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Home , Agilisaurus, Cerapoda, Ceratopsia, Heterodontosauridae, Marginocephalia, Pachycephalosauria

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Proc. R. Soc. B 1 doi:10.1098/rspb.2006.3566 65 2 66 3 67 4 68 5 A ceratopsian with transitional features from 69 6 70 7 the Late of northwestern 71 8 1,2,* 3 4 5,6 72 9 Xing Xu , Catherine A. Forster , James M. Clark and Jinyou Mo 73 10 1Institute of and Paleoanthropology, Chinese Academy of Sciences, 74 11 142 Xiwai Street, 100044, People’s Republic of China 75 12 2American Museum of Natural History, New York, NY 10024, USA 76 13 3Department of Anatomical Sciences, State University of New York, Stony Brook, NY 11794, USA 77 14 4Department of Biological Sciences, George Washington University, Washington, DC 20052, USA 78 15 5Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, People’s Republic of China 79 16 6Natural History Museum of Guangxi, Nanning 530012, People’s Republic of China 80 17 81 18 Although the and , two major ornithischian groups, are united as the 82 19 , few synapomorphies have been identified due to their highly specialized body-plans. 83 20 Several studies have linked the with either the Ceratopsia or Marginocephalia, but 84 21 evidence for these relationships is weak, leading most recent studies to consider the Heterodontosauridae 85 22 as the basal member of another major ornithischian radiation, the . Here, we report on a new 86 23 basal ceratopsian , downsi gen. et. sp. nov., from the upper part of the 87 24 of Xinjiang, China. This new ceratopsian displays a series of features transitional 88 25 between more derived ceratopsians and other ornithischians, shares numerous derived similarities with 89 26 both the heterodontosaurids and pachycephalosaurians and provides strong evidence supporting a 90 27 monophyletic Marginocephalia and its close relationship to the Heterodontosauridae. Character 91 28 distributions along the marginocephalian lineage reveal that, compared to the bipedal Pachycephalosauria, 92 29 which retained a primitive post-cranial body-plan, the dominantly quadrupedal ceratopsians lost many 93 30 marginocephalian features and evolved their own characters early in their . 94 31 Keywords: Ceratopsia; Marginocephalia; ; Late Jurassic; Shishugou Formation 95 32 96 33 97 34 98 35 1. INTRODUCTION Ceratopsia Marsh, 1890. 99 36 The Shishugou Formation of the Junggar Basin, Yinlong downsi gen et sp nov. 100 37 Xinjiang, China was deposited during the Middle–Late 101 38 Jurassic (Chen 1996; Eberth et al. 2001), a period that (a) Etymology 102 39 is critical to the origins and early evolution of the major ‘Yin’ and ‘long’ mean ‘hiding’ and ‘dragon’ in Chinese, 103 40 dinosaurian lineages, including (Sereno 1999). respectively, derived from the movie ‘Crouching Tiger, 104 41 Since 2001, we have prospected in this formation and Hidden Dragon’ which was filmed in the locality where the 105 42 collected numerous vertebrate specimens (Clark et al. was found; the specific name is in memory of Mr 106 43 2004), among them a nearly complete ornithischian Will Downs, who joined many palaeontological 107 44 skeleton we here identify as a basal ceratopsian expeditions in China including the one with us in 2003, 108 45 dinosaur. The two other possible Jurassic ceratopsians shortly before his death. 109 46 (Zhao et al. 1999, in press) are from the Tuchengzi and 110 47 Houcheng Formations of China. However, a recent (b) Holotype 111 48 radiometric sample from the IVPP V14530, a nearly complete skeleton missing only the 112 49 places its upper part in the Early (Swisher distal tail (figure 1 and figures 1 and 2 of electronic 113 50 et al. 2002). Consequently, the Shishugou ceratopsian supplementary material). 114 51 represents the first unquestionable Jurassic ceratopsian 115 . 52 (c) Locality and horizon 116 53 Wucaiwan, Junggar Basin, Xinjiang, China; upper part of 117 54 Shishugou Formation, correlated with the stage 118 55 2. SYSTEMATIC PALAEONTOLOGY of the early Late Jurassic (Chen 1996; Eberth et al. 2001). 119 56 Ornithischia Seeley, 1887. 120 57 Heterodontosauriformes new taxon. (d) Diagnosis 121 58 Marginocephalia, Sereno (1986). A small ceratopsian with the following unique features: a 122 59 distinct fossa along the midline of the frontals, trapezoidal 123 60 124 * Author for correspondence ([email protected]). quadratojugal longer than deep, sharp ridges and grooves 61 on the anterior surface of the proximal half of the 125 Q1 The electronic supplementary material is available at http://dx.doi. 62 paroccipital process, a long basipterygoid process oriented 126 Q2 org/10.1098/rspb.2006.3566 or via http://www.journals.royalsoc.ac. 63 uk. posteroventrally, slit-like carotid canal laterally bordered 127 64 128 Received 1 December 2005 1 q 2006 The Royal Society Accepted 29 March 2006

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129 lpe lis lt lf lm lu lh rib ls 193 130 194 131 195 132 196 133 197 134 198 135 199 136 200 137 201 138 202 139 203 140 204 141 205 142 206 143 207 144 208 145 209 5 cm 146 210 147 211 148 cv rpe rt ri rfi rf lp rm dv rh rs cev sk 212 149 213 Figure 1. Yinlong downsi (IVPP V14530, holotype). Abbreviations: cev, cervical vertebrae; cv, caudal vertebrae; dv, dorsal 150 vertebrae; lf, left ; lh, left ; lis, left ischium; lm, left ; lp, left pubis; lpe, left ; ls, left scapula; lt, left ; 214 151 lu, left ; rf, right femur; rfi, right fibula; rh, right humerus; ri, right ilium; rm, right manus; rpe, right pes; rs, right scapula; rt, 215 152 right tibia; sk, . Scale bar, 5 cm. 216 153 217 154 by a lamina, a prominent tubercle on the posteroventral that continues across the surface of the postorbital and 218 155 surangular and a vertical wear facet with a basal shelf on temporal bar. The large quadratojugal is trapezoidal and 219 156 the premaxillary teeth (figures 1 and 2; figures 1 and 2 of longer anteroposteriorly than dorsoventrally. As in 220 157 electronic supplementary material). Chaoyangsaurus (Zhao et al. 1999), the quadrate slopes 221 158 slightly anterodorsally. As in Liaoceratops, the quadrate has 222 159 (e) Description a longitudinal groove along its anterolateral margin and a 223 160 The holotype is probably a sub-adult as indicated by the paraquadratic foramen at mid-height on the posterior 224 161 open neurocentral sutures on the presacral vertebrae margin. A paraquadratic foramen is also seen in Hetero- 225 162 (closed on the caudal vertebrae). The holotype is dontosaurus (Norman et al. 2004). As in Chaoyangsaurus 226 163 estimated to be 120 cm in total body length. Relatively and an unnamed basal ceratopsian (Zhao et al. 1999), the 227 164 short and slender forelimbs and very robust and long distal quadrate condyles are separated by a deep groove, 228 165 hindlimbs (forelimb less than 40% the hindlimb length) which is shallow or absent in and more 229 166 suggest that Yinlong was a bipedal . derived ceratopsians ( You & Dodson 2003). Yinlong 230 167 The posterior part of the skull is proportionally deep in shares with (Norman et al. 2004)a 231 168 lateral view and extremely broad in the postorbital region post-orbital with a prominent ridge running along the 232 169 in dorsal view (figures 1 and 2). The pre-orbital region is jugal process and onto the postorbital process of the jugal. 233 170 shallow and, as in some other basal ceratopsians, is The squamosal process of the postorbital is long, forming 234 171 relatively short. The orbit lies entirely above the posterior nearly the whole length of the dorsal margin of the 235 172 maxillary tooth row. The supratemporal and infratem- infratemporal fenestra. As in pachycephalosaurians 236 173 poral fenestrae are proportionally large in size. The latter (Maryan´ska et al. 2004), the temporal bar has a broader 237 174 exceeds the orbit in diameter and is nearly circular in dorsal margin relative to the lateral margin. Similar to 238 175 shape. The small antorbital fossa has distinct margins, is most ornithischians, but differing from other ceratopsians 239 176 bordered ventrally by a prominent ridge, and is positioned (Sereno 2000; Makovicky 2001; You & Dodson 2004; 240 177 under the anterior orbit. Dodson et al. 2004), the posterior margin of the parietal is 241 178 As in all ceratopsians, a small rostral caps the positioned distinctly anterior to that of the squamosal and 242 179 anterior premaxillae. The has a distinct anterior its posterolateral wing has a deep distal ramus facing 243 180 process and a long posterior process reaching to the caudally rather than ventrally. These features suggest a 244 181 middle of the orbit. The skull roof bears two distinct lack of parietal contribution to the frill, though the parietal 245 182 midline fossae indenting the nasals and the frontals. The near the midline slightly overhangs over the occiput. The 246 183 Q3 former is seen in Heterodontosaurus (Barrett, personal squamosal overhangs the quadrate-squamosal articulation 247 184 communication) and some basal ornithopods (He & Cai laterally and posteriorly, forming a small squamosal frill. 248 185 1984; Peng 1992; Xu et al. 2000), and to a less degree, in Similar to pachycephalosaurians (Maryan´ska et al. 2004), 249 186 some basal ceratopsians (Xu et al. 2002). As in a row of distinct tubercles runs along the posterior margin 250 187 Heterodontosaurus (Norman et al. 2004) and some basal of the squamosal and the lateral margin of the squamosal 251 188 ceratopsians, the jugal, with a blunt anterior end, process of the post-orbital. Similar to basal pachycepha- 252 189 contributes significantly to the posterior margin of the losaurians (Maryan´ska et al. 2004), the articular fossa for 253 190 antorbital fossa. Posteriorly the jugal flares laterally and is the quadrate is separated from the main body of the 254 191 significantly thickened transversely but lacks a jugal flange. squamosal by a robust ventral process, such that the 255 192 The lateral surface of the jugal bears distinct sculpturing quadrate-squamosal articulation is relatively ventrally 256

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257 321 sq 258 (a) 322 259 323 260 en po 324 261 325 q 262 orb 326 263 itf 327 264 m 328 j 265 pm qj 329 266 330 267 sa 331 268 d an 332 269 333 270 334 271 pd emf ts 335 272 r 336 273 (b) fn pa ff tr 337 274 338 275 339 276 340 277 341 278 stf 342 279 343 280 344 p 281 f 345 282 346 283 stf 347 284 348 285 po sq 349 286 350 287 351 288 352 Figure 2. IVPP V14530. (a) Skull and in lateral view. (b) Skull in dorsal view. Abbreviations: an, angular; d, dentary; 289 emf, external mandibular fenestra; en, external naris; f, frontal; ff, fossa on frontals; fn, fossa on nasals; itf, infratemporal 353 290 fenestra; j, jugal; m, ; p, parietal; pa, palpebral; pd, predentary; pm, premaxilla; po, postorbital; q, quadrate; qj, 354 291 quadratojugal; r, rostral bone; sa, surangular; sq, squamosal; stf, supratemporal fenestra; tr, tubercle row; ts, tubercle on 355 292 surangular. Scale bar, 2 cm. 356 293 357 294 located. The braincase displays many pachycephalosaur- mandible and lacks a long retroarticular process. The 358 295 ian features (figure 1 of electronic supplementary predentary bears two small lateral processes and a long 359 296 material). The three exits for cranial nerves XI–XII open ventral process that is bifurcated distally. The oral margin 360 297 further ventrally than caudally on the posterior surface of of the predentary is somewhat bevelled. Similar to most 361 298 the proximal exoccipital, which is sharply defected ornithischians but differing from other ceratopsians 362 299 ventrally. The paroccipital process tapers distally, a feature ( You & Dodson 2004), the dentary is straight and shallow. 363 300 seen in many basal ornithischians and ornithopods. The As in pachycephalosaurians (Maryan´ska et al. 2004), the 364 301 distal paroccipital process is ventral to the occipital posterior dentary and to an even greater degree the 365 302 condyle whereas in most ceratopsians it is positioned anterior angular, is sculptured, a feature also seen in 366 303 dorsal to the occipital condyle. As in the ornithopod Liaoceratops and Archaeoceratops ( You & Dodson 2003), 367 304 Jeholosaurus and some basal ceratopsians, a longitudinal though less developed. The splenial is restricted to the 368 305 ridge is present under the occipital condyle. As in medial margin of the mandible with little ventral exposure. 369 306 pachycephalosaurians (Maryan´ska et al. 2004), the plate- As in other basal ceratopsians ( You & Dodson 2004), the 370 307 like basal tuber is compressed anteroposteriorly, with a posterior end of the mandible is medially expanded and 371 308 significant exposure of the basiosphenoid in posterior the glenoid fossa is elevated dorsally. 372 309 view. The posteroventrally oriented basipterygoid process Three premaxillary teeth and 13 maxillary teeth are 373 310 is long, with a large, flat, elongated elliptical articular facet present per side. The premaxillary tooth crowns are 374 311 facing more laterally than anteriorly. This basipterygoid notably larger than the maxillary ones and are symmetrical 375 312 process indicates a medial and posterior extension of the in lateral view with a mesiodistally convex labial surface 376 313 pterygoid, covering much of the basicranium in ventral and a flat lingual surface. The premaxillary teeth have a 377 314 view. The pterygoid has an anteriorly directed, elongate, vertical wear facet and a horizontal shelf on the lingual 378 315 vertical plate for contacting the vomer, a plate-like ventral surface (figure 1 of electronic supplementary material), 379 316 process and an extremely small posterior process. In indicating occlusion with the predentary, a feature 380 317 psittacosaurids and neoceratopsians ( You & D od so n also reported in the heterodontosaurid 381 318 2004), the vomer process is dorsally directed. (Thulborn 1970). The second premaxillary tooth bears 382 319 The mandible has a small external mandibular fenestra, fine serrations along its distal margin. There is a very short 383 320 a large fossa on the external surface of the posterior diastema. As in heterodontosaurids and most basal 384

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385 ceratopsians, the maxillary teeth are closely packed, 1999; Norman et al. 2004), which is the sistergroup to the 449 386 chisel-shaped and bear prominent mesial and distal ridges Marginocephalia (see electronic supplementary material 450 387 (figure 1 of electronic supplementary material). Relatively for further comments). A recent analysis suggests a 451 388 low-angled wear facets are present on both mesial and different ornithischian phylogeny: the Ceratopsia and 452 389 distal margins of the maxillary tooth crowns. Most dentary Pachycephalosauria are nested within the Ornithopoda, 453 390 teeth cannot be seen due to the closely occluded , but which conforms to the pre-cladistic ornithischian phylo- 454 391 the anteriormost ones are much smaller than the more geny (Butler 2005). As Yinlong has a combination of 455 392 posterior ones. derived character states found in the Ceratopsia, Pachy- 456 393 The cervical centra are short axially and tall dorsoven- cephalosauria and Heterodontosauridae, it provides 457 394 trally. The neural spines of the anterior and middle caudal substantial evidence for a close relationship of these 458 395 vertebrae are tall, approximating the corresponding three groups. A phylogenetic analysis places Y. downsi as 459 396 chevrons in length. The posterior sacral ribs are probably the most basal ceratopsian, strongly supports a mono- 460 397 long as inferred from the lateral deflection of the posterior phyletic Marginocephalia and suggests a sistergroup 461 398 end of the ilium. relationship between the Heterodontosauridae and Mar- 462 399 Three carpals are visible on the left manus. The manus ginocephalia (figure 3; see electronic supplementary 463 400 is much smaller than the pes, with robust phalanges and material). 464 401 both hoof-like unguals (flat and rounded distally) and Yinlong downsi has a skull proportionally larger relative 465 402 claw-like unguals (transversely narrower and distally to the body than most ornithischians but smaller than 466 403 pointed). The ilium displays several features shared with other ceratopsians (Sereno 2000), a thickened jugal but no 467 404 pachycephalosaurians (Sereno 2000; Maryan´ska et al. jugal flange, an elevated posterior margin of the skull roof 468 405 2004) including being longer than the femur and slightly but no parietal contribution to the frill and has a 469 406 S-shaped in dorsal view, having the distal half of the posteriorly elevated but anteriorly shallow and straight 470 407 preacetabular process twisted such that its lateral surface mandible. These features are intermediate in conditions 471 408 faces more dorsally than laterally and having a blunt between basal ornithischians and more derived ceratop- 472 409 anterior end. The dorsal margin of the posterior ilium sians, documenting the incremental appearance of cer- 473 410 flares laterally and the iliac blade expands ventrally to form atopsian diagnostic characters, a pattern also suggested by 474 411 a flat, narrow brevis shelf. The ischial peduncle is broad other basal ceratopsians (Xu et al. 2002). The Psittaco- 475 412 axially with little lateral expansion, and appears to bear a sauridae represents a lineage of the Ceratopsia that has 476 413 ventral notch. The pubis bears a short and blunt prepubic diverged from this incremental line. For example, 477 414 process and a distally flattened postpubic rod, which is Psittacosauridae has a relatively lower posterior margin 478 415 much shorter than the robust, plate-like ischium. As in of the skull roof and slender postorbital and temporal bars, 479 416 typical bipedal ornithischians, the femur is distinctly reversals to the more primitive conditions. 480 417 bowed anteriorly. A longitudinal ridge is present along A calibrated phylogeny of the Ceratopsia suggests that 481 418 the posterolateral edge of the proximal half of the femoral the group evolved the unique rostral bone and enlarged, 482 419 shaft and a robust, pendant fourth trochanter is located triangular head as early as the Oxfordian, but did not 483 420 along the posteromedial shaft. In most ceratopsians evolve the typical ceratopsian body-plan, characterized by 484 421 including Psittacosaurus, the femur is less bowed, a similar quadrupedal posture and with a parietosquamosal frill, 485 422 ridge is absent, and the fourth trochanter is thinner until the . Our character distribution 486 423 transversely and positioned mainly on the posterior suggests that ceratopsians first shortened their snout, 487 424 margin of the femoral shaft. Yinlong has very flat, pointed broadened their occipital region and elevated the posterior 488 425 pedal unguals. margin of the skull roof, then deepened their snout and 489 426 Seven , each 1–1.5 cm. in diameter, are mandible, and finally extended their frill posteriorly 490 427 exposed within the ribcage at the midlength of the trunk. (figure 3). 491 428 As in psittacosaurids (Xu 1997), they are proportionally The Heterodontosauridae–Marginocephalia is 492 429 large relative to the body size of the animal. characterized by many features related to the antorbital 493 430 and temporal regions and . Some dental features 494 431 (e.g. enlarged premaxillary teeth) were previously used to 495 432 3. DISCUSSION diagnose the Heterodontosauridae (Norman et al. 2004) 496 433 The discovery of Yinlong has significant implications for but now characterize the Heterodontosauridae–Margino- 497 434 ornithischian phylogeny. Although the Ceratopsia and cephalia clade, so we hereby name this clade Hetero- 498 435 Pachycephalosauria, two major ornithischian groups, are dontosauriformes (defined as a node-based group that 499 436 united as the Marginocephalia, only three synapomor- includes the most recent common ancestor of Hetero- 500 437 phies have been previously identified due to their highly dontosaurus and and all their descendents). 501 438 specialized body-plans (Sereno 2000; see electronic Some heterodontosauriform features, such as very large 502 439 supplementary material for further comments). Another infratemporal fenestra, are absent in known pachycepha- 503 440 problem concerning ornithischian phylogeny is the losaurian species due to their highly modified postorbital 504 441 systematic position of the Heterodontosauridae. Several region of the skull, but are expected in their basal 505 442 studies have linked the Heterodontosauridae with either members, which are currently poorly known. Noteworthy 506 443 the Ceratopsia or Marginocephalia (Norman 1984; is that our analysis posits , a 507 444 Cooper 1985; Maryan´ska & Osmo´lska 1985; Zhao et al. ornithischian taxon suggested to be either a basal 508 445 1999; Yo u et al. 2003), but evidence for these relationships ornithopod or a basal ornithischian by previous phyloge- 509 446 has been weak, leading most recent studies to consider the netic analyses (Barrett et al. 2005), as the to 510 447 Heterodontosauridae as the basal member of another the Heterodontosauriformes, though the evidence is not 511 448 major ornithischian radiation, the Ornithopoda (Sereno strong (a bootstrap value for this relationship is 21%; 512

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513 65 577 514 maa 578 515 74 579 cmp 516 L 580 83 517 san 581

518 582 Ceralopsidae cen 519 97 583 520 584 alb 521 Pachycephalosauria 585 522 586 Cretaceous

523 Archaeceratops 587 E apt 524 125 588 525 bm 589 Joholosaurus Psittacosaurus Liaoceratops 526 Hau 590 527 vlg 591 ber 528 145 592 529 tth 593 chaoyangsaurus L Yinlong 530 594 ctv 157 Agilisaurus 531 Ceratopsia 595 532 bth 596 M 533 baj 597 534 aal 178 598 535 Jurassic toa 599 536 600 plb 537 E Heterodontosauridae 601 538 sin 602 539 603 540 het 208 604 541 605 nor Heterodontosauriformes 542 606 543 L 607

544 cm 608 545 235 609 546 610 Figure 3. A single most parsimonious tree recovered showing the phylogenetic position of Yinlong downsi and the 547 611 interrelationships of the . The proposed tree plotted against geological time indicates the timing of the major cranial 548 changes among the Ceratopsia. The Marginocephalia node is supported by 22 synapomorphies and the Heterodontosaur- 612 549 iformes by nine synapomorphies (see electronic supplementary material). 613 550 614 551 see electronic supplementary material). Although the and tubercles. Conversely, the ceratopsians adopted 615 552 discovery of Y. downsi provides strong support for the quadrupedalism, gradually lost a number of plesio- 616 553 monophyly of the Marginocephalia (Sereno 1986, 2000), morphic marginocephalian characters, and evolved typical 617 554 the overhanging parietosquamosal frill, the most obvious ceratopsian features, notably the jugal horn and an 618 555 feature of the three previously recognized for the group, extensive parietosquamosal frill. Consequently, a classical 619 556 might have been independently developed in pachycepha- taxonomical dichotomy is documented by the divergence 620 557 losaurians and derived ceratopsians. Yinlong shares with leading to a more plesiomorphic pachycephalosaurian and 621 558 pachycephalosaurians a squamosal frill but lacks a parietal a more specialized ceratopsian lineage within the Margin- 622 559 contribution to the frill. The parietosquamosal frill ocephalia. The evolution of ceratopsians is particularly 623 560 evolved in more derived ceratopsians with the posterior interesting in terms of the presence of a remarkable 624 561 extension of the parietal and the reorientation of the number of both reversals and specializations: while the 625 562 posterolateral wing of the parietal. derived ceratopsians reversed to the primitive conditions 626 563 Meanwhile, several salient pachycephalosaurian in many characters, they evolved highly specialized 627 564 features, such as the sculptured angular and jugal, broad features related to a large frilled skull and a quadrupedal 628 565 and flat dorsal margin of the temporal bar, the squamosal- post-cranial skeleton, which make the group unique 629 566 postorbital tubercles and the anteroposteriorly com- among the ornithischians. 630 567 pressed basal tuber (Maryan´ska et al. 2004), are now 631 568 synapomorphies of the Marginocephalia. These features The authors thank H.-J. Wang and Z.-L. Tang for organizing 632 569 are shared by pachycephalosaurians and basal ceratop- the fieldwork, T. Yu for finding the specimen, R. S. Li for 633 570 sians, but are gradually reduced and lost in derived illustrations, R. F. Cao and X. Q, Ding for preparing the 634 571 ceratopsians (Dodson et al. 2004). Two distinctly different specimen, Dr H. L. You for constructive suggestions, Richard 635 R. Butler for sharing information on Agilisaurus, Q. Zhao, 572 evolutionary patterns are present after the splitting of the 636 C.-K. Jia and X.-Q. Ding for editing the illustrations and 573 Marginocephalia into a pachycephalosaurian and a measuring the specimen and members of Sino-American 637 574 ceratopsian lineage. The pachycephalosaurians retain a expedition team for collecting the . Fieldwork was 638 575 bipedal body-plan and further develop several margin- supported by the National Natural Science Foundation of 639 576 ocephalian features, such as the cranial sculptures China and National Geographic Society and this project 640

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