9volutions, it would be nearly identical to IMNH 30897.

Growth rate of tooth and whorl metrics are gradational amongspecimens although the two type H. bessonowi (TsNIGR 1/1865,3/1865) from the same locality show equivalent growth rates forvolution height. This suggests that growth rate may have aphylogenetic basis. IMNH 30900 also shows equivalent growthrate to the two type H. bessonowi specimens from Russia,however we will demonstrate later that IMNH 30900 ismorphologically distinct from this group. Therefore, growth rateon its own may not discriminate among Helicoprion species, butis useful in description and extrapolation. Variability of growthrate within a species may correspond either to extrinsic factors(e.g., environment and diet) or to sexual dimorphism, a commonphenomenon among modern euchondrocephalans (e.g., males aresmaller).

Growth rates measured from well preserved specimens can beuseful for modeling unknown metrics in poorer quality speci-mens. Strong correlations between volution height, tooth number,volution number, and maximum whorl diameter are presented asallometric equations, and are used to extrapolate unknown valuesfor some specimens. Volution height is the simplest metric tomeasure and often is the only reliable metric for incompletespecimens; therefore equations describing correlations in terms ofvolution height can be used to decipher a variety of importantwhorl characters. Table 6 lists the logarithmic and linearequations and parameters derived from ten elite Helicoprionspecimens (r2.0.94; equations are best for T#.40). Tables 4 and5 can be used similarly to model whorls from equations (Table 7).

Tooth count and angular measures.—Teeth are added as a non-linear function of volution. Although this pattern is very subtle(Fig. 6.2), teeth are added to whorls in either increasing ordecreasing number as the whorl grows larger. The change is bestobserved by plotting insertion angle by volution number (Fig.6.3). Tooth count is most similar among specimens prior tovolution ~1.5, and diverges with each volution thereafter.Specimens such as IMNH 30900 and TsNIGR 1/1865 decrease

the insertion angle from 98 to ~88, and thereby increase toothcount per volution. By contrast, IMNH 30897 and 36701 have aninsertion angle that increases with volution up to 118. These largewhorls have fewer teeth at the same volution number compared towhorls having lower insertion angles. Other specimens tend tostabilize after the first volution between 98 and 108.

Apical angle does not strongly correlate with tooth number, butis not constant either, showing a standard deviation of 4–88 of themean angle per specimen (Fig. 8; Table 8). Basal angle issimilarly variable (Table 9; S.D.¼4–68 of mean), although thereappears to be an inverse relationship between insertion angle andbasal angle, i.e., widely spaced teeth tend to have more acutebasal angles. In practice, both basal and apical angles are difficultto measure consistently. In the instance of basal angle, choosingthe midpoint at the lower part of the tooth is challenging andlikely contributes to the variability in our measurements.Similarly, apical angle is difficult to measure if the exposure ofteeth is not equivalent from specimen to specimen, and the trueapex of the tooth often is damaged or obscured. This affectsangular measures more than linear measures (e.g., volutionheight).

To summarize, number of teeth per volution is considered apoor taxobase because it is strongly dependent on the volutioninterval being counted. Instead, we propose using the change intooth count per volution or the insertion angle of teeth as criteria.The latter has the advantage because it can be quantified in

TABLE 5—Regression of volution height (A) and width (B) on tooth number (T# . 30) for select specimens of Helicoprion. Variables k, m are for exponentialequation, A or B¼kem(T#), where T# is tooth number.

Specimen

Volution height (A) in mm Tooth width (B) in mm

k m r2 k m r2

IMNH 348 6.7633 0.0154 0.94 1.3788 0.0163 0.77IMNH 37899 5.0778 0.0216 0.99 1.3534 0.0216 0.99UNMMPC 1002 5.5287 0.018 0.97 1.4778 0.0173 0.91IMNH 30897 5.0436 0.025 0.99 1.192 0.0276 0.99IMNH 36701 7.496 0.0243 0.98 1.2278 0.0296 0.97YIGM V 25147 7.3351 0.0183 0.98 1.1538 0.0252 0.91IMNH 30900 5.4308 0.0166 0.99 1.5216 0.0153 0.97TsNIGR 1/1865 6.0107 0.0169 0.99 3.2163 0.0111 0.85TsNIGR 3/1865 6.1138 0.0181 0.98

FIGURE 7—Relationship of volution height and maximum whorl diameter.Disparity among specimens is explained by the proportion of volution heightand encapsulating cartilage in comprising whorl diameter.


incomplete specimens using a tooth spacing ratio (D/A) as aproxy, where low D/A approximates low insertion angle (seebelow).

Tooth and root shape.—Six landmarks define the shape of 172teeth from a total of 23 Helicoprion specimens (Fig. 9; onlineSupplemental Table 2). Specimens figured in the principalcomponents analyses (PCA) are grouped by the new emendedspecies, which are listed in online Supplemental Table 2. A PCAof all specimens shows that the first two principal componentsaccount for 47 percent and 19 percent of variation, respectively(Fig. 9.1). Positive values of PC1 are attributed to an increase inthe cutting blade height and width with a reduction in the heightof the middle tooth, whereas positive values of PC2 describe anelongation of the middle section and compression of the root (Fig.9.2). Plotting this analysis in terms of known tooth number(N¼112, Fig. 9.3) demonstrates that PC1 varies inversely withtooth number. Teeth numbered ,85 for all specimens plot closeto each other, near to or greater than the mean value of PC1. Bycontrast, teeth numbering 85 and higher have a more negativePC1 value, with some specimens plotting below�0.1, and othersbetween mean and 0.1. Therefore, a greater degree of shapediscrimination associated with PC1 is possible by examining onlyT#�85.

Recalculating the six-landmark PCA to include only teethnumbered 85 or greater (N¼86, 19 specimens, online Supple-mental Table 2) demonstrates again that the dominant shapevariation (PC1, 65% variance) is explained by the proportion ofthe upper and middle tooth relative to volution height (Fig. 9.4,9.5). Teeth at or above mean PC1 values have an elongate upperpart and short middle (e.g., H. davisii and H. ergassaminon

groups) whereas negative PC1 values, more typical of the H.

bessonowi group, are characterized by short upper and elongatemiddle parts of the teeth. The most negative PC1 teeth belong tolarge specimens, IMNH 14095 and IMNH 49382.

The proportional height of the shaft has been used as a criterionfor nearly all Helicoprion species. To address this criterion, aseventh landmark (LM7) was added to denote the upper margin ofthe shaft for a select number of teeth from the previous analysis.Adding this Type III landmark limits our sample size of T#,85 to48 teeth from 19 specimens. PCA produces a dominant PC1 axisaccounting for 70 percent of shape variance (Fig. 10.1, onlineSupplemental Table 2). As in previous PCA, positive PC1corresponds to increased height of the upper tooth and only aslight increase in the relative height of the shaft. Segregation ofspecimen groups is similar to the six-landmark analyses (e.g., Fig.9.5). PC2 accounts for 14 percent variance and is related toincreased shaft height with increased tooth number, primarily forthe H. davisii group (Fig. 10.2, 10.3).

TAXOBASES

Results of meristic and geometric analyses indicate that theonly measure approaching constancy in the Helicoprion whorl isthe inner curvature of the root. Otherwise, measurements ofdimension, angle and shape vary as a function of volution andtooth number. Ontogenetic variation is identified for tooth shapeby plotting tooth number on the principal components analysis(Fig. 9.3), which demonstrates a decrease in PC1 with increasedtooth number for all specimens. Removal of juvenile teeth(T#,85) from the analysis highlights phylogenetic differencesin the sample, which are also delineated along the PC1 axis(Figs. 9.5, 10.3). Taxobases should therefore be designatedprimarily for the adult form, which is defined here as a whorlafter the second volution, or roughly the 85th tooth. Twoproportional measures, the upper ratio and the tooth spacingratio, are identified as significant taxobases that correspond tophylogenetic variation on PC1 (Table 9). A third proportion, theshaft ratio, is considered a useful secondary taxobase.

Upper ratio.—Results of PCA suggest that ontogenetic andphylogenetic variations in teeth can be explained primarily byPC1, which corresponds to height proportions of the tooth.Proportional width dimensions of the tooth are also described byPC1 as significant shape variables, i.e., positive PC1 correspondswith widening of the tooth. These shape variations associatedwith PC1 can be simplified by reporting the upper ratio of a tooth(vertical height between LM2 and LM3 divided by volutionheight), and using this as a primary criterion to define threeemended species of mature Helicoprion. Figure 10.4 illustratesthe tooth shape and upper ratios corresponding to PC1 values forthe seven-landmark analysis. Specimens ascribed to H. bessonowihave a distinctly smaller upper ratio (,0.35) than either theintermediate values of H. ergassaminon (~0.37) or the largeupper ratio (.0.38) of H. davisii. From a developmentalviewpoint, all juvenile whorls have similar upper ratio values.Helicoprion bessonowi is characterized by having the greatestontogenetic decrease in its upper tooth ratio, whereas the toothshape of H. davisii retains similar proportions of upper ratio fromjuvenile to adult. Helicoprion ergassaminon is intermediate.

Tooth spacing ratio.—Changes in the insertion angle of teethcontrols the number of teeth per volution in a whorl but itsmeasurement requires preservation of the spiral center. The toothspacing ratio (D/A) is related to this aspect of the whorl and doesnot require a complete specimen. Plotting D/A for mature teeth inthe seven-landmark analysis shows a strong positive correlation(r2¼0.8) with PC1, i.e., a greater upper ratio and tooth widthcorresponds with greater tooth spacing (Fig. 10.5). Specimens ofH. davisii have D/A typically .0.3 compared to H. ergassaminonand H. bessonowi that each have values below 0.3. The D/A¼0.3boundary is comparable to the 98 tooth insertion angle boundaryobserved in Figure 6.3 that separates wide-spaced teeth of H.

TABLE 6—Equations and parameters for modeling tooth number (T#), volution number (Vol), and maximum whorl diameter (Dmax). Parameters m, k, and b arefor exponential and linear relationships between T#, Vol, Dmax and volution height (A), as depicted in Figures 5 and 6. R-squared values are presented toshow strength of relationship. Reference (Ref) number corresponds to Table 8.

Ref Specimen

Estimated T# Estimated volutions Estimated Dmax

T# ¼ Ln(A/m)/k Vol ¼ (T#�b)/m Dmax ¼ (A�b)/m

m k r2 m b r2 m b r2

1 TsNIGR 1/1865 5.6610 0.018 0.99 43.709 �5.3181 0.99 2.2734 �1.7461 12 TsNIGR 3/1865 6.1138 0.018 0.983 IMNH 30900 4.5696 0.018 0.97 43.262 �2.8280 1 2.5961 0.9662 14 IMNH 348 5.3817 0.018 0.945 IMNH 37899 3.5418 0.026 0.97 35.790 1.8707 1 2.7525 0.9523 0.996 UNMMPC 1002 4.1150 0.022 0.97 43.189 �11.2510 17 IMNH 30897 4.0828 0.028 0.97 32.367 2.0251 1 3.0883 �1.4270 18 IMNH 36701 5.6225 0.027 0.98 27.890 13.7470 0.98 2.8045 7.8979 19 YIGM V 25147 7.3351 0.018 0.98 36.442 4.3720 1

10 UMNH 5131 36.566 4.9876 1 2.9655 �0.5616 1


davisii from the close-spaced teeth of H. bessonowi and H.ergassaminon. Plotting for a broader range of tooth numbers (Fig.11.1) shows that D/A is greater than 0.3 for nearly all H. davisii atany tooth number, and that it can be distinguished from H.

bessonowi after A¼40 mm.

Shaft ratio.—Shaft ratio (S/A) of mature teeth shows a veryweak positive correlation (r2¼0.13) with PC1 for the seven-landmark analysis (Fig. 10.6). The H. davisii group shows high

variability between shaft ratios of 1/5 and 1/10, although plottingthe ratio for a wide range of tooth numbers shows that large teethin this group uniformly have ratios between 1/4 and 1/5 (Fig.11.2). At A.40 mm, H. bessonowi specimens have a much lowershaft ratio than H. davisii. Mature teeth of H. ergassaminon plotwithin the overlapping ranges of H. davisii and H. bessonowi.

Shaft ratio values are highly variable within a specimen (e.g.,

H. ergassaminon [IMNH 30900] in Fig. 11.2), and this is most

TABLE 7—Modeled tooth number (T#), volution number, and maximum whorl diameter (Dmax) as a function of measured volution height (A) for specimens ofHelicoprion used for PCA. Index number corresponds to landmarked teeth for each specimen. Reference equations and specimens (Ref) correspond to Table 6.

Specimen Index

Estimated tooth number Estimated volutionEstimated Dmax

Reference 1 Reference 2 Reference 1 Reference 1

PIN 1988/38 0 17 12 0.5 41 34 30 0.9 52 39 35 1.0 63 43 40 1.1 64 47 43 1.2 75 47 43 1.2 76 49 45 1.2 7

Reference 5 Reference 6 Reference 5 Reference 6 Reference 5USNM 22577 9 90 99 2.5 2.6 13

10 94 103 2.6 2.7 1411 92 102 2.5 2.6 13

IMNH 36510 0 99 110 2.7 2.8 162 117 131 3.2 3.3 253 120 135 3.3 3.4 28

WAMAG 9080 0 73 79 2.0 2.1 81 76 83 2.1 2.2 92 77 84 2.1 2.2 93 77 84 2.1 2.2 94 80 88 2.2 2.3 105 80 88 2.2 2.3 106 80 88 2.2 2.3 107 81 88 2.2 2.3 108 81 89 2.2 2.3 109 84 93 2.3 2.4 11

10 85 93 2.3 2.4 1111 85 93 2.3 2.4 1112 87 96 2.4 2.5 1213 88 97 2.4 2.5 12

DG3 0 104 115 2.8 2.9 181 103 115 2.8 2.9 182 104 116 2.9 2.9 183 104 115 2.8 2.9 184 106 118 2.9 3.0 195 105 117 2.9 3.0 196 107 120 2.9 3.0 207 107 119 2.9 3.0 20

DG4 0 57 61 1.6 1.7 51 63 68 1.7 1.8 62 64 69 1.7 1.8 63 66 71 1.8 1.9 74 69 75 1.9 2.0 75 71 77 1.9 2.0 86 75 82 2.1 2.2 9

Reference 3 Reference 3 Reference 3IMNH 14095 0 187 4.4 53

1 188 4.4 552 188 4.4 54

DG5 0 189 4.4 561 190 4.5 572 190 4.5 57

mexicanus 0 148 3.5 261 144 3.4 242 146 3.4 253 151 3.5 27

PMO A-33961 0 78 1.9 71 79 1.9 72 82 2.0 83 81 1.9 74 161 3.8 335 163 3.8 34

IMNH 49382 0 167 3.9 371 166 3.9 372 168 4.0 383 166 3.9 364 171 4.0 405 169 4.0 38


likely a result of preservation and measurement fidelity ratherthan a primary anatomical signal. Shaft height is a relatively smalldimension compared to volution height, and its lower and upperboundaries can be difficult to consistently measure. The base ofthe root (i.e., shaft) is deeply concave in cross-section, such thatdifferential exposure of this part of the whorl can producevariable apparent shaft heights. In addition, recognizing the top ofthe shaft requires good preservation of the lower enameloidprojections of teeth, which is not always the case. Although not ofprimary discriminatory value, description of shaft ratio has somecomparative importance in defining end-members. It may alsohave value for the larger undiagnosed specimens in the sample.

SYSTEMATIC PALEONTOLOGY

Class CHONDRICHTHYES Huxley, 1880Subclass EUCHONDROCEPHALI Lund and Grogan, 1997

Order EUGENEODONTIFORMES Zangerl, 1981Family HELICOPRIONIDAE Karpinsky, 1911

Genus HELICOPRION Karpinksy, 1899

Type species.—Helicoprion bessonowi Karpinsky, 1899.Diagnosis.—Continuous root forming closed, multi-volution,

logarithmic spiral, supporting tooth crowns that extend down toroot shaft; abaxially, enameloid tooth crowns increase in heightand become crenulate at upper surface; basal projection of toothdirected tangential and adaxial to spiral; ventral surface of rootconcave; encasing cartilage surrounds inactive tooth crownsforming base of subsequent root volution. (Emended for the whorlapparatus).

Occurrence.—Early Permian (Cisuralian–early Guadalupian):Western and arctic Canada, western U.S.A., Mexico, Spitsbergen,central Russia, Kazakhstan, eastern China, Laos, Japan, andWestern Australia (see online Supplemental Table 1 forstratigraphic information).

Remarks.—All nine genera in the helicoprionid family(reviewed by Lebedev, 2009) have arched rows of symphyseal

FIGURE 8—Apical and basal angle measured by tooth number for select specimens.

TABLE 8—Descriptive statistics of apical angle for select specimens ofHelicoprion.

Specimen Mean S.D. Minimum Maximum N

IMNH 348 37.3 3.7 31.2 44.2 18IMNH 37899 37.9 8.2 27.9 54.7 34UNMMPC 1002 34.7 7.3 21.7 51.6 40IMNH 30897 42.9 6.7 36.2 55.2 7IMNH 36701 50.0 6.1 39.9 59.0 18YIGM V 25147 52.8 11.5 35.0 64.0 6IMNH 30900 32.8 6.5 22.4 56.5 85TsNIGR 1/1865 45.0 3.7 34.4 50.9 23

TABLE 9—Descriptive statistics of basal angle for select specimens ofHelicoprion.

Specimen Mean S.D. Min Max N

IMNH 30900 123.2 4.4 115 128 6IMNH 37899 129.8 4.0 123 133 5IMNH 36510 115.0 2.8 113 117 2USNM 22577 115.0 3.2 112 119 4FortHall 123.0 2.3 121 126 5DG3 119.7 6.0 114 126 3DG4 118.3 5.5 113 124 3IMNH 30897 113.0 9.9 106 120 2IMNH 36701 118.8 2.6 116 121 4TsNIGR 1/1865 128.6 4.2 123 134 10TsNIGR 3/1865 126.4 6.4 119 137 7PIN 1769/6 126.0 4.0 120 133 9PIN 1988/38 143.5 3.7 140 148 4IMNH 14095 131.0 2.8 129 133 2IMNH 49382 122.0 122 122 1WAMAG 9080 120.9 2.7 116 124 7mexicanus 125.0 125 125 1UNMMPC 1001 127.3 6.5 119 136 8UNMMPC 1002 126.0 4.1 121 131 6


teeth with anterior projections at the base of each tooth(Obruchev, 1964). Helicoprion is the only genus to form a closedspiral, by contrast to the open partial whorl of Shaktauites(Tchuvashov, 2001). The equiangular spiral has a near uniformalpha angle between 818 and 838, and is planispiral. Instances ofW.1/D are small in magnitude and uncommon, perhaps owing tominor taphonomic deformation of the spiral.

HELICOPRION DAVISII (Woodward, 1886)Figures 2.1–2.5, 12.1

1886 Edestus davisii WOODWARD, p. 1, pl. I.1902 Campyloprion davisii (Woodward); EASTMAN, p. 65.

1907 Lissoprion ferrieri HAY, p. 22, fig. 1.

1909 Lissoprion ferrieri; HAY, p. 52, fig. 7, pls. 14, 15.

1911 Helicoprion davisii (Woodward); KARPINSKY, p. 1122,

fig. 4.

1911 Helicoprion ferrieri, KARPINSKY, p. 1122, figs. 4, 6.

1939 Helicoprion sierrensis WHEELER, p. 112, fig. 4.

1940 Helicoprion davisii (Woodward); TEICHERT, p. 145, pl.

22, fig. 1.

1966 Helicoprion ferrieri, BENDIX-ALMGREEN, p. 35, pl. I–III,

V, X, XI.

1981 Helicoprion ferrieri, ZANGERL, p. 86, fig. 98H.

FIGURE 9—PCA for all specimens using six landmarks. 1–3, all teeth included, n¼172 teeth from 23 whorls: 1, PC variance and shape; 2, 3, tooth shape on PC1and PC2 plotted by specimen by known tooth number, respectively; gray outline denotes range of all teeth; 4, 5, only teeth numbered �85, n¼86 teeth from 19whorls; 4, PC variance and shape; 5, tooth shape on PC1 and PC2 plotted by specimen. Specimens listed in online Supplemental Table 2.


1982 shark; LARSON and BIRKELAND, p. 231, fig. 7–16H.2007 Helicoprion jingmenense CHEN, CHENG AND YIN, p.

2248, figs. 2, 3, 4c.2013 Helicoprion ferrieri TAPANILA, PRUITT, PRADEL, WILGA,

RAMSAY, SCHLADER AND DIDIER, p. 2, fig. 2.

Diagnosis.—Species of Helicoprion with tall cutting blade andwide tooth spacing that increases with maturity; anteriorcurvature of cutting blade pronounced; basal angle decreaseswith maturity.

Description.—For mature teeth, upper tooth height greater than0.38 of volution height and tooth apices spaced .0.28 of volutionheight; after second volution, tooth insertion angle increases to 98or higher and basal angle decreases.

Holotype.—WAMAG 9080, Woodward, 1886 (pl. I); EarlyPermian, Wandagee Beds, Australia.

Material.—YIGM V 25147 (Chen et al., 2007), UNMMPC1002 (Wheeler, 1939), USNM 22577, WAMAG 9080 (Wood-ward, 1886; Teichert, 1940), IMNH 36510 (¼‘‘Thiel P’’ ofBendix-Almgreen, 1966), IMNH 36701, IMNH 30897 andcounterpart GSA 30897 (Larson and Birkeland, 1982), IMNH37899 (¼‘‘Idaho 4’’ of Bendix-Almgreen, 1966; Tapanila et al.,2013), FortHall, DG3, DG4. Additional specimens listed indiagnosis column of online Supplemental Table 1.

Occurrence.—Cisuralian–early Guadalupian (Roadian). Aus-tralia: Wandagee Beds; Canada: Assistance Formation, VanHousen Formation, and Rocky Mountain Quartzite; China: QixiaFormation; Mexico: Patlanoaya Formation; U.S.A.: Bone SpringFormation, ?Goodhue Formation, Phosphoria Formation, SkinnerRanch Formation, and Word Formation.

Remarks.—This emended concept is a significant merger ofprevious species, and is demonstrated by a shape population thatis most distinct from H. bessonowi. A fair amount of variabilityexists within the group, most notably a range in maximum whorlsize that may include more than four volutions, and growth ratesthat vary widely among specimens. This species concept accountsfor size variations in terms of volution and tooth number. Therange of values for the shaft ratio is expanded here, by contrast toearlier studies that regarded this ratio as a significant taxobase.Instead, greater emphasis is placed on tooth spacing and toothshape in defining species. The synonymy disregards all previousspecies criteria using apical angle, height of shaft, or number ofteeth per volution that do not relate these variables to tooth orvolution number.

Tooth shape is distinct from other species after approximatelythe second volution (e.g., volution height of 26 mm at T#80 forIMNH 37899, or T#68 for IMNH 30897). The short, wide middlepart of the tooth is distinct from the tall, narrow teeth of H.bessonowi and H. ergassaminon. Width of tooth is 0.28 or greaterthan volution height. Associated with increased tooth width is anincrease in insertion angle, which results in a greater spacingbetween adjacent tooth apices. Also, the lower projections of theteeth decrease in height, giving a more acute basal angle.

Helicoprion davisii is the senior species of the synonymy,regarded by Karpinsky (1911) to be very similar to H. ferrieri.Wheeler (1939) argued that the shaft of H. davisii is too thin andmore resembles H. bessonowi, but Teichert (1940) showed H.davisii (WAMAG 9080 and 8523) shafts are 1/7 to 1/8 volutionheight. Our measures of shaft ratio for the holotype (WAMAG

FIGURE 10—PCA for all specimens from seven landmarks, T#�85, n¼48, 15

specimens. 1, PC variance and shape; 2, 3, tooth shape on PC1 and PC2plotted by number and species, respectively; 4–6, proportional measuresplotted on PC1: 4, diagram and values of upper ratio; 5, tooth spacing (D/A);6, shaft ratio (S/A). Specimens listed in online Supplemental Table 2.


9080) of H. davisii range from 0.11 to 0.18, which falls in therange for the new species concept of H. davisii.

Helicoprion ferrieri is the best described species of thissynonymized group, following Bendix-Almgreen’s (1966) effort.His differential diagnostic criteria of apical angle and shaft heightnotwithstanding, Bendix-Almgreen correctly recognized thesmall middle part of H. ferrieri teeth as similar to H. davisiiand H. sierrensis, and that these are dissimilar to H. bessonowi, H.nevadensis, and H. ergassaminon.

Wheeler (1939) differentiated Helicoprion sierrensis from H.ferrieri as having a shaft ratio of 1/10, which he regarded too low.Our measures demonstrate that the type H. sierrensis has a shaftratio closer to 1/9 and is within the range of type H. davisii.

The species characteristics of H. jingmenense rely especially onthe observation that teeth of the fourth and fifth volution growwider than they are tall, evidently because the height of the teethceases to increase. Chen et al. (2007, table 1) show that 13consecutive teeth have a constant volution height (~80 mm),which we also depict in our measurements by tooth and byvolution (Figs. 5, 6) from their original figure. We havedemonstrated that all Helicoprion, including the earlier volutionsof YIGM V 25147, have tooth metrics that grow exponentiallywith tooth number, including volution height. In this context, andwith closer inspection of the photograph of the final volution ofthe H. jingmenense holotype, we suggest that the tooth apices areconcealed in matrix and that the visible tooth height isunderestimated. Underestimation of tooth height at the apexfurther explains why Chen and colleagues report unusually highapical angles (56–658) for these teeth. This solution makes theholotype of H. jingmenense comparable in all respects to IMNH30897.

HELICOPRION BESSONOWI Karpinsky, 1899Figure 2.6, 12.2

1899 Helicoprion bessonowi KARPINSKY, p. 20, figs. 18–57,pls. 1–4, figs. 1–11.

1903 Helicoprion bessonowi, YABE, p. 8, pl. II.1911 Helicoprion bessonowi, KARPINSKY, p. 1105, fig. 2.1939 Helicoprion nevadensis WHEELER, p. 109, fig. 3.1964 Helicoprion bessonowi, OBRUCHEV, p. 375, pl. 3, fig. 1,

figs. 32, 33.1981 Helicoprion bessonowi, ZANGERL, p. 86, fig. 98I.2009 Helicoprion bessonowi, LEBEDEV, p.173, figs. 2, 3, 9.

Diagnosis.—Species of Helicoprion with short cutting bladeatop long parallel-sided middle part, and narrow tooth spacing;apex of cutting blade skewed posteriorly; basal angle obtuse;shaft diminutive for all volutions.

Description.—For mature teeth, upper tooth height less than0.35 of volution height and tooth apices spaced ,0.31 of volutionheight; tooth insertion angle decreases below 98 after the secondvolution.

Type.—Lectotype selected by Obruchev (1964), TsNIGR 1/1865, Karpinsky, 1899 (pl. I).

Material.—TsNIGR 1/1865 (Karpinsky, 1899), TsNIGR 3/1865 (Karpinsky, 1899), PIN 1769/6 (Lebedev, 2009), PIN 1988/38 (Lebedev, 2009), UNMMPC 1001 (Wheeler, 1939), UMUTPV 7214 (Yabe, 1903). Additional specimens listed in diagnosiscolumn of online Supplemental Table 1.

Occurrence.—Artinskian; Japan: Tanukihara Formation; Ka-zahkstan: Aktasty beds; Russia: Divya Formation; U.S.A.:formation unknown.

Remarks.—Tooth morphology is distinct from H. davisii andH. ergassaminon after the second volution, at around T#90(volution height¼30 mm, TsNIGR 1/1865). Although there issome overlap in the lower range of H. bessonowi values, shaftratio for this species is typically ,1/8, much smaller compared toother species (Fig. 11). Tooth spacing is similar to H.ergassaminon, with D/A values at or below 0.31, whichcorresponds to an insertion angle between 88 and 98. Thesevalues are smaller than H. davisii.

Width of tooth is approximately 0.24 of volution height. Lowinsertion angle and generally narrow tooth form is distinct from

FIGURE 11—Tooth spacing and shaft ratios by volution height for emended species. Specimens listed in Table 10.


FIGURE 12—Model whorls for Helicoprion species. 1, H. davisii; 2, H. bessonowi; 3, H. ergassaminon. ‘X’ marks spiral center and dashed line marks the originat T#2. The transition in tooth shape from juvenile (j) to adult (a) occurs at the start of the second volution. The 85th tooth is labeled for each whorl. Serrations onteeth not depicted.


H. davisii, but similar to H. ergassaminon. Helicoprion bessonowihas laterally flattened teeth and nearly parallel sides of the middlepart of the tooth, unlike the more conical and curved teeth of H.ergassaminon. Apical angle and presence of serrations, asproposed by Lebedev (2009), are rejected in this emendeddiagnosis.

Wheeler (1939) initially regarded H. nevadensis (UNMMPC1001) as similar to H. bessonowi, but suggested that a more rapidexpansion of the middle part of the teeth and taller shaftdistinguished H. nevadensis. Results of six- and seven-landmarkPCA show tooth shape of the H. nevadensis holotype to beconsistent with PIN 1769/6 and TsNIGR 3/1865 for similar toothnumber. The final full tooth exposed on the UNMMPC 1001 isT#87, and it has an upper ratio of 0.32, and a shaft ratio of 1/9,both consistent with H. bessonowi at this developmental stage.

HELICOPRION ERGASSAMINON Bendix-Almgreen, 1966Figures 2.7, 2.8, 12.3

1966 Helicoprion ergassaminon BENDIX-ALMGREEN, p. 39,fig. 25, 26, pl. XV.

Diagnosis.—Species of Helicoprion with tall cutting blade andnarrow tooth spacing; gently curved anterior and posterior cuttingsurface; basal angle obtuse.

Description.—For mature teeth, upper tooth height between0.35 and 0.38 of volution height and tooth apices spaced between0.25 and 0.30 of volution height; tooth insertion angle decreasesbelow 98 after the second volution.

Type.—Holotype, Idaho 5, Waterloo Mine, Idaho. Type

repository listed as University of Idaho, Moscow (Bendix-Almgreen, 1966). Specimen missing.

Material.—IMNH 30900, LACM 4251 (may be counterpart ofIMNH 30900), AMNH FF 8248, ‘‘Idaho 5’’ of Bendix-Almgreen,1966 (pl. XV). See online Supplemental Table 1 for morespecimen information.

Occurrence.—Kungurian–Roadian; Phosphoria Formation,Idaho, U.S.A.

Remarks.—Diagnosis of dentition follows Bendix-Almgreen(1966). Helicoprion ergassaminon is intermediate in characterswith H. davisii and H. bessonowi, and is distinct from both groupsafter T#85 using the seven-landmark PCA. With H. davisii, itshares having a high shaft and concave middle portion but differsin having an overall tall, narrow shape with a low insertion angle(8 to 98), which is also demonstrated by D/A intermediate withthe other species. Helicoprion ergassaminon has a much tallerand curved crown, even at high tooth number, by comparison toH. bessonowi. We do not observe evidence of pre-mortal toothbreakage in our referred specimens, as did Bendix-Almgreen inthe type specimen, Idaho 5.

LARGE SPECIMENS OF HELICOPRION

Large size is not a valid taxobase. It is a direct result ofincreased volution number and growth rate, as demonstrated forlarge specimens included with H. davisii, and may be connectedto environmental conditions. This study included five specimensof very large tooth whorls that cannot currently be assigned tothe three emended species concepts. In each case, the innervolutions of the whorls are absent or concealed, and so there is

TABLE 10—Tooth spacing and shaft ratio values corresponding to PC1 for the seven-landmark PCA in Figure 10. * ital.¼estimated tooth number from Table 7.

Specimen T#* PC1 A (mm) D (mm) S (mm) D/A S/A Species

USNM 22577 105 0.095 35.7 7.2 0.20 H. davisiiUSNM 22577 109 0.092 38.8 12.7 6.8 0.33 0.17 H. davisiiUSNM 22577 110 0.090 37.9 8.0 0.21 H. davisiiIMNH 30897 86 0.145 46.5 16.2 9.6 0.35 0.21 H. davisiiIMNH 30897 123 0.073 123.3 24.9 0.20 H. davisiiIMNH 37899 101 0.038 45.9 15.0 5.4 0.33 0.12 H. davisiiIMNH 36510 99 0.032 49.9 9.9 0.20 H. davisiiIMNH 36510 117 0.098 76.6 27.9 17.5 0.36 0.23 H. davisiiIMNH 36701 122 0.131 120.1 51.8 24.4 0.43 0.20 H. davisiiDG3 104 0.109 52.4 6.2 0.12 H. davisiiDG3 103 0.062 50.9 16.0 6.0 0.31 0.12 H. davisiiDG3 104 0.071 52.9 6.2 0.12 H. davisiiDG3 104 0.069 55.1 16.0 6.4 0.29 0.12 H. davisiiDG3 106 0.078 54.2 5.7 0.11 H. davisiiDG3 107 0.105 52.8 5.3 0.10 H. davisiiFortHall 98 0.110 46.2 15.0 7.2 0.32 0.16 H. davisiiFortHall 100 0.110 46.9 6.1 0.13 H. davisiiWAMAG 9080 85 0.143 31.4 11.4 3.3 0.36 0.11 H. davisiiWAMAG 9080 85 0.148 31.6 12.1 3.4 0.38 0.11 H. davisiiWAMAG 9080 87 0.146 33.2 13.4 3.6 0.40 0.11 H. davisiiUNMMPC 1002 92 0.061 28.0 8.7 3.1 0.31 0.11 H. davisiiPIN 1769/6 107 �0.095 41.7 9.0 5.5 0.22 0.13 H. bessoPIN 1769/6 109 �0.074 40.6 5.3 0.13 H. bessoTsNIGR 1/1865 88 �0.085 36.1 10.9 2.7 0.30 0.07 H. bessoTsNIGR 1/1865 100 �0.102 44.1 10.9 3.7 0.25 0.08 H. bessoTsNIGR 1/1865 106 �0.151 48.1 12.3 5.3 0.26 0.11 H. bessoTsNIGR 1/1865 123 �0.126 66.1 14.9 10.6 0.23 0.16 H. bessoIMNH 30900 100 �0.017 29.4 8.5 4.8 0.29 0.16 H. ergassIMNH 30900 102 0.028 31.3 4.5 0.14 H. ergassIMNH 30900 116 0.003 37.1 9.4 5.3 0.25 0.14 H. ergassIMNH 49382 167 �0.197 97.7 10.5 0.11 UnknownIMNH 49382 166 �0.194 95.8 13.0 0.14 UnknownIMNH 49382 168 �0.187 100.5 24.3 13.9 0.24 0.14 UnknownIMNH 49382 166 �0.205 104.9 13.9 0.13 UnknownIMNH 49382 171 �0.168 101.8 26.0 14.8 0.26 0.14 UnknownDG5 189 �0.012 144.1 27.6 0.19 UnknownDG5 190 0.007 148.2 43.5 30.2 0.29 0.20 UnknownDG5 190 �0.060 148.2 45.6 29.9 0.31 0.20 UnknownIMNH 14095 187 �0.262 154.6 15.2 0.10 UnknownIMNH 14095 188 �0.259 154.3 28.5 11.6 0.18 0.08 UnknownIMNH 14095 188 �0.258 153.5 28.5 11.0 0.19 0.07 Unknown


not a means to compare similar volutions with other smallerspecimens. A comparison of these five specimens to H. davisii

at similar volution height indicates that upper tooth and D/Aratios are distinct.

PMO A-33961.—The holotype of H. svalis from Norway isdistinguished from H. bessonowi as having narrow, non-abuttingteeth according to Siedlecki (1970). Review of this specimenshows that inadequate exposure and moldic preservation is thelikely reason that the teeth appear to be non-abutting, a conditionthat is unknown amongst other Helicoprion, and would result invery fragile teeth. Instead, we suggest that only the central,thickest part of the teeth is exposed or impressed in the matrix.The largest teeth of the specimen show an upper ratio of 0.23 for avolution height of approximately 72 mm, and the middle part ofthe tooth is elongate with parallel sides, reminiscent of H.bessonowi. Estimated tooth number, based on equations derivedfrom H. bessonowi, place the largest teeth at roughly T#150. Theshaft ratio is 1/10 for the smaller part of the whorl, but the shaft isnot apparent for the largest teeth of H. svalis. We therefore cannotmake a conclusive comparison to H. bessonowi but suspect thatH. svalis is a large member of this species. The largest volutionheight yet recorded among Russian H. bessonowi is TsNIGR 7/1865 (Karpinsky, 1899, fig. 22) with a height of 76 mm, which isvery close to PMO A-33961.

IMNH 14095.—A specimen from Idaho (Fig. 2.9) has sevenelongate teeth (A¼148 mm) with the shortest upper ratio (0.14)and D/A (0.18) of all specimens. Shaft height is 1/12 of volutionheight, also the lowest recorded among specimens. It is on trendwith H. bessonowi, but the apex of the teeth differs by having aflattened flange-like edge not observed in any other specimen.This variation may arise with increased volution, or it mayrepresent a separate taxon. In addition the inner concave surfaceof the root has a pair of prominent ridges not observed in otherspecimens.

IMNH 49382.—A large whorl from Idaho exposes only thefinal volution with a diameter of 56 cm, the largest whorl in ourdatabase. Conical teeth of the inner volutions are concealed belowconcretionary cemented limestone. Large teeth (A¼100 mm) havea very low upper ratio (0.22), with a modest shaft ratio (0.13). Theapices of exposed teeth are truncated, making D/A calculationsdifficult; however, the narrow teeth are very close together.Modeling this whorl after IMNH 30900 (H. ergassaminon) givesa maximum tooth number estimate of 170.

DG5.—A partial whorl from Idaho (Fig. 2.10) with even largerteeth (A¼148 mm) has an intermediate upper ratio (0.3), a highshaft ratio of 1/5, and intermediate D/A of 0.3. This combinationof traits is similar to both H. ergassaminon and H. davisii,although the narrow width of the tooth (B/A¼0.28) is moresimilar to the former.

Helicoprion mexicanus.—An unnamed and currently missingholotype of H. mexicanus has a volution height of more than 70mm but it lacks both the apex and base of root for all teeth. It ismost similar in appearance to IMNH 49382. Mullerreid (1945)suggested that tooth ornamentation and size were distinctive forthis new taxon, but crenulation and longitudinal striae are observedin many specimens, and absolute tooth size is rejected as a validtaxobase. We suggest that H. mexicanus be considered nomendubium until such time that new, more complete material fromMexico can describe characters unique to this species concept.

CONCLUSIONS

Morphometric analysis of Helicoprion whorls demonstrates awide range of size and shape variation in the genus, which helpsexplain the proliferation of species concepts over the pastcentury, especially given the rarity and poor preservation ofmany specimens. By controlling for ontogenetic variation, we

identify three distinct species concepts on the fundamental basisof tooth shape and tooth spacing in the adult whorl.

Helicoprion davisii is the dominant species in the PhosphoriaFormation of the western United States during Kungurian–Roadian time and it is the most common and widespread speciesof the genus. They have broad, widely spaced teeth with tallcutting surfaces.

Helicoprion bessonowi includes Artinskian whorls primarilyfrom Russia having closely spaced, narrow teeth with shortcutting surfaces.

Helicoprion ergassaminon is the least common species,known only from the Phosphoria Formation of Idaho. Closelyspaced teeth are narrow with tall cutting surfaces.

Several large specimens of Helicoprion, including theholotype of H. svalis, cannot be assigned to the three emendedspecies or be described fully until more complete material isrecovered. Results from this morphometric analysis providetaxobases that are frequently available in incomplete whorls,which should facilitate future diagnoses of specimens. Inaddition, allometric equations supplied from the analysis addvalue to previously collected Helicoprion by allowing unknownwhorl parameters, such as maximum diameter, tooth count andvolution number, to be modeled.

ACKNOWLEDGMENTS

We are grateful to museum collections and research personnelwho facilitated this project by providing images and informationabout Helicoprion specimens: H. A. Nakrem (Natural HistoryMuseum, University of Oslo); O. A. Lebedev (PalaeontologicalInstitute of the Russian Academy of Sciences); X.-H. Chen(Yichang Institute, China); A. Gishlick, A. Pradel (AmericanMuseum of Natural History); M. Brett-Surman, D. Bohaska(National Museum of Natural History); R. Dolbier (University ofNevada, Reno); R. Irmis (Natural History Museum of Utah); M.Currie (Museum of Nature, Ottawa); P. Isaacson (University ofIdaho, Moscow); D. Buehler (GSA, Boulder office), and D.George. We thank T. Clark and D. Carpenter (Monsanto Co.) forgranting access to active mines in Idaho. D. Sheets providedinitial assistance with tps software. We thank colleagues G. Hunt,R. Lockwood, J. Long and R. Troll for discussions that helped toadvance this project. Initial morphometry funded by ISUUndergraduate Research Council. We appreciate the constructivecomments from C. Ciampaglio and an anonymous reviewer aswell as the editorial staff that improved the final manuscript.

ACCESSIBILITY OF SUPPLEMENTAL DATA

Supplemental data deposited in Dryad repository: http://dx.doi.org/10.5061/dryad.q64g3.

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UNRAVELING SPECIES CONCEPTS FOR THE HELICOPRION …geology.isu.edu/Papers/TapanilaPruitt2013.pdf · teeth at the axis of the spiral (adaxial). Teeth gradually increase in size away