Comparative anatomy of selected basal ceratopsian dentitions

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  • Comparative anatomy of selected basal ceratopsian 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 of basal 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 and basal neoceratopsian Archaeoceratops oshimai exhibit wear facets and denticles along the carina, which imply use for feeding. Maxillary and dentary teeth of basal ceratopsians were probably not as effective in feeding as those in ceratopsids because 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. Re´sume´ : La structure dentaire des ce´ratopsiens basaux est de´crite. Parmi les tendances e´volutives des dents maxillaires et dentaires des ce´ratopsiens basaux figurent une diminution et une e´ventuelle disparition de l’e´mail sur la face occlusive des couronnes des dents, une augmentation de l’angle des facettes d’usure, l’apparition d’une creˆte primaire e´minente et de profondes indentations sur les coˆte´s me´sial et distal de la creˆte primaire, ainsi que l’augmentation de la taille des dents chez les ne´oce´ratopsiens. Les dents pre´maxillaires de Yinlong, le plus basal des ce´ratopsiens, et du ne´oce´ratopsien basal Archaeoceratops oshimai pre´sentent des facettes d’usure et des denticules le long de la care`ne, ce qui indique une fonction alimentaire. Les dents maxillaires et dentaires des ce´ratopsiens basaux n’e´taient probablement pas aussi efficaces pour l’alimentation que celles des ce´ratopside´s en raison de leurs creˆtes primaires relativement moins e´minentes. Certains carac- te`res dentaires peuvent eˆtre utilise´s pour de´terminer le taxon et la position de dents isole´es de ce´ratopsiens basaux. [Traduit par la Re´daction] Introduction A unique feature of ceratopsids, the most derived ceratop- sians, is the form of mastication. Derived ceratopsian jaws contain large numbers of teeth, which are mesiodistally compressed for close packing in dental batteries in which files 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 and lingual side of the dentary tooth crown. The dental batteries exhibit vertical cutting planes, in which the lingual sides of maxillary teeth occlude against the labial sides of dentary teeth. (Vertical shear of dental batteries is not found in any other herbivorous vertebrate taxa.) In contrast, basal ceratop- sians lack dental batteries; instead, the closely spaced teeth merely erupt in a single horizontal line. Examination of the tooth structure of basal ceratopsians should help to elucidate the evolutionary transformation of dentition within the Cera- topsia. However, compared with theropod dinosaurs, compa- rative studies on ornithischian dentitions, including those of ceratopsians, have rarely been done (Ostrom 1966; Weish- ampel 1984; Coombs 1990). Dental structure in the basal Ceratopsia has attracted little attention other than a few thor- ough descriptions (Zhao et al. 1999; Makovicky and Norell 2006; Godefroit and Lambert 2007). Diagnostic characters of neoceratopsian teeth from North America were identified by Chinnery et al. (1998). Moreover, premaxillary teeth are found only in basal ceratopsians, except for the Psittacosaur- idae, but the function of these teeth has not been discussed previously. 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 from the Upper Jurassic (Oxfordian; Xu et al. 2006) to the Upper Cretaceous (Campanian; Chinnery and Horner 2007). Subse- quent preparation of many of these specimens has revealed new 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. Published on the NRC Research Press Web site at cjes.nrc.ca on 12 August 2009. Paper handled by Associate Editor H.-D. Sues. K. Tanoue.1 Canadian Museum of Nature, P.O. Box 3443, Stn ‘‘D’’, Ottawa, ON K1P 6P4, Canada. H. You. Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China. P. Dodson. School of Veterinary Medicine and Department of Earth and Environmental Science, University of Pennsylvania, 3800 Spruce Street, Philadelphia, PA 19104-6045, USA. 1Corresponding author (e-mail: ktanoue@mus-nature.ca). 425 Can. J. Earth Sci. 46: 425–439 (2009) doi:10.1139/E09-030 Published by NRC Research Press
  • Institutional abbreviations AMNH, 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 University Paleontological Collections, Beijing, China. Materials and methods Dentitions of nine basal ceratopsian genera were exam- ined and measured. Phylogenetic relationships among these genera are shown in Fig. 1. In this study, Yinlong and Chaoyangsaurus are considered the basalmost Ceratopsia, which are here defined as outgroup to Psittacosauridae + Neoceratopsia (Xu et al. 2006). Psittacosaurus and Hon- gshanosaurus compose Psittacosauridae. Hongshanosaurus is a newly discovered psittacosaurid that differs from Psitta- cosaurus in having greater preorbital length relative to basal skull 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. in press). The adult specimen (IVPP V12617) examined in this study is attributed to Hongshanosaurus since the preorbital length is approximately half of the basal skull length and the orbit and infratemporal fenestra are elliptical, with the long axes sloping anteroventrally as in the holotype (IVPP V12704). IVPP V12617 is unlikely to have undergone dis- tortion, for the skull and mandible are symmetrical and the delicate palatal structure is preserved intact (Dodson et al. in press). In this study Liaoceratops, Archaeoceratops, Auroraceratops, Leptoceratops, and Protoceratops represent the basal Neoceratopsia. Dental terminology used is shown in Fig. 2. Description Yinlong Yinlong (IVPP V14530) has three premaxillary teeth on each side (Fig. 3A; Xu et al. 2006). The well-preserved left first and right first and second premaxillary teeth are larger than the maxillary teeth in labial view. The lengths of the crown and root are greater than the width. The tooth crown base widens for the first 2 to 3 mm apically in labial and mesial views, then tapers toward the tip (Fig. 3B). Most of the occlusal surface is flat, except at the base where it is lin- gually concave in the first teeth on both sides (Fig. 3C). The labial side is mesiodistally convex as in Chaoyangsaurus. Xu et al. (2006) reported serrations only on the distal carina of the right second tooth, but the mesial carina is also ser- rated (Fig. 3B). The serrated region is longer and more prominent 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 the labially convex right maxillary tooth row (Xu et al. 2006). The diastema between the right third premaxillary and the first maxillary teeth measures approximately 4 mm. The right tooth row is 59 mm long. The maxillary teeth are tightly packed with slight overlap of the crowns. The distal end of each tooth is lateral to the mesial end of the subse- quent tooth. The unworn crown of a left maxillary tooth is ovate (Figs. 3D, 3E). The low primary ridge is wide at the base and tapers apically toward the crown apex. The base of the primary ridge is confluent with the low cingulum. On Fig. 1. Phylogenetic tree of the Marginocephalia compiled from You 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. 426 Can. J. Earth Sci. Vol. 46, 2009 Published by NRC Research Press
  • both mesial and distal sides of the primary ridge are shallow depressions, bounded basally by the cingulum. Secondary ridges develop within both depressions. The secondary ridges on the mesial indentation are much longer and more prominent than those on distal depression. The secondary ridges extend toward the primary ridge near the occlusal margin 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 part of the right tooth row and the partial crown of an erupting tooth in the left dentary (Figs. 3F, 3G). This partial crown can be observed ventral to the middle of the dentary tooth row on the medial side of the left dentary. It is 5.7 mm wide and 6.0 mm high. The exposed portion is triangular, and its dorsal margin bears denticles. The primary ridge is poorly developed and only slightly wider than the secondary ridges. Secondary ridges are short. Four and five secondary ridges develop mesial and distal to the low primary ridge, respectively, supporting denticles on the occlusal edges of the tooth. Chaoyangsaurus The dentition of Chaoyangsaurus is relatively well pre- served in the holotype (Fig. 4A; IGCAGS V371). Two teeth compose the premaxillary tooth row (Figs. 4A, 4B). Although the ventral margin of the premaxilla just posterior to the second premaxillary tooth is not preserved on either side, it is unlikely that this specimen bore a third premaxil- lary tooth, since it lacks any evidence of an alveolus. The two 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 tooth crown 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 a small wear facet on the lingual side of the tip, which is sloped 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 preserved tooth row measures 36 mm in length. Nine teeth compose the 37 mm long right maxillary tooth row, from which the second tooth is missing. The maxillary tooth rows are slightly concave labially (Fig. 4C). There is a slight overlap in the maxillary teeth, with the distal end of a tooth labially overlapping 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 maxillary tooth crown is greater than the width. The height of the crown is greater than the length in the mesial teeth, and smaller in the distal teeth (Zhao et al. 1999). In occlusal view, the labial surface is more convex than the lingual sur- face. The occlusal edge of an unworn, erupting maxillary tooth preserves about 10 denticles. A low vertical ridge ex- tends at the midline on the labial surface (Fig. 4D). This ridge probably represents a remnant of the primary ridge (Zhao et al. 1999). However, this is uncertain due to the preservation of the specimen. On both sides of this low ridge 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 in right lateral view. (B) Left second premaxillary tooth in lingual view. (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, and F. Tanoue et al. 427 Published by NRC Research Press
  • cylindrical root, which is only slightly narrower than the crown (Zhao et al. 1999). Enamel covers both sides of the maxillary teeth. The enamel thickness appears to be about the same on both the labial and the lingual sides (Zhao et al. 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. The left tooth row, which is 39 mm long, preserves only nine teeth. Lack of an alveolus distal to the last tooth indicates that the distal teeth of the left tooth row are completely pre- served. The length of the dentary teeth increases distally up to the seventh tooth in the left tooth row and eighth tooth in the right tooth row, and decrease to the last tooth. The length 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 denticles are supported by short secondary ridges extending ventrally toward the tooth base. The wear facet is concave apicola- bially. Fully exposed dentary teeth including the root show that these teeth are single-rooted. Psittacosaurus The 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 this study. None of the specimens studied possess premaxillary teeth. 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 and dentary 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 all teeth. The tooth rows of most species are nearly straight in oc- clusal view, but in P. sinensis and P. lujiatunensis, they are concave labially (Sereno and Chao 1988; Zhou et al. 2006). The maxillary tooth crowns are aligned at a shallow angle to the long axis of the tooth row. The distal end of each crown labially overlaps the mesial end of the subsequent crown (Sereno et al. 1988; Sereno 1990; Zhou et al. 2006; You et al. 2008). Although no fully exposed unworn maxillary tooth crown was observed in this study, the crown is oval in labial view in P. meileyingensis and P. mongoliensis (Se- reno et al. 1988). A low primary ridge separates the mesial and 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 both sides of the primary ridge (You et al. 2008). The mesial lobe is flat and broader than the distal lobe. The distal lobe is swollen and can be more prominent than the primary ridge. Several secondary ridges, separated from each other by shallow longitudinal grooves, extend toward the tooth base from the occlusal margins of the two lobes. They do not reach the cingulum. The mesial lobe displays more sec- ondary ridges than the distal lobe. Denticles are present at the occlusal ends of the secondary ridges and can be ob- served in relatively unworn crowns (Fig. 5B; Sereno and Chao 1988; Sereno et al. 1988). In relatively well-preserved maxillary teeth, enamel layers cover both labial and lingual surfaces of the crown, with the thicker layer on the labial side (Sereno and Chao 1988; Sereno 1990; Averianov et al. Fig. 4. Chaoyangsaurus youngi (IGCAGS V371). (A) Skull in right lateral view. (B) Left premaxillary teeth in labial view. (C) Left maxillary tooth row in occlusal view. (D) Left maxillary tooth row in labial view. (E) Mandible in dorsal view. (F) Left dentary teeth in lingual view. dt, dentary teeth. See Fig. 3 for other abbreviations. Scale bars = 5 mm in B, C, D, and F. 428 Can. J. Earth Sci. Vol. 46, 2009 Published by NRC Research Press
  • 2006). The wear facet on the lingual side of the crown is oblique rather than vertical. The number of teeth in the dentary tooth row usually equals the number in the maxillary tooth row. The size of the dentary teeth is comparable to that of the maxillary teeth. Unworn dentary tooth crowns are circular to oval in outline in lingual view (Sereno and Chao 1988; Averianov et al. 2006; You et al. 2008). The crown is symmetrical, with the bulbous primary ridge in the middle of the lingual surface (Fig. 5E). The primary ridge is more prominent on the 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. Several secondary ridges supporting the denticles along the occlusal margin are present on both lobes, but they do not reach the basal half of the lobe (Fig. 5E). These short secondary ridges are visible on the labial surfaces of relatively unworn dentary teeth (Fig. 5C). The largest number of denticles, 10 in each lobe, has been reported in P. xinjiangensis (Sereno and Chao 1988). The enamel layer is thicker on the lingual side than on the labial side of the tooth (Sereno and Chao 1988; Sereno 1990; Averianov et al. 2006). The wear facet is sloped to occlude against that of maxillary tooth. Hongshanosaurus The adult specimen of Hongshanosaurus (IVPP V12617) was examined. As in all other psittacosaurids, Hongshano- saurus lacks premaxillary teeth (Fig. 6A). Juvenile specimen was not included in this study. The maxillary tooth rows are poorly preserved compared with the dentary tooth rows in IVPP V12617. The maxillary tooth 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 maxillary tooth crown is greater than its width. The primary ridge is low and widens toward the tooth base (Fig. 6B). The pri- mary ridge is shifted distally, resulting in a wider mesial lobe than distal lobe. Short vertical secondary ridges extend from 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, the occlusal margin bears denticles supported by the secondary ridges. The enamel covers the labial and lingual surfaces of the crown, but it is thicker on the labial side (You and Xu 2005). Both left and right dentary tooth rows bear 10 teeth and are 38 mm and 41 mm long, respectively (Fig. 6C). They extend along the medial side of the dentaries. The tooth rows are slightly concave labially in occlusal view. In most dentary 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 tooth crown is widest at the base and tapers dorsally. It is more prominent than that of a maxillary tooth, as in Psittacosau- rus (Fig. 6D; Sereno 1990). The primary ridge separates the mesial and distal lobes of the crown. It generally extends near the midline, but it is shifted distally in some teeth. As many as eight mesial and six distal denticles are present. A secondary ridge stretching toward the tooth base from the Fig. 5. Psittacosaurus. (A) Right maxillary teeth of Psittacosaurus lujiatunensis (PKUP V1053) in labial view. (B) Right maxillary tooth row of P. sinensis (IVPP V738) in labial view. (C) Left max- illary and dentary teeth of P. neimongoliensis (IVPP 12-0888-2) in labial view. (D, E) P. major (CAGS-IG-VD-004). (D) Right maxil- lary tooth row in labial view. (E) Right dentary tooth row in lingual view. Scale bars = 5 mm. pr, primary ridge; sr, secondary ridge. Tanoue et al. 429 Published by NRC Research Press
  • occlusal margin of the dentary tooth crown supports each denticle. The secondary ridges develop only in the apical third of the crown. Both labial and lingual surfaces of the crown are enameled, with the enamel layer being thicker on the lingual side (You and Xu 2005). The wear facet is flat and slopes labially. Liaoceratops The 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 other two are juvenile skulls, with CAGS-IG-VD-002 being the smallest of the three specimens. Two premaxillary teeth are preserved on both sides in the holotype (Fig. 7A). Sockets are present rostrolateral to the right first tooth and caudolateral to the second right tooth, which may represent alveoli for additional premaxillary teeth, but no alveoli are preserved for more than two teeth in 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-shaped with a cylindrical root (Fig. 7B). Both labial and lingual sides of the premaxillary tooth crown are covered with en- amel. The carina extends along the mesiodistal axis of the crown. 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 second tooth display denticles, as in Yamaceratops and a new spe- cies of Archaeoceratops (Fig. 7B; Makovicky and Norell 2006; You et al. in press). The serrated region is longer on the distal carina than on the mesial carina. No wear facets are present on any premaxillary tooth. The left and right maxillary tooth rows of the holotype (adult) skull consist of 11 and 12 teeth, respectively. All maxillary 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 is confluent with the cingulum at the base. The primary ridges are poorly differentiated from secondary ridges on the max- illary teeth of the juvenile skulls. Two or three secondary ridges are located mesial to the primary ridge and at least one lies distally. In occlusal view, the maxillary teeth show steep but not vertical wear facets. At least 12 teeth are preserved in the left dentary tooth row of the holotype (Fig. 7D). The right dentary tooth row consists of 15 teeth. Only the lingual surface of the oval crown is covered with enamel. The primary ridge is mesial to the midline in lingual view (Fig. 7E). The primary ridges in the dentary teeth of juvenile mandibles are only slightly more prominent than the secondary ridges, as is also the case 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 left lateral view. (B) Left maxillary teeth in labial view. (C) Mandible in dorsal view. (D) Left dentary teeth in lingual view. dt, dentary teeth. See Fig. 3 for other abbreviations. 430 Can. J. Earth Sci. Vol. 46, 2009 Published by NRC Research Press
  • all three specimens. Some dentary teeth exhibit wear facets with shelf structure. Archaeoceratops Well-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 described by You et al. (in press). Three teeth compose the premaxil- lary tooth row (Figs. 8A, 8B). All three right premaxillary teeth are preserved, but only the second premaxillary tooth remains on the left side. In occlusal view, the second tooth is slightly labial to the first tooth and the third tooth is slightly lingual to the first tooth. Both labial and lingual sides of the crown are covered with enamel (You and Dod- son 2003). The premaxillary teeth are peg-shaped. In labial view, the premaxillary teeth first narrow ventrally from the base, then widen, and the crowns then taper to a narrow apex. The length is greater than the width. The carina ex- tends mesiodistally along the occlusal edge. No denticles were observed along the carina, unlike in the new species of 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 wear facets 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 dorsal margin of the predentary (Fig. 8A). Thus, the lingual surfa- ces of the premaxillary crowns will have contacted the outer face of the lower beak. The left maxillary tooth row comprises 13 teeth and is 46 mm long. Fourteen teeth compose the 47 mm long right maxillary tooth row. In occlusal view, the tooth rows are lenticular, widest a little distal to the middle of the tooth rows and tapering both mesially and distally. Enamel covers both surfaces of the crown. Unworn crowns are oval in la- bial view. Unlike the new species of Archaeoceratops, in which the maxillary teeth lack a primary ridge, maxillary teeth of A. oshimai show a primary ridge (Fig. 8C; You et al. in press). However, it is only slightly broader than the secondary ridges. The primary ridge is slightly distal to the midline of the crown, and merges basally with the cingulum. In some teeth, the primary ridge curves ventrodistally. Three secondary ridges are present on each side of the primary ridge, forming denticles along the occlusal margin (Fig. 8C). The secondary ridges stretch dorsally toward the tooth base, and toward the primary ridge. Roots of some teeth are partially exposed. They narrow slightly toward the root apex. Replacement teeth for the left 10th and 12th, and right eighth and 10th tooth positions are partially exposed on the medial surface of the maxilla. Only one replacement tooth is present for each tooth position. The wear facets of maxillary teeth are steeply inclined. The labially concave dentary tooth rows are aligned along the medial borders of the dentaries in occlusal view. Four- Fig. 7. Liaoceratops yanzigouensis (IVPP V12738). (A) Skull in left lateral view. (B) Left premaxillary teeth in labial view. (C) Left maxillary teeth in labial view. (D) Mandible in dorsal view. (E) Right dentary teeth in lingual view. dt, dentary teeth. See Fig. 3 for other abbreviations. Scale bars = 5 mm in B, C, and E. Tanoue et al. 431 Published by NRC Research Press
  • teen teeth compose the 55 mm long left tooth row. The right tooth row comprises 14 teeth and is 53 mm long. Both the labial and lingual sides of the dentary teeth are covered with 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 tooth rows (Fig. 8D). When the skull and mandible are articulated, the first two dentary teeth are mesial to the first maxillary tooth. The first maxillary tooth occludes with the third den- tary tooth. In lingual view, the subtriangular crowns of the first two teeth lack both primary and secondary ridges and only show denticles along the apical margins. The third tooth also displays a subtriangular crown outline, but there are secondary ridges on the lingual surface. The fourth tooth preserves a wear facet, but it lacks the primary ridge. The primary ridges are preserved on teeth distal to the fifth tooth on both dentaries. With the exception of the third tooth, the teeth of the closely packed tooth row exhibit oval crowns. The primary ridges in teeth 5–14 extend vertically in the mesial third of the crown (Fig. 4E). However, the primary ridge is only slightly more prominent than the secondary ridges, which converge basally toward the primary ridge. Three secondary ridges are located mesial and four distal to the 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. Unworn erupting teeth display more denticles than the number of secondary ridges. Only one replacement tooth is present for each tooth position. The wear facet is sloped as in maxillary tooth. Auroraceratops The 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 second and third teeth are aligned along the lateral margin of the pre- maxilla in occlusal view, with the first alveolus and fourth tooth lingual to them. The length and width of the second tooth are 7.2 mm and 5.2 mm, respectively. The length of the third tooth is 7.6 mm, and the width is 4.5 mm. The heights of the second and third teeth are approximately 7 mm and 8 mm, respectively, in labial view. The third peg- shaped tooth is the best preserved in this specimen. The blunt mesial 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 in occlusal 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 to the first alveolus and to the third tooth. Fig. 8. Archaoeceratops oshimai (IVPP V11114). (A) Skull and mandible in right lateral view. (B) Right premaxillary tooth row in lingual view. (C) Left maxillary teeth in labial view. (D) Rostral dentary teeth in lingual view. Numbers indicate tooth positions. (E) Caudal dentary teeth in lingual view. dt, dentary teeth. See Fig. 3 for other abbreviations. Scale bars = 5 mm in B–E. 432 Can. J. Earth Sci. Vol. 46, 2009 Published by NRC Research Press
  • The axes of the maxillary teeth slope ventrodistally (Fig. 9C). Thirteen maxillary teeth form the 65 mm long left tooth row. The right maxillary tooth row consists of 12 teeth. It is 65 mm long. Both the length and width of the maxillary teeth increase distally, except for the distalmost two or three teeth in both maxillary tooth rows. The ovate crown 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 basally confluent with the cingulum. Mesial and distal to the pri- mary ridge are shallow depressions. The concave surface of the mesial lobe extends closer to the tooth base than does the distal one. Three secondary ridges are located mesial and two distal to the primary ridge. They stretch toward the base from the occlusal margin of the crown but are only about half the length of the primary ridge. Enamel covers only the labial surfaces of the maxillary teeth, but this may be due to preservation. Replacement teeth are not exposed in lingual view. Although small spaces are found between adja- cent roots, adjacent crowns are in contact and form a tightly packed tooth row. The tooth height decreases mesially and distally from tooth position 8 in lingual view. The wear facet is nearly vertical. The dentary teeth are poorly preserved. Twelve teeth form the dentary tooth row on either side (Fig. 9D). The length and the width of the teeth increase up to the seventh or eighth tooth and then decrease distally in both tooth rows. In labial view, the better preserved left tooth row is highest at the middle, with the seventh and eighth teeth being the highest. In dorsal view, the tooth rows are lingually convex (Fig. 9D). No unworn teeth are present. The roots of the first four teeth in the left tooth row are longer than they are wide. The crown is asymmetrical in lingual view, with the primary ridge developing mesial to the midline (Fig. 9E). The primary 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 the cingulum. Enamel is preserved only on the labial sides of the dentary teeth. The presence of an enamel layer on the lin- gual surface is uncertain because of poor preservation. The wear facet appears to be vertical. Two sockets are located mesial to the right dentary tooth row (Fig. 9D). The diameters of these sockets are approxi- mately 2 to 3 mm, with the interval between them of about the same length. These may be alveoli for small mesial teeth that were set off from each other and from the more distal tooth row. They would have fit in the diastema between pre- maxillary and maxillary teeth. Leptoceratops The 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 and among the largest in basal ceratopsians. Leptoceratops lacks premaxillary teeth (Sternberg 1951). Fig. 9. Auroraceratops rugosus (CAGS-2004-IG-VD-001). (A) Skull in left lateral view. (B) Left premaxillary teeth in labial view. (C) Left maxillary teeth in labial view. (D) Mandible in dorsal view. (E) Left dentary teeth in lingual view. dt, dentary teeth; r, root; s, sockets (possibly alveoli). See Fig. 3 for other abbreviations. Tanoue et al. 433 Published by NRC Research Press
  • Both maxillary tooth rows of CMN 8889 consist of 17 teeth. The left tooth row is 157 mm long and the right tooth row 160 mm. In occlusal view, the tooth rows are slightly concave labially. The width of the tooth crown exceeds the length (Sternberg 1951). The primary ridge is shifted distally and is curved in some teeth (Fig. 10B). Some primary ridges show striations parallel to the ridge axis. At least three sec- ondary ridges are situated mesial to, and one distal to, the primary ridge. The secondary ridges are subparallel to the primary 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 base of the primary ridge does not quite extend to the lingual edge 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 tooth row. The left 14th and 16th and the right 14th maxillary teeth exhibit narrow horizontal shelves at the base of the subvertical 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 tooth rows of Leptoceratops are much less curved than those of other basal neoceratopsians. The width of the dentary tooth is greater than the length. The primary ridge is mesial to the midline of the dentary tooth crown, as in other basal neoceratopsians (Fig. 10D). It is more prominent than that of a maxillary tooth and widens basally, just dorsal to the junction with the cingulum. The primary ridge is confluent with the cingulum, unlike the condition in maxillary teeth. Up to four mesial and three distal secondary ridges are present. The secondary ridges develop in a way that is unique 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 ridge and the cingulum and the secondary ridges are illustrated in Brown (1914, fig. 2) as those of a maxillary tooth; however, it appears to be a dentary tooth. Similarly, the dentary tooth illustrated by Brown (1914, fig. 6) appears to be a maxillary tooth. As on the maxillary teeth, wear facets are steeply in- clined or vertical (Sternberg 1951). At the base of this wear facet, there is a horizontal shelf, as is also seen in other North American basal neoceratopsians and some Asian forms, including Udanoceratops and Archaeoceratops (Sternberg 1951; Kurzanov 1992; Chinnery and Weishampel 1998; Chinnery 2004; Chinnery and Horner 2007; You et al. in press). It is difficult to confirm the distribution of enamel in the teeth of CMN 8889 since the entire specimen is stained black. However, isolated teeth of CMN 8888, whose dentine portion is brown, show distinct enamel layers on both labial and lingual surfaces of the crown. Protoceratops Two 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 between the two teeth (Fig. 11B). The long cylindrical root is nearly round in horizontal section. Among the observed premaxil- lary teeth, the right first tooth of AMNH 6433 is the largest with a width of 7.6 mm. The maxillary tooth row is labially concave mesially, but is straight distally. There are up to 15 maxillary tooth posi- tions (You and Dodson 2004). Each maxillary tooth crown is 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 dentary teeth in lingual view. dt, dentary teeth; n, notch. See Fig. 3 for other abbreviations. 434 Can. J. Earth Sci. Vol. 46, 2009 Published by NRC Research Press
  • the primary ridge is often basal to the distal cingulum. The indentations on both mesial and distal lobes of the crown are deep. The secondary ridges on both lobes extend from the occlusal margin toward the basal part of the primary ridge. Isolated teeth are single-rooted and show longitudinal grooves 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 apical half of the lingual surface of the crown; the lingual enamel band is only preserved on unworn teeth. The wear facet is steeply inclined or vertical. As in maxillary teeth, isolated dentary teeth are single- rooted, with longitudinal grooves extending on mesial and distal surfaces of the root. Unworn dentary tooth crowns are oval in lingual view. The primary ridge of the crown is situ- ated mesially (Fig. 11D). It is straight or slightly curved and flares toward the base. The primary ridge merges with the cingulum at the base. The bases of some primary ridges are striated. The cingulum often extends distobasally. The inden- tations mesial and distal to the primary ridge are shallower than those of maxillary tooth crowns. At least four secondary ridges are present both mesial and distal to the primary ridge. These secondary ridges extend obliquely toward the tooth base and toward the primary ridge. There is only one replacement tooth in each tooth position. The lingual surfa- ces, and the apical half of the labial surface of unworn tooth crowns, are enameled. The wear facet is nearly vertical. Discussion Premaxillary dentition Premaxillary teeth of basalmost ceratopsians and some basal neoceratopsians share some morphological features with those of Pachycephalosauria, the sister-taxon of Cera- topsia, including subconical crowns with oval cross-section and enamel on both the labial and lingual sides of the crown (Maryan´ska 1990). On the other hand, premaxillary tooth crowns in pachycephalosaurians are set off from the roots, whereas crowns of basal ceratopsian premaxillary teeth are only slightly longer and wider than the root (Figs. 3B, 7B, 8B). Premaxillary tooth crowns of pachycephalosaurians are also recurved apicodistally, but those of basal ceratopsians are 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 premaxillary teeth of Yinlong, Liaoceratops, Yamaceratops, and a new species of Archaeoceratops (Figs. 3B, 7B; Makovicky and Norell 2006; Xu et al. 2006; You et al. in press) show den- ticles along the carina. In Yinlong and Liaoceratops, a longer portion of the distal carina bears denticles than the mesial carina (Figs. 3B, 7B), suggesting that the premaxillary teeth were used to bite an object at an angle oblique to the vertical axis of the tooth (D. D’Amore, personal communication, 2008), associated with the motion of pulling the lowered head posterodorsally or the lifted head posteroventrally. Maxillary and dentary dentition Maxillary teeth of Yinlong resemble those of pachycepha- losaurs in that the low primary ridge of maxillary tooth is wide at the base (Figs. 3D, 3E). Crowns of unworn dentary tooth of Yinlong and mesial dentary teeth of A. oshimai are subtriangular as in pachycephalosaurians (Figs. 3F, 3G, 8D). All other maxillary and dentary teeth of basal ceratopsians examined, however, exhibit ovate crowns. Additionally, the labial surface of pachycephalosaurian maxillary tooth crowns is concave vertically (Sues and Galton 1987; Mar- yan´ska et al. 2004), in contrast to the convex labial surface of ceratopsian tooth crowns (Chinnery et al. 1998). The evolutionary transition toward the ceratopsid dental structure can also be observed in basal neoceratopsians. In most basal ceratopsians, enamel covers both sides of maxil- lary and dentary teeth. In Chaoyangsaurus, enamel layers on both sides of maxillary and dentary teeth exhibit almost the same thickness, but in other basal ceratopsians with tooth crowns enameled on both sides, the enamel layer on the la- bial side of the maxillary tooth crown and the lingual side of the dentary tooth crown is thicker than that on the opposing Fig. 11. Protoceratops andrewsi. (A) Skull and mandible in right lateral view (AMNH6425). (B) Right premaxillary tooth row in lin- gual view (AMNH 6433). (C) Right maxillary tooth row in labial view (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 B–D. Tanoue et al. 435 Published by NRC Research Press
  • side. Only the labial side of the maxillary and the lingual side of the dentary teeth in Auroraceratops and Liaocera- tops are enameled, but it may be due to poor preservation of the known specimens. The thickness of enamel layers in Leptoceratops is uncertain. In contrast, ceratopsid teeth de- velop the enamel only on the non-occluding side. Having the non-occluding side of the tooth crown covered with thicker enamel layer to resist abrasion and preserve the apex of the crown, while covering the occluding side with less or no enamel would have facilitated effective shearing in ceratopsians. In general, it is difficult to measure the inclination of wear facets on maxillary and dentary teeth of specimens be- cause of deformation during fossilization. However, among basal ceratopsians, the wear facets for all teeth seem to be steeper in Auroraceratops, Leptoceratops, and Protocera- tops. It appears that an increase of the wear facet angle took place in basal neoceratopsians, and it is possible that a bite producing a vertical wear pattern typical of ceratopsids may have evolved in basal Neoceratopsia. Although the condition is unclear in Chaoyangsaurus, the primary ridges of maxillary and dentary teeth in Yinlong and most psittacosaurids are relatively wider than those of basal neoceratopsians. This feature appears to have helped protect the apex of the tooth crown from abrasion, especially on the dentary teeth of psittacosaurids (Figs. 5E, 6D). However, these primary ridges taper apically and many of the deeply worn teeth with narrow or low primary ridges show nearly horizontal apical margins (Fig. 5D). As for basal neocera- topsians, in large individuals of Leptoceratops and Proto- ceratops, the teeth show pointed apical edges, even when worn, 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 prominent of any seen among all the taxa studied, still can exhibit hor- izontal apical margins (Fig. 10B). Overall, the primary ridges of basal ceratopsians are less developed than those of ceratopsids. The high primary ridges of ceratopsids remain as high points along the cutting edge of each tooth as the tooth is worn down. The mesial and distal lobes of each tooth slope away from the high point supported by the pri- mary ridge, to abut the distal or mesial lobe of the adjacent tooth in the dental battery. The net effect is that the cutting edge of the scissor-like occlusal surface is serrated; this ser- ration might have been more effective at slicing through vegetation than a horizontal occlusal edge would have been. The maxillary teeth of basalmost ceratopsians and the maxillary and dentary teeth of some basal neoceratopsians show shallow indentations on mesial and distal sides of the primary ridges (Figs. 3D, 4D, 7C, 8C). In contrast, the max- illary and dentary teeth of derived basal neoceratopsians, in- cluding Leptoceratops and Protoceratops, have deep indentations on both sides of the primary ridges, as in cera- topsids (Figs. 10B, 10D, 11C, 11D). Development of deep indentations in neoceratopsians is associated with that of prominent primary ridges. The function of deep indenta- tions, however, is uncertain. Shallow longitudinal grooves on the roots of maxillary and dentary teeth in Protoceratops, which have also been re- ported in Zuniceratops (Wolfe and Kirkland 1998), may be precursor of ceratopsid bifid roots if they became deeper and eventually split a root into two prongs. Replacement tooth fit between the bifid roots of ceratopsids, allowing each tooth position to accommodate more teeth and possibly increasing the rate of tooth replacement compared with that in basal ceratopsians (Ostrom 1966). However, dentition with 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 so far (Ostrom 1966; You and Dodson 2004). Further examina- tion of well-preserved teeth is required to understand the evolution of double-rooted teeth in Ceratopsidae. Tooth counts in maxillary and dentary tooth rows of adult basal ceratopsians range from eight in Psittacosaurus xin- jiangensis (Sereno and Chao 1988) to 17 in Leptoceratops (Sternberg 1951). In ceratopsids, the number of functional teeth forming tooth rows increased, and the maximum tooth count is 40 in Triceratops (Ostrom 1966). Increase in tooth size associated with that in absolute skull size also occurred among Neoceratopsia. Derived basal neoceratopsians, in- cluding Auroraceratops, Leptoceratops, and Protoceratops, have larger maxillary and dentary teeth than more basal forms, but the teeth are smaller than in ceratopsids. Increase in the number of teeth and tooth size resulted in the progres- sive elongation of tooth rows in neoceratopsians. Tooth rows extended 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 of ceratopsids in that the crowns are longer than they are wide; these proportions are the opposite in ceratopsids. Max- illary and dentary teeth of basal ceratopsians occlude indi- vidually with each other. Having a mesiodistally elongated crown would have increased the surface area for contact, presumably increasing the efficiency of mastication. In cera- topsids, mesiodistally narrow crowns form a tightly packed dental 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 crown bases and roots are not in contact with each other in basal ceratopsians. In some basal neoceratopsians observed, the first dentary tooth is mesial to the first maxillary tooth when the skull and mandible are articulated (Fig. 8A). This also seems to be the case with Chaoyangsaurus. Archaeoceratops and pos- sibly Auroraceratops possess dentary teeth mesial to and apart from the packed tooth rows. These isolated mesial teeth 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 the diastema between premaxillary and maxillary tooth row, which is distal to the upper beak. Hence, they may have been 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 in addition to vertical or subvertical wear facets (Sternberg 1951; Kurzanov 1992; Chinnery and Weishampel 1998; Chinnery 2004; Chinnery and Horner 2007; You et al. in press), which suggests that shearing and crushing functions were combined in these taxa (Sternberg 1951; Ostrom 1966). However, the apex of the maxillary tooth bears no 436 Can. J. Earth Sci. Vol. 46, 2009 Published by NRC Research Press
  • horizontal wear facet to occlude with the horizontal shelf of the dentary tooth. The horizontal shelf appears to be formed by food-to-tooth occlusion (Varriale 2008). Although isolated teeth are considered of little taxonomic utility, dental characters, including the notch on cingulum of maxillary teeth in Leptoceratops (Fig. 10B), V-shaped in- dentations of the maxillary tooth crown in Bagaceratops rather than U-shaped depressions in other basal ceratopsians (Maryan´ska and Osmo´lska 1975), and a primary ridge on the labial side of dentary tooth crown in Montanoceratops (Chinnery and Weishampel 1998), can be utilized for identi- fication. In addition, the various species of Psittacosaurus differ in the relative sizes of the primary ridges in maxillary and dentary tooth crowns (Sereno 1990). Some differences have been observed in the maxillary and dentary teeth of several other genera. In Hongshanosaurus, the primary ridges of the dentary teeth are more prominent than those of the maxillary teeth, as in Psittacosaurus (Figs. 6B, 6D). In Leptoceratops, the orientation of the secondary ridge is dif- ferent on the maxillary and dentary teeth. The secondary ridges in maxillary teeth are subparallel to the primary ridge (Fig. 10B). On the dentary teeth, however, secondary ridges mesial to the primary ridge converge toward the primary ridge, whereas those distal to the primary ridge extend paral- lel to it (Fig. 10D). In Protoceratops, the indentations on mesial and distal sides of the primary ridge are deeper on the maxillary teeth than on the dentary teeth (Figs. 11C, 11D). These features can be used to distinguish isolated maxillary and dentary teeth. Conclusions Some premaxillary teeth show wear facets and serrated carina, which imply that they were utilized for feeding in concert with the predentary beaks. Evolutionary trends in maxillary 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 be less effective for cutting than those of ceratopsids because of the shorter tooth rows and less developed primary ridges. In ceratopsids, the prominent primary ridges contribute to the serration of the dental battery when it is considered as a single blade, retaining pointed apices on the individual crowns. Basal ceratopsian dentition differs from ceratopsids in that the teeth occluded individually in general unlike the packed tooth row forming a dental battery in ceratopsids. Horizontal shelves in dentary teeth, which imply crushing function, are confined to some basal neoceratopsian genera. Additionally, some dental characters can be utilized to iden- tify isolated teeth of basal ceratopsians. Acknowledgements The 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 of Alberta, Edmonton, Alberta) kindly reviewed the early ver- sion of the manuscript. Thanks are due to D. D’Amore (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 improved the manuscript. K. Tanoue was funded by Summer Research Stipends 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-Doctoral Research Fellowship. Funding was provided by the Basic Outlay of Scientific Research Work and 973 Project from Ministry of Science and Technology, the National Natural Science Foundation of China (40672007), and Hundred Tal- ents Project of Ministry of Land and Resources of China to H.-L. You. P. Dodson thanks his chairman, N. Avadhani, for support. References Alifanov, V.R. 2003. Two new dinosaurs of the infraorder Neocer- atopsia (Ornithischia) from the Upper Cretaceous of the Nemegt depression, Mongolian People’s Republic. Paleontological Jour- nal, 37: 524–534. Averianov, A.O., Voronkevich, A.V., Leshchinskiy, S.V., and Fayngertz, A.V. 2006. A ceratopsian dinosaur Psittacosaurus si- biricus from the early cretaceous of West Siberia, Russia and its phylogenetic relationships. Journal of Systematic Palaeontology, 4(04): 359–395. doi:10.1017/S1477201906001933. Brown, B. 1914. Leptoceratops, a new genus of Ceratopsia from the Edmonton Cretaceous of Alberta. American Museum of Nat- ural History Bulletin, 33: 567–580. Brown, B., and Schlaikjer, E.M. 1940. The structure and relationships of Protoceratops. Annals of the New York Academy of Sciences, 40(3): 133–266. doi:10.1111/j.1749-6632.1940.tb57047.x. Chinnery, B.J. 2004. Description of Prenoceratops pieganensis gen. et sp. nov. (Dinosauria: Neoceratopsia) from the Two Medicine Formation of Montana. Journal of Vertebrate Paleontology, 24(3): 572–590. doi:10.1671/0272-4634(2004)024[0572:DOPPGE]2.0. CO;2. Chinnery, B.J., and Horner, J.R. 2007. A new neoceratopsian dino- saur linking North American and Asian taxa. Journal of Vertebrate Paleontology, 27(3): 625–641. doi:10.1671/0272-4634(2007) 27[625:ANNDLN]2.0.CO;2. Chinnery, B.J., Lipka, T.R., Kirkland, J.I., Parrish, J.M., and Brett- Surman, M.K. 1998. Neoceratopsian teeth from the Lower to middle Cretaceous of North America. In Lower and Middle Cre- taceous terrestrial ecosystems. New Mexico Museum of Natural History and Science, Bulletin 14, pp. 297–303. Chinnery, B.J., and Weishampel, D.B. 1998. Montanoceratops cer- orhynchus (Dinosauria: Ceratopsia) and relationships among ba- sal neoceratopsians. Journal of Vertebrate Paleontology, 18: 569–585. Coombs, W.P., Jr. 1990. Teeth and taxonomy in ankylosaurs. In Dinosaur systematics: approaches and perspectives. Edited by K. Carpenter, and P.J. Currie. Cambridge University Press, New York, N.Y., pp. 269–279. Dodson, P. 1996. The Horned Dinosaurs. Princeton University Press, Princeton, N.J. Tanoue et al. 437 Published by NRC Research Press
  • Dodson, P., You, H., and Tanoue, K. Comments on the palate and basicranium of basal ceratopsians. In New perspectives on horned dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium. Edited by M. J. Ryan, B. J. Chinnery-Allgeier, and D. A. Eberth. Indiana University Press, Bloomington, Ind. In press. Dong, Z.-M., and Azuma, Y. 1997. On a primitive neoceratopsian from the Early Cretaceous of China. In Sino-Japanese Silk Road Dinosaur Expedition. Edited by Z.-M. Dong. Ocean Press, Bei- jing, China, pp. 68–89. Godefroit, P., and Lambert, O. 2007. A re-appraisal of Craspedo- don lonzeensis Dollo, 1883 from the Upper Cretaceous of Bel- gium: the first record of a neoceratopsian dinosaur in Europe? Bulletin de L’institut Royal des Sciences Naturelles de Belgique, Sciences de la Terre, 77: 83–93. Gregory, W.K., and Mook, C.C. 1925. On Protoceratops, a primi- tive ceratopsian dinosaur from the Lower Cretaceous of Mongo- lia. American Museum Novitates, 156, pp. 1–9. Kurzanov, S.M. 1992. A gigantic protoceratopsid from the Upper Cretaceous of Mongolia. Paleontological Journal, 26: 103–116. Makovicky, P.J., and Norell, M.A. 2006. Yamaceratops dorngo- biensis, a new primitive ceratopsian (Dinosauria: Ornithischia) from the Cretaceous of Mongolia. American Museum Novitates, 3530(1), pp. 1–42. doi:10.1206/0003-0082(2006) 3530[1:YDANPC]2.0.CO;2. Maryan´ska, T. 1990. Pachycephalosauria. In The Dinosauria. Edi- ted by D.B. Weishampel, P. Dodson, and H. Osmo´lska. Univer- sity of California Press, Berkeley, Calif., pp. 564–577. Maryan´ska, T., and Osmo´lska, H. 1975. Protoceratopsidae (Dino- sauria) of Asia. Palaeontologica Polonica, 33: 133–182. Maryan´ska, T., Chapman, R.E., and Weishampel, D.B. 2004. Pa- chycephalosauria. In The Dinosauria. 2nd ed. Edited by D.B. Weishampel, P. Dodson, and H. Osmo´lska. University of Cali- fornia Press, Berkeley, Calif., pp. 464–477. Ostrom, J.H. 1964. A functional analysis of jaw mechanics in the dinosaur Triceratops. Postilla, 88: 1–35. Ostrom, J.H. 1966. Functional morphology and evolution of the ceratopsian dinosaurs. Evolution, 20: 290–308. Sereno, P.C. 1990. Psittacosauridae. In The Dinosauria. Edited by D.B. Weishampel, P. Dodson, and H. Osmo´lska. University of California Press, Berkeley, Calif., pp. 579–592. Sereno, P.C. 2000. The fossil record, systematics and evolution of pachycephalosaurs and ceratopsians from Asia. In The Age of dinosaurs in Russia and Mongolia. Edited by M.J. Benton, M.A. Shishkin, D.M. Unwin and E.N. Kurochkin. Cambridge Univer- sity Press, Cambridge, UK., pp. 480–516. Sereno, P.C., and Chao, S. 1988. Psittacosaurus xinjiangensis (Or- nithischia: Ceratopsia), a new psittacosaur from the Lower Cre- taceous of northwestern China. Journal of Vertebrate Paleontology, 8: 353–365. Sereno, P.C., Chao, S., Cheng, Z., and Rao, C. 1988. Psittaco- saurus meileyingensis (Ornithischia: Ceratopsia), a new psittaco- saur from the Lower Cretaceous of northeastern China. Journal of Vertebrate Paleontology, 8: 366–377. Sternberg, C.M. 1951. Complete skeleton of Leptoceratops gracilis Brown from the Upper Edmonton Member on Red Deer River, Alberta. Bulletin of the National Museum of Canada, 123: 225– 255. Sues, H.-D., and Galton, P.M. 1987. Anatomy and the classification of the North American Pachycephalosauria (Dinosauria: Or- nithischia). Palaeontographica A, 198: 1–40. Tanoue, K., You, H., and Dodson, P. Mandibular anatomy in basal Ceratopsia. In New perspectives on horned dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium. Edited by M.J. Ryan, B.J. Chinnery-Allgeier, and D.A. Eberth. Indiana University Press, Bloomington, Ind. In press. Varriale, F. 2008. Dental microwear and jaw mechanics in basal neoceratopsians.. Society of Vertebrate Paleontology, Program with Abstracts , 28: 76A. Weishampel, D.B. 1984. Evolution of jaw mechanisms in ornitho- pod dinosaurs. Advances in Anatomy, Embryology, and Cell Biology, 87: 1–109. Wolfe, D.G., and Kirkland, J.I. 1998. Zuniceratops christopheri n. gen., and n. sp., a ceratopsian dinosaur from the Moreno Hill Formation (Cretaceous, Turonian) of west-central New Mexico. In Lower and Middle Cretaceous terrestrial ecosystems. New Mexico Museum of Natural History and Science, Bulletin 14, pp. 303–317. Xu, X., Forster, C.A., Clark, J.M., and Mo, J. 2006. A basal cera- topsian with transitional features from the Late Jurassic of north- western China. Proceedings of the Royal Society of London. Series B. Biological Sciences, 273(1598): 2135–2140. doi:10. 1098/rspb.2006.3566. Xu, X., Makovicky, P.J., Wang, X.-L., Norell, M.A., and You, H.- L. 2002. A ceratopsian dinosaur from China and the early evolu- tion of Ceratopsia. Nature, 416(6878): 314–317. doi:10.1038/ 416314a. You, H.-L., and Dong, Z.-M. 2003. A new protoceratopsid (Dinosauria: Neoceratopsia) from the Late Cretaceous of Inner Mongolia. Acta Geologica Sinica, 77: 299–304. [English Edition.] You, H.-L., and Dodson, P. 2003. Redescription of neoceratopsian dinosaur Archaeoceratops and early evolution of Neoceratopsia. Acta Palaeontologica Polonica, 48: 261–272. You, H.-L., and Dodson, P. 2004. Basal Ceratopsia. In The Dino- sauria. 2nd ed. Edited by D.B. Weishampel, P. Dodson, and H. Osmo´lska. University of California Press, Berkeley, Calif., pp. 478–493. You, H.-L., and Xu, X. 2005. An adult specimen of Hongshano- saurus houi (Dinosauria: Psittacosauridae) from the Lower Cre- taceous of Western Liaoning Province, China. Acta Geologica Sinica, 79: 168–173. [English Edition.] You, H.-L., Xu, X., and Wang, X.-L. 2003. A new genus of Psitta- cosauridae (Dinosauria: Ornithopoda) and the origin and early evolution of marginocephalian dinosaurs. Acta Geologica Sinica, 77: 15–20. [English Edition.] You, H.-L., Li, D.-Q., Ji, Q., Lamanna, M.C., and Dodson, P. 2005. On a new genus of basal neoceratopsian dinosaur from the Early Cretaceous of Gansu Province, China. Acta Geologica Sinica, 79: 593–597. [English Edition.] You, H.-L., Tanoue, K., and Dodson, P. 2007. A new specimen of Liaoceratops yanzigouensis (Dinosauria: Neoceratopsia) from the Early Cretaceous of Liaoning Province, P. R. China. Acta Geologica Sinica, 81: 898–904. [English Edition.] You, H.-L., Tanoue, K., and Dodson, P. 2008. A new specimen of Psittacosaurus major (Dinosauria: Ceratopsia) from the Early Cretaceous Yixian Formation of Liaoning Province, China. Acta Palaeontologica Polonica, 53: 183–196. You, H.-L., Tanoue, K., and Dodson, P. A new species of Archae- oceratops (Dinosauria: Neoceratopsia) from the Early Cretac- eous of the Mazongshan area in northwestern China. In New perspectives on horned dinosaurs. The Royal Tyrrell Museum Ceratopsian Symposium. Edited by M. J. Ryan, B. J. Chinnery- Allgeier, and D. A. Eberth. Indiana University Press, Blooming- ton, Ind. In press. Zhao, X.-J., Cheng, Z.-W., and Xu, X. 1999. The earliest ceratop- sian from the Tuchengzi Formation of Liaoning, China. Journal of Vertebrate Paleontology, 19: 681–691. 438 Can. J. Earth Sci. Vol. 46, 2009 Published by NRC Research Press
  • Zhao, X.-J., Cheng, Z.-W., Xu, X., and Makovicky, P.J. 2006. A new ceratopsian from the Upper Jurassic Houcheng Formation of Hebei, China. Acta Geologica Sinica, 80: 467–473. [English Edition.] Zhou, C.-F., Gao, K.-Q., Fox, R.C., and Chen, S.-H. 2006. A new species of Psittacosaurus (Dinosauria: Ceratopsia) from the Early Cretaceous Yixian Formation, Liaoning, China. Palaeo- world, 15(1): 100–114. doi:10.1016/j.palwor.2005.11.001. Tanoue et al. 439 Published by NRC Research Press /ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 150 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Average /GrayImageResolution 225 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.00000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Average /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.00000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects true /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName (http://www.color.org) /PDFXTrapped /False /CreateJDFFile false /SyntheticBoldness 1.000000 /Description > >> setdistillerparams > setpagedevice