Lithobiotopes of the Nemegt Gobi Basin

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Canadian Journal of Earth Sciences

LITHOBIOTOPES OF THE NEMEGT GOBI BASIN

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2020-0148.R1

Manuscript Type: Article

Date Submitted by the 17-Nov-2020 Author:

Complete List of Authors: Jerzykiewicz, Tomasz; retired Currie, Philip J.; University of Alberta, Biological Sciences Fanti, Federico; Departmen of Earth and Geoenvironmental Sciences, University of Bologna Lefeld, Jerzy; retired

Keyword: Mongolia, UpperDraft Cretaceous, Lithobiotope, Dinosaurs

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

1 LITHOBIOTOPES OF THE NEMEGT GOBI BASIN

3 Tomasz Jerzykiewicz, Philip J. Currie, Federico Fanti and Jerzy Lefeld

5 T. Jerzykiewicz. Research scientist and exploration geologist, Montréal, PQ, Canada.

6 P.J. Currie. Department of Biological Sciences, University of Alberta, Edmonton, AB T6G

7 2E9, Canada.

8 F. Fanti. Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Alma Mater Studiorum,

9 Università di Bologna, Via Zamboni 67, 40126 Bologna, Italy.

10 J. Lefeld. Research scientist, Polish Academy of Sciences, Warszawa, Poland.

11 [email protected]. Draft

13 Corresponding authors. T. Jerzykiewicz (email: [email protected]) and P.J. Currie

14 (email: [email protected]).

24 Abstract: Three distinct but overlapping dinosaur-dominated faunas characterize the Upper

25 Cretaceous Djadokhta, Baruungoyot and Nemegt formations of the Nemegt Basin of Mongolia.

26 Documented faunal differences cannot be explained easily by temporal succession, but can be

27 understood in the light of physical processes controlling life, death, and burial of taxa. The

28 stratigraphy of the Gobi Desert region records tectonically driven geometries, clearly

29 documenting preservational processes different than those acting in most other dinosaur-

30 dominated beds worldwide. Small, asymmetric tectonic grabens were filled with Upper

31 Cretaceous, dinosaur bearing deposits showing asymmetric distributions of facies, here termed

32 Lithobiotopes. The water-lain fluvial and alluvial plain facies of the Nemegt Lithobiotope

33 supported and preserved a fauna dominated by gigantic dinosaurs, but had a preservational bias

34 against smaller animals. The Nemegt passedDraft laterally into interdune facies of the Baruungoyot

35 Lithobiotope, which represented a hostile environment for large species, but preserved smaller

36 animals. This in turn passed laterally into the aeolianite facies of the Djadokhta Lithobiotope,

37 which is characterized by remains of small dinosaurs and a rich fauna of other animals. The

38 Nemegt Gobi Basin can be visualized as an oasis with a central pond supplied with water from

39 ephemeral channels and surrounded by a semi-arid alluvial plain and dune fields.

41 Key words: Mongolia, Upper Cretaceous, Lithobiotope, dinosaurs

43 Résumé: To be translated by the journal.

44 Mots-clés: Mongolie, Crétacé supérieur, Lithobiotope, dinosaure

53 Introduction

54 The Late Cretaceous dinosaurs from the Gobi Desert have been the subject of enormous

55 interest since their discovery by American expeditions one hundred years ago (Andrews 1923,

56 1932; Berkey 1924a, 1924b). Unearthed rather unexpectedly in a desert, the dinosaurs from

57 southern Mongolian localities became anDraft instant sensation and the subject of original

58 palaeontological publications (Granger and Gregory 1923; Osborn 1924a, b; Gilmore 1933) and

59 popular books (Andrews 1921, 1926, 1953; Gallenkamp 2001; Jerzykiewicz 2018). The interest

60 of palaeontologists in exploration of the Gobi localities has not diminished. On the contrary,

61 palaeontologists from all over the world continue to be attracted to the region (for an extensive

62 and up-to-date review see Fanti et al. 2018). Of special interest are the world-renowned localities

63 in the Nemegt Basin where a rich and diverse dinosaur-dominated fauna and continuous

64 exposures have continued to attract multinational expeditions (Kielan-Jaworowska 1968; Lavas

65 1993; Novacek 1996; Fanti et al. 2018). The geological signatures of the dinosaur habitats from

66 the Gobi Desert have also been studied and publicized (Berkey and Morris 1927; Gradziński

67 1970; Gradziński and Jerzykiewicz 1974a, b; Jerzykiewicz and Russell 1991; Fastovsky et al.

68 1997; Hicks et al. 1999; Dingus et al. 2008; Eberth et al. 2009; Eberth 2018). Despite such

69 premises, the stratigraphy and sedimentary environments of the Gobi habitats are controversial

70 and, as a matter of fact, these controversies remain largely unresolved. The reasons are manifold.

71 Progress in sedimentary geology combined with new field observations have made most of the

72 initial interpretations obsolete. Insufficient and frequently localized observations resulted in

73 speculation rather than a comprehensive stratigraphic analysis of the basin. However, despite

74 many unfruitful attempts, the most outstanding issue is the lack of stratigraphical interpretations

75 on the genesis and correlativity of the Upper Cretaceous strata.

76 The geographic extension, nature and age of the boundaries dividing each of the major

77 fossil units (Djadokhta, Baruungoyot and Nemegt formations) remain poorly understood as the

78 lack of continuous exposures limits our possibilities to address these critical questions.

79 Makovicky (2008) found that the Djadokhta sites were probably progressively younger to the

80 west, and found weak support that UkhaaDraft Tolgod may even be contemporaneous with

81 Baruungoyot beds. This is a conclusion also suggested faunally by Kielan-Jaworowska et al.

82 (2003).

83 Furthermore, two irreconcilable classifications have been used to describe stratigraphy of

84 the Cretaceous of Mongolia. The Russian explorers, ignoring stratigraphy previously introduced

85 by the American expeditions of the 1920s, utilized their own stratigraphic classification and still

86 insist on their application (Shuvalov 2000). This fact is causing considerable confusion in the

87 correlation of the Upper Cretaceous dinosaur-bearing strata – starting with nomenclatural issues -

88 and controversies in understanding of the habitats in which the dinosaurs lived (for a

89 comprehensive discussion of the problem see Jerzykiewicz, 2000). Recent attempts to create a

90 single, stratigraphic framework within which detailed and more localized geological

91 observations could be placed provided useful insights into the lithostratigraphy of the area, and

92 also documented how different palaeoenvironments can be traced in the area. Nonetheless, such

93 interpretations are not supported by structural analyses at a basin scale, rely on stratigraphic

94 patterns rather than observable marker beds, lack data from key areas where exposures provide

95 crucial information on the reciprocal architecture of the sedimentary units, and do not improve

96 our understanding on the relationship between fossil spatial and stratigraphic occurrence and

97 depositional settings in the Cretaceous (Hicks et al. 1999; Eberth 2018).

98 The present situation can be described as a scientific crisis, especially in the light of the

99 relevance of the Gobi basin for our comprehension of Late Cretaceous ecosystems. It is the

100 purpose of this paper to offer a solution of this predicament by proposing a new genetic

101 classification of the dinosaur-bearing strata of the area. Instead of using either formation or the

102 Russian svita, we propose to use a unit called a lithobiotope. Lithobiotope integrates the

103 lithology of the sediments that entomb theDraft dinosaur remains with the habitats of the dinosaur

104 assemblage.

105 This paper is based on firsthand experience that all of the authors gained during fieldwork

106 in expeditions to the Gobi Desert. The first author served as the geologist of the 1971 Polish-

107 Mongolian Palaeontological Expedition, was the geologist of the 1988 Sino-Canadian

108 Palaeontological Expedition, and returned to the Ordos Basin of China in 1990 (sponsored by the

109 Canadian Museum of Nature). The second author was co-leader of the Sino-Canadian

110 Palaeontological Expeditions in the Chinese part of the Gobi Desert from 1985 to 1990, and was

111 the Canadian leader of various expeditions to collect dinosaurs in Mongolia every year since

112 1999. The third author was on a Dinosaurs of the Gobi expedition in 2007, and led two National

113 Geographic funded expeditions into the Nemegt Basin in 2016 and 2018. The fourth author is a

114 veteran of the first Polish-Mongolian Palaeontological Expeditions in the years 1964 and 1965.

115 The expertise of the authors covers all known dinosaur-bearing localities of southern Mongolia,

116 plus some localities in the Chinese Gobi Desert and the Ordos Basin of China.

117 Mongolian place and stratigraphic names have been spelled in many different ways. To

118 minimize confusion, the standards suggested by Benton et al. (2000) are used in this paper

119 (without accents), although alternative spellings are offered in brackets whenever there is the

120 potential for confusion.

121

122 Geological setting

123 The Gobi Desert is a large physiographic region that covers the area of central Asia

124 between the Khangai Mountains of Mongolia to the north and the Lang Shan Mountains of Inner

125 Mongolia of China to the south (Fig. 1).Draft But the name ‘Gobi’ also has an original geological

126 connotation. The Baga Bogdo Gobi Basin was described as an asymmetric syncline by Berkey

127 and Morris (1924, fig.10 on page 114). This subordinate fault-bounded tectonic unit of a half-

128 graben type is filled up with Upper Cretaceous dinosaur-bearing deposits that are overlain by

129 Tertiary strata. The Nemegt Basin described by Gradziński (1970) and Gradziński and

130 Jerzykiewicz (1972) is a similar tectonic graben located farther south between Altan Uul and

131 Noyon Uul. Structural geology of the Nemegt Basin was not understood in the 1970s because of

132 the lack of subsurface data.

133 The geological setting of the Upper Cretaceous dinosaur-bearing deposits within fault

134 bounded tectonic grabens has more recently been confirmed by petroleum exploration of

135 southern Mongolia with seismic reflection sections (Johnson 2004; Prost 2004). Seismic

136 stratigraphy helped to reconstruct the structural evolution of southern Mongolia. It appears that

137 the Gobi Desert region is indeed subdivided into subordinate tectonic grabens. One of them is the

138 Nemegt Basin, where most of the Upper Cretaceous dinosaur-bearing localities that are the

139 subject of this paper occur. Geologic evolution of the Nemegt Basin has been controlled by

140 tectonics and is bound by the late Cenozoic transpressional uplift of the Altan Uul (fig. 8 on page

141 330 in Cunningham et al. 2009), which is related to a major intercontinental Gobi-Tien-Shan

142 strike-slip fault system (Cunningham et al. 1996; Owen et al. 1999; Cunningham, 2007).

143 The subordinate Upper Cretaceous basins – known since the outset of exploration

144 of the southern Mongolia as the gobi basins – are filled by asymmetrically distributed facies.

145 Fluvial and alluvial plain facies (Nemegt strata) have been deposited in relative proximity to the

146 bounding faults, whereas the aeolian facies (Djadokhta strata) are much farther away.

147 Intermediate between the proximal and distal facies are the Baruungoyot strata deposited largely

148 in interdune settings. This lateral distributionDraft of facies reflects the control of two major factors –

149 tectonics and climate. Sea level changes have not been recorded in these semi-desert and desert

150 environments, which has caused significant difficulties in stratigraphic correlation of the

151 dinosaur-bearing deposits.

152

153 Controversy concerning stratigraphy of the dinosaur bearing deposits

154 The ages of the Upper Cretaceous formations of the Gobi Desert have never been

155 established unequivocally, and neither has their stratigraphic correlation within the basin.

156 Opinions about age and stratigraphic position of the Djadokhta Formation are perhaps the most

157 symptomatic of the problem. Originally the ‘Dja-doch-ta’ and Iren Dabasu formations were

158 classified as Upper Cretaceous deposits unconformably overlain by the Lower Tertiary (Eocene)

159 Gashato Formation (fig. 4 on p. 614 in Berkey, 1924b). Subsequently Morris (1936) on his

160 ‘Correlation diagram of the principal Cretaceous formations’ placed the Djadokhta Formation

161 both at the bottom (Cenomanian) and at the top (Danian) of the Upper Cretaceous (in both cases

162 with question marks). The differences stemmed from the previous opinions of Osborn (1924a)

163 who “at first regarded the Djadokhta life zone as ‘near the beginning of Upper Cretaceous time’

164 but in a later paper, he correlated it with the ‘Senonian, Danian, Edmonton’ chiefly because of

165 the resemblance of Protoceratops to Leptoceratops of Alberta.” (Quotation from Morris, 1936,

166 p. 1505).

167 Despite considerable research by subsequent multinational teams of palaeontologists and

168 geologists, the stratigraphic distribution of the dinosaur faunas, and the correlations of the

169 dinosaur-bearing deposits in the Gobi Desert have yet to be solved. None of the applied methods

170 (biostratigraphic, chronostratigraphic, lithostratigraphic, palaeomagnetic or phylogenetic) have

171 helped (Kielan Jaworowska 1974; SochavaDraft 1975; Gradziński et al. 1977; Martinson 1982;

172 Jerzykiewicz and Russell 1991; Hicks et al. 1999; Makovicky 2008) to resolve the

173 spatiotemporal relations amongst the main dinosaur bearing formations (Djadokhta, Baruungoyot

174 and Nemegt) of the Gobi Desert. The superposition of the Nemegt over the Baruungoyot

175 Formation has been inferred based on an observation that the top of the latter is dipping at a 1.5-

176 degree angle at the base of the Nemegt Formation (Gradziński 1970). Boundaries between layers

177 of continental deposits are generally not horizontal but as a rule are inclined at various angles,

178 and therefore it is rather unlikely that a 1.5-degree inclination indicates structural superposition.

179 Observations by Eberth et al. (2009) and by the authors of this paper at the Khulsan and Nemegt

180 localities suggest interfingering rather than superpositional relations between the formations in

181 question.

182 From a methodological point of view, the situation that developed in stratigraphy of the

183 dinosaur-bearing deposits of Mongolia may be described as a methodological impasse. It has

184 long-standing roots in conflicting stratigraphic classifications of American and Russian explorers

185 of the Gobi Desert. The original approach, which stems from the pioneer work of American

186 palaeontologists and geologists in Mongolia (Granger and Gregory 1923; Berkey and Morris

187 1927), relies on the ‘formation’ as the basic stratigraphic category. This approach makes a clear

188 distinction between ‘rock units’ and ‘time units’ and it is known as the dual stratigraphic

189 classification (e.g. Krumbein and Sloss 1963).

190 The subsequent approach, employed largely by authors publishing in Russian (Sochava

191 1975; Verzilin 1978, 1980; Shuvalov 2000), utilizes the ‘svita’ as a basic stratigraphic category.

192 Svita integrates lithological and temporal aspects of strata into one unit. ‘Formations’ and

193 ‘svitas’ cannot be compared. Application of these different units for the description of

194 stratigraphy has led to considerable confusionDraft in terminology of the stratigraphic record of the

195 dinosaur-bearing deposits of the Gobi Desert (for more comprehensive discussions see

196 Gradziński et al. 1977; Jerzykiewicz and Russell 1991; Jerzykiewicz 2000).

197 The conflicting stratigraphic classifications contributed also to controversies about the

198 sedimentary environments of the dinosaur-bearing strata. The debates centered on the question of

199 whether lacustrine, fluvial or aeolian depositional models better describe the Cretaceous

200 stratigraphic record of the Gobi Desert (Tverdokhlebov and Tsybin 1974; Gradziński and

201 Jerzykiewicz 1974b; Shuvalov 1982; Verzilin 1982). It is worth noting that Berkey and Morris

202 (1927) had no doubts that sandstone of the Djadokhta Formation was deposited by wind and that

203 some structureless beds represented settled dust or loess (Berkey and Morris 1927, p. 377). The

204 controversy concerning depositional environments of the Gobi region in Late Cretaceous times

205 began when Soviet expeditions in the 1940s discovered gigantic dinosaurs entombed in water-

206 lain sandstone at the localities of Altan Uul and Nemegt (Fig. 2). In their opinion these giants

207 inhabited the shores of large permanent lakes.

208 As indicated by more recent publications (Jerzykiewicz 2000), neither the stratigraphic

209 nor the sedimentological controversy has been definitely resolved. Some recent publications

210 even utilize both ‘formations’ and ‘svitas’ in the descriptions of dinosaur-bearing deposits of the

211 Gobi Desert. This conflict has some ideological overtones discussed by O’Rourke (1976).

212 Despite obvious superiority of the dual classification applied originally by American scientists

213 and then utilized and improved by Polish and Canadian stratigraphers (Gradziński 1970;

214 Gradziński and Jerzykiewicz 1974a; Jerzykiewicz and Russell 1991), there is a growing

215 awareness of the still unresolved problems (Jerzykiewicz 2000; Eberth et al. 2009; Fanti et al.

216 2012). It appears that even the methodicallyDraft correct dual stratigraphy used for the description of

217 the dinosaur-bearing beds does not adequately describe the complexity of the problem. This is

218 because stratigraphic classifications are subject to fundamental differences between the

219 completely continental dinosaur bearing deposits of the Gobi Desert and the more common

220 marine or coastal plain deposits.

221 In conclusion, the main dinosaur bearing formations (Djadokhta, Baruungoyot, Nemegt)

222 of the Nemegt Gobi Basin lack positive proof of their reciprocal spatiotemporal relations. From a

223 scientific point of view, the situation that developed in stratigraphy of these dinosaur-bearing

224 deposits of Mongolia may be described as a methodological impasse.

225

226 Understanding the stratigraphic record of desert environments

227 In order to fully understand the complexity of the problem and to find a solution, it is

228 necessary to look closely at the stratigraphic differences between a desert as opposed to marine

229 and marginal-marine environments. The stratigraphic classification based on formation as the

230 principal category works very well with the latter because the stratigraphic record is an interplay

231 between sea level changes, tectonics, and climates. The intensity of these factors changes in

232 space and time. However, extreme differences exist between strata laid down under open-to-

233 marginal marine conditions from those that are completely continental deposits of desert

234 environments. Both the physical and biological components of these depositional environments

235 are fundamentally different. As a result, the stratigraphic record of marine deposition is

236 dramatically different from deposition in deserts, and describing these differences requires

237 refinement of stratigraphic methods.

238 Stratigraphy of marine basins is typically based on the record of migration of the

239 shoreline and the evolution of the marineDraft fauna. Lateral extents of strata are rather large, and

240 sedimentation is considered almost continuous, especially in the open and deep marine

241 environments. In contrast, continental basins – such as the gobi basins of central Asia – that are

242 completely free of marine processes have strata that are limited laterally to individual tectonic

243 grabens, and sedimentation is discontinuous. Such discontinuous sedimentation is particularly

244 visible in the Gobi Desert where the physical processes of fluvial and aeolian deposition are

245 limited laterally to individual tectonic grabens influenced by changing climates. As a result,

246 lithologic correlation of strata is extremely difficult, if not virtually impossible. Biological

247 criteria of correlation are also uncertain due to laterally diversified conditions within different

248 parts of the gobi basins.

249 Accumulation of sediments over time in the gobi basins is of concern because a case can

250 be made that only a small fraction of Late Cretaceous time is represented in dinosaur-bearing

251 strata. We are dealing with an extreme example of the dictum ‘More gaps than record’ (Ager

252 1973, 1993). Indeed, for the entire duration of the Late Cretaceous, only very limited intervals of

253 time are recorded in central Asia. The entire 35 Ma of the Late Cretaceous is represented in the

254 gobi basins by only a few hundred meters of deposits. Most of the time is within the boundaries

255 rather than within the preserved sediments. Continuous depositions of clastic sediments in other

256 continental environments would produce much thicker intervals of strata. For example, the

257 Upper Cretaceous fluvial and alluvial plain deposits in the foothills of the Western Canada

258 Sedimentary Basin are approximately ten times thicker than the Upper Cretaceous deposits of the

259 Gobi region (Jerzykiewicz 1997). It is therefore justified to assume that most Late Cretaceous

260 time in the Gobi region has either not been recorded or was represented by erosion.

261 This produces an extreme example of event stratigraphy rather than gradual

262 sedimentation. Only certain events are preservedDraft in the stratigraphic record of the gobi basins.

263 The events preserved in the Djadokhta are different from the events in the Nemegt deposits.

264 They represent different sedimentary environments and produced extremely different lithological

265 and stratigraphic records. Temporal correlation between gobi basins is also not entirely certain as

266 there is no way to determine if they were being infilled at the same time, even though the

267 depositional environments may have been identical.

268 Because completely continental sediments are so different from those laid down in

269 marine environments, correlation of the Upper Cretaceous formations of Mongolia with marine

270 successions will remain hypothetical until some chronostratigraphic methods can be successfully

271 applied. So far in the Gobi region, this has only been possible for Cenomanian basalts at the base

272 of the Cretaceous dinosaur-bearing strata.

273

274 A genetic approach to the Gobi habitats

275 In the last decade, several scientific publications have dealt with the complex community

276 composition of the Nemegt dinosaur-dominated fauna. In so doing, new observations where

277 combined with extensive literature surveys, including the mandatory, painstaking process of

278 gathering information from original field notes and photographs of twentieth century expeditions

279 (e.g. Jerzykiewicz 2018). All authors observed a remarkable lack of taphonomic information for

280 most dinosaur quarries; during the collection of specimens very little attention was given to

281 detailed lithological, sedimentological and even spatial data. Although recent expeditions collect

282 such crucial information (accurate maps, relocation of historic quarries, accurate taphonomic

283 analyses), historically such neglect resulted in a paradoxical fragmentation of the Gobi fauna. A

284 detailed analysis of the vertebrate fauna and its composition is beyond the aim of this paper,

285 although the biological component preservedDraft in the gobi beds is essential in providing a solid,

286 genetic interpretation of such sedimentary units.

287 In keeping with the goal of developing a conceptual framework comparable with

288 present-day analogues, vertebrate concentrations in the Upper Cretaceous Gobi beds can be

289 grouped in two distinct assemblages: individuals (or groups of individuals) presenting evidence

290 of direct, permanent burial, and others showing clear reworking and postmortem transportation

291 (Rogers and Kidwell 2007). Clear examples of the first typologies include brooding dinosaurs

292 (Osborn 1924a; Norell et al. 1995; Dong and Currie 1996; Fanti et al. 2012; Norell et al. 2018),

293 undisturbed but isolated nests of eggs – sometimes in nesting grounds – (Andrews 1923; Sabath

294 1991; Mikhailov et al. 1994), massive ankylosaurs trapped in upright positions (Lefeld 1971;

295 Jerzykiewicz et al. 1993; Currie et al. 2011; Mallon et al. 2018; Jerzykiewicz 2018), dinosaurs

296 mired in mud (Currie et al. 2011; Lee et al. 2018), smaller dinosaurs in life positions trapped in

297 ‘escaping’ or even ‘fighting’ positions (Figs. 3, 4), and the thousands of footprints documented

298 in the area (Ishigaki 1999; Currie et al. 2003; Ishigaki et al. 2009). The second type includes

299 large bonebeds such as the Saurolophus-dominated Dragon’s Tomb (Efremov 1958;

300 Rozhdestvensky 1965; Bell et al. 2018), the isolated skeletons of large sauropods (Currie et al.

301 2018), and countless other individual skeletons that litter the surface of several key localities

302 (Fig. 5). Relevant to this study, such typologies – although statistically more common - are not

303 exclusive of any of the three stratigraphic units (Djadokhta, Baruungoyot, Nemegt), but rather

304 are found in specific facies within each unit. Such distribution clearly reflects a combination of

305 biological and physical controls, which indeed represent the key components in understanding

306 the origin of these deposits. One of the most diagnostic is the skeleton of the armored dinosaur

307 Saichania found at the transition between an aeolian dune and an ephemeral pond, which so

308 strongly suggested that the animal died Draftin search for water that the site was called ‘the agony of

309 an armored dinosaur’ (Fig. 6).

310 A second line of evidence is grounded in more comprehensive approaches on the

311 relations between diversity and spatial distribution of taxa. This results in ecological

312 interpretations of the areas being considered, and thus to the genesis of the fossil-bearing beds. In

313 the early studies on the diversity of dinosaurs and other vertebrates, taxa provided first a

314 chronostratigraphic tool to discriminate ‘formations’ or ‘svitas’ by documenting faunal turnovers

315 (Osborn 1924b; Osmólska 1980). This approach was not just controversial from a

316 methodological perspective, but also inadequate as new discoveries were made across the basin

317 documenting virtually no taxa distinctive for any specific unit (Gao and Norell 1998; Makovicky

318 2008; Funston et al. 2018; Czepiński 2020).

319 Recently, an increasing number of papers discuss potential habitat preferences within

320 complex ecosystems (Funston et al. 2016, 2018; Park et al. 2020) and urge for a definitive

321 interpretation on the reciprocal stratigraphic occurrences of fossil beds (Jerzykiewicz 2018). It is

322 also evident that detailed facies analysis (integrating sedimentological and palaeontological

323 information), support clear palaeoecological signatures in each of the main formations of the

324 Gobi Desert. The impact of changing climate and tectonics are clearly visibly in the character of

325 the facies (aeolian, fluvial, lacustrine, etc.).

326 However, facies supporting such interpretations do not occur in vertical successions but

327 rather represent lateral alternations across each individual gobi basin. The Nemegt Gobi Basin is

328 perhaps the best example of such lateral alternations of facies. Therefore, documented taxa

329 distributions also seem justifiable following the same lateral, patchy trends rather than

330 stratigraphic succession.

331 Draft

332 Proposed solution - Lithobiotopes

333 The principal stratigraphic problem is a simple question of whether the dinosaur-bearing

334 strata of the Gobi Desert should be classified as formations or facies. In fact, neither of these

335 terms is adequate, nor do they fully correspond to the complexity of the problem. Utilizing

336 formation is not appropriate because according to the International Stratigraphic Code, each

337 formation is meant to be laterally limited to one sedimentary basin. As explained in the

338 geological setting section of this paper and subsequently in the description of the Djadokhta

339 Lithobiotope, the dinosaur-bearing strata are not found in one sedimentary basin, but infill

340 individual gobi basins separated by tectonic movements. The interfingering relations between the

341 Baruungoyot and the Nemegt dinosaur-bearing strata of the gobi basins clearly preclude usage of

342 formations. In this situation, the authors propose introducing a new eclectic stratigraphic

343 category called a lithobiotope. This is a genetic term broader than sedimentary facies routinely

344 utilized in sedimentology (Anderton 1985; Walker and James 1992).

345 In the specific case of Mongolia, a lithobiotope subdivides sedimentary units on the basis

346 of a distinctive assemblage of organisms formed under one set of environmental and taphonomic

347 conditions (also with respect to nonbiologic features) as compared with other assemblages

348 formed at the same time but under different conditions. As such, a lithobiotope describes the

349 living space of fauna (and flora) adjusted to the physical conditions existing in that space during

350 their life spans.

351 This new stratigraphic category is similar to the original definition of sedimentary facies

352 in geology, which was an integration of both the lithology and palaeontology. Since that time,

353 the concept of facies has changed withinDraft the specialization of geology, and it is presently

354 understood differently in various branches of the earth sciences (e.g. sedimentary facies,

355 petrologic facies, tectonic facies). Most of these terms have some genetic connotations.

356 Furthermore, it is our opinion that the term lithobiotope is more effective in describing the Gobi

357 setting than other terms such as biofacies (Weller 1958), biosome (Wheeler 1958), biotope

358 (Wells 1947), lithosome (Wheeler and Mallory 1956) and magnafacies (Caster 1934). As

359 introduced in this paper, the concept of the lithobiotope also has genetic connotations in terms of

360 the origin of sediments and biota. As explained in this paper, this new concept in relation to

361 dinosaurs and associated faunas of the Gobi region is intended to help solve outstanding

362 problems of dinosaur life and extinction by the end of Mesozoic.

363

364 The Djadokhta Lithobiotope

365 The Bayan Zag locality (originally referred to as Shabarakh Usu and/or the Flaming

366 Cliffs, but has also been frequently called Bayn Dzak) is the type locality of the Djadokhta

367 Formation (originally spelled as Dja-doch-ta or Djadochta Formation, Berkey 1923). It is also

368 considered the type locality of the Djadokhta Lithobiotope. The Djadokhta Formation was

369 described by Berkey and Morris (1927) in terms of steeply cross-stratified aeolian dunes (Fig. 7,

370 showing cross-bedding from Tögrögiin Shiree). These authors interpreted the ‘uniformly’ fine-

371 grained Djadokhta red sandstone as a wind-blown deposit (loess). That interpretation was

372 confirmed by Lefeld (1971) and later analogous deposits were described from Alag Teeg,

373 Tögrögiin Shiree, and Ukhaa Tolgod localities (Fastovsky et al. 1997; Dingus et al. 2008;

374 Hasegawa et al. 2009) as well as from the Bayan Mandahu locality of Inner Mongolia

375 (Jerzykiewicz et al. 1993). The upper boundaryDraft of the Djadokhta strata has been identified at the

376 erosional surface overlain by gravels of the Palaeocene Khashaat (Gashoto) Formation

377 (Gradziński at al. 1968; Jerzykiewicz and Russell 1991; Hasegawa et al. 2009).

378 The lower boundary of the Djadokhta Formation has been identified in the northern

379 region of Bayan Zag, at Alag Teeg and at Abdrant Nuru, where the Djadokhta aeolianites are

380 underlain by layers of mudstone and small-scale trough cross-bedded and ripple drift sandstone

381 described as the Alagteeg Formation (Hasegawa et al. 2009, Averianov and Lopatin 2020).

382 Pinacosaurus is the most common taxon recovered in these beds, which suggests the formation

383 is more or less contemporary with the overlying Djadokhta beds. However, Abdarainurus

384 barsboldi and Plesiohadros djadochtaensis have not been recovered from any of the other

385 formations in the area, which suggests this formation is different in more than one way.

386 Unfortunately, little more can be said until more refined sedimentological and palaeontological

387 work is done in the formation. Therefore, the Alagteeg Formation will be largely ignored in this

388 paper, although it is included as a separate column in Table 2.

389 The Bayan Mandahu locality is the most extensive and instructive in terms of origins,

390 even though it is in China and lies outside of the Nemegt Gobi Basin of Mongolia (Fig. 8).

391 There, several hundred kilometers from Bayan Zag, are strata that are similar to the type locality

392 of the Djadokhta Formation in terms of the dinosaurs and associated fauna. The Sino-Canadian

393 Dinosaur Project excavated the site from 1988 to 1990 (Currie 1991; Jerzykiewicz et al. 1993;

394 Dong 1993; Grady 1993; You and Cui 2019), and it has also been worked by multiple

395 expeditions since (Xu et al. 2011). The significance of this locality is important because it

396 became clear that formational stratigraphy is not appropriate for the dinosaur-bearing strata of

397 the Gobi Desert; the specimen excavationsDraft led to central conclusions about their demise (Figs. 3,

398 4, 8), and sedimentary structures led to improvements of previous interpretations of the

399 depositional environments (Jerzykiewicz et al. 1993). Sedimentary structures at the Bayan

400 Mandahu locality confirm the interpretation of a semi-desert environment with well-developed

401 sand seas and dust accumulations (Fig. 8). Climatically sensitive caliche palaeosols, developed

402 either in the form of limestone nodules or as limestone layers, are present at Bayan Mandahu

403 (Fig. 9), but are also known from Bayan Zag. At the Flaming Cliffs, well developed caliche

404 horizons were described in terms of calcareous horizons (Lefeld, 1971, fig. 3).

405 The Djadokhta Lithobiotope differs from both the Nemegt and Baruungoyot

406 Lithobiotopes in both the physical and biological aspects of the habitat. Large-scale cross-

407 stratification of aeolian origin (aeolianites), and caliche palaeosols are the diagnostic sedimentary

408 structures. The aeolian stratification has been recognized since the American expeditions

409 (Berkey and Morris 1927), but was later confirmed in Bayan Zag by Lefeld (1971) and Bayan

410 Mandahu (Jerzykiewicz et al. 1993).

411 Both the caliche nodules and layers of limestone – referred to as ‘hardpan’ or ‘K

412 horizons’ – have been described from the soil profiles of semi-arid regions (Bretz and Horberg

413 1949; Reeves 1970) and deserts (Mayer et al. 1988). Mature caliche profiles capped with well-

414 developed hardpan horizons require at least tens of thousands of years of semi-aridity with rare

415 and episodic rainfall (Gile et al. 1981) as well as a flux of calcareous dust (Mayer et al. 1988).

416 Episodic rain not exceeding 400 mm/year may produce ephemeral standing bodies of water on

417 the hardpan surfaces. In contrast, excessive and/or continuous rain tends to dissolve the hardpan.

418 It is worth noting that Berkey and Morris (1927, p. 377) interpreted the limestone layers of

419 caliche type at Bayan Zag in terms of precipitationDraft at the bottom of ponds rather than ephemeral

420 ponds developed on hardpan horizons.

421 The presence of well-developed caliche profiles with hardpans clearly indicates that

422 water was in short supply and that rainfall was sporadic. As a result, the assemblage of dinosaurs

423 of this lithobiotope is rather impoverished and consists largely of small to medium-sized

424 individuals that browsed on low-growing plants. The indigenous species were adapted for

425 survival in this harsh, arid environment that was dominated by strong, blowing winds and

426 seasonal shortages of water. Many of the dinosaurs lost their lives buried by blowing sands or

427 collapsing foresets of aeolian dunes (Figs. 3, 4, 6, 8). Like modern arid regions (such as the

428 present-day Gobi Desert), there were clearly ephemeral sources of water that supported

429 specialized plants and animals. Unfortunately, plants and invertebrates tend not to be preserved

430 in these highly oxidized environments, so their nearly complete absence from the Djadokhta

431 Formation is predictable. Invertebrates are represented by trace fossils, including fossil burrows

432 that are abundant at Bayan Mandahu, Bayan Zag, Tögrögiin Shiree and other Djadokhta sites,

433 and fossil insect pupal chambers (Lefeld 1971; Tverdokhlebov and Tsybin 1974; Eberth 1993;

434 Jerzykiewicz et al. 1993; Johnston et al. 1996; Fastovsky et al. 1997). The prevalence of

435 invertebrate burrows is a clear indication that the dunes were not composed of dry sands (other

436 than on the surface), but were generally internally moist, otherwise the invertebrates could not

437 have lived there, nor been preserved (Ekdale et al. 2006; Good and Ekdale 2014). As in modern

438 dunes at Bayan Mandahu, the invertebrates were mining layers of organic material (probably

439 wind-blown, wind-deposited dead plants and dinosaur dung), and the burrows tend to either be

440 vertical through the layers, or follow the sloping surfaces of the foresets. Some species of insects

441 were specialized in scavenging the carcasses of dinosaurs and other vertebrates, and bones from

442 the Djadokhta Formation are frequentlyDraft bored or are completely destroyed at the epiphyses

443 (Saneyoshi et al. 2010; Fanti et al. 2012).

444 Nanhsiungchelyid turtles, plus several species of small crocodylians (Gobiosuchus

445 kielanae, Shamosuchus djadochtaensis) from the Djadokhta Formation suggest the nearby

446 presence of water, although none would have required permanent or large bodies of water.

447 Mlynarski (1972) considered the nanhsiungchelyid Zangerlia as a terrestrial turtle, whereas all of

448 the crocodylians are relatively small. Osmólska (1980) speculated that gobiosuchids with their

449 small sizes, anteriorly-positioned internal nares and long, slender limbs may have been terrestrial

450 predators that were feeding on lizards and mammals.

451 The abundance and high diversity of lizards and mammals in the Djadokhta is striking in

452 comparison with their near-absence in the Nemegt Formation. The depositional environments of

453 the Djadokhta strongly favour the preservation of small animals, usually as skulls in hard

454 calcareous nodules, but sometimes as more or less complete animals in the sandstones. Large

455 dinosaurs (tyrannosaurids, sauropods, and hadrosaurs), on the other hand, are neither abundant

456 nor well-preserved. They are generally only represented in the Djadokhta by rare isolated teeth

457 (Osmólska 1980), a few isolated bones, and extremely rare partial skeletons (Chinzorig et al.

458 2017). Although one could argue that the Djadokhta environment lacked the ability to bury and

459 preserve large skeletons, medium sized dinosaurs were buried rapidly in life poses. Furthermore,

460 ankylosaurs were large animals, are relatively common as articulated skeletons in the Djadokhta

461 environments, and were buried. This suggests the absence of other large dinosaurs is not likely a

462 preservational bias, but that it is more likely that the larger animals were infrequent visitors, and

463 were perhaps in transit and died in an area that was inhospitable for them. It is worth noting that

464 articulated but partial skeletons of the hadrosauroid Plesiohadros djadokhtaensis (Tsogtbaatar et

465 al. 2014), embryonic hadrosaurs (BarsboldDraft and Perle 1983) and the bones of other large

466 dinosaurs (Averianov and Lopatin 2020) were recovered from what were once considered as

467 Djadokhta sites (Abdrant Nuru, Alag Teeg) with evidence of fluvial activity. However, these

468 beds are now considered as the separate but older Alagteeg Formation (Hasegawa et al. 2009).

469 In modern environments, small vertebrates are always both more speciose and more

470 numerous than large animals. This was clearly the case in the Djadokhta Lithobiotope, where the

471 number of species of just lizards and mammals was more than double the number of dinosaur

472 species (Tables 1, 2). It is more complicated than just counting the number of species of course,

473 because many of the dinosaurs were diminutive as well. Furthermore, a true assessment would

474 need to also look at how many individuals of any one species were present for a given area.

475 Preservational biases clearly were present, and worked against the fossilization of plants and

476 invertebrates, both of which would have been at the base of the food chain. However, the point is

477 that the Djadokhta Lithobiotope looks like a normal vertebrate ecosystem, which suggests that a

478 relatively unbiased sample of the ecosystem was being preserved.

479 Eggshell, eggs and nests of dinosaur eggs were first recognized in the Djadokhta

480 formation (Anonymous 1923, Andrews 1924, 1926), and are quite common. Embryos have been

481 recovered from Djadokhta level beds (Sochava 1972; Dong and Currie 1993; Norell et al. 1994,

482 2001), but they are rare, and are seldom found within the confines of the eggs themselves. One

483 possible explanation is that as long as the eggs were intact for long enough after burial,

484 embryonic bones were dissolved in the internal microenvironments of decomposing fluids

485 (Currie 1988; Dong and Currie 1993). This would explain why so many of the intact eggs show

486 dissolution of the inner surfaces of the eggshell as though the embryo were developing normally

487 even though they did not fossilize. Furthermore,Draft it recently became evident that the absence of

488 identified Protoceratops eggs from the Djadokhta (and ceratopsian eggs in general) is because

489 ceratopsian eggshells were soft-shelled (Norell et al., 2020). Regardless of the rarity of embryos,

490 the Djadokhta depositional environment was alkali enough to preserve the eggshell, and both

491 eggs and nests were buried before hatching. And in some cases, even the brooding mothers were

492 buried on top of the nests (Osborn 1924; Norell et al. 1995; Dong and Currie 1996; Clark et al.

493 1999; Norell et al. 2018).

494 The largest animals in the Djadokhta Lithobiotope for which articulated skeletons have

495 been collected were the ankylosaurs Pinacosaurus (Gilmore 1933; Maleev 1952; Maryańska

496 1971, 1977; Tumanova 1987; Hill et al. 2003), which was up to 5 m long, and the neoceratopsian

497 Udanoceratops (Kurzanov 1992), which may have been 3-4 m long. Most of the dinosaurs living

498 in the Djadokhta Lithobiotope were considerably less than three metres long. Because the

499 depositional environments favoured the preservation of small animals, many of the dinosaur

500 skeletons recovered were juvenile animals. Protoceratops was one of the first dinosaurs known

501 with a good ontogenetic series (Brown and Schlaikjer 1940), and more than a hundred 1.5m long

502 skeletons of juvenile Pinacosaurus have been collected from Bayan Mandahu (Jerzykiewicz et

503 al. 1993; Burns et al. 2011), Bayan Zag (Maryańska 1971), and Alag Teeg (Tverdokhlebov and

504 Tsybin 1974; Suzuki et al. 2000, Currie et al. 2011; Burns et al. 2015).

505 In summary, the Djadokhta Lithobiotope is one of the richest dinosaur ecosystems that

506 we know of, although the diversity of dinosaurs is relatively low whereas the diversity of lizards

507 and mammals is amazingly high. Even with dinosaurs, however, the Djadokhta sites have

508 produced an amazing number of specimens with hundreds of skeletons of Protoceratops having

509 been collected. Unfortunately, plant fossils are unknown, and what must have been a rich

510 invertebrate fauna is abundantly representedDraft only by trace fossils. The depositional environment,

511 which for the most part is semi-arid to arid dune and interdune deposits, seems to have preserved

512 animals of all sizes, although the rarity of evidence for really large dinosaurs suggests that they

513 did not live in the immediate area, but would occasionally pass through it. Most of the animals

514 that lived there were medium-sized dinosaurs that were adapted to survive in a semi-arid

515 environment with sand dunes and ephemeral bodies of water, possibly small interdune lakes such

516 as are found in the Gobi Desert today. The dinosaurs were not diverse in body form, and the

517 small carnivores probably fed mostly on lizards, mammals, baby dinosaurs, and carrion (it is not

518 uncommon to find shed dromaeosaurid and troodontid teeth mixed in with the skeletons of

519 ankylosaurs and protoceratopsids). Occasionally, as suggested by the “fighting dinosaurs” (Fig.

520 4), predators may have tried to take down herbivorous dinosaurs that were as large, or larger

521 than, themselves.

522 Pinacosaurus, Protoceratops and Velociraptor are the most characteristic dinosaurs of

523 the Djadokhta Lithobiotope, and all have been recovered from virtually all of the Djadokhta

524 Formation sites (Table 2). More than a dozen lizard species, three crocodylians, and ten

525 mammals seem to have also characterized this Lithobiotope. Eggs, eggshells and nests are

526 common.

527

528 The Baruungoyot Lithobiotope

529 The Baruungoyot Lithobiotope is intermediate between the Nemegt and Djadokhta

530 Lithobiotopes, both in terms of the physical processes shaping the habitats, and their biota. The

531 transitional character of the Baruungoyot lithobiotope is perhaps best seen at the Khulsan

532 locality, which should be considered theDraft type locality for this lithobiotope. It is also represented

533 by the following localities: Altan Uul (2, 3, and 4), Nemegt (Central, Eastern, Northern and

534 Western Sayrs, Monadnocks and Red Walls), Hermiin Tsav (I, II) and Ulan Khushuu.

535 Both water-laid and the aeolian sedimentary structures (Fig. 10) are present in this

536 lithobiotope (Gradziński and Jerzykiewicz 1974a, b; Eberth et al. 2009; Fanti et al. 2012; Eberth

537 2018). Wind-blown sand and episodic flooding events were the major factors that shaped the

538 topography of the environment. It can be visualized as a marginal zone of a sand-sea covered by

539 aeolian dunes, and ephemeral interdune streams and ponds.

540 The biological aspects of these habitats correspond to the differences in the

541 environmental conditions. The Baruungoyot Lithobiotope consists of an impoverished

542 assemblage of dinosaurs, the herbivores of which fed on low-growing plants. There were perhaps

543 also transient specimens trying to adapt to rapidly changing environmental conditions.

544 Plants and invertebrates are poorly represented in the Baruungoyot Formation, as

545 expected in the highly oxidized beds. Invertebrate trace fossils include bone-boring insect traces

546 and non-marine invertebrate traces (Eberth et al. 2009; Fanti et al. 2012). Also similar to the

547 Djadokhta Formation, there is a preservational bias that favours small vertebrates like lizards and

548 mammals. Almost 40 species of lizards have been described from the Baruungoyot beds, and

549 more than a quarter of these are the same species as have been found in the Djadokhta (Table 2).

550 The known mammalian fauna of a dozen species is a little less diverse than the known Djadokhta

551 mammals, but again a quarter of the Baruungoyot species are known from both Lithobiotopes.

552 Terrestrial turtles like nanhsiungchelyids Bulganemys and Zangerlia are supplemented by more

553 aquatic forms like the lindholmemydines Gravemys and Linholmemys, and the trionychid

554 Trionyx (Table 2). The latter three generaDraft suggest the presence of larger, less ephemeral bodies

555 of water than the Djadokhta Lithobiotope. Only one gobiosuchid crocodylian (Artzosuchus

556 brachicephalys) is known from the Baruungoyot at present, but three species of frogs have been

557 discovered (Table 2). As in the Djadokhta, bird fossils are rare, but four species are also known

558 from the Baruungoyot Lithobiotope, one of which (Gobipteryx minuta) is known from both.

559 The dinosaurs in the Baruungoyot Lithobiotope are similar to those in the Djadokhta

560 Lithobiotope in that the majority are small to medium-sized forms that include dromaeosaurids,

561 oviraptorids, protoceratopsids and ankylosaurids. As pointed out by Osmólska (1980),

562 protoceratopsids are much rarer in the Baruungoyot, although theropods seem more common.

563 This is particularly true of oviraptorids (Funston et al. 2018), which are represented in the

564 Baruungoyot by three species (as they are in the Djadokhta) although multiple mass death sites

565 have been found at Khulsan and Hermiin Tsav for Conchoraptor and possibly Heyuannia

566 (formerly known incorrectly as Ingenia, see Funston et al. 2020).

567 Large dinosaurs like tyrannosaurids are represented by a few isolated teeth and bones,

568 and one partial skeleton that we examined in 2003 at Khulsan. Sauropod remains include a

569 partial skeleton that we collected in Eastern Sayr of the Nemegt Locality in 2003, and the

570 holotype of Quaesitosaurus orientalis Kurzanov and Bannikov, 1983, which was collected at

571 Shar Tsav (which is, however, considered by most other researchers as Djadokhta). As with the

572 Djadokhta Formation, the rare evidence of large dinosaurs suggests that they are probably the

573 remnants of animals that did not frequent this habitat.

574 Dinosaur eggs and eggshell fragments are as common in the Baruungoyot as they are in

575 the Djadokhta, and there has been at least one nest found with a brooding mother (Fanti et al.

576 2012). Dinosaur footprints are common in the Baruungoyot, but these are all in the interfingering

577 Baruungoyot-Nemegt zone, and documentDraft the presence of hadrosaurs, sauropods and

578 tyrannosaurids in the habitats that are marginal to both habitats.

579 The Baruungoyot Lithobiotope is similar to the Djadokhta Lithobiotope in most respects.

580 Both have preservational biases that favoured the fossilization of small vertebrates like lizards

581 and mammals, whereas the remains of large dinosaurs are rare, incomplete and poorly preserved.

582 Although the environments of the Djadokhta Formation consist mostly of aeolian deposits with

583 ephemeral streams and ponds, those of the Baruungoyot Formation include alluvial, lacustrine

584 and aeolian environments with more reliable sources of water. This allowed the presence of

585 anurans and aquatic turtles, which have not been found in the Djadokhta Lithobiotope. A quarter

586 of the lizard and mammal species are the same as those of the Djadokhta Formation, but at

587 present the only known shared non-avian dinosaur species is Avimimus portentosus and the only

588 avian is Gobipteryx minuta. Protoceratopsids are not as common as they are in the Djadokhta

589 Lithobiotope, whereas oviraptorids were more common in the Baruungoyot Lithobiotope.

590 Medium-sized dinosaurs include ankylosaurs, but there is no overlap of ankylosaur species

591 between the Djadokhta and Baruungoyot Lithobiotopes. With the exception of the sauropod

592 Quaesitosaurus, the partial skeletons and isolated bones of large dinosaurs are not identifiable to

593 generic or species level, but as in the Djadokhta Lithobiotope large dinosaurs seem to have been

594 in transit through the local environment rather than permanent residents. Overall, the evidence

595 suggests that the environments of the Djadokhta and Baruungoyot Formations were similar but

596 not the same. Because there are no localities known where the two formations are in physical

597 contact, then it is usually assumed that they are successional. However, given that there were

598 environmental differences between the two formations, differences in faunal composition would

599 be expected, even if the two formations were contemporaneous. The fact that a quarter of the

600 lizard and mammal diversity overlaps betweenDraft the two suggests that if the formations were not

601 coeval, then they were laid down close in time. The basin dynamics would support the idea that

602 they are at least partially overlapping in time.

603

604 The Nemegt Lithobiotope

605 The Nemegt locality on the south side of Nemegt Uul (Mountain) is considered the type

606 locality of the Nemegt Lithobiotope (Fig. 1). Other localities where the strata show analogous

607 lithological and biological characters are Altan Uul (I, II, III, IV), Bugiin Tsav (I, II), Guriliin

608 Tsav, Hermiin Tsav (I, II), Nogon Tsav, Tsaagan Khushuu, Ulan Khushuu, and smaller sites

609 dispersed amongst this group. The Altan Uul and Nemegt localities were discovered and

610 explored by the Mongolian Palaeontological Expeditions of 1946, 1948 and 1949 (Efremov

611 1949, 1954, 1955, 1958; Rozhdestvensky 1954, 1960, 1977; Lavas 1993), and were interpreted

612 in terms of lacustrine environments. This has been one of the most notorious dogmatic

613 interpretations of Russian geologists (Shuvalov 2000). The meandering channels documented by

614 Gradziński (1970) at the Nemegt locality (Fig. 11) were ignored by the Russian scientists, who

615 felt that a large lake system dominated the whole ‘Gobi Basin’ and assumed that the Gobi

616 Region in the Late Cretaceous was one large basin. As already argued in this paper, the region by

617 the Late Cretaceous was subdivided into individual gobi basins with topography similar to the

618 basin-and-range regions. The permanent lake similar to that described by Kalugina (1980) ceased

619 to exist before the end of early Cretaceous times.

620 The Nemegt Lithobiotope contains one of the most diverse assemblages of dinosaurs

621 known (Table 2). They inhabited an alluvial plain setting (Nemegt) passing laterally into a delta

622 plain (Altan Uul) drained largely by meandering channels into an ephemeral lake system. The

623 drainage was of internal type similar to Draftthe one presently existing in the Okavango Oasis of

624 Botswana (Jerzykiewicz 1998). Vegetation of this environment was abundant enough to support

625 both the animals that browsed high in the trees and those that ate low-growing plants. Predators

626 were also part of the dinosaur assemblage of this lithobiotope. The most recognizable sites of

627 dinosaur skeleton burials are meandering channels laterally filled by sand (Gradziński 1970).

628 The alluvial plains of this lithobiotope were episodically washed by violent flood events. One

629 such violent event produced the concentration of Saurolophus skeletons (Fig. 12) known as The

630 Dragon’s Tomb (Efremov 1958; Bell et al. 2018).

631 The Nemegt Formation is one of the richest (both in terms of numbers of articulated

632 dinosaur skeletons and diversity of the taxa represented) dinosaur sites in the World with close to

633 thirty species of non-avian dinosaurs described, and at least another 20 species of other

634 vertebrates identified (Table 2). Similar to Dinosaur Provincial Park in Alberta, Canada (Currie

635 and Koppelhus 2005), there is a preservational bias (Brown et al. 2013) that favours the

636 preservation and recovery of articulated skeletons of large (>500 kg) individuals. Large

637 articulated skeletons are more common in the Nemegt Formation than in the Dinosaur Park

638 Formation. However, isolated, disarticulated bones and teeth, as well as microvertebrate sites and

639 bonebeds (Rogers et al. 2007), number in the millions in the latter formation but are relatively

640 rare in the Nemegt. Similar to Dinosaur Provincial Park, skeletons are almost invariably

641 preserved lying on their sides or backs (Fig. 5), which contrasts with the majority of dinosaurs

642 recovered from the Djadokhta and Baruungoyot Lithobiotopes, which as articulated skeletons

643 tend to be found upright in life poses (Figs. 3, 4, 6, 8). And also like Dinosaur Provincial Park,

644 many of the skeletons – especially hadrosaurids and tyrannosaurids – have skin impressions

645 preserved on at least parts of their bodies (Currie et al. 2003; Bell 2012; Bell et al. 2018). The

646 preservational biases in the Nemegt FormationDraft against articulated skeletons of small vertebrates

647 also worked against isolated bones and teeth, microvertebrate sites, and bonebeds, all of which

648 are extremely rare. There are two major bonebeds worthy of note. The Dragon’s Tomb is a

649 monodominant bonebed (Fig. 12) consisting mostly of articulated skeletons of the multi-tonne

650 Saurolophus, many with associated skin impressions (Bell et al. 2018), whereas most of the

651 specimens in a bonebed of the chicken-sized Avimimus are disarticulated (Funston et al. 2016).

652 Both formations preserve dinosaur eggshell and footprints, although the latter are far more

653 common in the Nemegt Formation (Currie et al. 2003). Comparison with the Dinosaur Park

654 Formation suggests that small, non-avian dinosaurs should be much more diverse in the Nemegt

655 Formation, but that our knowledge of small dinosaurs (including juveniles of large species) is

656 limited. Furthermore, comparisons with the Baruungoyot and Djadokhta formations (Tables 1, 2)

657 demonstrates that our knowledge of most small, non-dinosaurian vertebrates (especially lizards

658 and mammals) is impoverished for the Nemegt Formation.

659 Plant and invertebrate fossils are rare throughout the Nemegt Formation, and consist of

660 ostracods (Szczechura and Blaszyk 1970), charophytes (Karczewska and Ziembinska-Tworzydlo

661 1970), gastropods, palynomorphs, wood fragments (Efremov 1955; Kielan-Jaworowska and

662 Dovchin 1968; Gradziński 1970), and unionid clams (Martinson 1982). Invertebrate trace fossils

663 (simple tubular feeding traces) are relatively common but have not been studied in detail (Eberth

664 et al. 2009; Eberth 2018).

665 Layered concentrations of fish vertebrae are common at Nemegt Formation sites, and

666 partial skeletons of a hiodontid teleost have been recovered from Ulaan Khushuu (Newbrey et al.

667 2013). Like fish, even partial skeletons of other small vertebrates – including amphibians,

668 lizards, crocodylians, mammals and birds – are rare and incomplete (Table 2), even though they

669 were probably the most common animalsDraft in the fauna. Small dinosaurs (including the young of

670 large species) are also extremely rare and are seldom represented by complete specimens

671 (Tsuihiji et al. 2011).

672 Dinosaur eggshell is relatively rare in the Nemegt Formation (Graf et al. 2018), whereas

673 eggs and nests of eggs are extremely rare (Weishampel et al. 2008). Dinosaur footprint levels are

674 common (Currie et al. 2003; Nakajima et al. 2018), and literally thousands of footprints can be

675 seen throughout the formation. Because they are mostly found in hard, indurated layers that are

676 resistant to erosion, the footprint layers tend to form cliffs that erode vertically. Therefore, the

677 footprints erode out individually, footprint bedding planes are rarely seen, and tracks arranged

678 into trackways are almost non-existent. Furthermore, although Nemegt footprints are common,

679 they do seem to be biased towards larger animals, and there is no indication at this time of small

680 vertebrates from the footprint record.

681 The Nemegt Lithobiotope is therefore incompletely known but is characterized by

682 dinosaurs of limited diversity (Alioramus remotus (which probably includes Alioramus altai),

683 Avimimus nemegtensis, Deinocheirus mirificus, Elmisaurus rarus, Gallimimus bullatus,

684 Mononykus olecranus, Nemegtosaurus mongoliensis (which probably includes

685 Opisthocoelicaudia skarzynskii), Nomingia gobiensis, Prenocephale prenes, Saurolophus

686 angustirostris (which probably includes Barsboldia sicinskii), Tarbosaurus bataar, Tarchia

687 kielanae, Therizinosaurus cheloniformis), each of which has been found at two or more of the

688 Nemegt Formation sites. This is also true for the turtles Mongolemys efremovi and Mongolemys

689 elegans, where there are numerous sites with hundreds to thousands of complete specimens

690 concentrated in what have been referred to as ‘ponds’. Small, chicken- to man-sized dinosaurs

691 (Adasaurus mongoliensis, AnserimimusDraft planinychus, Borogovia gracilicrus, Homalocephale

692 prenes, Rinchenia mongoliensis, Tochisaurus nemegtensis, Zanabazar junior) are presently only

693 known from single specimens from single sites in the Nemegt Formation, but may also be

694 characteristic of the Nemegt Lithobiotope. This is also true for the anuran Altanulia alifanovi,

695 the lizard Shinisauroides latipalatum, the crocodilians Shamosuchus ancestralis and

696 Shamosuchus tarsus, the mammal Buginbaatar transaltaiensis, and the birds Gurilynia nessovi,

697 Judinornis nogontsavensis and Teviornis gobiensis, all of which are only known from single

698 specimens and/or Nemegt sites. The environment strongly suggests that smaller animals should

699 have been much more diverse than the limited number of species that are presently known, and

700 unidentifiable fragments show there was at least a greater diversity of fish, amphibians, turtles,

701 crocodilians and birds.

702 An unusual characteristic of the Nemegt Lithobiotope is the number of tyrannosaurid

703 skeletons found at every Nemegt locality. There is clearly a preservational bias in the fluvial

704 beds that favours the preservation of Tarbosaurus bataar skeletons, which can make up to a third

705 of the articulated skeletons (Osmólska 1980; Currie 2000, 2009, 2016). Based on other dinosaur

706 sites, one would expect Tarbosaurus to make up 5 to 10% of the dinosaurs (Currie 2016). The

707 footprint levels of the Nemegt are interspersed between fluvial levels of strata that produce the

708 Nemegt skeletons, and the number of Tarbosaurus footprints is close to the expected figure of

709 5% (Currie et al. 2003).

710 Nemegtomaia barsboldia is one of the few animals identifiable to species that is known

711 from both the Nemegt and Baruungoyot Lithobiotopes (Fanti et al. 2012). However, unidentified

712 hadrosaur, ornithomimid, tyrannosaurid and sauropod bones found in the Baruungoyot

713 Formation probably respectively represent individuals of Saurolophus, Gallimimus, Tarbosaurus

714 and Nemegtosaurus that wandered fromDraft their normal environments in the Nemegt Lithobiotopes

715 into the more hostile (for large animals) Baruungoyot Lithobiotopes and died there. Because of

716 their large sizes, their bodies would rarely be buried either fast enough to avoid scavenging or

717 deep enough to be fossilized as articulated skeletons. Their rarity compared with other fossils

718 suggests that they were rare visitors to the Baruungoyot Lithobiotopes, but that there was nothing

719 to physically bar them from doing so. Ankylosaurs on the other hand are found as articulated

720 skeletons in both formations/lithobiotopes, which suggests they may have preferred living in the

721 environments that were in the transitional areas between the two. Both the ankylosaurs Tarchia

722 and Saichania have been recovered from both the Baruungoyot and Nemegt Formations (Arbour

723 et al. 2014).

724 In summary, all localities of the Nemegt Lithobiotopes have produced skeletons of

725 Gallimimus, Saurolophus and Tarbosaurus, but another eight species of dinosaurs are found in

726 multiple localities where the Nemegt Formation is exposed. Ultimately, about a dozen dinosaurs

727 may represent the Nemegt Lithobiotopes. Smaller dinosaurs and birds, plus fish, amphibians,

728 lizards, turtles, crocodilians and mammals were clearly adapted for this environment but are

729 poorly represented by specimens (because of preservational biases). Many of their species are

730 represented by only single, incomplete specimens from single sites, so it is difficult to say

731 anything definitive about their environmental, geographic or stratigraphic distributions. There are

732 dinosaurs like Nemegtomaia, Saichania and Tarchia that are found in both the Baruungoyot and

733 Nemegt Lithobiotopes, and along with the sedimentary evidence of the interfingering nature of

734 the two formations (Eberth et al. 2009; Fanti et al. 2012), there is no doubt that the formations

735 are coeval.

736

737 Discussion Draft

738 The departure from the application of formations for description of stratigraphy and

739 correlation of the Upper Cretaceous dinosaur-bearing strata of the Gobi region is not a

740 resignation. In contrast, it is a conceptual advantage because it opens new vistas into the

741 conditions of life and death of dinosaurs and other Late Cretaceous biota of the region (Fig. 13).

742 From this viewpoint, the Nemegt Basin is perhaps the most revealing dinosaur area presently

743 known in that the Djadokhta, Baruungoyot and Nemegt strata are presented in lateral transition

744 order rather than in superposition. The fact that the age of the Djadokhta has always been

745 uncertain is rather symptomatic and needs to be carefully considered. The solution proposed in

746 this paper is based on integration of knowledge about physical processes controlling the

747 sedimentation of the dinosaur-bearing strata of the Gobi region and palaeontology of the biota.

748 Sedimentation of very fine sand and dust of the Djadokhta strata has a very different

749 dynamic than the deposition of water lain sandstone of the Nemegt strata. Furthermore, these two

750 most fossiliferous types of Upper Cretaceous sediments are diametrically different in terms of

751 the habitats/environments. Using the name lithobiotope with the Djadokhta and Nemegt strata

752 instead of formation reflects both the lithology and biota of these different habitats. As far as the

753 lithological aspects are concerned, the most revealing seem to be the palaeosols of caliche type.

754 In situ, developed caliche palaeosol profiles are one of the most diagnostic indicators of climate.

755 Caliche palaeosols preserved in the Djadokhta strata are analogous to the ones developed in the

756 Rio Grande River Valley of New Mexico. The measurements of the amount of rain and timing

757 required to form these soil profiles are in the order of tens to hundreds of thousands of years of

758 aridity (Gile et al., 1981).

759 The fact that well-developed mature caliche palaeosol profiles occur only in the

760 Djadokhta Lithobiotope does not indicateDraft climatic differences between the lithobiotopes

761 described in this paper. The lack of fully developed caliche profiles in the Baruungoyot and

762 Nemegt Lithobiotopes should be explained either in terms of differences in preservation potential

763 or different conditions of accumulation. It is possible that mature caliche profiles developed in

764 all the described lithobiotopes. Relics of these profiles in the form of dispersed calcareous

765 nodules occur in both Baruungoyot and Nemegt strata. The rest of the caliche profiles may have

766 been eroded away, and only the nodular parts of the profiles are still preserved in lags. It is also

767 possible that the caliche profiles in both the Baruungoyot and Nemegt lithobiotopes were not

768 fully developed because the high energy environments prevented a sufficient flux of calcareous

769 dust during sedimentation (Mayer et al. 1988).

770 As far as the time aspect of the Djadokhta and Nemegt strata is concerned, one needs to

771 realize that the rates of their deposition were very different. The rate of accumulation of the

772 aeolian sands and loess is much slower than accumulation of alluvial plain deposits. The

773 preservation potentials of both the Djadokhta and Nemegt strata as discussed at the beginning of

774 the paper are very low. For that reason, an interpretation of the stratigraphic record of the Late

775 Cretaceous of the Gobi Region in terms of event stratigraphy is appropriate. Introduced and

776 explained by Ager (1973, 1993), the concept of event stratigraphy applies well to the stratigraphy

777 of the gobi basins. Only some events have been recorded, some of which were cataclysmic (Figs.

778 3, 4). It appears that the dinosaurs and associated animals of the Late Cretaceous of the Gobi

779 Region were trying to adjust to semi-arid desert conditions. However, progressive desertification

780 of the whole region caused water deficiencies, and frequent dust and sand storms made their

781 struggle for life progressively more difficult.

782 Spatial distribution of the Djadokhta and Nemegt strata within the Gobi Region relates to

783 the differences in the physical processesDraft of transport and deposition between sand and dust

784 particles. Stratification of the Nemegt sandstone indicates transport by meandering channels with

785 a tendency for them to overflow onto the alluvial plain and form ephemeral ponds. Cataclysmic

786 flood events were rather sporadic but also appeared deadly for dinosaurs. The lateral extent of

787 fluvial and alluvial plain deposits would have been characteristic of individual gobi basins.

788 Structures and textures of the Djadokhta strata indicate they were formed mostly by

789 blowing sand and dust. The former is expressed by the high angle cross stratification of fine

790 grained sandstone and the latter by the structureless intervals of dust. The lateral extent of

791 Djadokhta strata is much larger than – and extends far beyond – each individual gobi basin. The

792 lithological similarity between Djadokhta strata in different gobi basins demonstrates the

793 extensive nature of wind transport. The different gobi basins might be of the same age, but there

794 is no chronostratigraphic evidence to support this claim. On the other hand, the fact that the

795 Djadokhta Lithobiotopes at both Bayan Zag and at Bayan Mandahu are overlain by gravels of

796 Tertiary age (Gradziński et al. 1968; Jerzykiewicz and Russell 1991; Hasegawa et al. 2009)

797 strongly suggests not only their synchronicity but also their uppermost Cretaceous stratigraphic

798 position.

799 In summary, the Upper Cretaceous sediments of the Gobi Desert of China and Mongolia

800 are a Graben-dominated system. Each Graben developed (and was filled) independently from the

801 others, although most likely they had the same source of sediments. The available geological

802 information is not reliable to support neither unequivocal mapping nor reciprocal architecture

803 between current formations. There are no tools available at this time to delimit current

804 formations laterally, vertically, or even temporally. This is why the term formation cannot be

805 used in the Nemegt and nearby basins. Data provided in this paper demonstrate that these units,

806 termed lithobiotopes, were in part coeval,Draft change remarkably from one locality to another in

807 terms of depositional settings, and were at least partially inhabited by the same creatures. The

808 observed differences are derived from the taphonomic potential of each unit. To our knowledge,

809 there is no other region where all of these parameters apply at the same time, and the term

810 lithobiotope solely describes the deposits in the Gobi Desert.

811

812 Acknowledgements

813 This paper is dedicated to the memory of our colleague and friend Dale A. Russell,

814 whose interest in central Asian dinosaur palaeobiogeography (Russell 1993; Jerzykiewicz and

815 Russell 1991) led to many influential publications on the subject. Those who were lucky enough

816 to work with him will always admire Dale’s original and unconventional attitude towards

817 research on dinosaur palaeobiology, and his tremendous enthusiasm.

818 Our research on Gobi dinosaurs has been possible thanks to the institutions and

819 organizations that sponsored expeditions: the ExTerra Foundation, the Canadian Museum of

820 Nature, the Chinese Academy of Sciences, Dinosaurs of the Gobi (Nomadic Expeditions),

821 Korea-Mongolia International Dinosaur Project, the Institute of Paleontology of the Mongolia

822 Academy of Sciences, National Geographic Society (2016, 2018, 2019), NSERC, and the Royal

823 Tyrrell Museum of Palaeontology.

824 Expeditions sponsored by the institutions mentioned took place during the second phase

825 of investigation of Gobi dinosaurs that was initiated by Canadian and Chinese scientists in the

826 late 1980s. Needless to say, we also benefited from the experience of the first, opening phase of

827 the expeditions organized by American Museum of Natural History, the Soviet Academy of

828 Sciences, and the Polish Academy of SciencesDraft between the 1920s and early 1970s. The

829 expertise, hard work and dedication of all the members of these expeditions over the last century

830 led to the publications that contributed tremendously to our work presented in this paper. Many

831 of the previous publications are cited because without their inspiration, the submission of this

832 manuscript would simply not have been possible.

833 Table 2 was greatly improved by comments from Lukasz Czepiński (University of

834 Warsaw, Poland). Photographs in Figures 3, 4, 6, 8 and 9 were taken by the first author in 1971

835 (Figs. 3, 4, 6) and 1988 (Figs. 8, 9). Figures 2, 5 and 7 were shot by PJC, and Figures 10 and 11

836 by FF. Figure 1 was compiled by FF, and Figure 12 was drawn by Matt Rhodes (UALVP) on the

837 basis of a quarry sketch by Efremov (1955). The first author prepared Figure 13. Content and

838 readability were improved by comments from Yuong-Nam Lee (Seoul University, Korea) and

839 Chinzorig Tsogtbaatar (North Carolina Museum of Natural Sciences).

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1261

1263 Figures

1264 Fig. 1. A, major localities within the Nemegt Basin in southern Mongolia with exposed

1265 Djadokhta, Baruungoyot and Nemegt beds. This area is confined within the bounds of the

1266 rectangle on the reference map. B, reference map showing major structural and orthographic

1267 features referred to in the text. Figure 1A was created using ESRI ArcMap® 10.8 and assembled

1268 from the following data sources: ASTER GDEM v2 Worldwide Elevation Data (1 arc-second

1269 Resolution). Figure 1B was created using ESRI ArcMap® 10.8 and assembled from the following

1270 data source: GobiSurvey2018.shp, base map imagery ESRI Word Imagery.

1271

1272 Fig. 2. Saurolophus rib cage at the Dragon’s Tomb (quarry AU2002 of the Altan Uul 2 locality,

1273 Nemegt Lithobiotope). This slab was turnedDraft on end like this by the Mongolian Palaeontological

1274 Expedition of 1949, and the upper part is visible in photographs taken by members of that

1275 expedition, and were even turned into a drawing for a book by Efremov (1958).

1276 Fig. 3. Protoceratops trapped in an aeolian sand dune. The majority of specimens of this genus

1277 are found as articulated skeletons in upright poses. Djadokhta Lithobiotope, Tögrögiin Shiree

1278 locality.

1279 Fig. 4. The ‘Fighting dinosaurs’ (Velociraptor MPC-D100/25 to the right in both photographs,

1280 Protoceratops MPC 100/501 to the left), Djadokhta Lithobiotope, Tögrögiin Shire locality. A,

1281 closeup showing the Velociraptor arms clasping the head of the Protoceratops. The top of the

1282 Protoceratops skull had been eroded away before discovery, and the right forearm of the

1283 Velociraptor was clasped by the Protoceratops beak. The left hand of the predator is wrapped

1284 around the back of the skull of the supposed prey. B, the Protoceratops is in life pose with its

1285 limbs tucked underneath, whereas the Velociraptor is lying on its right side.

1286 Fig. 5. Sauropod sacrum (visible in ventral view) as seen in 2016 at quarry Nem034 (also

1287 referred to as “Sauropod 23”) 400 metres from the 1970 Polish Mongolian camp in the North

1288 Sayr of the Nemegt locality, Nemegt Lithobiotope. The specimen was partially excavated in

1289 1970 by the Polish-Mongolian Palaeontological Expedition when some of the appendicular

1290 bones were still present.

1291 Fig. 6. ‘Agony of an armored dinosaur’. Baruungoyot Lithobiotope exposed at Khulsan. The

1292 skeleton of the holotype specimen of Saichania chulsanensis was found at the transition between

1293 an aeolian dune and an ephemeral pond. A, white arrows point to the bottoms of individual flood

1294 layers covered with natural moulds of desiccation cracks. B, dorsal view of the skull and neck

1295 plates of Saichania. Abbreviations: Exb, aeolian, cross-bedded sandstone: Wld, water-laid

1296 interbedded sandstones and mudstones.Draft

1297 Fig. 7. Section showing cross-bedding of the Djadokhta Lithobiotope at Tögrögiin Shiree.

1298 Fig. 8. A, cross-bedded aeolian sand and dust deposits of the Djadokhta Lithobiotope, Bayan

1299 Mandahu locality, Nei Mongol. B, excavation of Pinacosaurus ‘babies’ from aeolian dune. C,

1300 detail of an articulated skull and neck plates of a Pinacosaurus individual from the site.

1301 Fig. 9. Caliche palaeosol profiles with hardpan horizons (white arrows). Djadokhta Lithobiotope,

1302 Bayan Mandahu locality, Inner Mongolia.

1303 Fig. 10. Large-scale cross-stratified aeolian sandstone. Baruungoyot Lithobiotope, Khulsan

1304 locality.

1305 Fig. 11. Point bar stratification (indicated by dashed lines) of fluvial sandstones. Dinosaur

1306 footprints (white arrows) distort the lower surfaces of the harder, cliff-forming sandstones,

1307 especially the layer at midheight of the exposure. Nemegt locality of the Nemegt Lithobiotope.

1308 Fig. 12. Saurolophus angustirostris skeleton, skulls, and distribution of specimens in the

1309 Dragon’s Tomb in 1949. A, skeletal drawing of MPC-D 100/764. B, juvenile skull (PIN 551-

1310 359) scaled and adapted from Bell (2011). C, D, adult skull (PIN 551-356) in lateral and dorsal

1311 views (after Rozhdestvensky 1957). E, map of the Saurolophus-dominated assemblage of the

1312 ‘Dragon’s Tomb’ at Altan Uul 2, Nemegt Lithobiotope, redrawn from Efremov, 1955. Cross-

1313 hatching shows area of the bonebed that was eroded, and lines in upper left corner are petrified

1314 logs.

1315 Fig. 13. Lithobiotopes of the Nemegt Gobi Basin. Early Cretaceous environments with extensive

1316 lake deposits and volcanism represented on the left side of the drawing.

1317

1318 Draft

1319 Table 1. Characteristics of the Djadokhta, Baruungoyot and Nemegt Lithobiotopes.

Characteristics Djadokhta Baruungoyot Nemegt

Mostly aeolian yes no no

Aeolian and alluvial no yes no

Mostly alluvial no no yes

Eggs, nests common yes yes no

Fish, aquatic turtles no no yes

Footprints few common common

Lizard diversity 40 40 1

Terrestrial crocodilians 2 1 0

Mammal diversity 40 speciesDraft40 species 1 species

Dinosaur diversity 38 species 19 species 32 species

Small to medium dinosaurs articulated articulated partial

Large dinosaurs fragmentary fragmentary common

Dinosaurs in life poses common common no

Preservation of skin rare Not known common

1320

1321 Table 2. Distribution of vertebrate taxa in the Nemegt Basin according to our most recent

1322 understanding. Dinosaurs are listed and sorted at family level. Barsboldia is synonymous with

1323 Saurolophus; Eopelobates is Gobiates; Gobibaatar is considered to be Kryptobaatar;

1324 Gobiceratops, Lamacertops, Magnirostris and Platyceratops are all synonymized with

1325 Bagaceratops (Czepiński 2019); Ingenia is now Heyuannia; Minotaurosaurus is subsumed under

1326 Saichania (Arbour et al. 2014, but see Penkalski and Tumanova (2016) for a different

1327 conclusion); Nanantius is considered synonymous with Gobipteryx (Chiappe et al. 2001);

1328 Opisthocoelicaudia is included under Nemegtosaurus (Currie et al. 2018, but see Averianov and

1329 Lopatin, 2019 for a different viewpoint); three species of Shamosuchus are considered as

1330 Paralligator gradilifrons (Turner 2015); Shinsauroides is considered to be the same as Carusia

1331 (Gao and Norell 1996) and Tugrigbaatar is synonymized with Kryptobaatar.

1332 Macrocephalosaurus was a preoccupied name that has been changed to Gilmoreteius (Langer

1333 1998). The Alagteeg Formation, listed as a separate column following Hasegawa et al. (2009),

1334 has a very limited exposure of fluvially derived beds at Alag Teeg and Abdrant Nuru that have

1335 previously been included in the Djadokhta Formation. Abbreviations: G., genus; indet.,

1336 indeterminant, although it could be a new taxon; sp., species; Thero, theropod: Localities: A,

1337 Bayan Mandahu; B, Bayan Zag; C, TögrögiinDraft Shire; D, Ukhaa Tolgod; E, Hermiin Tsav,

1338 Baruungoyot level; F, Nemegt Locality, Baruungoyot level; G, Khulsan; H, Altan Uul, Nemegt

1339 Formation levels; I, Bugiin Tsav; J, Guriliin Tsav; K, Hermiin Tsav, Nemegt level; L, Nemegt

1340 Locality, Nemegt level; M, Tsaagan Khushuu; N, Ulaan Khushuu.

Higher taxon Genus Species Alagteeg Djadokhta A B C D Baruungoyot E F G Nemegt H I J K L M N Tyrannosauridae Genus indet. Species indet. ------x ------Tyrannosauridae Tarbosaurus bataar ------x x x x x x x x Tyrannosauridae Alioramus altai ------x - - - - - x - Tyrannosauridae Alioramus remotus ------x - - - - x - - Tyrannosauridae Bagaraatan ostromi ------x - - - - x - - Large theropod A Genus indet. Species indet. - x - - - x ------Large theropod B Genus indet. Species indet. - x - - - x ------Dromaeosauridae Adasaurus mongoliensis ------x - x - - - - - Dromaeosauridae Linheraptor exquisitus - x x ------Dromaeosauridae Mahakala omnogovae - x - - x ------Dromaeosauridae Tsaagan mangas - x - - - x ------Dromaeosauridae Velociraptor osmolskae - x x ------Dromaeosauridae Velociraptor mongoliensis - x - x x ------Dromaeosauridae Velociraptorine n.sp. - x ------Dromaeosauridae Velociraptorine Species indet. ------x - - - x ------Dromaeosauridae Halskaraptor escuilliei Draft- x - - - x ------Dromaeosauridae Hulsanpes perlei ------x - - x ------Troodontidae Almas ukhaa - x - - - x ------Troodontidae Arachaeornithoides deinosauriscus - x - x ------Troodontidae Borogovia gracilicrus ------x x ------Troodontidae Byronosaurus jaffeii - x - - - x ------Troodontidae Gobivenator mongoliensis - x ------Troodontidae Linhevenator tani - x x ------Troodontidae Philovenator curriei - x x ------Troodontidae Saurornithoides mongoliensis - x - x ------Troodontidae Tochisaurus nemegtensis ------x ------Troodontidae Genus indet. Species indet. - x - - - x ------Troodontidae Zanabazar junior ------x ------Ornithomimidae Genus indet. Species indet. ------x ------Ornithomimidae ornithomimid A Species indet. - x - - - x ------Ornithomimidae ornithomimid B Species indet. - x - - - x ------Ornithomimidae Aepyornithomimus tugrikinensis - x - - x ------Ornithomimidae Anserimimus planinychus ------x - x - x - - - Ornithomimidae Deinocheirus mirificus ------x x x - - - x - Ornithomimidae Gallimimus bullatus ------x x x x x x x x Alvarezsauridae Ceratonykus oculatus ------x x ------

Alvarezsauridae Kol ghuva - x - - - x ------Alvarezsauridae Linhenykus monodactylus - x x ------Alvarezsauridae Mononykus olecranus ------x - x x - - - Alvarezsauridae Nemegtonykus citus ------x x ------Alvarezsauridae Parvicursor remotus ------x - - x ------Alvarezsauridae Shuvuuia deserti - x - - x x ------Alvarezsauridae Genus indet. Species indet. - x ------Avimimidae Avimimus nemegtensis ------x - x - - x - Avimimidae Avimimus portentosus - x ------Caenagnathidae Elmisaurus rarus ------x x x - x x - - Caenagnathidae Nomingia gobiensis ------x - x - - x - - Oviraptoridae Citipati osmolskae - x - - - x ------Oviraptoridae Conchoraptor gracilis ------x x - x ------Oviraptoridae Gobiraptor minutus ------x x ------Oviraptoridae Heyuannia (=Ingenia) yanshini ------x x ------Oviraptoridae Khaan mckennai - x - - - x ------Oviraptoridae Machairasaurus leptonychus Draft- x x ------Oviraptoridae Nemegtomaia barsboldia ------x - x - x - - - - x - - Oviraptoridae Oksoko avarsan ------x - x x - - - - Oviraptoridae Oviraptor philoceratops - x - x ------Oviraptoridae Rinchenia mongoliensis ------x x ------Oviraptoridae Wulatelong gobiensis - x x ------Therizinosauria Therizinosaurus cheloniformis ------x x x - x x x - Therizinosauria Genus indet. Species indet. ------x ------Titanosauria Abdarainurus barsboldi x ------Titanosauria Nemegtosaurus mongoliensis ------x x - - x x x - Titanosauria Opisthocoelicaudia skarzynskii ------x x ------Titanosauria Quaesitosaurus orientalis - - - - - x ------Ankylosauridae Pinacosaurus grangeri x x x x - x x ------Ankylosauridae Pinacosaurus mephistocephalu x x x ------Ankylosauridae Saichania chulsanensis - x - - - x x x - x x x ------Ankylosauridae Tarchia kielanae ------x x - x x x - - x x - - Ankylosauridae Tarchia teresae ------x - - - x - - - Ankylosauridae Dyoplosaurus nom.dub. giganteus nom. ------x - - - - x - - Ankylosauridae Zaraapelta nomadis ------x - - - x - - - Hadrosauridae Genus indet. Species indet. - x - - - - x ------Hadrosauridae Barsboldia sicinskii ------x ------

Hadrosauridae Plesiohadros djadochtaensis x x ------Hadrosauridae Saurolophus angustirostris ------x x x x x x x x Pachycephalosauridae Goyocephale lattimorei - x ------Pachycephalosauridae Homalocephale calathocercos ------x - - - - x - - Pachycephalosauridae Prenocephale prenes ------x - x x x x - Pachycephalosauridae Tylocephale gilmorei ------x ------Protoceratopsidae Bagaceratops rozhdestvenskyi - x x - - - x x x ------Protoceratopsidae Bainoceratops efremovi - x - x ------Protoceratopsidae Breviceratops kozlowskii ------x - - x ------Protoceratopsidae Protoceratops andrewsi - x - x x ------Protoceratopsidae Protoceratops hellenikorhinus - x x ------Protoceratopsidae Udanoceratops tschizhovi - x ------Protoceratopsidae Udanoceratops sp. Indet. - x x ------Protoceratopsidae New genus New species - x - - - x ------Fish indeterminate indeterminate ------x - - - x x x x - x - - Amphibian Altanulia alifanovi ------x x ------Amphibian Cretsalia tsybini Draft------x x ------Amphibian Gobiates khermeentsavi ------x x ------Amphibian Gobiates (= Eopelobates) leptocolaptus ------x x ------Turtle "Neurankylus" Species indet. ------x ------Turtle Amyda' menenri ------x ------Turtle Basilemys Species indet. ------x ------Turtle Bulganemys jaganchobili - x - - - - x ------Turtle Genus indet. Species indet. x ------x ------Turtle Gobiapalone breviplastra ------x - x x - - Turtle Gravemys barsboldi ------x - - - x - - - x - - - Turtle Haichemys ulensis ------x ------Turtle Jiangxichelys neimongolensis - x x ------Turtle Lindholmemys sp. ------x ------Turtle Mongolochelys efremovi ------x x x x x x x x Turtle Mongolemys elegans ------x - x - - - x - Turtle Mongolemys Species indet. ------x ------Turtle Nemegtemys conflata ------x - x - - - - - Turtle Trionychidae Species indet. x - - - - - x ------Turtle Trionyx sp. A ------x ------Turtle Trionyx sp. B ------x ------Turtle Yumenemys inflatus ------x ------

Turtle Zangerlia dzamynchondi ------Turtle Zangerlia testudinimorpha ------x x x ------Turtle Zangerlia ukhaachelys - - - - - x ------Lizard Adamisaurus magnidentatus - x x x x x x x - x ------Lizard Anchaurosaurus gilmorei - x x ------Lizard Aiolosaurus oriens - x - - - x ------Lizard Altanteius facilis ------x x ------Lizard Bainguis parvus - x x x ------Lizard Barungoia vasta ------x - - x ------Lizard Carusia intermedia - x x x - x x x - x x ------Lizard Chamaeleognathus iordanskyi ------x x ------Lizard Cherminotus longifrons - x - - x x x x ------Lizard Cherminsaurus kozlowskii ------x x ------Lizard Conicodontosaurus djadochtaensis - x x x ------Lizard Cretagama bialynickae ------Lizard Ctenomastax parva - x - - - - x - - x ------Lizard Darchansaurus estesi Draft------x x ------Lizard Dzhadochtosaurus giganteus - x - - x ------Lizard Eoxanta lacertifrons ------x x ------Lizard Erdenetosaurus robinsonae ------x x ------Lizard Estesia mongoliensis - x - x - x x x - x ------Lizard Exoanta lacertifrons - x - - - x - x ------Lizard Flaviagama dzerzhiniskii - x - - x ------Lizard Genus indet. Species indet. - x ------Lizard Gladidenagama semiplena ------x x ------Lizard Globaura venusta - x x x - x x x - x ------Lizard Gobekko cretacicus - x - x ------Lizard Gobiderma pulchrum - x x - x x x x - x ------Lizard Gobinatus arenosus - x - - - x x x - x ------Lizard Gurvansaurus canaliculatus ------x - - x ------Lizard Gurvansaurus potissimus - x - - x ------Lizard Hymenosaurus clarki - x - - - x ------Lizard Igua minuta ------x - - x ------Lizard Genus indet. Species indet. - x ------Lizard Isodontosaurus gracilis - x x x x x ------Lizard Genus indet. Species indet. - x ------Lizard macrocephalosaurid indet. ------x ------

Lizard Gilmoreteius chulsanensis ------x - x x ------Lizard Gilmoreteius ferrugenous - x - x ------Lizard Gilmoreteius gilmorei cf. ------x x ------Lizard Gilmoreteius Species indet. - x - - - x ------Lizard Mimeosaurus crassus - x x x - x ------Lizard Mimeosaurus tugrikinensis - x x - x ------Lizard Mongolochamops reshetovi ------x x ------Lizard Morunasius modestus ------x x ------Lizard Myrmecodaptri microphagos - x - - - x ------Lizard Genus indet. Species indet. - x x ------Lizard Ovoo gurvel - x - - - x ------Lizard Parameiva oculea ------Lizard Paravaranus angustrifrons ------x - - x ------Lizard Parmeosaurus scutatus - x - - - x ------Lizard Phrynosomimus asper ------x x ------Lizard Parviderma inexacta ------x ------Lizard Phrynosomimus asper Draft- x - - - x x x - x ------Lizard Piramicephalosaurus cherminicus ------x x ------Lizard Polrussia mongoliensis ------x - - x ------Lizard Pleurodontagama aenigmatodes - x x ------Lizard Priscagama gobiensis - x x x x x x - x ------Lizard Prodenteia ministra ------x x ------Lizard Proplatynotia longiostrata ------x - - x ------Lizard Pyramicephalosaurus cherminicus ------x x - x ------Lizard Saichangurvel davidsoni - x - - - x ------Lizard Saniwides mongoliensis ------x - - x ------Lizard Genus indet. Species indet. - x - - - x ------Lizard Sineoamphisbaena hexatabularis - x x - - - x - - x ------Lizard Slavoia darevskyi - x - - - x x x - x ------Lizard Tchingisaurus multivagus - x - - - x x x ------Lizard Telmasaurus grangeri - x - x - - x ------Lizard Temujinia ellisoni - x - - x x ------Lizard Xihaina aquilonia - x x ------Lizard Zapsosaurus sceliphros - x - - x ------Crocodile Artzosuchus brachicephalus ------x ------Crocodile Gobiosuchus kielanae - x - x ------Crocodile Paralligator gradilifrons ------x - - - - x - -

Crocodile Shamosuchus djadochtaensis - x - x x ------Crocodile Zaraasuchus shepardi - x ------Crocodile Zosuchus davidsoni - x ------Pterosaur Genus indet. Species indet. ------x x - - - - - x Bird Genus indet. Species indet. - x ------Bird Genus indet. Species indet. ------x ------Bird Genus indet. Species indet. ------x - - - - - x - Bird Apsaravis ukhaana - x - - - x ------Bird Brodavis mongoliensis ------x - x - - - - - Bird Elsornis keni - x ------Bird Gobipteryx minuta - x x - - x x x - x ------Bird Gurilynia nessovi ------x - - x - - - - Bird Hollanda luceria ------x x ------Bird Judinornis nogontsavensis ------x ------Bird Teviornis gobiensis ------x - - x - - - - Mammal Asiatherium reshetovi ------x ------Mammal Aioryctes nemegtensis Draft------x x x x ------Mammal Genus indet. Species indet. - x - - - x ------Mammal Barunlestes butleri ------x x - x ------Mammal Buginbaatar transaltaiensis ------x - x - - - - - Mammal Bulganbaatar nemegtbaataroid - x - x ------Mammal Catopsbaatar catopsaloides ------x x ------Mammal Chulsanbaatar vulgaris - x - - - x x x x x ------Mammal Deltatheridium pretrituberculare - x - x - x x x x ------Mammal Deltatheroides cretacicus - x - x ------Mammal Djadochtatherium matthewi - x - x x x ------Mammal Genus indet. Species indet. - x ------Mammal Genus indet. Species indet. - x ------Mammal Guibaatar castellanus - x x ------Mammal Hyotheridium dobsoni - x - x ------Mammal Kamptobaatar kuczynskii - x - x ------Mammal Kamptobaatar Species indet. ------x ------Mammal Kennalestes gobiensis - x x x ------Mammal Kryptobaatar dashezvegi - x - x x x x x ------Mammal Kryptobaatar gobiensis - x - - - x ------Mammal Kryptobaatar mandahuensis - x x ------Mammal Kryptobaatar saichanensis - x - - x x ------

Mammal Maelestes gobiensis - x - - - x ------Mammal Mangasbaatar udanii ------x ------Mammal Nemegtbaatar gobiensis - x - - - x x x x x ------Mammal Nesovbaatar multicostatus ------x x ------Mammal Sloanbaatar mirabilis - x - x ------Mammal Tombaatar sabuli - x - - - x ------Mammal Ukhaatherium nessovi - x - - - x ------Mammal Zalambdalestes lechei - x - x - x ------Mammal Zalambdalestes Species indet. ------x ------Mammal Zofialestes longidens ------x - x ------

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Fig. 1. A, major localities within the Nemegt Basin in southern Mongolia with exposed Djadokhta, Baruungoyot and Nemegt beds. This area is confined within the bounds of the rectangle on the reference map. B, reference map showing major structural and orthographic features referred to in the text. Figure 1A was created using ESRI ArcMap® 10.8 and assembled from the following data sources: ASTER GDEM v2 Worldwide Elevation Data (1 arc-second Resolution). Figure 1B was created using ESRI ArcMap® 10.8 and assembled from the following data source: GobiSurvey2018.shp, base map imagery ESRI Word Imagery.

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Fig. 2. Saurolophus rib cage at the Dragon’s Tomb (quarry AU2002 of the Altan Uul 2 locality, Nemegt Lithobiotope). This slab was turned on end like this by the Mongolian Palaeontological Expedition of 1949, and the upper part is visible in photographs taken by members of that expedition, and were even turned into a drawing for a book by Efremov (1958).

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Fig. 3. Protoceratops trapped in an aeolian sand dune. The majority of specimens of this genus are found as articulated skeletons in upright poses. Djadokhta Lithobiotope, Tögrögiin Shiree

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Fig. 4. The ‘Fighting dinosaurs’ (Velociraptor MPC-D100/25 to the right in both photographs, Protoceratops MPC 100/501 to the left), Djadokhta Lithobiotope, Tögrögiin Shire locality. A, closeup showing the Velociraptor arms clasping the head of the Protoceratops. The top of the Protoceratops skull had been eroded away before discovery, and the right forearm of the Velociraptor was clasped by the Protoceratops beak. The left hand of the predator is wrappedDraft around the back of the skull of the supposed prey. B, the Protoceratops is in life pose with its limbs tucked underneath, whereas the Velociraptor is lying on its right side.

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Fig. 5. Sauropod sacrum (visible in ventral view) as seen in 2016 at quarry Nem034 (also referred to as “Sauropod 23”) 400 metres from the 1970 Polish Mongolian camp in the North Sayr of the Nemegt locality, Nemegt Lithobiotope. The specimen was partially excavated in 1970 by the Polish-Mongolian Palaeontological Expedition when some of the appendicular bones were still present.

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Fig. 6. ‘Agony of an armored dinosaur’. Baruungoyot Lithobiotope exposed at Khulsan. The skeleton of the holotype specimen of Saichania chulsanensis was found at the transition between an eolian dune and an ephemeral pond. A, white arrows point to the bottoms of individual flood layers covered with natural moulds of desiccation cracks. B, dorsal view of the skull and neck plates of Saichania. Abbreviations: Exb, eolian, cross-bedded sandstone: Wld, water-laid interbedded sandstones and mudstones.

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Fig. 7. Section showing cross-bedding at Tögrögiin Shire.

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Fig. 8. A, cross-bedded aeolian sand and dust deposits of the Djadokhta Lithobiotope, Bayan Mandahu locality, Nei Mongol. B, excavation of Pinacosaurus ‘babies’ from aeolian dune. C, detail of an articulated skull and neck plates of a Pinacosaurus individual from the site.

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Fig. 9. Caliche palaeosol profiles with hardpan horizons (white arrows). Djadokhta Lithobiotope, Bayan Mandahu locality, Inner Mongolia.

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Fig. 10. Large-scale cross-stratified aeolian sandstone. Baruungoyot Lithobiotope, Khulsan locality.

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Fig. 11. Point bar stratification (indicated by dashed lines) of fluvial sandstones. Dinosaur footprints (white arrows) distort the lower surfaces of the harder, cliff-forming sandstones, especially the layer at midheight of the exposure. Nemegt locality of the Nemegt Lithobiotope.

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Fig. 12. Saurolophus angustirostris skeleton, skulls, and distribution of specimens in the Dragon’s Tomb in 1949. A, skeletal drawing of MPC-D 100/764. B, juvenile skull (PIN 551-359) scaled and adapted from Bell (2011). C, D, adult skull (PIN 551-356) in lateral and dorsal views (after Rozhdestvensky 1957). E, map of the Saurolophus-dominated assemblage of the ‘Dragon’s Tomb’ at Altan Uul 2, Nemegt Lithobiotope, redrawn from Efremov, 1955. Cross-hatching shows area of the bonebed that was eroded, and lines in upper left corner are petrified logs.

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Fig. 13. Lithobiotopes of the Nemegt Gobi Basin. Early Cretaceous environments with extensive lake deposits and volcanism represented on the left side of the drawing.

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