Computed tomographic analysis of dental system of three Jurassic ceratopsians: implications for the evolution of the tooth replacement pattern and diet in early-diverging ceratopsians

The dental system of ceratopsids is among the most specialized structure in Dinosauria, and includes high angled wear surfaces, split tooth roots, and multiple teeth in each tooth family. However, the early evolution of this unique dental system is generally poorly understood due to a lack of knowledge of the dental morphology and development in early-diverging ceratopsians. Here we study the dental system of three of the earliest-diverging Chinese ceratopsians: Yinlong and Hualianceratops from the early Late Jurassic of Xinjiang, and Chaoyangsaurus from the Late Jurassic of Liaoning. By using micro-computed tomographic analyses, our study has revealed significant new information regarding the dental system of these early ceratopsians, including no more than five replacement teeth in each jaw quadrant; at most one generation of replacement teeth in each alveolus; nearly full resorption of the functional tooth root during tooth replacement; and occlusion with low-angled, concave wear facets that differs significantly from the shearing occlusal system seen in ceratopsids. Yinlong displays an increase in the number of maxillary tooth alveoli and a decrease in the number of replacement teeth during ontogeny as well as the retention of remnants of functional teeth in the largest individual. Early-diverging ceratopsians thus display a relatively slow tooth replacement rate compared to late-diverging ceratopsians. Combined with paleobotany and palaeoenvironment data, Yinlong likely uses gastroliths to triturate foodstuffs, and the difference in diet strategy might have influenced the pattern of tooth replacement in later-diverging ceratopsians.

Among them, IVPP V18637 is the largest specimen and IVPP V18638 is the smallest one. IVPP 77 V18636 is slightly smaller than the holotype. The holotype of Hualianceratops (IVPP V18641) 78 was also CT scanned, but its teeth were unidentified due to poor preservation. An additional 79 specimen, IVPP V28614 (field number WCW-05A-2), which only preserves the left dentary is 80 described here for comparison (Fig. 6). We assigned this specimen to Hualianceratops based on 81 the deep and short dentary which measures 83.46 mm in length and has a depth of 33.38 mm at 82 the rostral end (40% length) and strongly rugose sculpturing present on the lateral surface of the 83 dentary ( Fig. 6A) . The holotype of Chaoyangsaurus (IGCAGS V371) includes 84 the dorsal part of a skull and a nearly complete mandible (Fig. 5) (Zhao et al., 1999).

Computed tomography 86
High-resolution X-ray micro-computed tomography was used to reveal internal anatomical   The reconstruction of Zahnreihen 95 Edmund (1960) hypothesized that teeth in reptiles are replaced in an ordered, alternating 96 segmented pattern called a Zahnreihe. Each Zahnreihe consists of a series of teeth, including 97 unerupted teeth, where a more mesial tooth is more mature than a more distal one (Hanai & 98 Tsuihiji, 2019). The distance between two successive Zahnreihe is the Z-spacing (Demar, 1972). 99 Previous researches usually defined the Zahnreihen by measurements of teeth or by applying a  Based on stage division, each functional tooth and replacement tooth was plotted on a graph 115 whose vertical axis is the growth stage and horizontal axis is the tooth position. In the graph, 116 these teeth show a regular pattern that the growth stage decreases progressively and periodically 117 over several-tooth positions. Each degressive sequence represents a Zahnreihe indicated by a 118 series of teeth linked with each other as black lines (Fig. 8). The distance between adjacent 119 Zahnreihen is Z-spacing and the Z-spacing of Yinlong is described by the mean of all 120 measurements.

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Premaxillary teeth. All premaxillae bear three alveoli (Figs. 2-4), and all three teeth are 128 preserved in IVPP V14530 (Fig. 3D). In IVPP V18636, the anterior two functional teeth are 129 preserved in the left premaxilla and the second functional tooth is shown in the right premaxilla 130 ( Fig. 2A-2D). In the largest specimen (IVPP V18637), the second left functional premaxillary 131 tooth has been lost and a replacement tooth remains in the alveolus ( Fig. 4E and 4G). The right 132 premaxilla is incomplete and the first tooth is poorly preserved but the second and third are 133 present with minor damage to their roots ( Fig. 4D and 4F).

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The digital reconstructions show that the second functional premaxillary tooth is the largest 135 among all maxillary or predentary teeth, and the third premaxillary tooth crown is quite short 136 (Fig. 3A, 3C and 3H). The lateral surface of the premaxillary teeth is convex (Fig. 2G, 3E, 4B).

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Compared with the functional teeth on the maxilla, the long axes of the roots of the premaxillary 138 teeth incline more dorsolingually ( Fig. 2G and 2H, 3E, 4B).   3 empty alveoli (Fig. 1A). The left and right maxillae of IVPP V18636 contain 7 functional teeth 169 and 8 functional teeth respectively with some empty sockets ( Fig. 2C and 2D). According to ). However, in the largest specimen IVPP V18637, the incomplete maxillae contain 7 174 functional teeth and 14 functional teeth on the left and right sides respectively ( Fig. 4D and 4E).

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The left maxilla of IVPP V18637 contains seven empty sockets, suggesting that the maxilla bears 176 14 or more teeth in adult Yinlong. 177 The maxillary tooth row is curved inwards (Fig. 3E, 4B). Generally, the length of functional 178 teeth increases to a maximum in the middle part of the maxillary tooth row and then decreases thinner layer of dentine (Fig. 1D). The elongate pulp cavity in the functional tooth nearly extends 185 over the whole root (Fig. 1D). In all specimens, strong root resorption is seen on the lingual 186 surface of some functional teeth adjacent to replacement teeth (Fig, 1D, 2F, 3C and 3D). In these 187 cases, the dentine has been resorbed by the replacement teeth such that the root base has been 188 hollowed (Fig. 3C). The root of M4 on the right maxilla of the holotype is also hollowed, but no 189 replacement tooth is present on both sides (Fig. 3C). M4s are hollowed less than D8 which is except for M9 whose lingual surface is convex (Fig. 1D). In addition, M10, which is in the 203 replacement process, has a more concave lingual surface than other functional teeth that have not 204 undergone resorption. Therefore, we hypothesize that the lingual surfaces of the crowns are flat 205 and gradually become concave as the wear facet develops (Fig. 1D). Similar wear facets can be 206 seen in Heterodontosaurus tucki (Sereno, 2012).

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The count of the replacement teeth in the maxilla of Yinlong is 1 out of 13 functional teeth in 208 the holotype. The smallest specimen IVPP V18638 has the most replacement teeth in the maxilla 209 with and CT data reveal 3 replacement teeth out of 10 functional teeth inside the right maxilla 210 (Fig. 1D). The replacement (rM10) occurs lingual to M10 whose root has been almost completely 211 resorbed with only a fragmentized layer of dentine remaining. This replacement tooth is well 212 developed and consists of the complete crown and partial root. The apex of rM10 meets the base 213 of the crown of the functional tooth. Compared with the functional teeth, the crowns of the 214 replacement teeth are rhomboidal in labial view, compressed labiolingually, and the denticles 215 extend nearly the entire margin of the crown (Fig. 1D). In IVPP V18636, there are two 216 replacement teeth preserved in the right maxilla ( Fig. 2C and 2E). The first replacement tooth, 217 preserving only the crown, is attached to the dorsal side of M3. The base of the corresponding 218 functional tooth has been hollowed and the root has been resorbed although the crown is still 219 functional (Fig. 2D). In IVPP V18636, the crown of rM10 (rM10?) is positioned posterior to M10 220 and is similar to the premaxillary replacement tooth of V18637in having a triangular outline in 221 lingual view (Fig. 4G). It may suggest that the replacement tooth with a compressed shape is 222 relatively common in Yinlong. 223 Remnants of roots of resorbed functional teeth occur in IVPP V18637. The remnants are 224 positioned labiodistal to functional M11 and M14 in the right maxilla ( Fig. 4A and 4D).

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Remnants of resorbed functional teeth preserve a thin layer of dentine and exhibit a crescent   The morphologies of dentary roots are similar to the maxillary teeth with a nearly conical 241 shape and oval cross-sections ( Fig. 2E and 2F, 3D). Most functional teeth in the dentary have 242 complete crowns ( Fig. 2C and 2D). In labial view, the outline of functional teeth on the dentary is 243 similar to maxillary teeth but its labial surface is concave (Fig. 2D). This concave surface has 244 never been found in other ceratopsians, suggesting that Yinlong had relatively precise occlusion. 245 The roots of the dentary teeth are similar to those of the maxillary teeth.

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Two replacement teeth can be seen in the dentary of the holotype (Fig. 3D). Among them, 247 the crowns of D9 in the left dentary and D12 in the right dentary have been hollowed although no 248 replacement tooth is preserved. However, the cavity caused by the resorption is similar to its 249 corresponding functional tooth on the contralateral side ( Fig. 3D and 3F). It can be concluded that 250 they are at a similar stage of the replacement progress. In addition, D8 on the right side also 251 exists a replacement tooth. Patterns of symmetry in replacement patterns can be seen in Yinlong, 252 but the replacement stage between two dentaries is slightly different.

Dentition of Chaoyangsaurus 254
The holotype of Chaoyangsaurus (IGCAGS V371) preserves the last two premaxillary  (Fig. 5E). The long axis of the replacement tooth in the premaxilla retains the same 260 angle of tilt with its corresponding functional tooth (Fig. 5E).

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CT reconstructions reveal that the maxillary teeth of Chaoyangsaurus possess different 262 crown morphology from Yinlong. In Chaoyangsaurus, the primary ridges are located more 263 distally ( Fig. 5E and 5G) and the basal ridge extends over more than 70% of the crown with 264 denticles spread over the mesial and distal margins (Fig. 5F). The lingual surfaces of the 265 maxillary crowns are concave and the crowns in the dentary also show concave surfaces similar 266 to the situation in Yinlong (Fig. 5E, 5I and 5J). The concave surface in the lingual side of open at their top and these teeth show less wear than others ( Fig. 5F and 5H). Therefore, the 271 functional teeth with open pulp cavities may be newly erupted. There are three replacement teeth 272 out of nine functional teeth on both maxillary tooth rows (Fig. 5E and 5G).

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The morphology of the dentary teeth is similar to that of maxillary teeth although no primary 274 ridge or denticles exist on the dentary crowns ( Fig. 5I and 5J). The left dentary of 275 Chaoyangsaurus possesses three replacement teeth out of nine functional teeth and on the other 276 side there are five replacement teeth out of 11 functional teeth ( Fig. 5I and 5J). According to 3D 277 reconstructions of maxillary and dentary teeth, the pulp cavity is gradually reduced through time 278 after tooth eruption ( Fig. 5F and 5H). The number of replacement teeth in Chaoyangsaurus is 279 slightly more than that of Yinlong.

Dentition of Hualianceratops 281
The crowns of the teeth in Hualianceratops are similar to those of Yinlong, but include more 282 denticles along the margins (Fig. 6C and 6D). The dentary preserves the complete morphology of 283 the crowns. They are subtriangular in labial view and the mesial and distal margins bear about 284 seven denticles respectively (Fig. 6C). Ten functional dentary teeth are identified. The tooth 285 crowns are slightly imbricated with the distal margin of each tooth overlapping the lingual side of 286 the mesial margin of the preceding tooth. The first functional tooth is broken with only part of the 287 root remains. Five replacement teeth are exposed on the lingual aspect of their corresponding 288 functional teeth and exposed at the border of the alveoli (Fig. 6D).

Replacement progress and tooth development in Yinlong and Chaoyangsaurus 290
In Yinlong and Chaoyangsaurus, the resorption of the functional tooth is initiated before the 291 successional tooth has germinated (Fig. 1D, 5G and 5I). The functional tooth roots are resorbed 292 resulting in a depression on the middle part of the roots (Fig. 1D and 5I). After the depression 293 extends enough, the replacement teeth form lingual to the functional tooth roots with the crown 294 situated a small distance away from the middle part of the roots. The replacement tooth crown 295 then gradually grows out towards the margin of the alveolus. The most immature replacement 296 teeth are represented by small cusps (Fig. 1C, 5E and 5G). With ontogeny, the crowns of more 297 mature teeth become fully developed and largely resorb the lingual aspects of the roots of the 298 functional teeth, part of which are housed in their pulp cavities (Fig. 1C, 3D, 4G and 5G). In this 299 stage, some crowns in Yinlong and Hualianceratops of replacement teeth were flat labiolingually 300 and possibly kept this morphology until erupted (Fig. 1D, 2C, 4G and 6D). However, the 301 replacement crowns in Chaoyangsaurus were inflated and the morphology was almost the same 302 as that of the functional teeth ( Fig. 5E and 5G). Differing from the maxillary teeth, the crowns of 303 the premaxillary replacement teeth are housed in the more apical part of the functional tooth and 304 a similar situation occurs in Chaoyangsaurus (Fig. 4E, 4G and 5E). As the lingual surface of the 305 functional teeth becomes heavily resorbed, the replacement teeth reach about 60% or more of 306 their predicted full size (Fig. 1D). When the replacement tooth grows to its final size, most of the 307 roots of the predecessors have faded through heavy resorption and may leave the small root 308 remnants on the labial surface of its successor's tooth ( Fig. 7A and 7B, 7D and 7E).

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The Zahnreihen in Yinlong 310 In the Zahnreihen graph of IVPP V18638, these teeth show the regular pattern that the 311 growth stage decreases progressively over a two-tooth position or three-tooth position period and 312 hence at least four Zahnreihe are possibly identified (Fig. 8). The resulting Zahnreihen are 313 formed by M1 to M3, M5 to M6, M8 to rM10 and M10 to M11 respectively and run more or less 314 parallel to each other (Fig. 8). The M1-M3 and M8-M10 are well-defined tooth replacement 315 series and the exceptions are rM1, rM2, and M13. In Yinlong, Z-spacing is between 2.5 and 3.0, 316 and the average Z-spacing is 2.83. Edmund (1960) suggested that the Z-spacing in reptilian 317 dentitions is higher in the anterior region of the tooth row generally. This pattern is also present 318 in Yinlong, whereas Z-spacing is higher in the posterior region of the tooth row in Liaoceratops 319 . Fastnacht (2008) suggested that the replacement ratio of tooth formation against 320 tooth resorption can be directly derived by the Z-spacing. Although the replacement ratio cannot 321 yield the time information, it represents the replacement rate to a certain extent and is only 322 comparable within a single taxon. The lower the value is, the higher is the tooth replacement rate 323 (Fastnacht, 2008). Therefore, Z-spacing provides an index to compare the replacement rate in one 324 taxon or jaw element.

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The lower Z-spacing in the posterior region of Yinlong may suggest that the posterior region 326 of the tooth row has a higher replacement rate. To maintain the efficiency of chewing, it is 327 advantageous to replace more worn teeth more rapidly. Therefore, this may indicate that the  tooth rows, and regular occlusal surfaces. There are also some differences between these taxa.

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The crowns of unworn teeth in Yinlong and Hualianceratops are subtriangular and bear primary 378 ridges located at the midline of the crowns (Fig. 1C and 6C). Unlike Yinlong and 379 Hualianceratops, the maxillary dentitions of Chaoyangsaurus developed ovate crowns and the 380 relatively prominent primary ridge located at relatively distal to the midline of the crowns as in 381 most neoceratopsians (Fig. 5F and 5H). In addition, the roots in Yinlong are straight, unlike 382 Chaoyangsaurus whose functional roots are curved lingually (Fig. 9A-C). Overall, the dentitions neoceratopsians. In addition, the primary ridges are located at the midline of the crowns in 389 P.lujiatunensis (IVPP V12617).

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The dental structures in neoceratopsians differ from early-diverging ceratopsians. In  in Leptoceratops (Brown & Schlaikjer, 1940), which may be a primitive feature of ceratopsids.

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As such the roots of the preceding functional teeth became two-rooted in ceratopsids (Erickson et  instead of mostly resorbed as they do in early-diverging ceratopsians (Edmund, 1960).

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Tooth replacement pattern. Besides the morphological differences, a high rate of tooth 439 replacement characterizes ceratopsids, identified by more replacement teeth in each vertical 440 series (Erickson, 1996). In early-diverging neoceratopsians (Liaoceratops, Auroraceratops), an 441 alveolus bears at most two replacement teeth with a relatively lower replacement rate (Tanoue et (Table. 3).

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Overall, the evolution of dentitions from the earliest-diverging ceratopsians to ceratopsids  (Wings, 2007). Previously, 480 unambiguous evidence supporting the presence of lithophagia is known from Psittacosaurus 481 among ceratopsians (Osborn, 1923;Ignacio, 2008). Therefore, some early-diverging ceratopsians 482 that show relatively slow tooth replacement rates and lack evidence of heavy tooth wear likely 483 used gastroliths to triturate foodstuffs to cope with the stringent requirements for digestion of 484 plant materials.