Metatarsals of a Large Caenagnathid Cf. Anzu Wyliei (Theropoda: Oviraptorosauria) from the Hell Creek Formation in South Dakota, U.S.A

Home , Caenagnathidae, Chirostenotes, Deinonychus, Elmisaurus, Gigantoraptor, Oviraptoridae

Canadian Journal of Earth Sciences

Metatarsals of a large caenagnathid cf. Anzu wyliei (Theropoda: Oviraptorosauria) from the Hell Creek Formation in South Dakota, U.S.A.

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2020-0171.R1

Manuscript Type: Article

Date Submitted by the 04-Jan-2021 Author:

Complete List of Authors: Tsujimura, Kousuke; The University of Tokyo, Dept. of Earth and Planetary Science Manabe, Makoto; National Museum of Nature and Science, Geology & PalaeontologyDraft Chiba, Yumiko; The University of Tokyo, Dept.of Earth and Planetary Science Tsuihiji, Takanobu; National Museum of Nature and Science, Dept. of Geology and Paleontology

Keyword: Dinosauria, Theropoda, Maastrichtian, Hell Creek Formation

Is the invited manuscript for consideration in a Special Tribute to Dale Russell Issue? :

1 Metatarsals of a large caenagnathid cf. Anzu wyliei (Theropoda: Oviraptorosauria) from the

2 Hell Creek Formation in South Dakota, U.S.A.

4 Kousuke Tsujimura1, Makoto Manabe2, Yumiko Chiba1 and Takanobu Tsuihiji1,2*

5 1Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo,

6 Bunkyo-ku, Tokyo 113-0033 Japan

7 2Department of Geology and Paleontology, National Museum of Nature and Science, 4-1-1

8 Amakubo, Tsukuba, Ibaraki 305-0005 Japan

9 *Corresponding author. Email: [email protected]

20 Abstract

21 Isolated metatarsals III and IV of a caenagnathid theropod likely referable to Anzu wyliei are

22 described from a locality of the Hell Creek Formation in northwestern South Dakota of the

23 U.S.A. These bones are missing from the holotype and only partial shafts have been

24 described for a specimen referable to this species. Accordingly, the present description adds

25 further anatomical information on this already well-known species of Caenagnathidae. The

26 present finding also demonstrates the significance of isolated or fragmentary specimens

27 found in multitaxic bone beds.

28 Draft

29 Key Words: Dinosauria, Theropoda, Caenagnathidae, Hell Creek Formation, Maastrichtian

41 Introduction

42 The caenagnathid theropod Anzu wyliei has been described from localities of the

43 Maastrichtian Hell Creek Formation in North and South Dakota (Lamanna et al. 2014). It is a

44 large-bodied member of Oviraptorosauria and, with the femoral length ranging from 500 to

45 525 mm and the total length estimated to be 3.5 m (Lamanna et al. 2014), is surpassed only

46 by Gigantoraptor erlianensis in body size for this clade. In addition, with most skeletal

47 elements known based on multiple specimensDraft referred to the species (Lamanna et al. 2014),

48 A. wyliei is one of the reference species among Caenagnathidae for its morphological

49 information. Among the bones missing from the holotype and referred specimen of this

50 species, however, are the weight-bearing metatarsals, i.e., metatarsals II through IV.

51 Although Cullen et al. (2020) recently described associated left metatarsals II through IV in a

52 large caenagnathid referred to A. wyliei (ROM VP 65884) found in the Hell Creek Formation

53 in Montana, these bones consist mostly of broken shafts and lack morphological information

54 on their proximal and distal articular ends.

55 In describing the skeletal morphology of Gigantoraptor erlianensis, Xu et al. (2007)

56 suggested a potentially high cursorial ability of Gigantoraptor erlianensis compared to

57 theropods of similar body sizes based on a relatively slender femur and longer tibia/fibular

58 and metatarsals relative to the femur. Although this observation was based on a limited

59 amount of data, it would suggest possible locomotor aptation specific to Caenagnathidae. To

60 further examine this hypothesis, information on the hindlimb bones of other species

61 belonging to this clade is crucial.

62 In the present paper, we describe isolated metatarsals III and IV likely referable to

63 Anzu wyliei that were found in the Hell Creek Formation exposed in northwestern South

64 Dakota of the U.S.A. These specimens contribute new anatomical information on this

65 currently well-known species of Caenagnathidae. In addition, the importance of such isolated

66 or fragmentary bone bed specimens inDraft evaluating the local species diversity of dinosaurs

67 before the K/Pg extinction event is discussed.

69 Institutional abbreviations

70 CMN, Canadian Museum of Nature, Ottawa, Ontario, Canada; NSM, National Museum of

71 Nature and Science, Tsukuba, Ibaraki, Japan; ROM, Royal Ontario Museum, Toronto,

72 Ontario, Canada; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta,

73 Canada; ZPAL, Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland.

75 Materials and methods

76 Measurements on the specimens described herein were made using a digital caliper

77 to the nearest hundredth mm and rounded to the nearest tenth millimeter (for those smaller

78 than 200 mm) or with a taper measure to the nearest mm (for those larger than 200 mm).

79 These measurements are summarized in Table 1.

80 The specimens were also subjected to X-ray computed tomographic (CT) imaging.

81 They were scanned on a Shimadzu inspeXio SMX-225 CT FPD HR scanner at the National

82 Museum of Nature and Science, Tsukuba, Ibaraki, Japan, with a slice thickness of 365 μm at

83 200 kV and 70 μA. Observations on image slices and three-dimensional (3D) visualization

84 were done using the software package Amira 6.4.0 (FEL, Hillsboro, Oreg.).

85 Draft

86 Geological setting

87 The specimens described herein were discovered from a single fossil locality in the Hell

88 Creek Formation called the Sandy Site (Triebold, 1997; Russell and Manabe, 2002; Bartlett,

89 2004) situated in Harding County in northwestern South Dakota, U.S.A (Fig. 1). The section

90 cropping out at this locality is located approximately 50 m below the boundary between the

91 Hell Creek and Fort Union formations and belongs to the megafloral zone HCII (Johnson,

92 2002; K. Johnson, pers. comm., cited in Bartlett, 2004). A recent survey, however, found that

93 the fossiliferous horizon is located only 30 m below the formational contact (T. R. Lyson,

94 pers. comm., October 2020).

95 These bones described herein are among more than 3000 isolated or fragmentary

96 vertebrate skeletal elements collected from a single layer. This fossil-bearing layer is spread

97 out over an approximately 50-m-long outcrop. It overlies a tabular mudstone and varies in

98 thickness between approximately 1 and 1.5 m. The fossiliferous horizon consists of a

99 structure-less unit with layers of rip-up clasts of 1–2 cm in diameter and is overlain by a

100 coarse-grained, yellowish sandstone. The fossils were found on top of, and mixed in with, the

101 rip-up clast horizon (Bartlett, 2004). These sedimentological observations suggest a crevasse

102 splay deposit within a meandering river system (Fastovsky, 1987; Bartlett, 2004). Such

103 multitaxic bonebed localities are not uncommonDraft within the Hell Creek Formation and provide

104 important insights into the paleoecology of the latest Cretaceous North American ecosystems

105 (Lyson and Joyce, 2009; Horner et al., 2011; Lyson et al., 2019).

106 The specimens described herein, as well as other bones collected at this fossiliferous

107 unit by Triebold Paleontology, Inc., are housed at the National Museum of Nature and

108 Science, Japan (Russell and Manabe, 2002). In addition, specimens collected at this locality

109 after acquisition of the collection by the National Museum of Nature and Science are housed

110 at the Denver Museum of Nature & Science, Denver, Colorado, U.S.A. (with the locality

111 number DMNH Loc. 1492; T. R. Lyson, pers. comm., October 2020).

112

113 Systematic paleontology

114 Dinosauria Owen, 1842

115 Theropoda Marsh, 1881

116 Maniraptora Gauthier, 1986

117 Oviraptorosauria Barsbold, 1976

118 Caenagnathidae Sternberg, 1940

119 Cf. Anzu wyliei Lamanna, Sues, Schachner, and Lyson, 2014

120

121 Material—NSM PV 21086, isolated left metatarsal III, and NSM PV 21055, isolated right

122 metatarsal IV. Draft

123 Locality—Sandy Site, Harding County, South Dakota, U.S.A.

124 Formation/Age—Hell Creek Formation (Maastrichtian).

125

126 Description

127 Third metatarsal (NSM PV 21086; Figs. 2, 4A)

128 This specimen, especially the distal portion, is fractured. A small portion of the

129 proximal end is missing whereas the proximal end of the distal condyle is badly damaged and

130 partially restored with plaster. This specimen is identified a bone of the left side for the

131 following reasons. Firstly, Currie (1989), Currie et al. (2016) and Funston et al. (2016)

132 described that the articulation between the metatarsals III and II is more intimate than the one

133 between the metatarsals III and IV in the caenagnathids Elmisaurus rarus and Citipes

134 elegans. As described below, the articular surface on the left side seen in dorsal or anterior

135 view is better developed than the one on the right. If it could be assumed that the relationship

136 among these three metatarsals in E. rarus and C. elegans holds true for Caenagnathidae in

137 general, NSM PV 21086 would be a left metatarsal because the larger articular surface on the

138 left would be for tighter articulation for the metatarsal II. Secondly, in caenagnathids, the

139 metatarsal III dorsally overlies the metatarsal IV more extensively than it does the metatarsals

140 II, resulting in the articular surface of the metatarsal III for the metatarsal IV more angled

141 than the one for the metatarsal II. ThisDraft condition is reflected by the medial cruciate ridge on

142 the posterior aspect of the shaft demarcating the articulation surface for the metatarsal II

143 being sharper and lying closer to the edge of the shaft than the lateral cruciate ridge

144 demarcating the one for the metatarsal IV (G. Funston pers. comm., October 2020). These

145 observations suggest that the articular surface for the metatarsal IV in NSM PV 21086 is the

146 one on left side seen in dorsal view, thus indicating that this is a bone of the left side.

147 In general, the shaft is dorsoplantarly (or anteroposteriorly) flat (Figs. 2, 4A). The

148 proximal end of the bone, however, is thickened along this axis. At approximately one half of

149 the shaft length from the proximal end and more distally, the articular surface for the adjacent

150 metatarsal (metatarsal II or IV) is developed as a distinct facet, the plantar edge of which

151 forms a distinct, longitudinal ridge on each side of the shaft (Figs. 2A, C, D, E, 4A). The

152 same pair of ridges, termed cruciate ridges by Funston et al. (2016), are present in the

153 metatarsals III of caenagnathids such as Chirostenotes pergracilis (Currie and Russell, 1988),

154 Elmisaurus rarus (Currie et al., 2016) and Citipes elegans (Funston et al., 2016). Between

155 these ridges on the plantar surface of the shaft developed is a longitudinal, concave surface

156 (Figs. 2D, E, 4A) as described in caenagnathids such as Chirostenotes pergracilis (Currie and

157 Russell, 1988), Elmisaurus rarus (Currie et al., 2016) and Citipes elegans (Funston et al.,

158 2016).

159 The facet of each articular surface for the metatarsal II and IV is roughly fusiform,

160 with the broader end distally and taperingDraft proximally and facing medioplantarly (the one for

161 the metatarsal II) or lateroplantarly (the one for the metatarsal IV; Fig. 2A, C). The distal end

162 of each articular surface extends distally as a rugose surface beyond that of the longitudinal

163 ridge onto the plantar aspect of the shaft. While tapering proximally, the articular surface for

164 the metatarsal IV appears twisted, coming to face laterally and then dorsolaterally along the

165 lateral margin of the shaft at approximately one third of the shaft length from the proximal

166 end. More proximally, the articular surface for the metatarsal IV expands, occupying the

167 dorsolateral surface of the shaft and extending onto the lateral aspect of the dorsoplantarly-

168 expanded proximal end. The articular surface for the metatarsal II, on the other hand,

169 becomes only slightly twisted proximally and extends along the medial edge of the shaft

170 toward the proximal end of the bone.

171 The distal condyle appears to be only barely ginglymoid, lacking a prominent groove

172 separating the medial and lateral hemicondyles, although cortical bone of the distal articular

173 condyle is mostly eroded away, obscuring detailed morphology (Fig. 2F). The ligament pits

174 are deep on both sides.

175

176 Fourth metatarsal (NSM PV 21055; Figs. 3, 4B)

177 NSM PV 21055 is a right metatarsal IV. In proximal view, the proximal end of this

178 bone appears strongly convex laterodorsally and possesses a concavity medioplantarly (Fig.

179 3H). The latter concavity would have accommodatedDraft the thickened proximal end of the

180 metatarsal III. A bulbous prominence projects dorsally, producing an overhang. CT scan data

181 show that this is not a distal tarsal or any other bones fused to the metatarsal IV. It appears to

182 be overgrowth of bone, possibly a pathological feature.

183 The shaft is bowed dorsally as preserved (Fig. 3A, B, D). Although most likely

184 affected through postmortem processes to some extent, the dorsal bow appears to be a

185 genuine condition in life. In general, the plantar surface of the shaft is slightly concave

186 whereas the dorsal surface is strongly convex dorsally (Fig. 4B). Accordingly, the cross

187 section at the proximal one fifth of the shaft is roughly D-shaped with a convex dorsal surface

188 and a flat to slightly concave plantar surface that is bounded by well-defined longitudinal

189 ridges medially and laterally. More distally, another ridge appears on the medial aspect of the

190 shaft, extends obliquely in the distolateral direction across the dorsal aspect of the shaft. The

191 same ridge is present in the metatarsal IV of Citipes elegans (Funston, 2020). This ridge

192 eventually disappears at the dorsolateral corner of the shaft proximal to the distal articular

193 condyle (olrd in Figs. 3 and 4). At this level, the cross section is trapezoidal-shaped, with this

194 ridge forming one of the corners (Fig. 4B)

195 The shaft lacks a posterolateral expansion that characterizes the metatarsals IV

196 described for Elmisaurus rarus (Currie et al., 2016). However, the posterolateral edge of the

197 shaft is characterized by a distinct, longitudinal ridge that bears a rugose surface distally (plr

198 in Fig. 3A, B). A similar ridge is describedDraft in the same bone of Citipes elegans by Funston

199 (2020). At approximately one seventh of the shaft length from its distal end, this ridge

200 extends onto the lateral aspect of the shaft. It terminates proximally to the distal articular

201 condyle.

202 A fusiform and rugose articular surface for the metatarsal III occupies most of the

203 medial aspect of the distal two fifths of the shaft (Fig. 3D). The main part of this articular

204 surface is divided by a low, longitudinal ridge (lras in Figs. 3D, 4B) into a weakly concave

205 dorsal part and a slightly convex plantar part, corresponding to the articular surface for the

206 metatarsal IV on the metatarsal III that is convex dorsally and concave plantarly.

207 The plantar margin of this articular surface is demarcated by a prominent,

208 longitudinal ridge, which is distally continuous with an oblique ridge (orp in Fig. 3) that

209 extends distolaterally toward the lateroplanter corner of the shaft across its planar surface and

210 bears rugosity. Finally, it is continuous with the plantarly-expanded margin of the distal

211 articular condyle. Between this oblique ridge on the one hand and the posterolateral edge of

212 the shaft and a ridge continuing from it described above on the other is a shallow depression

213 (sd in Fig. 3A, B) on the lateral aspect of the bone that continues proximally to the slightly

214 concave plantar surface of the shaft. A likely homologous depression was described as a

215 “shallow but wide groove” in the metatarsal IV in Elmisaurus rarus by Currie et al. (2016, p.

216 151).

217 In the proximal two fifths of theDraft shaft, on the other hand, the articular surface for the

218 metatarsal III tapers rapidly and becomes restricted to the medioplantar edge of the shaft

219 before slightly expanding again to form a rugose, mound-like articular surface. This

220 morphology suggests that the metatarsals IV and III would not have been appressed to each

221 other very strongly on this part.

222 The distal articular condyle is generally convex and ball-like, lacking a groove

223 separating the medial and lateral hemicondyles, appearing roughly trapezoidal in distal view

224 (Fig. 3G). This is similar to the same condyle of the metatarsal IV of Elmisaurus rarus

225 depicted by Currie et al. (2016) and Citipes elegans described by Funston (2020) but unlike

226 that of the metatarsal II of E. rarus or Chirostenotes sp. (CMN 9570) described in Currie and

227 Russell (1988) that is dorsoplatarly higher than mediolaterally wide and possesses a deep

228 cleft dividing the medial and lateral hemicondyles plantarly. Plantarly, the lateral margin of

229 the articular condyle forms a wing-like expansion on which a broad ligament pit lies. This

230 expansion is separated from the rest of the articular condyle only by a shallow depression

231 distoventrally. The ligament pit on the medial aspect is more deeply developed than the on

232 the lateral aspect.

233

234 Discussion

235 Systematic/taxonomic assessment of the specimens

236 The taxonomic referral of eachDraft described bone is herein discussed. First, the most

237 systematically distinguishing feature observed on the metatarsal III (NSM PV 21086) is a

238 proximally-pinched and dorsoplantarly-flattened shaft. Among theropods with the

239 arctometatarsalian condition (Holtz, 1995), a dorsoplantarly-flat shaft of the metatarsal III is

240 regarded a feature unique to Caenagnathidae (Funston et al., 2016), present in such species as

241 Chirostenotes pergracilis, Elmisaurus rarus and Citipes elegans (Osmólska, 1981; Currie

242 and Russell, 1988; Currie, 1989; Funston et al., 2016). In addition, cruciate ridges developed

243 on the plantar aspect of the shaft are also considered a caenagnathid characteristic (Funston et

244 al., 2016). These features present in NSM PV 21086 indicates that this specimen belongs to a

245 caenagnathid.

246 The basis for taxonomic identification of the metatarsal IV (NSM PV 21055) is

247 mainly the presence of characteristics shared with one or more caenagnathid species. Such

248 characteristics include a posterolateral ridge on the plantar aspect of the shaft, a longitudinal,

249 oblique ridge on the anterior surface of the shaft, and a shallow and wide groove/depression

250 positioned proximally to the lateral ligament pit of the distal condyle (Currie et al., 2016;

251 Funston et al., 2016; Funston, 2020). These features are not observed in other Late

252 Cretaceous theropods other than caenagnathids (G. Funston pers. comm., October 2020).

253 In addition, NSM PV 21055 is distinct from the metatarsal IV of other Late

254 Cretaceous non-avialan coelurosaurianDraft clades in the following aspects. It differs from the

255 same bone in Dromaeosauridae in having a very narrow articular surface for the metatarsal

256 III, mostly restricted to medioplantar edge of the shaft, in the proximal two fifths of the shaft.

257 In Dromaeosauridae, in contrast, the articular surface for the metatarsal III is well-developed

258 proximally, reflecting the condition that the metatarsals IV and III are appressed strongly to

259 each other proximally (Ostrom, 1969). NSM PV 21055 is also different from the metatarsal

260 IV in Tyrannosauroidea in lacking a pronounced lateral divergence of the distal condyle

261 present in the latter (e.g., Brochu, 2003). The articular surface for the metatarsal III is almost

262 entirely restricted to the medial aspect of the shaft in NSM PV 21055 unlike the one on the

263 metatarsal IV in Ornithomimidae (e.g., fig. 8 in Claessens and Loewen, 2015) or

264 Tyrannosauroidea (e.g., fig. 101 in Brochu, 2003) that expands onto the dorsomedial aspect

265 of the distal shaft and is thus visible in dorsal view.

266 NSM PV 21055 also lacks a flange along its posteromedial edge underlying the shaft

267 of the metatarsal III that characterizes derived Late Cretaceous members of Troodontidae

268 such as Troodon and Talos (Zanno et al. 2011). Finally, the metatarsal IV in Alvarezsauridae,

269 another clade of coelurosaurians present in the Upper Cretaceous, has a distal articular

270 condyle that is plantarly divided by a cleft into the lateral and medial rims or hemicondyles,

271 the latter of which is directed plantarly (e.g., Perle et al., 1994; Turner et al., 2009). In NSM

272 PV 21055, in contrast, the distal articularDraft condyle lacks such a cleft plantarly separating

273 lateral and medial rims and the medial rim is not projected plantarly. Accordingly,

274 Caenagnathidae is the most likely clade to which NSM PV 21055 belongs.

275 For oviraptorosaurian metatarsals, the metatarsals III and IV described herein with

276 lengths of 355 mm and 328 mm, respectively, are rather large. For comparison, the lengths of

277 metatarsals III and IV are 207 mm and 186 mm in Chirostenotes pergracilis (TMP

278 1979.020.0001; Funston, 2020), 157 mm and 147 mm in Elmisaurus rarus (ZPAL MgD-I/98;

279 Osmólska, 1981) and 172 mm and 161 mm in Citipes elegans (TMP 1982.016.0006; Funston

280 et al., 2016), respectively. The large oviraptorosaurian Anzu wyliei, of which three known

281 femora range from 500 to 525 mm in length, was described from localities of the same

282 formation exposed in North and South Dakota (Lamanna et al., 2014). Accordingly, it is

283 reasonable to tentatively refer these large metatarsals to this species.

284

285 Implications for caenagnathid anatomy

286 Xu et al. (2007) showed that the largest-known caenagnathid Gigantoraptor

287 erlianensis, of which the femur is 1100 mm in length, has a femoral length/metatarsal length

288 ratio (0.53) similar to the one found in much smaller oviraptorids (0.55). This observation led

289 to Xu et al. (2007) to suggest that G. erlianensis is an outlier for a general allometric trend of

290 the distal hind limb bones becoming shorter relative to the femoral length as the body size

291 increases in theropods. However, the caenagnathidDraft Chirostenotes pergracilis (TMP

292 1979.020.0001) with the femoral length of 310 mm has 0.67 for the metatarsal III/femur

293 length ratio (Currie and Russell, 1988; Holtz, 1995). Accordingly, it is possible that

294 caenagnathids in general tend to have a longer metatarsus relative to the femur length than

295 oviraptorids and thus that G. erlianensis is still on the trend of larger-bodied species having

296 relatively shorter distal hind limb bones within a clade. Although the femoral length of the

297 individual from which NSM PV 21086 or 21055 is derived is not known, the ratio of the

298 length of the former metatarsal III to the femoral lengths (between 500 and 525 mm) of

299 individuals of Anzu wyliei described by Lamanna et al. (2014) ranges between 0.68 and 0.71,

300 which are values similar to that in C. pergracilis. Although clarification of such allometric

301 relationships among hindlimb bones within Caenagnathidae obviously requires more data, it

302 is at least suggestive of a clade-specific pattern different from that for Oviraptoridae.

303

304 Implications for dinosaur species diversity before the K/Pg boundary event

305 Specimens collected at the Sandy Site locality including NSM PV 21055 and 21086

306 are predominantly disarticulated, isolated skeletal elements accumulated in a high

307 concentration (Bartlett, 2004). Such isolated and/or bonebed materials represent taphonomic

308 modes and, in turn, preservation biases that are different from those for articulated skeletons

309 (Dodson, 1983). Such differences indicateDraft that isolated specimens can potentially provide

310 data on the taxonomic diversity of dinosaurs that are not captured based on complete or

311 nearly complete skeletons, particularly for taxa that are rare, as is the case with vertebrate

312 microsite materials (e.g., Brinkman et al., 2005).

313 Whereas the hypothesis that the asteroid impact was the major cause for the K/Pg

314 mass extinction event has been strongly supported (e.g., Schulte et al., 2010; Hull et al.,

315 2020), the multiple cause scenario for this extinction event, e.g., the global species diversity

316 had already been declining during the latest Cretaceous due to environmental changes, is still

317 suggested especially by researchers working on terrestrial vertebrates (Archibald et al., 2010).

318 In fact, studies on non-reptilian clades found in the Hell Creek Formation proposed that

319 ecological parameters such as species diversity and relative abundance in the ecosystem had

320 been changing before the impact (e.g., Wilson, 2014; Wilson et al., 2014). As for non-avian

321 dinosaurs, a lingering question is whether their taxonomic and/or morphological diversity

322 was on decline during the latest Cretaceous or they were at the helm when the asteroid struck

323 (e.g., Russell, 1984; Person et al., 2001; Brusatte et al., 2012).

324 The dinosaurian fauna in the Hell Creek Formation has been playing a pivotal role in

325 this debate (Clemens and Hartman, 2014). Since the early 2000’s, in particular, studies

326 focusing on ontogenetic changes of the Hell Creek dinosaurs have tended to regard formerly

327 recognized different species merely representing intraspecific, ontogenetic variations and thus

328 synonymize them, leading to the hypothesisDraft of an extremely depleted dinosaur diversity in

329 this formation (Carr and Williamson, 2004; Horner and Goodwin, 2009; Scannella and

330 Horner, 2010; Campione and Evans, 2011). In contrast, Chiarenza et al. (2019), using

331 ecological niche modelling, proposed that dinosaur diversity in the Maastrichtian of North

332 America is likely to be underestimated, with the apparent decline during the latest Cretaceous

333 being caused by reduction of the spatial sampling window, and found no support for the

334 multiple cause scenario for extinction.

335 Because the studies focusing ontogenetic changes listed above necessarily depended

336 on well-preserved specimens, especially skulls, isolated and/or fragmentary specimens may

337 compliment data on the taxonomic diversity of dinosaurs for the reason provided above. In

338 fact, at least one caenagnathid species that is distinct from the one represented by NSM PV

339 21055 and 21086 and is likely similar to the Citipes-like form described by Varricchio (2011)

340 has been identified based on several bones collected at the Sandy Site (Tsujimura, 2020).

341 Additionally, Tsujimura (2020) identified two, potentially distinct forms of ornithomimids, in

342 addition to one form each for Tyrannosauridae, Troodontidae and Dromaeosauridae, in the

343 Sandy Site theropod assemblage housed at NSM. This is in contrast to the result of Horner et

344 al. (2011), which recognized only one ornithomimid taxon, Ornithomimus, in their survey of

345 the Hell Creek dinosaur fauna. Identification by Tsujimura (2020) was based on isolated

346 bones and could thus be confounded by intraspecific morphological variations more easily

347 than the study by Horner et al. (2011) Draftbased on articulated or associated skeletons.

348 Nonetheless, such a result shows premise that detailed studies on “poorly-preserved”

349 specimens in the Hell Creek Formation may contribute to changing the picture of the local

350 dinosaurian diversity right before the asteroid impact and, in turn, to the debate on the degree

351 of impact by the asteroid on the eventual extinction of non-avian dinosaurs.

352

353

354 Acknowledgements

355 It is our great honor to contribute a paper to the present volume in memory for Dale

356 Russell. Dale was an inspiring mentor and a good friend of MM for more than 30 years. He

357 was the one who recognized the importance of the Sandy Site material and came to Japan to

358 do a preliminary study of the material housed in NSM, resulting in Russell and Manabe

359 (2002). We thank C. Sakata (NSM) for her assistance in the collection. Thanks are also due to

360 S. Nomura G. Shinohara and T. Kutsuna (NSM) for access to the CT scan facility under their

361 care. This is a part of master’s thesis projects by K.T. and Y.C. conducted at the Department

362 of Earth and Planetary Science, The University of Tokyo. G.F. Funston and T.R. Lyson

363 provided valuable information and made constructive suggestions that greatly improved the

364 quality of the manuscript.

365

366 References Draft

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538

539

540

541

542

543

544

545 Figure captions

546 Figure 1. Map of the northwestern part of U.S.A. showing the Sandy Site locality (modified

547 from Johnson, 2002).

548

549 Figure 2. Caenagnathid left metatarsal III (NSM PV 21086). A, lateral view; B, dorsal view;

550 C, medial view; D, plantar view; E, 3D surface rendering image based on CT scan data in

551 dorsal view; F, distal view (with the dorsal side up); G, proximal view (with the dorsal side

552 up). Scale bar equals 10 cm. Abbreviations: asm, articular surface for the adjacent metatarsal;

553 lp, ligament pit. Draft

554

555 Figure 3. Caenagnathid right metatarsal IV (NSM PV 210550). A, lateral view; B, 3D surface

556 rendering image in lateral view; C, dorsal view; D, medial view; E, plantar view; F, 3D

557 surface rendering image in plantar view; G, distal view (with the dorsal side up); H, proximal

558 view (with the dorsal side up). Surface rendering images of B and F were based on CT scan

559 data. Scale bar equals 10 cm. Abbreviations: asm, articular surface for the metatarsal III; lp,

560 ligament pit; lras, longitudinal ridge dividing the articular surface for the metatarsal III ; olrd,

561 oblique longitudinal ridge on the dorsal aspect of the shaft; orp, oblique ridge on the plantar

562 aspect of the shaft; plr, posterolateral ridge of the shaft; sd, shallow depression.

563

564 Figure 4. Cross sectional images of the shaft of the caenagnathid metatarsals III (NSM PV

565 21086, A) and IV (NSM PV 210550, B) based on CT scan data with 3D surface rendering

566 images in dorsal view showing the position of each cross section. The plantar side is up for

567 each cross-sectional image. Scale bars are 1 cm for cross sectional images and 5 cm for

568 surface rendering images. Abbreviations: asm, articular surface for the adjacent metatarsal;

569 lras, longitudinal ridge dividing the articular surface for the metatarsal III; olrd, oblique

570 longitudinal ridge on the dorsal aspect of the shaft; plr, posterolateral ridge of the shaft.

Draft

TABLE 1. Measurements of the caenagnathid metatarsals III (NSM PV 21086) and IV (NSM

PV 21055) described herein in millimeters.

Left metatarsal III Right metatarsal IV

(NSM PV 21086) (NSM PV 21055)

Maximum length 355x 328

Mediolateral width of 47.2 28.6 the shaft1

Dorsoplantar depth of 21.4 22.3 the shaft2

Mediolateral width of -* 53.1 the proximal end Draft

Dorsoplantar depth of -* 55.1 the proximal end

Mediolateral width of 49.9 42.8 the distal condyle

Dorsoplantar depth of 47.2 47.8y the distal condyle

1 maximum width for the metatarsal III and minimum width for the metatarsal IV measured

2 maximum depth for the metatarsal III and minimum depth for the metatarsal IV measured x preserved maximum length y including the ventral protrusion of the lateral hemicondyle

* not measured because the proximal end is not completely preserved

Draft

Figure 1. Map of the northwestern part of U.S.A. showing the Sandy Site locality (modified from Johnson, 2002).

91x60mm (300 x 300 DPI)

Draft

Figure 2. Caenagnathid left metatarsal III (NSM PV 21086). A, lateral view; B, dorsal view; C, medial view; D, plantar view; E, 3D surface rendering image based on CT scan data in dorsal view; F, distal view (with the dorsal side up); G, proximal view (with the dorsal side up). Scale bar equals 10 cm. Abbreviations: asm, articular surface for the adjacent metatarsal; lp, ligament pit.

181x170mm (300 x 300 DPI)

Draft

Figure 3. Caenagnathid right metatarsal IV (NSM PV 210550). A, lateral view; B, 3D surface rendering image in lateral view; C, dorsal view; D, medial view; E, plantar view; F, 3D surface rendering image in plantar view; G, distal view (with the dorsal side up); H, proximal view (with the dorsal side up). Surface rendering images of B and F were based on CT scan data. Scale bar equals 10 cm. Abbreviations: asm, articular surface for the metatarsal III; lp, ligament pit; lras, longitudinal ridge dividing the articular surface for the metatarsal III ; olrd, oblique longitudinal ridge on the dorsal aspect of the shaft; orp, oblique ridge on the plantar aspect of the shaft; plr, posterolateral ridge of the shaft; sd, shallow depression.

181x171mm (300 x 300 DPI)

Draft

Figure 4. Cross sectional images of the shaft of the caenagnathid metatarsals III (NSM PV 21086, A) and IV (NSM PV 210550, B) based on CT scan data with 3D surface rendering images in dorsal view showing the position of each cross section. The plantar side is up for each cross-sectional image. Scale bars are 1 cm for cross sectional images and 5 cm for surface rendering images. Abbreviations: asm, articular surface for the adjacent metatarsal; lras, longitudinal ridge dividing the articular surface for the metatarsal III; olrd, oblique longitudinal ridge on the dorsal aspect of the shaft; plr, posterolateral ridge of the shaft.

181x124mm (300 x 300 DPI)