1

2

3

4 Revised geochronology, correlation, and dinosaur stratigraphic

5 ranges of the Santonian-Maastrichtian (Late Cretaceous)

6 formations of the Western Interior of North America

7

8

9 Denver Fowler 1*

10

11

12

13 1Dickinson Museum Center, Dickinson ND, USA

14

15

16 *Corresponding author

17

18 E-mail: [email protected]

1

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 19 Abstract

20

21 Interbasinal stratigraphic correlation provides the foundation for all consequent

22 continental-scale geological and paleontological analyses. Correlation requires synthesis

23 of lithostratigraphic, biostratigraphic and geochronologic data, and must be periodically

24 updated to accord with advances in dating techniques, changing standards for radiometric

25 dates, new stratigraphic concepts, hypotheses, fossil specimens, and field data. Outdated

26 or incorrect correlation exposes geological and paleontological analyses to potential error.

27

28 The current work presents a high-resolution stratigraphic chart for terrestrial Late

29 Cretaceous units of North America, combining published chronostratigraphic,

30 lithostratigraphic, and biostratigraphic data. 40 Ar / 39 Ar radiometric dates are newly

31 recalibrated to both current standard and decay constant pairings. Revisions to the

32 stratigraphic placement of most units are slight, but important changes are made to the

33 proposed correlations of the Aguja and Javelina Formations, Texas, and recalibration

34 corrections in particular affect the relative age positions of the Belly River Group,

35 Alberta; Judith River Formation, Montana; Kaiparowits Formation, Utah; and Fruitland

36 and Kirtland formations, New Mexico.

37

38 The stratigraphic ranges of selected clades of dinosaur species are plotted on the

39 chronostratigraphic framework, with some clades comprising short-duration species that

40 do not overlap stratigraphically with preceding or succeeding forms. This is the expected

2

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 41 pattern that is produced by an anagenetic mode of evolution, suggesting that true

42 branching (speciation) events were rare and may have geographic significance. The

43 recent hypothesis of intracontinental latitudinal provinciality of dinosaurs is shown to be

44 affected by previous stratigraphic miscorrelation. Rapid stepwise acquisition of display

45 characters in many dinosaur clades, in particular chasmosaurine ceratopsids, suggests that

46 they may be useful for high resolution biostratigraphy.

47

48 Introduction

49

50 In 1952, Cobban and Reeside published a grand correlation of Cretaceous rocks

51 of the Western Interior of central and southern North America, including both marine and

52 terrestrial units, and biostratigraphic ranges for a variety of invertebrates and vertebrates.

53 Such interbasinal correlation diagrams are enormously useful for making stratigraphic

54 comparisons between units and similar style diagrams have become commonplace in the

55 geological literature. Recent, broad-scale correlations akin to that of Cobban and Reeside

56 (1952) are less common, but examples include Krystinik and DeJarnett (1995), Sullivan

57 and Lucas (2003; 2006); Miall et al. (2008), and Roberts et al., (2013). Construction of

58 these kinds of correlation charts is built upon a great wealth of literature; the product of

59 dedicated work by generations of stratigraphers working in the Western Interior.

60 Individual papers doubtless number in the thousands, and there are far too many to

61 mention directly here, although many are cited in the supporting information (see S1

62 Table and S1 Text).

3

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 63

64 Interbasinal correlation charts are not just of use to geologists; more frequently

65 than ever, paleontologists are using high-resolution chronostratigraphic data to formulate

66 and test evolutionary hypotheses. A simple example is that of time-calibrated phylogenies,

67 where the stratigraphic positions of individual taxa are superimposed on phylogenetic

68 trees. These are becoming much more prominent in the dinosaur literature (e.g., Sampson

69 and Loewen, 2010; Sampson et al., 2010; Campione and Evans, 2011; Evans et al., 2013),

70 and are used to deduce the timing of important phylogenetic branching events, infer ghost

71 ranges, and potentially to calculate rates of evolution. A more nuanced application is

72 assessment of whether two sister taxa are contemporaneous (thereby inferring a genuine

73 speciation event), or whether they form a succession of stratigraphically separated

74 morphologies (potentially supportive of anagenesis; e.g. Horner et al., 1992; Campione

75 and Evans, 2011; Scannella et al., 2014; Freedman Fowler and Horner, 2015). The value

76 of such analyses is inherently dependent upon the accuracy of the plotted taxa, which in

77 turn depend upon the accuracy of the stratigraphic correlations of the formations from

78 which their fossils were recovered. Herein lies the problem. Precise dating of geological

79 formations is especially critical for testing anagenesis or cladogenesis in dinosaurs

80 (Horner et al., 1992; Sampson et al., 2010), but when specimens are very similar in age,

81 imprecision of only a few hundred thousand years is often enough to completely reverse

82 paleobiological interpretation.

83

84 The Upper Cretaceous deposits of the North American Western Interior represent

85 a rare opportunity to make a high-resolution chronostratigraphic framework within which

4

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 86 to study dinosaur evolution. This is due to the serendipitous combination of large areas of

87 outcrop, interfingering marine units with biostratigraphically informative fossils, and a

88 consistent scattering of radiometric dates due to synorogenic volcanic activity, not to

89 mention a vast literature detailing over a century’s worth of research and an ever

90 increasing collection of fossils. However, despite the large amount of data available,

91 some problems persist that strongly affect paleobiological interpretations:

92

93 1. It is difficult to find the reasoning behind some correlations

94 In paleontological papers especially, correlation charts are typically presented as a series

95 of geological columns, and rarely contain detailed justifications for the stratigraphic

96 positions of the depicted horizons. Usually a few citations are given for stratigraphic

97 position, and radiometric dates may be marked (also including citations), but important

98 details may be lacking. This can create many problems, including circular citation of

99 incorrect or unknown stratigraphic data or unknowingly mismatching old outdated

100 stratigraphic data with new interpretations or calibrations (see Discussion for detailed

101 explanation of examples). Admittedly, justifying every boundary or horizon in a

102 stratigraphic column is an arduous task, but without detailed work like this, precise

103 stratigraphic placement of taxa can be either impossible or plotted incorrectly.

104

105 2. Depositional hiatuses are not depicted

106 The normal method of illustrating stratigraphic columns often does not include

107 illustration of the depositional hiatuses (lacunae) that exist within formations, and this

108 can affect the perception of unit duration, conformability, and magnetostratigraphic

5

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 109 relationships. For example, under conventional lithostratigraphic practice, prominent

110 sandstones are sometimes chosen as uppermost units for formational contacts (e.g. the

111 Capping Sandstone Member, Wahweap Formation, Utah; Eaton, 1991). However, under

112 the conventional sequence stratigraphic model, amalgamated channel sandstones often

113 form the basalmost unit of depositional cycles, resting upon a surface of erosion or

114 depositional hiatus; i.e., the basal bed of a conformable cycle might simultaneously be

115 considered the uppermost unit of a lithostratigraphic formation. For this and other reasons,

116 formation members and the lacunae between them should be plotted on correlation charts

117 where possible.

118

119 3. Radiometric dates may be incorrect or incomparable

120 Many currently cited radiometric dates are not properly comparable, because from the

121 early 1980's to the current day radiometric analyses have used a variety of standards,

122 decay constants, or different methods. In order to rectify this, historical dates have been

123 recalibrated by previous workers (e.g. for the Western Interior, Eberth, 2005; Kuiper et

124 al., 2008; Schmitz, 2012; Roberts et al., 2013; Sageman et al., 2014; Freedman Fowler

125 and Horner, 2015). There is also an emerging issue that analyses performed in different

126 laboratories under slightly different methodologies produce slightly different results, and

127 this is being investigated internally by those labs

128

129 This current work presents a comprehensive stratigraphic correlation chart

130 comprising the major terrestrial geological formations of the Santonian through

131 Maastrichtian of the North American Western Interior (S1 Table). The chart is plotted

6

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 132 based on extensive reference to the stratigraphic literature on each formation (which is

133 reviewed and cited in detailed notes for each unit), and on the recalibration of 40 Ar / 39 Ar

134 radiometric dates. Over 200 recalibrated radiometric dates are presented as a separate

135 excel sheet (S2 Table), and are recalibrated to both currently accepted 40 Ar / 39 Ar

136 standards and decay constant pairings (Kuiper et al., 2008, combined with the decay

137 constant values of Min et al., 2000; and Renne et al., 2011). The resultant stratigraphic

138 framework is used in combination with locality data for individual dinosaur specimens to

139 plot the stratigraphic ranges for dinosaur taxa (currently restricted to Neoceratopsia,

140 Sauropoda, Pachycephalosauridae, and Hadrosauridae). This replotting of dinosaur taxa

141 is discussed with regard to current hypotheses of dinosaur biogeography and evolution.

142

143 Methods

144

145 Abbreviations used

146

147 Gp, Group; Fm, Formation; Mbr, Member; Ma, millions of years ago; Ka thousands of

148 years ago; m.y. million years; k.y. thousand years; c.z., coal zone; FCT, Fish Canyon

149 Tuff; TCR, Taylor Creek Rhyolite.

150

151 Display format - excel sheets

152

7

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 153 The recalibration sheet and stratigraphic correlation chart are offered as two separate

154 excel files (S1 and S2 Tables). They are kept separate for ease of cross referencing.

155

156 S1 Table - Stratigraphic correlation chart

157

158 The stratigraphic correlation chart is arranged as an Excel spreadsheet (S1 Table),

159 and is intended to be used directly in this format as it offers a number of advantages over

160 a graphic embedded within a PDF or printed page. The grid of cells naturally permit

161 precise plotting of stratigraphic boundaries, with each vertical cell height representing 0.1

162 m.y. Most usefully, each cell, (or group of cells) can be tagged with a pop-up note that is

163 activated by simply hovering the mouse cursor over any cell with a red triangle in the

164 upper right corner. These pop-up notes comprise the bulk of the results of this study,

165 providing the information that supports each depicted stratigraphic position or boundary

166 of the geologic unit or taxon, along with introductory text. For ideal formatting, the

167 reader is advised to view the chart in native resolution, at 22% zoom level.

168

169 Some disadvantages of the Excel format include the limited range of line styles

170 and orientations, such that (for example) it is not possible to represent unconformities by

171 a wavy line, and cell borders necessarily are straight. Due to the need to keep font size

172 small (to increase available space), taxon names are not produced in italics as it makes

173 them much less readable. The reader is advised that under some levels of zoom, a note

174 box might not be fully readable; if so, right click and select edit note, then either read the

175 note in place, or resize the note box such that all the text is visible.

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PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 176

177 References used in the construction of the chart are available as a separate

178 document (Text S1).

179

180 S2 Table - Recalibration sheet

181

182 The recalibration sheet (S2 Table) is also made available in the form of an Excel

183 sheet. This is due to its large size, but also benefits from the pop-up note function,

184 providing additional information on radiometric dates and the original publications.

185 Maintaining the recalibrations as an Excel sheet also permits the retention of the active

186 formulae used to calculate the new dates.

187

188 The recalibration sheet is adapted from the EARTHTIME excel recalculation

189 sheet kindly provided by Noah McLean at the earthtime.org website. Unfortunately, the

190 main homepage of the earthtime.org website is currently listed as "under construction";

191 however, the direct link to the recalibration spreadsheet and instructions download page

192 is still active as of Jan 30th 2017: http://www.earth-time.org/ar-ar.html Note that a

193 similar recalibration sheet was provided by Paul Renne (pers. comm. 2012)

194

195 The original recalibration formulae were duplicated across into S2 Table such that

196 this is a "live" document that independently recalculates dates based on the input data on

197 each line of the sheet. The source lines for each recalculation have been adapted from the

198 original EARTHTIME recalulation sheet such that in S2 Table all the original input data

9

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 199 (standards, decay constant, etc.) are visible for each recalculation. This way the sheet

200 shows all the "working" for all of the ~200 recalculations, and each can be properly

201 independently assessed.

202

203 There is an issue with the recalculation of error in the original formulae present in

204 the McLean EARTHTIME sheet. This has the result that for some recalibrations, the

205 excel sheet will only produce a "!VALUE" statement for the recalibrated

206 uncertainty/error (caused by the formula attempting to divide by zero). As a result, the

207 uncertainty/error for many recalibrations cannot be computed (an additional problem is

208 the lack of J-value data in most analyses). To overcome this, for analyses in which the

209 new error cannot be directly computed, the original error has been multiplied by the %

210 change output factor; errors calculated by this method are shown in red (normally

211 calculated error values are shown in black). Comparison to normally calculated error

212 values show that this method produces comparable results such that the new stated error

213 values are not significantly different from what would be calculated if J-values (etc) were

214 known.

215

216 There are two tabs of recalibrations. The first, labeled "Kuiper et al 2008",

217 recalibrates all the dates to the Kuiper et al. (2008) FCT standard, coupled with the Min

218 et al. (2000) decay constant. Dates from this first tab are plotted on the stratigraphic chart

219 (S1 Table). The second tab, labeled "Renne et al 2011", recalibrates all dates to the

220 standard and decay constant pairing of Renne et al. (2011). This second set of

10

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 221 recalibrations is provided for comparison. Both tables of recalibrations have the same

222 formatting for ease of comparison.

223

224 Stratigraphic chart (S1 Table)

225

226 Construction of the chart is complex and depends upon many different

227 stratigraphic methods. The following text explains the underlying definitions that provide

228 the base framework for the chart, and highlight some of the issues surrounding its

229 construction.

230

231 Definitions: stage and substages, magnetostratigraphy, and ammonite

232 biostratigraphy

233

234 The chart follows The Geological Time Scale 2012 (GTS2012; Gradstein et al.,

235 2012) for definitions of stage and substage boundaries (Ogg and Hinnov, 2012),

236 magnetostratigraphic boundaries (Ogg, 2012), and ammonite biostratigraphy (Ogg and

237 Hinnov, 2012). Although more recent revisions of these definitions are available,

238 GTS2012 integrates all these defined units with chronostratigraphic dates that use the

239 40 Ar / 39 Ar standard and decay constant pairing of Kuiper et al. (2008) and Min et al.

240 (2000), which are also used here. A second magnetostratigraphic column is also offered

241 containing some revised chron boundaries and includes many of the very short duration

242 cryptochrons that have not yet been officially recognised, but are often named in

11

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 243 magnetostratigraphic analyses (e.g. Lerbekmo and Braman, 2002). Individual definitions

244 and discussion (where appropriate) can be found in the pop-up notes in the respective

245 parts of the chart.

246

247 In some places it is necessary to provide a compromise in stratigraphic placement,

248 generally where a magnetostratigraphic assertion does not match, say, the ammonite

249 zonation (e.g. age of the Dorothy bentonite in the Drumheller Member, Horseshoe

250 Canyon Formation, Alberta; Lerbekmo, 2002; Eberth and Braman, 2012). In such cases,

251 the pop-up note text boxes provide explanation of the problem, and references.

252

253 Positioning of geological units and dinosaur taxa

254

255 The stratigraphic ranges of geological units and fossil taxa are plotted as a solid

256 bordered white cell with the lower and upper borders representing the lower and upper

257 contacts of the geological unit, or first and last documented taxon occurrences

258 (respectively).

259

260 If stratigraphic position is not sufficiently documented, the possible or likely

261 stratigraphic range is illustrated as a block arrow. For example, if we know a taxon

262 occurs in a given formation, but not the precise stratigraphic position within that

263 formation (or if the age of the formation itself is only roughly known), then the block

264 arrow would show the possible range being equivalent to the full duration of the

265 formation. A combination of a solid cell and block arrow may be used if a taxon or

12

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 266 geological unit comprises some specimens or horizons for which stratigraphic position is

267 precisely known (depicted by the solid cell) and some specimens or horizons for which

268 stratigraphic position is unknown (block arrows). Periods of non-material time (lacunae)

269 are represented by blank spaces. In the aforementioned cases, explanation for the plotting

270 of geological units, lacunae, and taxa is given in the corresponding note A graded block

271 arrow is used for units which may continue for a long time below the period of interest

272 (typically used for thick marine shales). The ranges and boundaries of each taxon or

273 geological unit are discussed on a case-by-case basis in S1 Table.

274

275

276 Issues with lithostratigraphy

277

278 Some features of typical lithostratigraphic units are not possible to depict properly

279 on the stratigraphic chart format. In the Western Interior, many terrestrial packages form

280 clastic wedges thinning basinward. Where possible, it is attempted to represent this in the

281 chart, although for the most part depicted stratigraphic sections are based on single well-

282 sampled sections, cores, or geographic areas, so the wedge-shaped overall geometry

283 might not be visible.

284

285 Limitation of scope & future versions

286

287 There are some limitations of scope for this initial version of the correlation chart.

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PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 288

289 The chart is currently mostly limited to units of Santonian age (86.3 Ma) up to the

290 K-Pg boundary (66.0 Ma). There are a few exceptions (e.g. Moreno Hill Formation, New

291 Mexico; Straight Cliffs Formation, Utah), which are included because they have yielded

292 important specimens, or provide stratigraphic context for overlying units.

293

294 Geological units featured in the correlation chart are currently limited to those for

295 which dinosaurian material has been collected, or which provide contextual information

296 for surrounding units (e.g. intertonguing marine units with biostratigraphically

297 informative fauna, and overlying or underlying units with chronostratigraphic marker

298 beds).

299

300 Dinosaurian fossils are limited to Neoceratopsia, Pachycephalosauridae,

301 Sauropoda, and Hadrosauridae. This is partly to limit the amount of data in this first

302 published version of the chart. Thus, the chosen clades represent the most abundant taxa,

303 and also include taxa considered biostratigraphically informative by previous workers

304 (e.g., Cobban and Reeside, 1952; Lawson, 1976; Sullivan and Lucas, 2003; 2006; Lucas

305 et al., 2012).

306

307 Future versions of the chart are intended to extend the stratigraphic range down to

308 the Jurassic-Cretaceous boundary. However, the plans for the first expansion concern

309 inclusion of more Upper Cretaceous formations from North America, and also similarly

310 aged deposits in Asia. Initial work on expanding faunal coverage has already begun

14

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 311 concerning the addition of all remaining dinosaur taxa (including birds), crocodylians,

312 and mammals, with the intent of eventually incorporating all useful taxa if possible.

313

314 Institutional abbreviations

315

316 A list of institutional abbreviations used in the correlation chart are provided in a

317 separate tab of the correlation chart Excel sheet (S1 Table) labeled "repository codes".

318

319 Taxa display format- phylogenies and lineages

320

321 It is not the intention of this project to make significant comment on phylogenies

322 per se . However, precise stratigraphic placement of taxa permits testing of speciation

323 hypotheses (see discussion), and so the arrangement of taxa on the chart should reflect

324 up-to-date phylogenies or other hypotheses of descent. In this current version, this only

325 affects Ceratopsidae and Hadrosauridae. For ceratopsids, the general arrangement follows

326 the chasmosaurine phylogeny from Fowler (2016), and for centrosaurines the

327 arrangement follows the phylogeny of Evans and Ryan (2015). For hadrosaurids, the

328 general arrangement of hadrosaurines follows Freedman Fowler and Horner (2015), and

329 lambeosaurines follows Evans and Reisz (2007).

330

15

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 331 Magnetostratigraphy

332

333 The conventional methodology used for delineating magnetostratigraphic chron

334 boundaries can create problems. In magnetostratigraphic analysis, if two stratigraphically

335 adjacent sample points yield opposite polarities (i.e., they are recognisable as different

336 chrons), then it is convention to draw the chron boundary stratigraphically halfway

337 between the two points. However an issue can arise if these lower and upper sample

338 points are separated by a sandstone from which it is difficult or impossible to extract a

339 magnetostratigraphic signal. In terrestrial floodplain deposition (typical of the units

340 studied in this work), the bases of depositional cycles are characterized by a surface of

341 non-deposition or erosion overlain by a low accommodation systems tract, typically

342 comprising an amalgamated channel sandstone. The combination of the depositional

343 hiatus at the base of the sandstone, and the sandstone itself, means that there may be a

344 considerable time gap (up to millions of years) between the last sampled horizon

345 immediately below the sandstone, and the first sampled horizon immediately above the

346 sandstone. If opposite polarities are recorded for the two sampled horizons on either side

347 of the unsampled sandstone, then the chron boundary would be drawn halfway, within

348 the sandstone, whereas it might be more likely to occur at the base of the sandstone, as

349 this is where the hiatus occurs. This would have the effect of making a unit appear older

350 or younger than it really is. For example, the mudstone immediately beneath the Apex

351 sandstone (basal unit of the upper Hell Creek Formation, Montana; Hartman et al., 2014)

352 is of normal polarity, assigned to C30n, whereas the mudstone immediately above the

353 Apex Sandstone is of reversed polarity (assigned to C29r; LeCain et al., 2014). The

16

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 354 C30n-C29r boundary is therefore drawn within the Apex Sandstone, whereas it is more

355 likely that it occurs at the hiatus at the base of the sandstone. A more significant case

356 arises with the contact between the Laramie Formation and overlying D1 sequence in

357 central Colorado: here, because of the halfway convention, the uppermost part of the

358 Laramie is drawn as being within the lowermost C31r zone (Hicks et al., 2003), whereas

359 in actuality, all magnetostratigraphic samples recovered by Hicks et al. from the Laramie

360 are normal, and it might therefore be entirely C31n. The effects of this issue are best

361 examined on a case by case basis; the reader is referred to the stratigraphic chart (S1

362 Table) where more examples are highlighted in pop-up text boxes. It should be noted that

363 this issue is purely an artifact of conventional methodology, not any mistake by a given

364 researcher. So long as the reader is careful and remains cautious of this issue, then

365 mistaken correlation and / or artificial age extension can be avoided.

366

367 Radiometric dating

368

369 This analysis recalibrates nearly 200 radiometric dates (S2 Table), most of which

370 are 40 Ar / 39 Ar dates that have been recalibrated to the standard and decay constant

371 pairing of Kuiper et al. (2008), and Min et al. (2000). It is not the intention here to

372 provide a thorough review of all radiometric dating methods (see Villeneuve, 2004);

373 however, given the large number of 40 Ar / 39 Ar dates used here, and given discrepancies

374 in past recalibrations, a cursory overview is given to the method. This text is also

375 included (and expanded) in the chart itself (S1 Table).

376

17

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 377 U-Pb and K-Ar

378

379 Most radiometric dates reported for Upper Cretaceous units use either U-Pb, K-Ar,

380 or 40 Ar / 39 Ar dating methods. U-Pb and K-Ar are primary dating methods, which directly

381 determine the age of a sample and do not require recalibration (unless decay constants

382 change, which is rare); whereas relative or secondary methods (such as 40 Ar / 39 Ar dating)

383 require use of a monitor mineral of known or presumed age ("standard"). It is the recent

384 changes to the recognized age of these standards that has been the cause of changing 40 Ar

385 / 39 Ar dates.

386

235 207 238 387 U-Pb dating actually analyses two decay series ( U decay to Pb, and U

388 decay to 206 Pb), such that there are two independent measures of age, the overlap of

389 which is the concordant age of the sample (Villeneuve, 2004). Recent improvements in

390 analytical techniques (High-Resolution–Secondary Ion Mass Spectrometry: SHRIMP;

391 and Chemical Abrasion Thermal Ionization Mass Spectrometry, CA TIMS) have brought

392 greater precision and accuracy to U-Pb dating, and it remains one of the best

393 methodologies currently available (Villeneuve, 2004). The decay constant for U-Pb

394 analysis is well established (Steiger and Jaeger, 1977), and known to better than 0.07%

395 accuracy (Villeneuve, 2004).

396

397 K-Ar dating is an older method of radiometric dating that was commonplace up

398 until the end of the 1980's when it was essentially replaced by the more precise and

399 accurate 40 Ar / 39 Ar method (Villeneuve, 2004). K-Ar had a range of benefits, including a

18

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 400 large number of possible datable minerals (due to the common occurrence of potassium

401 in many rock-forming minerals), but among its drawbacks was a relative lack of precision,

402 largely due to the requirement to run two separate analyses per sample for K and 40 Ar. As

403 such, analytical precision was never better than 0.5%, and with the development of new

404 technologies K-Ar dating was quickly replaced by 40 Ar / 39 Ar in the early 1990's

405 (Villeneuve, 2004). Even so, some K-Ar dates are still the only dates available for a given

406 unit, and so are included in the chart. K-Ar dates typically have error in the region of 1-2

407 m.y. for Upper Cretaceous units, so are useful indicators as to a general age range for a

408 unit, but not for precise correlation.

409

410 40 Ar / 39 Ar

411

412 Detailed reviews of 40 Ar / 39 Ar dating have been published elsewhere (e.g.,

413 McDougall and Harrison, 1999; Renne et al., 2010). Notes given here are for the purpose

414 of aiding the reader in understanding the recalculation of radiometric dates reported in

415 this work, how 40 Ar / 39 Ar dates are affected by changing standards and decay constants,

416 and comparability of radiometric dates recovered by different methods (e.g., 40 Ar / 39 Ar

417 vs U-Pb).

418

419 Standards (neutron fluence monitor)

420 As 40 Ar / 39 Ar dating is a secondary dating method, every unknown sample needs

421 to be analysed alongside a sample of known age: a standard. Primary standards are

422 minerals from specific rock samples that have been directly dated by 40 K / 40 Ar dating or

19

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 423 another method; whereas secondary standards are based on 40 Ar / 39 Ar intercalibration

424 with a primary standard (Renne et al., 1998). The following list includes (but is not

425 limited to) some of the more popular standards that have been used historically (see

426 McDougall and Harrison, 1999, for a more complete list):

427

428 MMhb-1 McClure Mountain hornblende, primary standard: ~420 Ma

429 GA-1550 Biotite, monazite, NSW, Australia, primary standard: ~98 Ma

430 TCR Taylor Creek Rhyolite (or sanidine, TCs), secondary standard: ~28 Ma

431 FCT Fish Canyon Tuff (or sandine, FCs), secondary standard: ~28 Ma

432 ACR Alder Creek Rhyolite (or sanidine, ACs), secondary or tertiary standard:

433 ~1 Ma

434

435 Standards are chosen depending on availability, and should be of an age

436 comparable to the unknown sample (Renne et al., 1998). Hence, for Upper Cretaceous

437 deposits, usually the secondary standards TCR or FCT have been used, typically

438 themselves being calibrated against a primary standard (historically, the MMhb-1 is

439 commonly used, although this depends on the preference of the particular laboratory).

440 Many historically popular standards are no longer used, as repeated calibration studies

441 have found the original sample to give inconsistent dates. For example, Baksi et al. (1996)

442 found the widely used MMhb-1 primary standard to be inhomogenous, making its use as

443 a standard no longer tenable. Further, intercalibration studies have continually honed and

444 refined the ages of standards (especially the more widely used secondary standards), with

445 the result that radiometric dates published years apart are typically not precisely

20

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 446 comparable without recalibration (e.g., Samson and Alexander, 1987; Renne et al., 1994;

447 1998).

448

449 For 40 Ar / 39 Ar analysis, a significant issue concerns the changing age of the Fish

450 Canyon Tuff (FCT: the relative standard used for most 40 Ar / 39 Ar analyses of Cretaceous

451 rocks), and to a lesser extent, the associated decay constants ( λβ : β- decay of 40K to 40Ca;

452 and λε : electron capture or β+ of 40K to 40Ar; which combined are referred to as λT or

453 λtotal; Beckinsale and Gale, 1969).

454

455 Cebula et al. (1986) first proposed an age of 27.79 Ma for the Fish Canyon Tuff.

456 This was quickly refined to 27.84 Ma by Samson and Alexander (1987), which remained

457 the standard used by 40 Ar / 39 Ar analyses published up to the mid 1990’s (e.g., Rogers et

458 al., 1993). Renne et al. (1994) revised the FCT to 27.95 Ma (although this new figure was

459 not commonly used at the time). The next major update was that of Renne et al. (1998),

460 whereupon the FCT was revised to 28.02 Ma, which was widely accepted up to 2008

461 when Kuiper et al. published the current standard of 28.201 Ma. This also brought 40 Ar /

462 39 Ar dates into line with U-Pb dates, unifying these two major chronostratigraphic dating

463 systems (Kuiper et al., 2008). Two further revisions have been offered by Renne et al. in

464 2010 and 2011, of 28.305 Ma, and 28.294 Ma (respectively). Rivera et al. (2011), Meyers

465 et al. (2012), Singer et al. (2012), and Sageman et al. (2014) all found independent

466 support for Kuiper et al. (2008)'s 28.201 Ma age for the Fish Canyon Sanidine (and

467 therefore rejected Renne et al.'s (2010) further revised 28.305 Ma standard as too old).

21

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 468 These analyses also used three methods ( 40 Ar / 39 Ar, U-Pb, cyclostratigraphy) to reach

469 consensus, confirming alignment of U-Pb and 40 Ar / 39 Ar dates.

470

471 When applied to Upper Cretaceous units, a ~0.2 m.y. difference between the age

472 of two different standards corresponds to ~0.4 - 0.5 m.y. difference in the 40 Ar / 39 Ar age

473 of the analysed sample, and this is exacerbated if the standards used were further apart.

474 For example, using the 27.84 Ma standard of Samson and Alexander (1987), Rogers et al.

475 (1993) published an 40 Ar / 39 Ar date of 74.076 Ma for a bentonite at the top of the Two

476 Medicine Formation, MT. This becomes 75.038 Ma if using the current Kuiper et al.

477 (2008) standard, and 75.271 Ma under the less commonly used Renne et al. (2011)

478 standard, a difference of 1.28 m.y. from the originally published date.

479

480 Decay constants

481 The 40 Ar / 39 Ar method depends upon the β- decay of 40 K to 40 Ca ( λβ ), and

482 electron capture or β+ of 40 K to 40 Ar ( λε ), which combined are referred to as λT or λtotal

483 (Beckinsale and Gale, 1969). The value of the decay constant λT (and its components)

484 has historically been subject to fewer changes than the standards listed above, but has

485 come under increased scrutiny since the late 1990's. The currently (2014) accepted

486 standard is 5.463 E-10/y (Min et al., 2000), although alternatives are available, and

487 refinement of this figure is the subject of active research (see S1 Table).

488

489 The decay constant used for an analysis is not always reported, although it has much less

490 effect on the final calculated age than variations in fluence monitor mineral ages. For

22

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 491 example, the difference between using 5.543 E-10/y (Steiger & Jaeger, 1977) and 5.463

492 E-10/y (Min et al., 2000) is 0.02%, equating to a difference of 0.013 Ma for a sample

493 from the late Campanian (~75 Ma). It should be noted that different values of λT have

494 been used historically by geochronologists compared to physicists and chemists; this is

495 pointed out by Renne et al., (1998) who note that (for example) Endt (1990) used a λT

496 value of 5.428 +/- 0.032 E-10 /y which "is more than 2% different from the values

497 recommended by Steiger and Jaeger (1977)". Thus, there is no guarantee that, unless

498 otherwise stated, a lab that performed an 40 Ar / 39 Ar analysis in the 1990's will be using

499 the λT of 5.543 E -10 /y of Steiger and Jaeger (1977), although all dates recalibrated here

500 use either this, Min et al. (2000), or Renne et al. (2011). Further details and a history of

501 decay constant values can be found in the corresponding note within S1 Table .

502

503 Recalibration & current standards

504 In order to compare 40 Ar / 39 Ar dates, it is essential to ensure that the same

505 standards and decay constants were used in their calculation, which may require

506 recalibration. If the standards used are different (for example, if an old analysis used the

507 TCR standard, and a more recent one used the FCT), then it will be necessary to find

508 what the equivalent FCT value was to the TCR used in the original analysis. This is

509 usually achieved by referencing either the original publication of the standard, or the

510 relevant published intercalibration analysis (e.g., Renne et al. 1994). It is critical to

511 understand that recalculation cannot simply be performed by entering the original

512 standard used (e.g., TCR = 28.32 Ma) into the equation provided on the recalculation

23

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 513 sheet from McLean and EARTHTIME (or the adapted spreadsheet used here); the

514 equivalent FCT value is what must be entered, as the formula only uses FCT.

515

516 The decay constant absolute value has only a small effect on the absolute age of a

517 sample, but decay constants contribute a greater amount to the error range of a

518 radiometric date.

519

520 There are two current prominently used pairings of standard and decay constant.

521 Kuiper et al. (2008) combined an FCT standard age of 28.201 +/-0.023 Ma, with the

522 decay constant of Min et al. (2000), λT = 5.463 +/- 0.214 E-10/y. Renne et al. (2011) use

523 an FCT standard age of 28.294 +/- 0.036 Ma, with a λT of 5.5305 E-10/y. The dates used

524 here in the correlation chart (S1 Table) are calibrated to the Kuiper et al. (2008) standard,

525 paired with the Min et al. (2000) decay constant. This is not a reflection on the reliability

526 of one method over another; rather it is out of convenience, because the various

527 ammonite biozones and magnetochrons detailed in The Geological Time Scale 2012

528 (Gradstein et al., 2012; upon which this chart is based) use the Kuiper et al. (2008) FCT

529 standard, and Min et al. (2000) decay constant.

530

531 Choice of mineral

532 Direct comparisons between 40 Ar / 39 Ar dates require not only the same standard

533 and decay constant pairing, but also that the subject mineral is the same. Although it is

534 theoretically possible that a date obtained from biotite crystals might be comparable with

535 one from sanidine, in practice the difference in closure temperature (the temperature at

24

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 536 which the mineral no longer loses any products of radioactive decay; Villeneuve, 2004)

537 and other factors such as recoil effects (Obradovich, 1993) mean that (for example)

538 biotite dates are typically ~0.3% older than sanidine dates (e.g., see Rogers et al., 1993).

539 The current "gold standard" mineral for 40 Ar / 39 Ar dating is sanidine, and most modern

540 analyses use this mineral exclusively; however, plagioclase and biotite dates are quite

541 common in literature from the 1990's. Here these non-sanidine dates are recalibrated, and

542 they are comparable to each other (i.e., biotite dates can be directly compared with other

543 biotite dates), but caution is advised when comparing non-sanidine dates with those of

544 sanidine (although this is sometimes unavoidable).

545

546 Reporting of uncertainty / error

547 Reporting of error associated with 40 Ar / 39 Ar derived ages is not standardized and

548 varies in the inclusiveness of sources of error, the statistical method used to calculate

549 error, the type of error, and in the amount of analytical information provided.

550

551 Sources of error in 40 Ar / 39 Ar analyses include analytical error (e.g., J-value),

552 uncertainty in the standard used (e.g. age of the Fish Canyon Tuff, FCT is 28.201 +/- 0.23

553 Ma at 1 σ; Kuiper et al., 2008), uncertainty in the decay constant (e.g., λT of 5.463 +/-

554 0.214 E-10/y; Min et al., 2000), and geological processes that may lead to post-

555 crystallization alteration of isotope ratios (Villeneuve, 2004). Most older publications do

556 not explicitly state what is included in the reported error, but newer studies (e.g., Sprain

557 et al., 2014) report both analytical and systematic error.

558

25

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 559 The statistical method used to report error is not standardized, and is typically

560 given in one of three forms; some authors report 1 or 2 standard deviations ( σ); Standard

561 Error is also commonly reported (especially for population means); finally, some authors

562 report the 95% confidence interval for the population mean, which is roughly equivalent

563 to 2 σ (=95.45% confidence interval).

564

565 It is common for published radiometric dates to lack associated details of the

566 analysis, by either the date being given as a personal communication, or simply the

567 omission of analytical details. Consequently, it is sometimes unclear as to whether (for

568 example) a stated error of +/- 0.15 Ma refers to 1 σ, 2 σ, Standard Error, or whether it

569 includes analytical and systematic error.

570

571 As such, it is not possible to make the error consistent between each recalibration

572 (although the effects are relatively minor). Where possible, recalibrated error is reported

573 to 1 σ analytical error, but generally the original reported error is simply processed

574 through the recalibration spreadsheet, noting wherever possible all details and any issues

575 that may arise. Direct comparison of error between dates (both recalibrated and

576 unrecalibrated) should therefore be approached with caution.

577

578 Agreement of 40 Ar / 39 Ar dates with U-Pb dates

579 40 Ar / 39 Ar dates have historically tended to be younger than U-Pb dates by about

580 1% (Schoene et al., 2006), equating to ~750 k.y. difference in a 75 m.y. old sample (i.e.,

581 the approximate age of the units studied here). Possible explanations include longer

26

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 582 zircon magma residence times prior to an eruption (Villeneuve, 2004), error in the 40 K

583 decay constant (Schmitz and Bowring, 2001), or interlaboratory bias and geological

584 complexities (Kuiper et al., 2008). Recent revisions of standards and decay constants for

585 40 Ar / 39 Ar dating have closed the gap to within ~0.3% (Kuiper et al., 2008; Renne et al.,

586 2011). This led Kuiper et al. (2008) to suggest that 40 Ar / 39 Ar dating has improved

587 "absolute uncertainty from ~2.5% to 0.25%"

588 It should be noted (Renne et al., 1998; Villeneuve, 2004), that when comparing

589 dates within the same system (i.e., 40 Ar / 39 Ar compared to 40 Ar / 39 Ar; and U-Pb dates

590 compared to other U-Pb dates) then it is accepted practice to not include internal error

591 (such as data uncertainties in K-Ar, decay constants, and intercalibration factors; Renne

592 et al., 1998) as both dates are subject to the same uncertainty, effectively canceling it out.

593 However, when directly comparing dates derived from different systems (i.e., 40 Ar / 39 Ar

594 dates with U-Pb dates), then internal error should be included. An example from Renne et

595 al. (1998) showed that when reported separately, and therefore without internal error, the

596 age of a biotite-derived 40 Ar / 39 Ar date for the Permo-Triassic Siberian Trap basalt was

597 250.0 +/- 0.1 Ma , whereas a zircon and baddeleyite U-Pb date from the same intrusion

598 was 251.2 +/- 0.2 Ma. When properly compared with the internal error included, the 40 Ar

599 / 39 Ar dates became 250.0 +/- 2.3 Ma, whereas the U-Pb date was recalculated as 251.2

600 +/- 0.3 Ma, such that the error ranges of the dates now overlap. In the case of this current

601 work, only three U-Pb dates are plotted in S1 Table, all of which are from the Javelina

602 and Aguja formations of Texas. The reader should therefore take care when comparing

603 these units directly with other units based on 40 Ar / 39 Ar geochronology.

604

27

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 605 Other general comments

606

607 The number of decimal places for reported dates and error are left in their original

608 published form where possible.

609

610 In previous publications, a number of radiometric dates are reported as personal

611 communication or featured only in abstracts. Such references typically lack any analytical

612 data, so original standards (etc.) must be assumed based on the year in which the analysis

613 was (likely) conducted, and any details of the typical standards used by the scientist and

614 laboratory that carried out the analysis (if known; see individual notes for details of

615 sleuthing).

616

617 Results

618

619 The results of this study are presented as separate documents in the Supporting

620 Information; the stratigraphic chart (S1 Table), and the recalibration sheet (S2 Table).

621 These documents contain a large amount of information in the various pop-up notes, most

622 of which is not repeated here as it is best viewed in stratigraphic context.

623

624 Notes on recalibrations by other authors

625

28

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 626 Various analyses published by J. D. Obradovich

627

628 Many critical 40 Ar / 39 Ar dates have been published by J. D. Obradovich (United

629 States Geological Survey, Colorado), not the least of which his 1993 work, "a Cretaceous

630 time scale" which presented over 30 40 Ar / 39 Ar dates for many key horizons or ammonite

631 biozones, establishing a robust framework for the Late Cretaceous of the U.S. Western

632 Interior. As such, recalibration of Obradovich radiometric dates is of great importance,

633 but requires special caution due to the particular methodology of Obradovich during the

634 1990's (and possibly early 2000's), which differs slightly from what might be expected.

635 During this time, Obradovich typically used the TCR as the standard for his analyses, but

636 the equivalent age of the FCT (required for recalibration) is not typical. Indication of this

637 is noted by Hicks et al. (2002, p.43) who state:

638

639 "The TCR (Duffield & Dalrymple, 1990) has been used exclusively since 1990 by

640 one of us (Obradovich) with an assigned age of 28.32 Ma normalized to an age of 520.4

641 Ma for MMhb-1 (Samson & Alexander, 1987). This age differs from that of 27.92 Ma

642 assigned by Sarna-Wojcicki and Pringle (1992). The choice of 28.32 Ma was entirely

643 pragmatic because this monitor age provided the best comparison with ages delivered by

644 Obradovich and Cobban (1975). In an intercalibration study [...] Renne et al. (1998)

645 obtained ages of 28.34 Ma for TCR and 28.02 Ma for FCT when calibrated against

646 GA1550 biotite as their primary standard with an age of 98.79 Ma. This value of 28.02

647 agrees quite well with […] 28.03 Ma obtained through calibration based on the

648 astronomical time scale (Renne et al., 1994). On the basis of unpublished data, one of us

29

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 649 (Obradovich) obtained an age of 28.03 Ma for the FCT [...] of W, McIntosh (Geoscience

650 Dept. New Mexico Institute of Mining and Technology, Socorro), calibrated against an

651 age of 28.32 Ma for TCR."

652

653 However, Obradovich-published analyses from this time do not exclusively use

654 the TCR at 28.32 Ma, as Izzett and Obradovich (1994) state that they use FCT sanidine at

655 27.55 Ma, and TCR sanidine at 27.92 Ma, both relative to MMhb-1 at 513.9 Ma (in

656 conjunction with λT = 5.543 E-10/y). They note that the 513.9 Ma age of MMhb-1 differs

657 from the then standardized age of 520.4 Ma (Samson and Alexander, 1987) as the former

658 age was calibrated in the lab where their current samples were analysed (Lanphere et al.,

659 1990; Dalrymple et al., 1993).

660

661 This creates a problem when recalibrating 40 Ar / 39 Ar ages that used TCR as the

662 fluence monitor (standard). The "official" TCR age of 27.92 Ma has a corresponding

663 FCT age of 27.84 Ma (Samson and Alexander, 1987; Renne et al., 1998). However, since

664 most analyses by Obradovich use TCR at 28.32 Ma, then the question remains as to what

665 number to use for the equivalent FCT when performing recalibrations. Renne et al. (1998)

666 provide an intercalibration factor for FCT:TCR of 1:1.00112 +/- 0.0010, which simply

667 calculated is FCT = 28.32 / 1.100112 = 28.006 Ma. This agrees well with an FCT

668 equivalent of 28.03 Ma (as calculated by Obradovich; see above; Hicks et al., 2002;

669 Obradovich, 2002) and a value of 28.02 Ma of Renne et al. (1998). In The Geological

670 Time Scale 2012 (Gradstein et al., 2012), Schmitz (2012) recalibrates a selection of dates

671 from Obradovich (1993), and Hicks et al. (1995, 1999) using a legacy FCT age of 28.00

30

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 672 Ma (not stated, but retrocalculated here). Sageman et al. (2014; cited as Siewert et al., in

673 press, by Schmitz, 2012) recalibrate Obradovich's older dates using a legacy FCT age of

674 28.02 Ma (thereby agreeing with Renne et al., 1998).

675

676 In this analysis, when recalibrating an 40 Ar / 39 Ar date that was calculated by

677 Obradovich using TCR = 28.32, I use an FCT value of 28.03, as this is the equivalent

678 FCT explicitly stated by Obradovich (2002). This is a very close value to 28.02 (Renne et

679 al., 1998; where the TCR equivalent is 28.34 +/- 0.16 Ma; 1 σ, ignoring decay error) so

680 confusion between the two should be avoided, although the difference between ages

681 calculated using 28.03 or 28.02 Ma standards would correspond to only 0.02 to 0.04 m.y.

682 for ages in the Late Cretaceous (100.5 - 66 Ma; Ogg and Hinnov, 2012)

683

684 Roberts et al. (2013)

685

686 Roberts et al. (2013) present a table of recalibrated radiometric dates from a

687 selection of important dinosaur-bearing formations of the North American Western

688 Interior. Unfortunately, 11 out of 18 dates are incorrectly recalibrated, producing dates

689 that are incorrect by up to a million years.

690

691 For recalibrated dates of the Judith River Formation (originally published by

692 Goodwin and Deino, 1989), the study (Roberts et al., 2013) utilizes an incorrect original

693 (legacy) FCT standard of 28.02 Ma (i.e., from Renne et al., 1998, published after the

694 original 1989 analysis). For the recalibration to be correct, the legacy standard must be

31

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 695 the value of FCT that was equivalent to the MMhb-1 at 420.4 Ma, which is FCT = 27.84

696 Ma (Samson and Alexander, 1987; see Renne et al., 1998). This produces recalibrations

697 for the Judith River Formation that are nearly half a million years different from the

698 corrected recalibrations calculated in the current article. For example, the sample

699 84MG8-3-4 was originally published as 78.2 Ma (Goodwin and Deino, 1989); Roberts et

700 al. (2013) recalibrate it as 78.71 Ma, whereas the recalibration offered in the current work

701 (see S1 and S2 Tables) is 79.22 Ma.

702

703 The same error was made for recalibrations from the Bearpaw, Dinosaur Park,

704 and Oldman formations as the Renne et al. (1998) FCT date of 28.02 was also input as

705 the legacy FCT for dates originally published by Eberth and Hamblin (1993) and Eberth

706 and Deino (1992); i.e. before the 1998 paper was published. The correct legacy standard

707 to be used for these recalibrations is again, FCT = 27.84 Ma (Samson and Alexander,

708 1987; confirmed by Eberth, pers. comm., 2017; in prep.)

709

710 When recalibrating 40 Ar / 39 Ar dates for the Fruitland and Kirtland formations,

711 New Mexico (originally published by Fassett and Steiner, 1997), incorrect values are

712 input for the original (legacy) decay constant ( λ) and standard (Roberts et al., 2013). First,

713 the legacy λ used (Roberts et al., 2013) is 4.962E-10/y, which was presumably copied

714 from the bottom of the chart on p. 243 of Fassett and Steiner (1997), where it is labeled as

715 the value of λβ (ie. the probability of β- decay of 40K to 40Ca), and is printed below the

716 value of λε (0.581 E-10/y; probability of electron capture or β+ of 40Kto 40Ar). In this

717 case, the correct λ value to use for recalibration is 5.543 E-10/y (Steiger and Jaeger,

32

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 718 1977), which is the total ( λT) of λβ plus λε . Second, Roberts et al. (2013) correctly state

719 that the legacy standard used by Fassett and Steiner (1997) for fluence monitoring was

720 the TCR at 28.32 Ma; however, this number is then input directly into the recalibration

721 formula with the new FCT standard (28.201; Kuiper et al., 2008). This is incorrect as

722 recalculation must use the same standard mineral (e.g., FCT) for both legacy and

723 recalibrated dates. For the recalculation to be correct, the legacy standard must therefore

724 be the value of FCT that was equivalent to the TCR at 28.32 at the time of the 1997

725 analysis, which is either FCT = 27.84 Ma or ~28.03 (see S1 Table; above note on

726 Obradovich), both of which produce recalibrated ages ~1 million years older than the

727 dates presented by Roberts et al. (2013). The resultant misrecalibrated dates are actually

728 younger than the original legacy dates, which should have been more difficult to overlook

729 as the standards for 40 Ar / 39 Ar dating have been getting progressively older, so all

730 recalibrations should produce older dates.

731

732 The recalibrated dates of Roberts et al. (2013) were replicated (therefore

733 confirmed) by rerunning the legacy values through the recalibration spreadsheet provided

734 by the EARTHTIME institute.

735

736 Seven recalibrations were performed correctly; four from the Kaiparowits

737 Formation, Utah, one from the Wahweap Formation, Utah, and two from the Two

738 Medicine Formation, Montana. All other recalibrated dates are incorrect and should be

739 discarded.

740

33

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 741 Discussion

742

743 It is beyond the scope of this short work to summarize the implications of

744 everything in the stratigraphic chart; only some enduring issues and important changes

745 that might affect geological or paleontological interpretation are discussed here, as

746 examples of potential uses of this chart’s correlations.

747

748 Geology

749

750 Judith River Formation

751

752 The Judith River Formation is a middle to (possibly) upper Campanian terrestrial

753 unit exposed across north and central Montana. It has been studied since the mid-19th

754 century (Stanton and Hatcher, 1905; Bowen, 1915; Sahni, 1972; Gill and Cobban, 1973),

755 but many aspects of its stratigraphy remain unresolved, especially regional correlation

756 (Rogers et al., 2016). This is in part due to the Judith River Formation being a clastic

757 wedge that thins west to east such that its upper and lower contacts change

758 geographically, and in part due to the uneven geographic distribution of published

759 measured sections. The stratigraphy of some areas of outcrop is well documented, such as

760 the type area in central Montana (Sahni, 1972; Rogers, 1993; Rogers et al. 2016) and near

761 Rudyard in northernmost Montana (e.g., Goodwin and Deino, 1989; Freedman Fowler

34

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 762 and Horner, 2015). However, few measured sections have been published for many other

763 equally excellent exposures. For example, despite the recovery of many fine fossil

764 specimens from near the north-central Montana town of Malta (e.g., Prieto-Marquez,

765 2005), the only measured sections available for this area are in unpublished MS theses

766 (Malik, 1990; LaRock, 2000).

767

768 The Judith River Formation in the type area (central Montana; Sahni, 1972) has

769 recently been formally subdivided into a series of members (Rogers et al., 2016), notably

770 a sand-dominated basal McClelland Ferry Member and overlying mud-dominated Coal

771 Ridge Member, distinguished in subsurface Spontaneous Potential (SP) logs by a

772 distinctive "kick" named the Mid Judith Discontinuity (Rogers et al., 2016). However,

773 despite the naming of these members for the type section in central Montana, it is not

774 clear how they might be applied to exposures of the Judith River Formation in

775 northernmost Montana along the U.S.-Canada border, from which most of the diagnostic

776 vertebrate fossils have been recovered.

777

778 In contrast to the type area, exposures of the Judith River Formation close to the

779 U.S.-Canada border are already recognized as direct lithostratigraphic equivalents to parts

780 of the Belly River Group of Alberta (formerly called the Judith River Formation or Group;

781 Jerzykiewicz and Norris, 1994; Hamblin and Abrahamson, 1996). Specifically,

782 subdivisions of the Foremost and Oldman formations of Alberta (Taber Coal Zone,

783 Herronton Sandstone Zone, Unit 1, Unit 2, and Unit 3; Eberth, 2005) have been identified

35

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 784 in outcrop in northernmost Montana (Eberth, 2005; Schott et al., 2009; Ryan et al., 2010;

785 Freedman Fowler and Horner, 2015).

786

787 The distinctive well-log "kick" that defines the boundary between the McClelland

788 Ferry and Coal Ridge members is present in the subsurface in northernmost Montana

789 (near Havre) and through into southern Canada (Rogers et al., 2016). Therefore, the

790 discontinuity should occur among the defined subunits of the Belly River Group already

791 identified in this part of southern Canada and their equivalents in the U.S. (Eberth, 2005;

792 Schott et al., 2009; Ryan et al., 2010; Freedman Fowler and Horner, 2015). However,

793 only the subsurface data is referenced for this area by Rogers et al. (2016), so it is not

794 explicitly clear in which Canadian unit the Mid Judith Discontinuity occurs. Rogers et al.

795 (2016) state that it occurs higher in section than the exposures in Kennedy Coulee, near

796 Rudyard, Montana, which comprise direct lithostratigraphic equivalents of the Foremost

797 Formation (Taber Coal Zone and Herronton Sandstone Zone) and the lower part of the

798 Oldman Formation (Unit 1 and the leached zone; sensu Eberth, 2005; Schott et al., 2009;

799 Ryan et al., 2010; Freedman Fowler and Horner, 2015; Evans, pers. comm.). As such, the

800 "kick" must therefore occur stratigraphically higher than Unit 1 of the Oldman Formation,

801 possibly correlating with the top of the Comrey Sandstone Zone (Unit 2, middle of the

802 Oldman Formation; Evans. pers. comm. 2016; see S1 Table). The only suggestion in

803 Rogers et al. (2016, p. 126) states, "the Oldman Formation [is an ...] approximate age

804 equivalent to the McClelland Ferry Member [... and ...] the overlying Dinosaur Park

805 Formation [is an ...] approximate age equivalent to the Coal Ridge Member". However,

806 40 Ar / 39 Ar dates from immediately below the discontinuity (76.24 and 76.17 Ma; Rogers

36

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 807 et al., 2016) are younger than an 40 Ar / 39 Ar date from the middle of the Dinosaur Park

808 Formation (76.39 Ma; Eberth, pers. comm.; see individual entry in S1 Table). This would

809 make the suggested correlation unlikely, and would also exclude the possibility of the

810 Mid Judith Discontinuity correlating with the top of the Comrey Sandstone Zone.

811 However, accuracy or incompatibility of the radiometric dates may be the cause of this

812 issue; as such it is only likely to be resolved if samples from both the Judith River

813 Formation and Canadian units are analysed by the same laboratory under identical

814 conditions.

815

816 A potential opportunity to correlate the type section of the Judith River Formation

817 with exposures in northern Montana and the equivalent units in southern Alberta is

818 provided by biostratigraphy of rhinobatid rays, the teeth of which are a common

819 component of microsites in Late Cretaceous units of the Western Interior. In Alberta, ray

820 teeth recovered from the Foremost Formation, and Unit 1 of the overlying lower Oldman

821 Formation pertain to the smooth-sided form Pseudomyledaphus sp., whereas the

822 overlying Comrey Sandstone Zone (Unit 2) and successively overlying units of the upper

823 Oldman Formation bear only Myledaphus bipartitus (Peng et al., 2001; Brinkman et al.,

824 2004; Kirkland et al., 2013). This biostratigraphic pattern was used to corroborate

825 lithostratigraphic correlation of exposures of the Judith River Formation north of

826 Rudyard, Montana, with the upper Foremost and lower Oldman formations, and also that

827 the lowermost exposures of the Judith River Formation near Malta, Montana, correlate

828 with the Comrey Sandstone Zone of the Oldman Formation (Freedman Fowler and

829 Horner, 2015). The large number of microsites recorded in the type section of the Judith

37

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 830 River Formation (Rogers and Kidwell, 2000), and indeed potentially elsewhere in

831 similarly aged units in the Western Interior (e.g. Mesa Verde Group, Wyoming and

832 Colorado; Aguja Formation, Texas), mean that this biostratigraphic change may be easily

833 detectable from existing collections and useful for correlation.

834

835 In summary, lack of a defined correlation or lithostratigraphic definition for the

836 Mid Judith Discontinuity in U.S.-Canada border exposures of the Judith River Formation

837 mean that the newly defined members are currently of limited use for surficial regional

838 correlation beyond the type area. Furthermore, as the McClelland Ferry Member is

839 equivalent to at least the combined Foremost Formation and Units 1-2 (and possibly 3) of

840 the Oldman Formation, it offers reduced stratigraphic resolution than simply using the

841 Canadian terminology when referring to these equivalent units for Judith River

842 Formation exposures along the U.S.-Canada border sections where correlations are

843 readily apparent. Due to these limitations, use of the newly defined members outside of

844 the type area of the Judith River Formation is problematic.

845

846 A final remaining issue is that the lower part of the Judith River Formation as

847 currently defined is strongly variable in age, being ~80 Ma in the area north of the town

848 of Rudyard (where it is equivalent to the Foremost Formation, Alberta; Godwin and

849 Deino, 1989; Eberth, 2005; Schott et al., 2009; Ryan et al., 2010; Freedman Fowler and

850 Horner, 2015), but perhaps as young as 77.5 Ma ~200km to the east near the town of

851 Malta (where it is equivalent to the Comrey Sandstone, Oldman Formation, Alberta;

852 Freedman Fowler and Horner, 2015). This is mostly an artifact of the lack of subdivision.

38

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 853 A solution to this issue would be to follow Canadian stratigraphic definition in raising the

854 Judith River Formation to group status, and subdivide it into constituent formations and

855 members that have direct correlates in Canada. In order to maintain the resolution offered

856 by the Canadian terms, here the lower-Oldman Formation (Herronton Sandstone and Unit

857 1; Eberth, 2005) equivalent of the Judith River Formation is referred to as the "Rudyard

858 beds", and the upper Oldman Formation (Units 2-3; Eberth, 2005) equivalent as the

859 "Malta beds" in reference to the geographic locations where these particular parts of

860 section are well exposed.

861

862 Age of the Aguja Formation, Texas

863

864 The Aguja Formation of southwest Texas is subdivided into Lower and Upper

865 Shale Members, which are separated by a marine tongue (Rowe et al., 1992; Sankey and

866 Gose, 2001). The Upper Shale Member has yielded important vertebrate fossils (Lehman,

867 1989; Wagner and Lehman, 2009) and is often considered as an upper Campanian or

868 even Maastrichtian aged unit (e.g., Longrich et al., 2010; Sankey, 2010) based on

869 interpretation of magnetostratigraphy (Sankey and Gose, 2001), phreatomagmatic

870 volcanism (Longrich et al., 2010), and chemostratigraphic correlation (Nordt et al., 2003).

871 Magnetostratigraphic analysis by Sankey and Gose (2001) shows that the base of the

872 Upper Shale Member is of reversed polarity, and is overlain by a short normal polarity

873 interval, another reversed interval, then another normal interval. The uppermost part of

874 the Upper Shale Member is shown as normal polarity by Lehman (1990). Sankey and

875 Gose (2001) correlate the basal part of the Upper Shale Member with C32r (74.309 -

39

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 876 73.649 Ma; Ogg, 2012), asserting a late Campanian age. Using chemostratigraphic and

877 magnetostratigraphic data, Nordt et al. (2003) and Sankey (2010) placed the upper part of

878 the Upper Shale Member as early Maastrichtian in age, ~69-68 Ma. However, more

879 recently published stratigraphic evidence is supportive of a middle Campanian age for the

880 Upper Shale Member.

881

882 Ammonites and radiometric dates constrain the age of the Upper Shale Member

883 as between ~80.2 and 76.9 Ma. Ammonite remains (Rowe et al., 1992; Lehman and

884 Tomlinson, 2004) from the marine tongue that separates the Lower and Upper Shale

885 members indicate that the Lower Shale Member should be no younger than the earliest

886 middle Campanian Baculites mclearni zone (80.67 - 80. 21 Ma; Ogg and Hinnov, 2012),

887 and the Upper Shale Member should be no older than this. A U-Pb date of 69.0 Ma

888 recovered from the overlying Javelina Formation, ~60 m above the formational contact

889 (Lehman et al., 2006) demonstrates that the upper part of the Upper Shale Member of the

890 Aguja Formation is unlikely to be any younger than this. Similarly, phreatomagmatic

891 volcanic deposits occur within the Upper Shale Member, and are U-Pb dated at 76.9 +/-

892 1.2 Ma (Befus et al., 2008) and 72.6 +/- 1.5 Ma (Breyer et al., 2007). In which case the

893 base of the Upper Shale Member should not be any younger than 76.9 +/- 1.2 Ma.

894

895 Further refinement of the age of the Upper Shale Member is problematic. Befus et

896 al. (2008; p. 262) proposed a formational model where the phreatomagmatic volcanic

897 deposits were emplaced into a crater formed within Aguja Formation sediment; i.e., the

898 76.9 Ma volcanic unit must have been deposited after deposition of the Upper Shale

40

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 899 Member (see Fig. 22 in Befus et al., 2008). As such the Upper Shale Member should not

900 itself be considered as 76.9 Ma in age. Sankey and Gose's (2001) assignment of the base

901 of the Upper Shale Member to magnetochron C32r (shown above to no longer be

902 possible) was based in part on the lack of short-duration polarity fluctuations below C32r

903 in the accepted magnetostratigraphic record (then represented by Gradstein et al., 1995).

904 However, a number of short duration 'cryptochron' reversals were detected at the base of

905 C33n by Montgomery et al. (1998). Although these are not yet officially accepted (i.e. in

906 GTS 2012; Ogg, 2012), if these short reversal cryptochrons are considered valid (as in

907 many publications by J. F. Lerbekmo; see S1 Table) then the base of the Upper Shale

908 Member might be more precisely correlated within the lower part of C33n, which would

909 be consistent with the constraints of radiometric dates, ammonite biostratigraphy, and the

910 assertion of Wagner and Lehman (2009). However, this is only speculative, and more

911 data is needed.

912

913 Age of the Javelina Formation, Texas

914

915 The age of the Javelina Formation (and basal part of the overlying Black Peaks

916 Formation, sensu Lehman and Coulson, 2002) of southwest Texas has often been

917 considered as late Maastrichtian, even latest Maastrichtian (e.g., Lawson, 1976; Lehman

918 and Coulson, 2002; Atchley et al., 2004), and is based on comparison of the Javelina

919 Formation dinosaur fauna with that of the Hell Creek and Lance formations (Lawson,

920 1976; Lehman, 2001). From this perspective then, it was perhaps surprising when

921 Lehman et al. (2006) published a U-Pb date of 69.0 +/- 0.9 Ma for the middle of the

41

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 922 Javelina Formation. Assuming it is accurate, this plots in the lowermost part of the late

923 Maastrichtian (69.1 - 66.0 Ma; Ogg and Hinnov, 2012). The Javelina Formation is often

924 considered to represent continuous deposition up to and through the K-Pg boundary (e.g.

925 Atchley et al., 2004). If this were the case, then it would mean that the ~90m of deposits

926 overlying the 69 Ma datum (Lehman et al., 2006) represented the 3 m.y. leading up to the

927 K-Pg boundary, and that the ~60 m below might represent 2 m.y. (if average rates of

928 deposition were assumed). This would seem to be an unusually long period of time for

929 such a thin unit, though not impossible. Alternatively, considerable hiatuses (up to 2 m.y.)

930 are suggested to occur within the Javelina Formation by Nordt et al. (2003). This is

931 important regarding regional correlation and warrants further consideration.

932

933 Paleontology

934 Thinking about taxa as lineages

935

936 There has been a recent re-emergence of study regarding the mode of evolution

937 of dinosaurs. Many analyses have found that dinosaur sister taxa form (typically) short-

938 duration species that do not overlap stratigraphically; a pattern especially common

939 within single depositional basins (e.g., Mateer, 1981; Horner et al., 1992; Holmes et al.,

940 2001, although see; Ryan and Russell, 2005; Campione and Evans, 2011; Evans et al.,

941 2011; Mallon et al., 2012; Gates et al., 2013; Scannella et al., 2014; Freedman Fowler

942 and Horner, 2015; Fowler, 2016). In some cases, it has been suggested that this may

943 represent anagenesis (or the synonym, 'phyletic evolution'; Horner et al., 1992; Campione

42

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 944 and Evans, 2011; Scannella et al., 2014; Freedman Fowler and Horner, 2015; Fowler,

945 2016), the evolutionary mode whereupon lineages or populations transform

946 morphologically through time without branching into multiple contemporaneous species

947 (cladogenesis; also technically, speciation; sensu Cook, 1906).

948

949 The stratigraphic correlations and taxonomic plots presented here (S1 Table)

950 facilitate more broad investigations of the mode of evolution in Western Interior

951 dinosaurs. One of the striking results of the replotting of both geological formations and

952 dinosaur taxa is that many dinosaur clades form columns of short-duration species (or

953 perhaps more accurately, metaspecies; Archibald, 1994; Lee, 1995) which do not overlap

954 stratigraphically, a pattern which, if real, fails to reject the hypothesis of anagenesis. This

955 suggests that in Western Interior dinosaurs, much of the morphological change that we

956 observe through time might not all be related to the multiplication of species. If

957 cladogenesis was the most important driver of morphological change (Eldredge and

958 Gould, 1972), then we would expect to see stratigraphic overlap between different

959 morphologies. Although this does occur, it is not ubiquitous, and anagenesis should be

960 equally considered alongside cladogenesis as a valid hypothesis explaining

961 morphological change. Further support for anagenesis is that many stratigraphically

962 intermediate forms are also intermediate in morphology; this is best shown in

963 chasmosaurine ceratopsids of the upper Maastrichtian Hell Creek Formation ( Triceratops ;

964 Scannella et al., 2014), but also in the Pentaceratops lineage chasmosaurines of the

965 middle to upper Campanian (Kaiparowits Formation, Utah; Fruitland and Kirtland

43

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 966 formations, New Mexico), and brachylophosaurin hadrosaurines of the middle

967 Campanian (Freedman Fowler and Horner, 2015).

968

969 It could be noted that if the error associated with radiometric dates is included,

970 then for some species it may not be possible to determine with total confidence whether

971 or not they were contemporaneous with their sister taxon (which would falsify

972 anagenesis). This criticism however, applies equally to any evolutionary hypothesis that

973 depends upon precise stratigraphic data for closely related taxa that are stratigraphically

974 similar ages, including the hypothesis of cladogenesis (the alternative to anagenesis), and

975 even biogeographic hypotheses (e.g., Sampson et al., 2010; 2013). It also could be argued

976 that a lack of stratigraphic overlap of sister taxa is simply an artifact of inadequate

977 sampling. These might well be true, but it does not mean that hypotheses should not be

978 formed and tested. In some examples where large datasets have been accumulated

979 (Scannella et al., 2014) the pattern has still been one of stratigraphic replacement rather

980 than overlap, with an increased number of stratigraphically intermediate specimens

981 exhibiting more intermediate morphologies. Increased sampling is evidently the best way

982 to test all hypotheses of anagenesis, cladogenesis, and biogeography in Western Interior

983 dinosaurs, but it would be " pusillanimous to avoid making our best efforts today because

984 they may appear inadequate tomorrow" (Simpson, 1944; p. xviii).

985

986 North-south biogeography and intracontinental faunal endemism

987

44

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 988 It has been proposed that during the Campanian, the Western Interior of North

989 America was divided into relatively small latitudinally arrayed faunal provinces, each

990 with a unique fauna (Sampson et al., 2010; 2013). This is based primarily on the

991 description of new genera and species of dinosaur collected from the Kaiparowits

992 Formation, Utah (e.g., Gates and Sampson, 2007; Sampson et al., 2010; 2013), and the

993 perception that the Kaiparowits Formation was deposited contemporaneously with other

994 dinosaur-bearing deposits (e.g., the Dinosaur Park Formation, Alberta; Fruitland and

995 Kirtland formations, New Mexico). However, review of the data used in the original

996 publication (Sampson et al., 2010) and recalibrations performed here reduce support for

997 this hypothesis.

998

999 In 2005, Roberts et al. presented a thorough stratigraphic and sedimentological

1000 description of the Kaiparowits Formation, including three 40 Ar / 39 Ar dates (75.96 Ma;

1001 75.02 Ma; and 74.21 Ma) from a series of volcanic ashes throughout the unit. This

1002 provided a welcome opportunity to more precisely correlate the Kaiparowits Formation

1003 with similarly aged units in the Western Interior, permitting the testing of paleontological

1004 hypotheses regarding the biogeography, phylogeny, and mode of evolution of their

1005 dinosaur fauna.

1006

1007 These themes were later explored by the hypothesis of 'intracontinental faunal

1008 endemism' (Sampson et al., 2010; 2013), which proposed that taxonomic differences

1009 among the dinosaurs of the Kaiparowits Formation, Dinosaur Park Formation, (Alberta);

1010 Two Medicine Formation (Montana) and Fruitland and Kirtland formations (New Mexico)

45

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 1011 were representative of different species being endemic to small geographic ranges. Key

1012 evidence for this hypothesis was the presentation and discussion of the stratigraphic

1013 ranges of chasmosaurine ceratopsid dinosaurs, of which many taxa were shown to have

1014 overlapped (Sampson et al., 2010). This would mean that these taxa were

1015 contemporaneous, but apparently segregated geographically, thereby forming key support

1016 for intracontinental faunal endemism (Sampson et al., 2010).

1017

1018 However, the chronostratigraphic data used to plot the stratigraphic ranges of

1019 chasmosaurine taxa (Sampson et al., 2010) contained an unexplained inconsistency

1020 related to the mixed use of unrecalibrated and recalibrated 40 Ar / 39 Ar dates. The

1021 stratigraphic ranges of chasmosaurines from the Kaiparowits Formation ( Utahceratops

1022 and Kosmoceratops ) were plotted as occurring from 76.3 to 75.5 Ma, and regarding the

1023 duration of the formation itself, Sampson et al. (2010, p.6) state "Laser-fusion 40Ar/39Ar

1024 ages indicate a late Campanian range for the formation, spanning 76.6–74.5 Ma and

1025 corresponding to the Judithian land vertebrate age (Fig. 7)", and cite Roberts et al. (2005)

1026 as the source for these ages. However, as shown above, the dates in Roberts et al. (2005)

1027 range from 75.96 to 74.21 Ma, i.e. the youngest date given by Roberts et al. (2005), 74.21

1028 Ma, is younger than the upper age limit of the entire formation (74.5 Ma) given by

1029 Sampson et al. (2010), which is clearly impossible. Furthermore, Roberts et al. (2005, p.

1030 312) explicitly state that "utilizing an average rock accumulation rate of 41 cm/ka, the ca.

1031 860-m-thick Kaiparowits Formation accumulated for ca. 2.1 Ma, from ca. 76.1 - 74.0

1032 Ma". This is therefore inconsistent with the taxon and formational ranges of Sampson et

46

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 1033 al. (2010; 76.6-74.5 Ma), and at the time of publication the origin of these dates remained

1034 unexplained

1035

1036 More information was later provided in a generalized stratigraphic column of the

1037 Kaiparowits Formation (Zanno et al., 2011), which presented four 40 Ar / 39 Ar dates

1038 (76.46 Ma; 75.97 Ma; 75.51 Ma; and again 75.51 Ma), three of which corresponded

1039 stratigraphically with the same horizons dated by Roberts et al. (2005), but with different

1040 numerical ages. Zanno et al. (2011) do not state that these are recalibrated dates, and

1041 instead cite Roberts (2007) as their source for three of these dates, but the dates in

1042 Roberts (2007), are the same as in Roberts et al. (2005), and do not correspond with the

1043 numbers given by Zanno et al. (2011). It is notable that the dates given by Zanno et al.

1044 (2011) are consistent with the age range given by Sampson et al. (2010), i.e. that they

1045 probably had the same, unexplained source.

1046

1047 The source of the new dates was only officially published in 2013, when Roberts

1048 et al. published a series of dates from the Kaiparowits Formation that were recalibrated

1049 (using the FCT standard and decay constant pairing of Kuiper et al., 2008; 28.2 Ma; and

1050 Min et al., 2000) from those published by Roberts et al. (2005; which used the 28.02 Ma

1051 age for the FCT standard; Renne et al., 1998). That the Sampson et al. (2010)

1052 Kaiparowits dates are indeed recalibrated is confirmed by Roberts et al. (2013; p.85) who

1053 state, "recalibration of Kaiparowits Formation ash beds demonstrates that the formation is

1054 approximately half a million years older than previously suggested, deposited ~76.6–74.5

1055 Ma.", i.e., exactly the same age duration as given by Sampson et al. (2010).

47

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 1056

1057 This demonstrates unequivocally that Sampson et al. (2010) used a mixture of

1058 40 Ar / 39 Ar dates calibrated to different standards to plot the stratigraphic occurrence of

1059 chasmosaurine taxa, creating the illusion that certain ataxa overlapped. Utahceratops and

1060 Kosmoceratops from the Kaiparowits Formation were the only taxa that were plotted

1061 based on radiometric dates recalibrated to the current standard (Kuiper et al., 2008).

1062 Other taxa from different units (Dinosaur Park Formation, Alberta; Fruitland and Kirtland

1063 formations, New Mexico) were plotted based on unrecalibrated dates which used

1064 previous standards, mostly that of Samson and Alexander (1987). This results in taxa

1065 from the Kaiparowits Formation being shown ~0.5 m.y. relatively older (Roberts et al.,

1066 2013) than they would have been if they had been plotted to the same standard as the taxa

1067 from the other units.

1068

1069 When all the available dates are recalibrated to the same standards (as in the current

1070 work), the stratigraphic overlap between key taxa is no longer recovered. Only the lower

1071 part of the Kaiparowits Formation stratigraphically overlaps with the fossiliferous portion

1072 of the Dinosaur Park Formation (see S1 Table). This is important as the lower

1073 Kaiparowits Formation does not yield the taxa purportedly endemic to southern Utah, and

1074 fragmentary specimens suggest that taxa are shared between the upper part of the

1075 Dinosaur Park and lower Kaiparowits Formations (Gates et al., 2013). Here it is

1076 considered more likely that differences between dinosaur species found in the Dinosaur

1077 Park Formation and middle Kaiparowits Formation are mostly an artifact of sampling

1078 different stratigraphic levels, rather than biogeographic segregation (also see Lucas et al.,

48

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 1079 2016). Similarly, differences between the middle Kaiparowits taxa and those of the

1080 Fruitland and Kirtland formations, New Mexico, are also more parsimoniously explained

1081 by the slight difference in age of the units, with the Fruitland and Kirtland formations

1082 being slightly younger than the middle Kaiparowits Formation (Sullivan and Lucas, 2006;

1083 Lucas et al., 2016). Moreover, the recent identification of purportedly southern

1084 Pentaceratops -lineage chasmosaurines within the Dinosaur Park Formation, Alberta

1085 (Longrich, 2014; Fowler, 2016), demonstrates that this lineage was able to move between

1086 northern and southern regions in the middle Campanian.

1087

1088 Biostratigraphy

1089

1090 Cobban and Reeside (1952) used the ceratopsid dinosaur Triceratops as an index

1091 taxon of the latest Maastrichtian. Similarly, dinosaurs were part of the original Land

1092 Vertebrate Ages (LVA; Aquilian; Judithian; Edmontonian; Lancian) described by Russell

1093 (1964) before revision into North American Land Mammal Ages (NALMA; Russell,

1094 1975; Lillegraven & McKenna 1986; Cifelli et al., 2004). More recently, dinosaurs have

1095 been used to stratigraphically correlate Campanian and Maastrichtian units of the United

1096 States (Lawson, 1976; Lehman 1987, 2001; Sullivan, 2003), and were utilized by

1097 Sullivan and Lucas (2003, 2006) in their definition of the “Kirtlandian”: an additional

1098 LVA roughly equivalent to the early deposition of the Bearpaw Shale and positioned in

1099 the gap between the Judithian and Edmontonian identified by Russell (1964; 1975).

1100 Dinosaurs were also strongly utilised for biostratigraphy in the definition or redefinition

1101 of 10 vertebrate biochrons for the Cretaceous of the Western Interior (Lucas et al., 2012).

49

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 1102

1103 The demonstration that individual dinosaur species form stratigraphically stacked

1104 sequences of non-overlapping taxa could make them useful for biostratigraphy. This

1105 might be seen as controversial, since generally dinosaur taxa are known from relatively

1106 few specimens and are arguably less abundant than mammals or other groups typically

1107 used in terrestrial biostratigraphy. However, at least some clades of dinosaurs would

1108 seem ideal for biostratigraphic correlation, especially if current hypotheses of rapid

1109 evolution are correct (e.g. Horner et al., 1992; Holmes et al., 2001; Scannella et al., 2014;

1110 Fowler, 2016). For example, the chasmosaurine dinosaur Triceratops has been

1111 demonstrated to evolve at least three different metaspecies through the duration of the

1112 Hell Creek Formation in Montana (Scannella et al., 2014). Although the duration of the

1113 Hell Creek Formation is not precisely known, two stratigraphically separated metaspecies

1114 of Triceratops ( T. prorsus and T. sp.; Scannella et al., 2014) are recorded from the

1115 uppermost 30m, which has been recently demonstrated by Ar / Ar dates as representing

1116 ~300 k.y. of deposition (Sprain et al., 2014; see S1 Table). If we are able to understand

1117 the stratigraphic distribution and ontogenetic variation of dinosaurs well enough, then

1118 conceivably many more clades may be biostratigraphically informative at resolutions of

1119 ~300Ka (or less; see S1 Table).

1120

1121 Conclusions

1122

50

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 1123 Understanding the paleobiology of extinct organisms requires explicit knowledge

1124 of their relative positions in time. In turn, this depends upon the accurate correlation of

1125 the geological formations from which fossil remains are recovered.

1126

1127 Here, recalibrated radiometric dates are combined with existing stratigraphic data

1128 to create a comprehensive stratigraphic correlation chart for terrestrial units of the U.S

1129 Western Interior. This revised stratigraphic framework is intended to be a tool for use by

1130 other researchers to investigate dinosaur evolution. Recalibration of radiometric dates to

1131 the same standard should remove artifacts of miscorrelation, permitting a clearer search

1132 for evolutionary patterns. Conflicts between different kinds of stratigraphic data are

1133 highlighted, particularly where they may affect paleontological understanding.

1134

1135 Future expansions of the chart will increase the geographic scope of formations covered,

1136 and include additional taxa.

1137

1138 Acknowledgements

1139

1140 Special thanks to my supervisor John Horner, David Eberth, Liz Freedman Fowler, Jack

1141 Wilson, Paul Renne, and Robert Sullivan. Thanks to David Bowen, Dennis Braman, Ray

1142 Butler, Peter Dodson, Federico Fanti, Jim Fassett, Joe Hartman, Rebecca Hunt-Foster,

1143 Neil Landman, Spencer Lucas, Jay Nair, Jason Noble, Ray Rogers, Julia Sankey, John

1144 Scannella, Courtney Sprain, David Varricchio, Anton Wroblewski, and everyone at the

51

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 1145 library project for discussion and sending me many essential papers. This manuscript was

1146 improved by comments from David Evans, Jim Kirkland, Andrew McDonald, and

1147 reviews from Spencer Lucas, Robert Sullivan, and two anonymous referees. Thanks to

1148 various people at SVP2006 for their helpful comments and suggestions.

1149

1150 References

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1152 1.

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1663

1664 Supporting Information

1665

1666 S1 Table. Stratigraphic Chart. Stratigraphic correlation of Upper Cretaceous terrestrial

1667 strata of the North American Western Interior from the Santonian through to the K-Pg

1668 boundary. Dinosaur taxon ranges plotted on to correlated geological units.

1669

1670 S2 Table. Recalibration Sheet . This sheet shows recalibration calculations for over 200

1671 published Ar / Ar radiometric dates. These are recalibrated to the two current standards

1672 (Kuiper et al., 2008; Renne et al., 2011), shown on separate tabs. References are given

1673 within pop up notes for the respective recalibrated date(s).

1674

75

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017 1675 S1 Text. References for Stratigraphic Chart. This text file lists all the references used

1676 in construction of the stratigraphic chart (Table S1).

1677

1678 S2 Text. Comment Boxes for Stratigraphic Chart. This text file provides transcripts of

1679 all the pop-up comment boxes featured in the stratigraphic chart (Table S1). This file

1680 should be of use to readers who prefer the text in this larger format.

1681

1682 S1 Figure. Stratigraphic Chart, graphic version. This is a image file version of the

1683 stratigraphic chart (S1 Table). It is an image only, provided for quick reference, and does

1684 not have embedded pop-up comments.

1685

1686 S2 Figure. Geographic Location of Stratigraphic Sections featured on Stratigraphic

1687 Chart Table S1. This map shows the geographic location of the different stratigraphic

1688 sections shown in stratigraphic chart Table S1.

1689

1690

76

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2554v2 | CC BY 4.0 Open Access | rec: 4 Sep 2017, publ: 4 Sep 2017