Comparative anatomy of selected basal ceratopsian dentitions

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  • Comparative anatomy of selected basalceratopsian dentitions

    Kyo Tanoue, Hai-Lu You, and Peter Dodson

    Abstract: The dental structure of basal ceratopsians is described. Evolutionary trends in maxillary and dentary teeth ofbasal ceratopsians include decrease and possible loss of enamel on the occluding side of tooth crowns, increase in the an-gle of wear facet, development of a prominent primary ridge and deep indentations on mesial and distal sides of the pri-mary ridge, and increase in tooth size in neoceratopsians. Premaxillary teeth in the basalmost ceratopsian Yinlong andbasal neoceratopsian Archaeoceratops oshimai exhibit wear facets and denticles along the carina, which imply use forfeeding. Maxillary and dentary teeth of basal ceratopsians were probably not as effective in feeding as those in ceratopsidsbecause of the relatively less prominent primary ridges. Some dental characters can be used to identify taxon and tooth po-sition of isolated basal ceratopsian teeth.

    Resume : La structure dentaire des ceratopsiens basaux est decrite. Parmi les tendances evolutives des dents maxillaires etdentaires des ceratopsiens basaux figurent une diminution et une eventuelle disparition de lemail sur la face occlusive descouronnes des dents, une augmentation de langle des facettes dusure, lapparition dune crete primaire eminente et deprofondes indentations sur les cotes mesial et distal de la crete primaire, ainsi que laugmentation de la taille des dentschez les neoceratopsiens. Les dents premaxillaires de Yinlong, le plus basal des ceratopsiens, et du neoceratopsien basalArchaeoceratops oshimai presentent des facettes dusure et des denticules le long de la care`ne, ce qui indique une fonctionalimentaire. Les dents maxillaires et dentaires des ceratopsiens basaux netaient probablement pas aussi efficaces pourlalimentation que celles des ceratopsides en raison de leurs cretes primaires relativement moins eminentes. Certains carac-te`res dentaires peuvent etre utilises pour determiner le taxon et la position de dents isolees de ceratopsiens basaux.

    [Traduit par la Redaction]

    IntroductionA unique feature of ceratopsids, the most derived ceratop-

    sians, is the form of mastication. Derived ceratopsian jawscontain large numbers of teeth, which are mesiodistallycompressed for close packing in dental batteries in whichfiles of teeth interlock both vertically and horizontally (Os-trom 1964; Dodson 1996). In ceratopsids the enamel is con-fined to the labial side of the maxillary tooth crown andlingual side of the dentary tooth crown. The dental batteriesexhibit vertical cutting planes, in which the lingual sides ofmaxillary teeth occlude against the labial sides of dentaryteeth. (Vertical shear of dental batteries is not found in anyother herbivorous vertebrate taxa.) In contrast, basal ceratop-sians lack dental batteries; instead, the closely spaced teethmerely erupt in a single horizontal line. Examination of the

    tooth structure of basal ceratopsians should help to elucidatethe evolutionary transformation of dentition within the Cera-topsia. However, compared with theropod dinosaurs, compa-rative studies on ornithischian dentitions, including those ofceratopsians, have rarely been done (Ostrom 1966; Weish-ampel 1984; Coombs 1990). Dental structure in the basalCeratopsia has attracted little attention other than a few thor-ough descriptions (Zhao et al. 1999; Makovicky and Norell2006; Godefroit and Lambert 2007). Diagnostic charactersof neoceratopsian teeth from North America were identifiedby Chinnery et al. (1998). Moreover, premaxillary teeth arefound only in basal ceratopsians, except for the Psittacosaur-idae, but the function of these teeth has not been discussedpreviously.

    Recent discoveries of skulls of basal Ceratopsia include Ar-chaeoceratops (Dong and Azuma 1997), Zuniceratops(Wolfe and Kirkland 1998), Chaoyangsaurus (Zhao et al.1999), Liaoceratops (Xu et al. 2002), Hongshanosaurus(You et al. 2003), Magnirostris (You and Dong 2003),Lamaceratops, Platyceratops (Alifanov 2003), Prenoceratops(Chinnery 2004), Auroraceratops (You et al. 2005), Yinlong(Xu et al. 2006), Xuanhuaceratops (Zhao et al. 2006),Yamaceratops (Makovicky and Norell 2006), and Cerasinops(Chinnery and Horner 2007), whose occurrences range fromthe Upper Jurassic (Oxfordian; Xu et al. 2006) to the UpperCretaceous (Campanian; Chinnery and Horner 2007). Subse-quent preparation of many of these specimens has revealednew information of the teeth and jaws, allowing us to con-duct a comparative study of the early evolution of the cera-topsian dentition.

    Received 3 December 2008. Accepted 20 June 2009. Publishedon the NRC Research Press Web site at cjes.nrc.ca on 12 August2009.

    Paper handled by Associate Editor H.-D. Sues.

    K. Tanoue.1 Canadian Museum of Nature, P.O. Box 3443, StnD, Ottawa, ON K1P 6P4, Canada.H. You. Institute of Geology, Chinese Academy of GeologicalSciences, 26 Baiwanzhuang Road, Beijing 100037, China.P. Dodson. School of Veterinary Medicine and Department ofEarth and Environmental Science, University of Pennsylvania,3800 Spruce Street, Philadelphia, PA 19104-6045, USA.1Corresponding author (e-mail: ktanoue@mus-nature.ca).

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    Can. J. Earth Sci. 46: 425439 (2009) doi:10.1139/E09-030 Published by NRC Research Press

  • Institutional abbreviationsAMNH, American Museum of Natural History, New

    York, N.Y., USA; CAGS, IG, IGCAGS, Institute of Geol-ogy, Chinese Academy of Geological Sciences, Beijing,China; CMN, Canadian Museum of Nature, Ottawa, Ontario,Canada; IVPP, Institute of Vertebrate Paleontology and Pa-leoanthropology, Beijing, China; PKUP: Peking UniversityPaleontological Collections, Beijing, China.

    Materials and methodsDentitions of nine basal ceratopsian genera were exam-

    ined and measured. Phylogenetic relationships among thesegenera are shown in Fig. 1. In this study, Yinlong andChaoyangsaurus are considered the basalmost Ceratopsia,which are here defined as outgroup to Psittacosauridae +Neoceratopsia (Xu et al. 2006). Psittacosaurus and Hon-gshanosaurus compose Psittacosauridae. Hongshanosaurusis a newly discovered psittacosaurid that differs from Psitta-cosaurus in having greater preorbital length relative to basalskull length (approximately one half) than Psittacosaurus(less than 40%; Sereno 2000) and elliptical external naris,orbit, and infratemporal fenestra, whose long axes are ori-ented posterodorsally (You et al. 2003; You and Xu 2005).The mandible of Hongshanosaurus is deeper than the man-dible in any species of Psittacosaurus (Tanoue et al. inpress). The adult specimen (IVPP V12617) examined in thisstudy is attributed to Hongshanosaurus since the preorbitallength is approximately half of the basal skull length andthe orbit and infratemporal fenestra are elliptical, with thelong axes sloping anteroventrally as in the holotype (IVPPV12704). IVPP V12617 is unlikely to have undergone dis-tortion, for the skull and mandible are symmetrical and thedelicate palatal structure is preserved intact (Dodson et al.in press). In this study Liaoceratops, Archaeoceratops,Auroraceratops, Leptoceratops, and Protoceratops representthe basal Neoceratopsia. Dental terminology used is shownin Fig. 2.

    Description

    YinlongYinlong (IVPP V14530) has three premaxillary teeth on

    each side (Fig. 3A; Xu et al. 2006). The well-preserved leftfirst and right first and second premaxillary teeth are largerthan the maxillary teeth in labial view. The lengths of thecrown and root are greater than the width. The tooth crownbase widens for the first 2 to 3 mm apically in labial andmesial views, then tapers toward the tip (Fig. 3B). Most ofthe occlusal surface is flat, except at the base where it is lin-gually concave in the first teeth on both sides (Fig. 3C). Thelabial side is mesiodistally convex as in Chaoyangsaurus.Xu et al. (2006) reported serrations only on the distal carinaof the right second tooth, but the mesial carina is also ser-rated (Fig. 3B). The serrated region is longer and moreprominent on the distal carina than on the mesial carina.

    The left maxillary tooth row of the type specimen of Yin-long downsi is not fully exposed. Thirteen teeth make up thelabially convex right maxillary tooth row (Xu et al. 2006).The diastema between the right third premaxillary and thefirst maxillary teeth measures approximately 4 mm. The

    right tooth row is 59 mm long. The maxillary teeth aretightly packed with slight overlap of the crowns. The distalend of each tooth is lateral to the mesial end of the subse-quent tooth. The unworn crown of a left maxillary tooth isovate (Figs. 3D, 3E). The low primary ridge is wide at thebase and tapers apically toward the crown apex. The baseof the primary ridge is confluent with the low cingulum. On

    Fig. 1. Phylogenetic tree of the Marginocephalia compiled fromYou et al. (2003, 2005), Makovicky and Norell (2006), Xu et al.(2006), and Chinnery and Horner (2007).

    Fig. 2. Schematic diagram of a ceratopsian tooth with dental termi-nology. (A) Dentary tooth in lingual view. (B) Dentary tooth in oc-clusal view.

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  • both mesial and distal sides of the primary ridge are shallowdepressions, bounded basally by the cingulum. Secondaryridges develop within both depressions. The secondaryridges on the mesial indentation are much longer and moreprominent than those on distal depression. The secondaryridges extend toward the primary ridge near the occlusalmargin of the crown, but curve to be subparallel to the pri-mary ridge dorsally, near the tooth base.

    Dentary teeth are not exposed, except for the rostral partof the right tooth row and the partial crown of an eruptingtooth in the left dentary (Figs. 3F, 3G). This partial crowncan be observed ventral to the middle of the dentary toothrow on the medial side of the left dentary. It is 5.7 mmwide and 6.0 mm high. The exposed portion is triangular,and its dorsal margin bears denticles. The primary ridge ispoorly developed and only slightly wider than the secondaryridges. Secondary ridges are short. Four and five secondaryridges develop mesial and distal to the low primary ridge,respectively, supporting denticles on the occlusal edges ofthe tooth.

    ChaoyangsaurusThe dentition of Chaoyangsaurus is relatively well pre-

    served in the holotype (Fig. 4A; IGCAGS V371). Two teethcompose the premaxillary tooth row (Figs. 4A, 4B).Although the ventral margin of the premaxilla just posteriorto the second premaxillary tooth is not preserved on eitherside, it is unlikely that this specimen bore a third premaxil-lary tooth, since it lacks any evidence of an alveolus. Thetwo peg-shaped premaxillary teeth are close to each other,with little or no space between them (Zhao et al. 1999).The root is cylindrical. The base of the premaxillary toothcrown is slightly wider than the root. The crown tapers api-cally. It is enameled on both the labial and lingual surfaces.Along the occlusal margin, the carina extends in a mesiodis-tal direction. The left second premaxillary tooth preserves asmall wear facet on the lingual side of the tip, which issloped ventrolabially.

    The left maxillary tooth row preserves seven distal teeth(Figs. 4C, 4D). Mesial teeth are missing due to poor preser-vation of the rostral portion of left maxilla. The preservedtooth row measures 36 mm in length. Nine teeth composethe 37 mm long right maxillary tooth row, from which thesecond tooth is missing. The maxillary tooth rows areslightly concave labially (Fig. 4C). There is a slight overlapin the maxillary teeth, with the distal end of a tooth labiallyoverlapping the mesial end of the subsequent tooth in oc-clusal view. The maxillary tooth crown is ovoid in labial(Fig. 4D) and lingual views. The length of the maxillarytooth crown is greater than the width. The height of thecrown is greater than the length in the mesial teeth, andsmaller in the distal teeth (Zhao et al. 1999). In occlusalview, the labial surface is more convex than the lingual sur-face. The occlusal edge of an unworn, erupting maxillarytooth preserves about 10 denticles. A low vertical ridge ex-tends at the midline on the labial surface (Fig. 4D). Thisridge probably represents a remnant of the primary ridge(Zhao et al. 1999). However, this is uncertain due to thepreservation of the specimen. On both sides of this lowridge are shallow depressions (Fig. 4D; Zhao et al. 1999:Fig. 4B). A low cingulum separates the crown from the

    Fig. 3. Yinlong downsi (IVPP V14530). (A) Skull and mandible inright lateral view. (B) Left second premaxillary tooth in lingualview. (C) Left first premaxillary tooth in mesial view. (D, E)Photograph (D) and line drawing (E) of left maxillary tooth in la-bial view. (F, G) Photograph (F) and line drawing (G) of left den-tary tooth in lingual view. cin, cingulum; den, denticles; mxt,maxillary teeth; pmxt, premaxillary teeth; pr, primary ridge; sr,secondary ridge; wf, wear facet. Scale bars = 5 mm in B, C, D, andF.

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  • cylindrical root, which is only slightly narrower than thecrown (Zhao et al. 1999). Enamel covers both sides of themaxillary teeth. The enamel thickness appears to be about

    the same on both the labial and the lingual sides (Zhao etal. 1999). The wear facet is sloped.

    The dentary tooth rows are concave labially (Fig. 4E).The right tooth row, bearing 11 teeth, is 46 mm long. Theleft tooth row, which is 39 mm long, preserves only nineteeth. Lack of an alveolus distal to the last tooth indicatesthat the distal teeth of the left tooth row are completely pre-served. The length of the dentary teeth increases distally upto the seventh tooth in the left tooth row and eighth tooth inthe right tooth row, and decrease to the last tooth. Thelength is greater than the width, as on the maxillary teeth.The tooth crown is round or oval in lingual view (Fig. 4F).The occlusal margin bears several denticles. The denticlesare supported by short secondary ridges extending ventrallytoward the tooth base. The wear facet is concave apicola-bially. Fully exposed dentary teeth including the root showthat these teeth are single-rooted.

    PsittacosaurusThe dentitions of five of the eight named species of Psit-

    tacosaurus, P. lujiatunensis, P. major, P. mongoliensis, P.neimongoliensis, and P. sinensis, were examined in thisstudy. None of the specimens studied possess premaxillaryteeth. The maxillary and dentary teeth are single-rooted.There is only one replacement tooth in each tooth position,as in other basal ceratopsians. Although the maxillary anddentary teeth are closely spaced, there is a small gap be-tween the basal portions of the tooth crowns in some speci-mens. The crown length is greater than the width in allteeth.

    The tooth rows of most species are nearly straight in oc-clusal view, but in P. sinensis and P. lujiatunensis, they areconcave labially (Sereno and Chao 1988; Zhou et al. 2006).The maxillary tooth crowns are aligned at a shallow angle tothe long axis of the tooth row. The distal end of each crownlabially overlaps the mesial end of the subsequent crown(Sereno et al. 1988; Sereno 1990; Zhou et al. 2006; You etal. 2008). Although no fully exposed unworn maxillarytooth crown was observed in this study, the crown is ovalin labial view in P. meileyingensis and P. mongoliensis (Se-reno et al. 1988). A low primary ridge separates the mesialand distal lobes on the labial side of the crown. It tapers to-ward the apex, except for P. lujiatunensis (PKUP V1053)and P. sinensis (IVPP V738), in which the widths of the pri-mary ridges remain constant (Figs. 5A, 5B). In P. neimongo-liensis (Fig. 5C; IVPP 12-0888-2) and P. major (Fig. 5D;CAGS-IG-VD-004), long deep grooves are present on bothsides of the primary ridge (You et al. 2008). The mesiallobe is flat and broader than the distal lobe. The distal lobeis swollen and can be more prominent than the primaryridge. Several secondary ridges, separated from each otherby shallow longitudinal grooves, extend toward the toothbase from the occlusal margins of the two lobes. They donot reach the cingulum. The mesial lobe displays more sec-ondary ridges than the distal lobe. Denticles are present atthe occlusal ends of the secondary ridges and can be ob-served in relatively unworn crowns (Fig. 5B; Sereno andChao 1988; Sereno et al. 1988). In relatively well-preservedmaxillary teeth, enamel layers cover both labial and lingualsurfaces of the crown, with the thicker layer on the labialside (Sereno and Chao 1988; Sereno 1990; Averianov et al.

    Fig. 4. Chaoyangsaurus youngi (IGCAGS V371). (A) Skull in rightlateral view. (B) Left premaxillary teeth in labial view. (C) Leftmaxillary tooth row in occlusal view. (D) Left maxillary tooth rowin labial view. (E) Mandible in dorsal view. (F) Left dentary teethin lingual view. dt, dentary teeth. See Fig. 3 for other abbreviations.Scale bars = 5 mm in B, C, D, and F.

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  • 2006). The wear facet on the lingual side of the crown isoblique rather than vertical.

    The number of teeth in the dentary tooth row usuallyequals the number in the maxillary tooth row. The size ofthe dentary teeth is comparable to that of the maxillaryteeth. Unworn dentary tooth crowns are circular to oval inoutline in lingual view (Sereno and Chao 1988; Averianovet al. 2006; You et al. 2008). The crown is symmetrical,with the bulbous primary ridge in the middle of the lingualsurface (Fig. 5E). The primary ridge is more prominent onthe dentary teeth than on the maxillary teeth (Sereno 1990).It narrows toward the apex, as in the maxillary tooth crown.Mesial and distal lobes are of about the same size. Severalsecondary ridges supporting the denticles along the occlusalmargin are present on both lobes, but they do not reach thebasal half of the lobe (Fig. 5E). These short secondaryridges are visible on the labial surfaces of relatively unworndentary teeth (Fig. 5C). The largest number of denticles, 10in each lobe, has been reported in P. xinjiangensis (Serenoand Chao 1988). The enamel layer is thicker on the lingualside than on the labial side of the tooth (Sereno and Chao1988; Sereno 1990; Averianov et al. 2006). The wear facetis sloped to occlude against that of maxillary tooth.

    HongshanosaurusThe adult specimen of Hongshanosaurus (IVPP V12617)

    was examined. As in all other psittacosaurids, Hongshano-saurus lacks premaxillary teeth (Fig. 6A). Juvenile specimenwas not included in this study.

    The maxillary tooth rows are poorly preserved comparedwith the dentary tooth rows in IVPP V12617. The maxillarytooth row probably consists of eight or nine teeth. Five func-tional teeth are preserved in both maxillary tooth rows(Fig. 6A). Several replacement teeth are partially exposed,with one for each tooth position. The length of the maxillarytooth crown is greater than its width. The primary ridge islow and widens toward the tooth base (Fig. 6B). The pri-mary ridge is shifted distally, resulting in a wider mesiallobe than distal lobe. Short vertical secondary ridges extendfrom the occlusal margin of the crown. At least three secon-dary ridges are present on the mesial lobe, and one secon-dary ridge on the distal lobe. In unworn maxillary teeth, theocclusal margin bears denticles supported by the secondaryridges. The enamel covers the labial and lingual surfaces ofthe crown, but it is thicker on the labial side (You and Xu2005).

    Both left and right dentary tooth rows bear 10 teeth andare 38 mm and 41 mm long, respectively (Fig. 6C). Theyextend along the medial side of the dentaries. The toothrows are slightly concave labially in occlusal view. In mostdentary teeth, the distal end of the tooth crown labially over-laps the mesial end of the subsequent crown. Unworn den-tary tooth crowns are round or oval in lingual view(Fig. 6D). The bulbous primary ridge of a dentary toothcrown is widest at the base and tapers dorsally. It is moreprominent than that of a maxillary tooth, as in Psittacosau-rus (Fig. 6D; Sereno 1990). The primary ridge separates themesial and distal lobes of the crown. It generally extendsnear the midline, but it is shifted distally in some teeth. Asmany as eight mesial and six distal denticles are present. Asecondary ridge stretching toward the tooth base from the

    Fig. 5. Psittacosaurus. (A) Right maxillary teeth of Psittacosauruslujiatunensis (PKUP V1053) in labial view. (B) Right maxillarytooth row of P. sinensis (IVPP V738) in labial view. (C) Left max-illary and dentary teeth of P. neimongoliensis (IVPP 12-0888-2) inlabial view. (D, E) P. major (CAGS-IG-VD-004). (D) Right maxil-lary tooth row in labial view. (E) Right dentary tooth row in lingualview. Scale bars = 5 mm. pr, primary ridge; sr, secondary ridge.

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  • occlusal margin of the dentary tooth crown supports eachdenticle. The secondary ridges develop only in the apicalthird of the crown. Both labial and lingual surfaces of thecrown are enameled, with the enamel layer being thicker onthe lingual side (You and Xu 2005). The wear facet is flatand slopes labially.

    LiaoceratopsThe dentition of Liaoceratops was examined in three

    specimens: IVPP V12738, IVPP V12633, and CAGS-IG-VD-002 (Xu et al. 2002; You et al. 2007: Figs. 1C, 2C).The holotype (IVPP V12738) is an adult skull and the othertwo are juvenile skulls, with CAGS-IG-VD-002 being thesmallest of the three specimens.

    Two premaxillary teeth are preserved on both sides in theholotype (Fig. 7A). Sockets are present rostrolateral to theright first tooth and caudolateral to the second right tooth,which may represent alveoli for additional premaxillaryteeth, but no alveoli are preserved for more than two teethin the left premaxillary tooth row. In both juvenile speci-mens the premaxillary tooth rows each contain three teeth(You et al. 2007). The premaxillary teeth are peg-shapedwith a cylindrical root (Fig. 7B). Both labial and lingualsides of the premaxillary tooth crown are covered with en-amel. The carina extends along the mesiodistal axis of thecrown. The first right premaxillary tooth exhibits the ser-rated distal carina reported by Makovicky and Norell(2006). Both the mesial and distal carinae of the left secondtooth display denticles, as in Yamaceratops and a new spe-cies of Archaeoceratops (Fig. 7B; Makovicky and Norell2006; You et al. in press). The serrated region is longer onthe distal carina than on the mesial carina. No wear facetsare present on any premaxillary tooth.

    The left and right maxillary tooth rows of the holotype(adult) skull consist of 11 and 12 teeth, respectively. Allmaxillary tooth rows of juvenile specimens contain 10 teeth.No unworn tooth is preserved in the holotype. Enamel cov-ers the labial surface of the crown, but not the lingual sur-face. The primary ridge is shifted distally (Fig. 7C) and isconfluent with the cingulum at the base. The primary ridgesare poorly differentiated from secondary ridges on the max-illary teeth of the juvenile skulls. Two or three secondaryridges are located mesial to the primary ridge and at leastone lies distally. In occlusal view, the maxillary teeth showsteep but not vertical wear facets.

    At least 12 teeth are preserved in the left dentary toothrow of the holotype (Fig. 7D). The right dentary tooth rowconsists of 15 teeth. Only the lingual surface of the ovalcrown is covered with enamel. The primary ridge is mesialto the midline in lingual view (Fig. 7E). The primary ridgesin the dentary teeth of juvenile mandibles are only slightlymore prominent than the secondary ridges, as is also thecase in the maxillary teeth. As many as six and seven secon-dary ridges develop mesial and distal to the primary ridge,respectively. The wear facets are concave dorsolabially in

    Fig. 6. Hongshanosaurus houi (IVPP V12617). (A) Skull in leftlateral view. (B) Left maxillary teeth in labial view. (C) Mandiblein dorsal view. (D) Left dentary teeth in lingual view. dt, dentaryteeth. See Fig. 3 for other abbreviations.

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  • all three specimens. Some dentary teeth exhibit wear facetswith shelf structure.

    ArchaeoceratopsWell-preserved teeth can be observed in the holotype of

    Archaeoceratops oshimai (Fig. 8A; IVPP V11114). The den-tition of a new species of Archaeoceratops will be describedby You et al. (in press). Three teeth compose the premaxil-lary tooth row (Figs. 8A, 8B). All three right premaxillaryteeth are preserved, but only the second premaxillary toothremains on the left side. In occlusal view, the second toothis slightly labial to the first tooth and the third tooth isslightly lingual to the first tooth. Both labial and lingualsides of the crown are covered with enamel (You and Dod-son 2003). The premaxillary teeth are peg-shaped. In labialview, the premaxillary teeth first narrow ventrally from thebase, then widen, and the crowns then taper to a narrowapex. The length is greater than the width. The carina ex-tends mesiodistally along the occlusal edge. No denticleswere observed along the carina, unlike in the new speciesof Archaeoceratops, Liaoceratops, and Yamaceratops (Ma-kovicky and Norell 2006; You et al. in press). The right sec-ond and third teeth exhibit ventrolabially sloping, oval wearfacets on the lingual side of the apex of the crown (Fig. 8B).When the skull and mandible are articulated, the premaxil-lary tooth rows are lateral to the posterior half of the dorsalmargin of the predentary (Fig. 8A). Thus, the lingual surfa-ces of the premaxillary crowns will have contacted the outerface of the lower beak.

    The left maxillary tooth row comprises 13 teeth and is46 mm long. Fourteen teeth compose the 47 mm long rightmaxillary tooth row. In occlusal view, the tooth rows arelenticular, widest a little distal to the middle of the toothrows and tapering both mesially and distally. Enamel coversboth surfaces of the crown. Unworn crowns are oval in la-bial view. Unlike the new species of Archaeoceratops, inwhich the maxillary teeth lack a primary ridge, maxillaryteeth of A. oshimai show a primary ridge (Fig. 8C; You etal. in press). However, it is only slightly broader than thesecondary ridges. The primary ridge is slightly distal to themidline of the crown, and merges basally with the cingulum.In some teeth, the primary ridge curves ventrodistally. Threesecondary ridges are present on each side of the primaryridge, forming denticles along the occlusal margin(Fig. 8C). The secondary ridges stretch dorsally toward thetooth base, and toward the primary ridge. Roots of someteeth are partially exposed. They narrow slightly toward theroot apex. Replacement teeth for the left 10th and 12th, andright eighth and 10th tooth positions are partially exposed onthe medial surface of the maxilla. Only one replacementtooth is present for each tooth position. The wear facets ofmaxillary teeth are steeply inclined.

    The labially concave dentary tooth rows are aligned alongthe medial borders of the dentaries in occlusal view. Four-

    Fig. 7. Liaoceratops yanzigouensis (IVPP V12738). (A) Skull inleft lateral view. (B) Left premaxillary teeth in labial view. (C) Leftmaxillary teeth in labial view. (D) Mandible in dorsal view.(E) Right dentary teeth in lingual view. dt, dentary teeth. See Fig. 3for other abbreviations. Scale bars = 5 mm in B, C, and E.

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  • teen teeth compose the 55 mm long left tooth row. The righttooth row comprises 14 teeth and is 53 mm long. Both thelabial and lingual sides of the dentary teeth are coveredwith enamel, as is the case on the maxillary teeth. Each den-tary preserves two or three small rostral teeth, which are iso-lated from each other and from the closely packed toothrows (Fig. 8D). When the skull and mandible are articulated,the first two dentary teeth are mesial to the first maxillarytooth. The first maxillary tooth occludes with the third den-tary tooth. In lingual view, the subtriangular crowns of thefirst two teeth lack both primary and secondary ridges andonly show denticles along the apical margins. The thirdtooth also displays a subtriangular crown outline, but thereare secondary ridges on the lingual surface. The fourth toothpreserves a wear facet, but it lacks the primary ridge. Theprimary ridges are preserved on teeth distal to the fifth toothon both dentaries. With the exception of the third tooth, theteeth of the closely packed tooth row exhibit oval crowns.The primary ridges in teeth 514 extend vertically in themesial third of the crown (Fig. 4E). However, the primaryridge is only slightly more prominent than the secondaryridges, which converge basally toward the primary ridge.Three secondary ridges are located mesial and four distal tothe primary ridge. In the new species of Archaeoceratops,the dentary teeth lack primary ridges (You et al. in press).The cingulum is poorly developed in A. oshimai. Unwornerupting teeth display more denticles than the number ofsecondary ridges. Only one replacement tooth is present foreach tooth position. The wear facet is sloped as in maxillarytooth.

    AuroraceratopsThe teeth of Auroraceratops (Fig. 9A; IG-2004-VD-001)

    are the largest among the known Chinese basal ceratopsians.The premaxillary teeth are better preserved on the left side(Fig. 9B). Three or four left premaxillary teeth are present.There is a shallow depression mesial to the preserved teeth,which possibly is an alveolus for the first tooth. The secondand third teeth are aligned along the lateral margin of the pre-maxilla in occlusal view, with the first alveolus and fourthtooth lingual to them. The length and width of the secondtooth are 7.2 mm and 5.2 mm, respectively. The length ofthe third tooth is 7.6 mm, and the width is 4.5 mm. Theheights of the second and third teeth are approximately7 mm and 8 mm, respectively, in labial view. The third peg-shaped tooth is the best preserved in this specimen. The bluntmesial and distal carinae extend lingually toward the tip,unlike in Liaoceratops. As a result, the tip of the third pre-maxillary tooth is situated on the lingual side of the crown inocclusal view. Serrations are not present along the carina.Only three premaxillary teeth are present in the right premax-illa, represented by two alveoli and the root of the third pre-maxillary tooth. The second alveolus is slightly labial tothe first alveolus and to the third tooth.

    Fig. 8. Archaoeceratops oshimai (IVPP V11114). (A) Skull andmandible in right lateral view. (B) Right premaxillary tooth row inlingual view. (C) Left maxillary teeth in labial view. (D) Rostraldentary teeth in lingual view. Numbers indicate tooth positions.(E) Caudal dentary teeth in lingual view. dt, dentary teeth. SeeFig. 3 for other abbreviations. Scale bars = 5 mm in BE.

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  • The axes of the maxillary teeth slope ventrodistally(Fig. 9C). Thirteen maxillary teeth form the 65 mm longleft tooth row. The right maxillary tooth row consists of 12teeth. It is 65 mm long. Both the length and width of themaxillary teeth increase distally, except for the distalmosttwo or three teeth in both maxillary tooth rows. The ovatecrown of the left third tooth displays an unworn labial sur-face. The primary ridge is situated distal to the midline(Fig. 9C). It widens toward the tooth base and is basallyconfluent with the cingulum. Mesial and distal to the pri-mary ridge are shallow depressions. The concave surface ofthe mesial lobe extends closer to the tooth base than doesthe distal one. Three secondary ridges are located mesialand two distal to the primary ridge. They stretch toward thebase from the occlusal margin of the crown but are onlyabout half the length of the primary ridge. Enamel coversonly the labial surfaces of the maxillary teeth, but this maybe due to preservation. Replacement teeth are not exposed inlingual view. Although small spaces are found between adja-cent roots, adjacent crowns are in contact and form a tightlypacked tooth row. The tooth height decreases mesially anddistally from tooth position 8 in lingual view. The wearfacet is nearly vertical.

    The dentary teeth are poorly preserved. Twelve teeth formthe dentary tooth row on either side (Fig. 9D). The lengthand the width of the teeth increase up to the seventh oreighth tooth and then decrease distally in both tooth rows.In labial view, the better preserved left tooth row is highestat the middle, with the seventh and eighth teeth being thehighest. In dorsal view, the tooth rows are lingually convex(Fig. 9D). No unworn teeth are present. The roots of the firstfour teeth in the left tooth row are longer than they are wide.The crown is asymmetrical in lingual view, with the primaryridge developing mesial to the midline (Fig. 9E). Theprimary ridge is confluent with the cingulum. Three secon-dary ridges lie distal to the primary ridge in relatively well-preserved teeth. The secondary ridges do not reach thecingulum. Enamel is preserved only on the labial sides ofthe dentary teeth. The presence of an enamel layer on the lin-gual surface is uncertain because of poor preservation. Thewear facet appears to be vertical.

    Two sockets are located mesial to the right dentary toothrow (Fig. 9D). The diameters of these sockets are approxi-mately 2 to 3 mm, with the interval between them of aboutthe same length. These may be alveoli for small mesial teeththat were set off from each other and from the more distaltooth row. They would have fit in the diastema between pre-maxillary and maxillary teeth.

    LeptoceratopsThe description of the dentition of Leptoceratops is based

    primarily on CMN 8889 (Fig. 10A). Isolated teeth of Lepto-ceratops (CMN 8889, CMN 52781) are single-rooted andamong the largest in basal ceratopsians. Leptoceratops lackspremaxillary teeth (Sternberg 1951).

    Fig. 9. Auroraceratops rugosus (CAGS-2004-IG-VD-001).(A) Skull in left lateral view. (B) Left premaxillary teeth in labialview. (C) Left maxillary teeth in labial view. (D) Mandible in dorsalview. (E) Left dentary teeth in lingual view. dt, dentary teeth; r, root;s, sockets (possibly alveoli). See Fig. 3 for other abbreviations.

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  • Both maxillary tooth rows of CMN 8889 consist of 17teeth. The left tooth row is 157 mm long and the right toothrow 160 mm. In occlusal view, the tooth rows are slightlyconcave labially. The width of the tooth crown exceeds thelength (Sternberg 1951). The primary ridge is shifted distallyand is curved in some teeth (Fig. 10B). Some primary ridgesshow striations parallel to the ridge axis. At least three sec-ondary ridges are situated mesial to, and one distal to, theprimary ridge. The secondary ridges are subparallel to theprimary ridge, and some of them turn away from the pri-

    mary ridge as they approach the tooth base. They are rela-tively long in Leptoceratops, but do not reach the cingulum.The cingulum is well developed. In some maxillary teeth,the cingulum is notched in the middle (Fig. 10B). The baseof the primary ridge does not quite extend to the lingualedge of the cingulum (Godefroit and Lambert 2007). In oc-clusal view, the wear facet is steeply inclined in most teeth,but vertical in a few teeth in the second quarter of the toothrow. The left 14th and 16th and the right 14th maxillaryteeth exhibit narrow horizontal shelves at the base of thesubvertical wear facets.

    Sixteen teeth compose both dentary tooth rows(Fig. 10C). The tooth row is slightly concave labially in oc-clusal view, as in the maxillary tooth row, but the toothrows of Leptoceratops are much less curved than those ofother basal neoceratopsians. The width of the dentary toothis greater than the length. The primary ridge is mesial tothe midline of the dentary tooth crown, as in other basalneoceratopsians (Fig. 10D). It is more prominent than thatof a maxillary tooth and widens basally, just dorsal to thejunction with the cingulum. The primary ridge is confluentwith the cingulum, unlike the condition in maxillary teeth.Up to four mesial and three distal secondary ridges arepresent. The secondary ridges develop in a way that isunique among the basal ceratopsians. Secondary ridges me-sial to the primary ridge converge toward the primary ridge,whereas those distal to the primary ridge are parallel to it.These features of the junction between the primary ridgeand the cingulum and the secondary ridges are illustrated inBrown (1914, fig. 2) as those of a maxillary tooth; however,it appears to be a dentary tooth. Similarly, the dentary toothillustrated by Brown (1914, fig. 6) appears to be a maxillarytooth. As on the maxillary teeth, wear facets are steeply in-clined or vertical (Sternberg 1951). At the base of this wearfacet, there is a horizontal shelf, as is also seen in otherNorth American basal neoceratopsians and some Asianforms, including Udanoceratops and Archaeoceratops(Sternberg 1951; Kurzanov 1992; Chinnery and Weishampel1998; Chinnery 2004; Chinnery and Horner 2007; You et al.in press). It is difficult to confirm the distribution of enamelin the teeth of CMN 8889 since the entire specimen isstained black. However, isolated teeth of CMN 8888, whosedentine portion is brown, show distinct enamel layers onboth labial and lingual surfaces of the crown.

    ProtoceratopsTwo peg-shaped teeth are present in each premaxilla in

    Protoceratops (Figs. 11A, 11B; Gregory and Mook 1925;Brown and Schlaikjer 1940). The second tooth is slightly la-bial to the first tooth. There is little or no space in betweenthe two teeth (Fig. 11B). The long cylindrical root is nearlyround in horizontal section. Among the observed premaxil-lary teeth, the right first tooth of AMNH 6433 is the largestwith a width of 7.6 mm.

    The maxillary tooth row is labially concave mesially, butis straight distally. There are up to 15 maxillary tooth posi-tions (You and Dodson 2004). Each maxillary tooth crownis oval in labial view. The primary ridge is prominent(Fig. 11C). It is shifted distally from the midline. The pri-mary ridge widens at the base and merges with the cingu-lum. In some teeth, it is sinuous. The cingulum mesial to

    Fig. 10. Leptoceratops gracilis (CMN 8889). (A) Skull in left lat-eral view. (B) Left maxillary teeth in labial view. (C) Right man-dibular ramus without predentary in dorsal view. (D) Left dentaryteeth in lingual view. dt, dentary teeth; n, notch. See Fig. 3 forother abbreviations.

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  • the primary ridge is often basal to the distal cingulum. Theindentations on both mesial and distal lobes of the crown aredeep. The secondary ridges on both lobes extend from theocclusal margin toward the basal part of the primary ridge.Isolated teeth are single-rooted and show longitudinalgrooves on mesial and distal surfaces of the root about one-third the width of the root (Brown and Schlaikjer 1940).These grooves would have accommodated the distal and me-sial edges of preceding and subsequent teeth, respectively.Enamel covers the labial surface of the crown and the apicalhalf of the lingual surface of the crown; the lingual enamelband is only preserved on unworn teeth. The wear facet issteeply inclined or vertical.

    As in maxillary teeth, isolated dentary teeth are single-rooted, with longitudinal grooves extending on mesial anddistal surfaces of the root. Unworn dentary tooth crowns areoval in lingual view. The primary ridge of the crown is situ-ated mesially (Fig. 11D). It is straight or slightly curved andflares toward the base. The primary ridge merges with thecingulum at the base. The bases of some primary ridges arestriated. The cingulum often extends distobasally. The inden-

    tations mesial and distal to the primary ridge are shallowerthan those of maxillary tooth crowns. At least four secondaryridges are present both mesial and distal to the primaryridge. These secondary ridges extend obliquely toward thetooth base and toward the primary ridge. There is only onereplacement tooth in each tooth position. The lingual surfa-ces, and the apical half of the labial surface of unworn toothcrowns, are enameled. The wear facet is nearly vertical.

    Discussion

    Premaxillary dentitionPremaxillary teeth of basalmost ceratopsians and some

    basal neoceratopsians share some morphological featureswith those of Pachycephalosauria, the sister-taxon of Cera-topsia, including subconical crowns with oval cross-sectionand enamel on both the labial and lingual sides of the crown(Maryanska 1990). On the other hand, premaxillary toothcrowns in pachycephalosaurians are set off from the roots,whereas crowns of basal ceratopsian premaxillary teeth areonly slightly longer and wider than the root (Figs. 3B, 7B,8B). Premaxillary tooth crowns of pachycephalosaurians arealso recurved apicodistally, but those of basal ceratopsiansare symmetrical in labial view.

    Wear facets on the mesial surface of the apex of premax-illary tooth crowns in Yinlong and Archaeoceratops oshimai(Figs. 3C, 8B) indicate that they were used during feeding,probably as they contacted against the lower beaks (F. Var-riale, personal communication, 2008). Some premaxillaryteeth of Yinlong, Liaoceratops, Yamaceratops, and a newspecies of Archaeoceratops (Figs. 3B, 7B; Makovicky andNorell 2006; Xu et al. 2006; You et al. in press) show den-ticles along the carina. In Yinlong and Liaoceratops, a longerportion of the distal carina bears denticles than the mesialcarina (Figs. 3B, 7B), suggesting that the premaxillary teethwere used to bite an object at an angle oblique to the verticalaxis of the tooth (D. DAmore, personal communication,2008), associated with the motion of pulling the loweredhead posterodorsally or the lifted head posteroventrally.

    Maxillary and dentary dentitionMaxillary teeth of Yinlong resemble those of pachycepha-

    losaurs in that the low primary ridge of maxillary tooth iswide at the base (Figs. 3D, 3E). Crowns of unworn dentarytooth of Yinlong and mesial dentary teeth of A. oshimai aresubtriangular as in pachycephalosaurians (Figs. 3F, 3G, 8D).All other maxillary and dentary teeth of basal ceratopsiansexamined, however, exhibit ovate crowns. Additionally, thelabial surface of pachycephalosaurian maxillary toothcrowns is concave vertically (Sues and Galton 1987; Mar-yanska et al. 2004), in contrast to the convex labial surfaceof ceratopsian tooth crowns (Chinnery et al. 1998).

    The evolutionary transition toward the ceratopsid dentalstructure can also be observed in basal neoceratopsians. Inmost basal ceratopsians, enamel covers both sides of maxil-lary and dentary teeth. In Chaoyangsaurus, enamel layers onboth sides of maxillary and dentary teeth exhibit almost thesame thickness, but in other basal ceratopsians with toothcrowns enameled on both sides, the enamel layer on the la-bial side of the maxillary tooth crown and the lingual side ofthe dentary tooth crown is thicker than that on the opposing

    Fig. 11. Protoceratops andrewsi. (A) Skull and mandible in rightlateral view (AMNH6425). (B) Right premaxillary tooth row in lin-gual view (AMNH 6433). (C) Right maxillary tooth row in labialview (AMNH 6433). (D) Left dentary tooth row in lingual view(AMNH 6460). dt, dentary teeth. See Fig. 3 for other abbreviations.Scale bars = 5 mm in BD.

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  • side. Only the labial side of the maxillary and the lingualside of the dentary teeth in Auroraceratops and Liaocera-tops are enameled, but it may be due to poor preservationof the known specimens. The thickness of enamel layers inLeptoceratops is uncertain. In contrast, ceratopsid teeth de-velop the enamel only on the non-occluding side. Havingthe non-occluding side of the tooth crown covered withthicker enamel layer to resist abrasion and preserve theapex of the crown, while covering the occluding side withless or no enamel would have facilitated effective shearingin ceratopsians.

    In general, it is difficult to measure the inclination ofwear facets on maxillary and dentary teeth of specimens be-cause of deformation during fossilization. However, amongbasal ceratopsians, the wear facets for all teeth seem to besteeper in Auroraceratops, Leptoceratops, and Protocera-tops. It appears that an increase of the wear facet angletook place in basal neoceratopsians, and it is possible that abite producing a vertical wear pattern typical of ceratopsidsmay have evolved in basal Neoceratopsia.

    Although the condition is unclear in Chaoyangsaurus, theprimary ridges of maxillary and dentary teeth in Yinlong andmost psittacosaurids are relatively wider than those of basalneoceratopsians. This feature appears to have helped protectthe apex of the tooth crown from abrasion, especially on thedentary teeth of psittacosaurids (Figs. 5E, 6D). However,these primary ridges taper apically and many of the deeplyworn teeth with narrow or low primary ridges show nearlyhorizontal apical margins (Fig. 5D). As for basal neocera-topsians, in large individuals of Leptoceratops and Proto-ceratops, the teeth show pointed apical edges, even whenworn, because of the presence of prominent primary ridges(Figs. 11C, 11D). However, some maxillary teeth of Lepto-ceratops, in which the primary ridge is the most prominentof any seen among all the taxa studied, still can exhibit hor-izontal apical margins (Fig. 10B). Overall, the primaryridges of basal ceratopsians are less developed than those ofceratopsids. The high primary ridges of ceratopsids remainas high points along the cutting edge of each tooth as thetooth is worn down. The mesial and distal lobes of eachtooth slope away from the high point supported by the pri-mary ridge, to abut the distal or mesial lobe of the adjacenttooth in the dental battery. The net effect is that the cuttingedge of the scissor-like occlusal surface is serrated; this ser-ration might have been more effective at slicing throughvegetation than a horizontal occlusal edge would have been.

    The maxillary teeth of basalmost ceratopsians and themaxillary and dentary teeth of some basal neoceratopsiansshow shallow indentations on mesial and distal sides of theprimary ridges (Figs. 3D, 4D, 7C, 8C). In contrast, the max-illary and dentary teeth of derived basal neoceratopsians, in-cluding Leptoceratops and Protoceratops, have deepindentations on both sides of the primary ridges, as in cera-topsids (Figs. 10B, 10D, 11C, 11D). Development of deepindentations in neoceratopsians is associated with that ofprominent primary ridges. The function of deep indenta-tions, however, is uncertain.

    Shallow longitudinal grooves on the roots of maxillaryand dentary teeth in Protoceratops, which have also been re-ported in Zuniceratops (Wolfe and Kirkland 1998), may beprecursor of ceratopsid bifid roots if they became deeper

    and eventually split a root into two prongs. Replacementtooth fit between the bifid roots of ceratopsids, allowingeach tooth position to accommodate more teeth and possiblyincreasing the rate of tooth replacement compared with thatin basal ceratopsians (Ostrom 1966). However, dentitionwith transitional morphology, such as tooth with deep longi-tudinal grooves or single-rooted teeth with more than one re-placement tooth at each position has not been discovered sofar (Ostrom 1966; You and Dodson 2004). Further examina-tion of well-preserved teeth is required to understand theevolution of double-rooted teeth in Ceratopsidae.

    Tooth counts in maxillary and dentary tooth rows of adultbasal ceratopsians range from eight in Psittacosaurus xin-jiangensis (Sereno and Chao 1988) to 17 in Leptoceratops(Sternberg 1951). In ceratopsids, the number of functionalteeth forming tooth rows increased, and the maximum toothcount is 40 in Triceratops (Ostrom 1966). Increase in toothsize associated with that in absolute skull size also occurredamong Neoceratopsia. Derived basal neoceratopsians, in-cluding Auroraceratops, Leptoceratops, and Protoceratops,have larger maxillary and dentary teeth than more basalforms, but the teeth are smaller than in ceratopsids. Increasein the number of teeth and tooth size resulted in the progres-sive elongation of tooth rows in neoceratopsians. Tooth rowsextended distally, even beyond the coronoid process in cera-topsids (Ostrom 1964; Ostrom 1966).

    Maxillary and dentary teeth of examined basal ceratop-sians (except in Leptoceratops) clearly differ from those ofceratopsids in that the crowns are longer than they arewide; these proportions are the opposite in ceratopsids. Max-illary and dentary teeth of basal ceratopsians occlude indi-vidually with each other. Having a mesiodistally elongatedcrown would have increased the surface area for contact,presumably increasing the efficiency of mastication. In cera-topsids, mesiodistally narrow crowns form a tightly packeddental battery that functions as a single unit thus maximiz-ing the area available to process food. There is no space be-tween adjacent teeth in ceratopsids, whereas adjacent crownbases and roots are not in contact with each other in basalceratopsians.

    In some basal neoceratopsians observed, the first dentarytooth is mesial to the first maxillary tooth when the skulland mandible are articulated (Fig. 8A). This also seems tobe the case with Chaoyangsaurus. Archaeoceratops and pos-sibly Auroraceratops possess dentary teeth mesial to andapart from the packed tooth rows. These isolated mesialteeth lack primary ridges and are structurally somewhat sim-ilar to the premaxillary teeth with no wear facets (Fig. 8D).When the mouth of the animal was closed, they fit in thediastema between premaxillary and maxillary tooth row,which is distal to the upper beak. Hence, they may havebeen vestigial teeth.

    Worn dentary teeth of North American basal neoceratop-sians, as well as Udanoceratops and a new species of Ar-chaeoceratops from Asia, exhibit horizontal shelves inaddition to vertical or subvertical wear facets (Sternberg1951; Kurzanov 1992; Chinnery and Weishampel 1998;Chinnery 2004; Chinnery and Horner 2007; You et al. inpress), which suggests that shearing and crushing functionswere combined in these taxa (Sternberg 1951; Ostrom1966). However, the apex of the maxillary tooth bears no

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  • horizontal wear facet to occlude with the horizontal shelf ofthe dentary tooth. The horizontal shelf appears to be formedby food-to-tooth occlusion (Varriale 2008).

    Although isolated teeth are considered of little taxonomicutility, dental characters, including the notch on cingulum ofmaxillary teeth in Leptoceratops (Fig. 10B), V-shaped in-dentations of the maxillary tooth crown in Bagaceratopsrather than U-shaped depressions in other basal ceratopsians(Maryanska and Osmolska 1975), and a primary ridge on thelabial side of dentary tooth crown in Montanoceratops(Chinnery and Weishampel 1998), can be utilized for identi-fication. In addition, the various species of Psittacosaurusdiffer in the relative sizes of the primary ridges in maxillaryand dentary tooth crowns (Sereno 1990). Some differenceshave been observed in the maxillary and dentary teeth ofseveral other genera. In Hongshanosaurus, the primaryridges of the dentary teeth are more prominent than those ofthe maxillary teeth, as in Psittacosaurus (Figs. 6B, 6D). InLeptoceratops, the orientation of the secondary ridge is dif-ferent on the maxillary and dentary teeth. The secondaryridges in maxillary teeth are subparallel to the primary ridge(Fig. 10B). On the dentary teeth, however, secondary ridgesmesial to the primary ridge converge toward the primaryridge, whereas those distal to the primary ridge extend paral-lel to it (Fig. 10D). In Protoceratops, the indentations onmesial and distal sides of the primary ridge are deeper onthe maxillary teeth than on the dentary teeth (Figs. 11C,11D). These features can be used to distinguish isolatedmaxillary and dentary teeth.

    ConclusionsSome premaxillary teeth show wear facets and serrated

    carina, which imply that they were utilized for feeding inconcert with the predentary beaks. Evolutionary trends inmaxillary and dentary teeth of basal ceratopsians include(1) decrease and possible loss of enamel on the occluding

    side of tooth crowns,(2) increase in the angle of wear facets,(3) development of a prominent primary ridge,(4) development of deep indentations on the mesial and dis-

    tal sides of the primary ridge, and(5) increase in tooth size in neoceratopsians.Overall, the dentitions of basal ceratopsians appear to beless effective for cutting than those of ceratopsids becauseof the shorter tooth rows and less developed primary ridges.In ceratopsids, the prominent primary ridges contribute tothe serration of the dental battery when it is considered as asingle blade, retaining pointed apices on the individualcrowns. Basal ceratopsian dentition differs from ceratopsidsin that the teeth occluded individually in general unlike thepacked tooth row forming a dental battery in ceratopsids.Horizontal shelves in dentary teeth, which imply crushingfunction, are confined to some basal neoceratopsian genera.Additionally, some dental characters can be utilized to iden-tify isolated teeth of basal ceratopsians.

    AcknowledgementsThe authors are grateful to C. Mehling (AMNH), K.

    Shepherd and M. Feuerstack (CMN), X. Xu (IVPP), and K.-

    Q. Gao (Peking University, Beijing, China) for providing ac-cess to their collections. B. Grandstaff (University of Penn-sylvania, Philadelphia, Pa.) and R. Holmes (University ofAlberta, Edmonton, Alberta) kindly reviewed the early ver-sion of the manuscript. Thanks are due to D. DAmore(Rutgers University, New Brunswick, N.J.) and F. Varriale(Rowan University, Glassboro, N.J.) for valuable discus-sions. The authors also appreciate Associate Editor H.-D.Sues and reviewers A. Averianov, B. Chinnery-Allgeier,and M. Ryan for helpful suggestions which greatly improvedthe manuscript. K. Tanoue was funded by Summer ResearchStipends in Paleontology (University of Pennsylvania),School of Arts and Sciences Dissertation Research Fellow-ship (University of Pennsylvania), Jurassic Foundation Re-search Grant, and Government of Canada Post-DoctoralResearch Fellowship. Funding was provided by the BasicOutlay of Scientific Research Work and 973 Project fromMinistry of Science and Technology, the National NaturalScience Foundation of China (40672007), and Hundred Tal-ents Project of Ministry of Land and Resources of China toH.-L. You. P. Dodson thanks his chairman, N. Avadhani, forsupport.

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