4 Revised geochronology, correlation, and dinosaur ... · 4 Revised geochronology, correlation, and dinosaur stratigraphic 5 ranges of the Santonian-Maastrichtian (Late Cretaceous)

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Revised geochronology, correlation, and dinosaur stratigraphic 4

ranges of the Santonian-Maastrichtian (Late Cretaceous) 5

formations of the Western Interior of North America 6

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Denver Fowler1* 9

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1Dickinson Museum Center, Dickinson ND, USA 13

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*Corresponding author 16

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E-mail: [email protected] 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

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Abstract 19

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Interbasinal stratigraphic correlation provides the foundation for all consequent 21

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

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

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

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 Outdated 25

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

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

Cretaceous units of North America, combining published chronostratigraphic, 29

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

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

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

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

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

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

and Kirtland formations, New Mexico. 36

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

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

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


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pattern that is produced by an anagenetic mode of evolution, suggesting that true 41

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

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

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

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

they may be useful for high resolution biostratigraphy. 46

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Introduction 48

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

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

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

Such interbasinal correlation diagrams are enormously useful for making stratigraphic 53

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

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

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

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

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

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

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

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

Table and S1 Text). 62


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

paleobiological interpretation. 82

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

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


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to study dinosaur evolution. This is due to the serendipitous combination of large areas of 86

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

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

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

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

some problems persist that strongly affect paleobiological interpretations: 91

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

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

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

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

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

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

incorrect or unknown stratigraphic data or unknowingly mismatching old outdated 99

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

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

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

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

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2. Depositional hiatuses are not depicted 105

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

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

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


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relationships. For example, under conventional lithostratigraphic practice, prominent 109

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

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

the conventional sequence stratigraphic model, amalgamated channel sandstones often 112

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

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

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

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

where possible. 117

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3. Radiometric dates may be incorrect or incomparable 119

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

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

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

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

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

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

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

this is being investigated internally by those labs 127

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This current work presents a comprehensive stratigraphic correlation chart 129

comprising the major terrestrial geological formations of the Santonian through 130

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


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based on extensive reference to the stratigraphic literature on each formation (which is 132

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

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

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

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

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

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

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

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

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

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Methods 143

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Abbreviations used 145

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

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

Tuff; TCR, Taylor Creek Rhyolite. 149

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Display format - excel sheets 151

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The recalibration sheet and stratigraphic correlation chart are offered as two separate 153

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

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S1 Table - Stratigraphic correlation chart 156

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

document (Text S1). 178

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S2 Table - Recalibration sheet 180

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

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

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

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

formulae used to calculate the new dates. 186

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The recalibration sheet is adapted from the EARTHTIME excel recalculation 188

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

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

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

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

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

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

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

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

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


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(standards, decay constant, etc.) are visible for each recalculation. This way the sheet 199

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

independently assessed. 201

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

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

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

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

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

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

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

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

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

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

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

known. 214

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

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

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

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

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


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recalibrations is provided for comparison. Both tables of recalibrations have the same 221

formatting for ease of comparison. 222

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Stratigraphic chart (S1 Table) 224

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

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

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

construction. 229

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Definitions: stage and substages, magnetostratigraphy, and ammonite 231

biostratigraphy 232

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

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

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

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

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

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

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

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

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


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magnetostratigraphic analyses (e.g. Lerbekmo and Braman, 2002). Individual definitions 243

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

parts of the chart. 245

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

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

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

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

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

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Positioning of geological units and dinosaur taxa 253

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

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

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

(respectively). 258

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

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

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

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

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

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


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geological unit comprises some specimens or horizons for which stratigraphic position is 266

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

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

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

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

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

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

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

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Issues with lithostratigraphy 276

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

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

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

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

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

might not be visible. 283

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Limitation of scope & future versions 285

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


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

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

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

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

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

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

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

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

beds). 298

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Dinosaurian fossils are limited to Neoceratopsia, Pachycephalosauridae, 300

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

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

and also include taxa considered biostratigraphically informative by previous workers 303

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

et al., 2012). 305

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

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

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

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


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concerning the addition of all remaining dinosaur taxa (including birds), crocodylians, 311

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

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Institutional abbreviations 314

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

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

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Taxa display format- phylogenies and lineages 319

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

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

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

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

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

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

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

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

lambeosaurines follows Evans and Reisz (2007). 329

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Magnetostratigraphy 331

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The conventional methodology used for delineating magnetostratigraphic chron 333

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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C30n-C29r boundary is therefore drawn within the Apex Sandstone, whereas it is more 354

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

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

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

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

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

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

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

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

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

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

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

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Radiometric dating 367

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

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

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

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

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

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

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

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U-Pb and K-Ar 377

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

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

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

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

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

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

/ 39Ar dates. 385

386

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

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

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

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

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

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

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

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

accuracy (Villeneuve, 2004). 395

396

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

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

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


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large number of possible datable minerals (due to the common occurrence of potassium 400

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

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

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

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

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

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

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

unit, but not for precise correlation. 408

409

40Ar / 39Ar 410

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

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

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

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

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

vs U-Pb). 417

418

Standards (neutron fluence monitor) 419

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

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

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


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another method; whereas secondary standards are based on 40Ar / 39Ar intercalibration 423

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

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

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

427

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

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

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

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

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

~1 Ma 433

434

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

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

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

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

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

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

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

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

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

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

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


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comparable without recalibration (e.g., Samson and Alexander, 1987; Renne et al., 1994; 446

1998). 447

448

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

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

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

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

λtotal; Beckinsale and Gale, 1969). 453

454

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

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

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

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

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

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

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

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

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

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

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

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

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


22

These analyses also used three methods (40Ar / 39Ar, U-Pb, cyclostratigraphy) to reach 468

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

470

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

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

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

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

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

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

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

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

479

Decay constants 480

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

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

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

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

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

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

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

488

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

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


23

example, the difference between using 5.543 E-10/y (Steiger & Jaeger, 1977) and 5.463 491

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

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

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

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

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

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

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

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

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

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

502

Recalibration & current standards 503

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

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

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

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

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

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

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

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

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


24

sheet from McLean and EARTHTIME (or the adapted spreadsheet used here); the 513

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

515

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

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

radiometric date. 518

519

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

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

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

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

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

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

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

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

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

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

530

Choice of mineral 531

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

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

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

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


25

which the mineral no longer loses any products of radioactive decay; Villeneuve, 2004) 536

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

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

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

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

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

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

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

sanidine (although this is sometimes unavoidable). 544

545

Reporting of uncertainty / error 546

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

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

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

550

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

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

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

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

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

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

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

558


26

The statistical method used to report error is not standardized, and is typically 559

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

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

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

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

564

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

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

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

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

includes analytical and systematic error. 569

570

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

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

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

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

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

unrecalibrated) should therefore be approached with caution. 576

577

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

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

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

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


27

zircon magma residence times prior to an eruption (Villeneuve, 2004), error in the 40K 582

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

604


28

Other general comments 605

606

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

published form where possible. 608

609

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

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

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

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

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

sleuthing). 615

616

Results 617

618

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

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

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

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

623

Notes on recalibrations by other authors 624

625


29

Various analyses published by J. D. Obradovich 626

627

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

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

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

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

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

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

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

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

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

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

638

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

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

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

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

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

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

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

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

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

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


30

(Obradovich) obtained an age of 28.03 Ma for the FCT [...] of W, McIntosh (Geoscience 649

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

age of 28.32 Ma for TCR." 651

652

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

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

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

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

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

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

1990; Dalrymple et al., 1993). 659

660

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

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

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

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

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

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

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

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

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

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

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


31

Ma (not stated, but retrocalculated here). Sageman et al. (2014; cited as Siewert et al., in 672

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

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

675

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

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

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

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

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

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

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

683

Roberts et al. (2013) 684

685

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

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

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

that are incorrect by up to a million years. 689

690

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

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

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

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


32

the value of FCT that was equivalent to the MMhb-1 at 420.4 Ma, which is FCT = 27.84 695

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

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

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

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

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

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

702

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

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

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

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

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

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

709

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

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

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

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

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

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

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

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


33

1977), which is the total (λT) of λβ plus λε. Second, Roberts et al. (2013) correctly state 718

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

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

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

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

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

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

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

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

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

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

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

recalibrations should produce older dates. 730

731

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

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

by the EARTHTIME institute. 734

735

Seven recalibrations were performed correctly; four from the Kaiparowits 736

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

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

discarded. 739

740


34

Discussion 741

742

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

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

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

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

747

Geology 748

749

Judith River Formation 750

751

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

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

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

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

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

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

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

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

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

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


35

and Horner, 2015). However, few measured sections have been published for many other 762

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

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

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

(Malik, 1990; LaRock, 2000). 766

767

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

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

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

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

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

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

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

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

vertebrate fossils have been recovered. 776

777

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

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

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

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

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

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


36

in outcrop in northernmost Montana (Eberth, 2005; Schott et al., 2009; Ryan et al., 2010; 784

Freedman Fowler and Horner, 2015). 785

786

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


37

et al., 2016) are younger than an 40Ar / 39Ar date from the middle of the Dinosaur Park 807

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

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

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

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

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

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

conditions. 814

815

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

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

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

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

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

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

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

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

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

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

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

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

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

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


38

River Formation (Rogers and Kidwell, 2000), and indeed potentially elsewhere in 830

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

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

detectable from existing collections and useful for correlation. 833

834

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

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

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

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

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

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

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

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

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

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

845

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

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

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

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

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

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

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


39

A solution to this issue would be to follow Canadian stratigraphic definition in raising the 853

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

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

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

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

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

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

section are well exposed. 860

861

Age of the Aguja Formation, Texas 862

863

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

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

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

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

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

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

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

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

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

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

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

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


40

73.649 Ma; Ogg, 2012), asserting a late Campanian age. Using chemostratigraphic and 876

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

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

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

Upper Shale Member. 880

881

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

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

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

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

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

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

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

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

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

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

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

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

894

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

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

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

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


41

Member (see Fig. 22 in Befus et al., 2008). As such the Upper Shale Member should not 899

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

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

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

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

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

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

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

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

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

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

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

data is needed. 911

912

Age of the Javelina Formation, Texas 913

914

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

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

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

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

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

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

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


42

Javelina Formation. Assuming it is accurate, this plots in the lowermost part of the late 922

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

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

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

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

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

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

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

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

important regarding regional correlation and warrants further consideration. 931

932

Paleontology 933

Thinking about taxa as lineages 934

935

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

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

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

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

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

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

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

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


43

and Evans, 2011; Scannella et al., 2014; Freedman Fowler and Horner, 2015; Fowler, 944

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

morphologically through time without branching into multiple contemporaneous species 946

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

948

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

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

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

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

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

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

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

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

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

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

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

equally considered alongside cladogenesis as a valid hypothesis explaining 960

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

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

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

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

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


44

formations, New Mexico), and brachylophosaurin hadrosaurines of the middle 966

Campanian (Freedman Fowler and Horner, 2015). 967

968

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

985

North-south biogeography and intracontinental faunal endemism 986

987


45

It has been proposed that during the Campanian, the Western Interior of North 988

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

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

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

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

perception that the Kaiparowits Formation was deposited contemporaneously with other 993

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

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

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

this hypothesis. 997

998

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

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

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

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

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

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

dinosaur fauna. 1005

1006

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

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

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

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


46

were representative of different species being endemic to small geographic ranges. Key 1011

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

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

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

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

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

1017

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

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

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

stratigraphic ranges of chasmosaurines from the Kaiparowits Formation (Utahceratops 1021

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

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

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

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

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

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

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

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

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

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

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


47

al. (2010; 76.6-74.5 Ma), and at the time of publication the origin of these dates remained 1033

unexplained 1034

1035

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

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

(76.46 Ma; 75.97 Ma; 75.51 Ma; and again 75.51 Ma), three of which corresponded 1038

stratigraphically with the same horizons dated by Roberts et al. (2005), but with different 1039

numerical ages. Zanno et al. (2011) do not state that these are recalibrated dates, and 1040

instead cite Roberts (2007) as their source for three of these dates, but the dates in 1041

Roberts (2007), are the same as in Roberts et al. (2005), and do not correspond with the 1042

numbers given by Zanno et al. (2011). It is notable that the dates given by Zanno et al. 1043

(2011) are consistent with the age range given by Sampson et al. (2010), i.e. that they 1044

probably had the same, unexplained source. 1045

1046

The source of the new dates was only officially published in 2013, when Roberts 1047

et al. published a series of dates from the Kaiparowits Formation that were recalibrated 1048

(using the FCT standard and decay constant pairing of Kuiper et al., 2008; 28.2 Ma; and 1049

Min et al., 2000) from those published by Roberts et al. (2005; which used the 28.02 Ma 1050

age for the FCT standard; Renne et al., 1998). That the Sampson et al. (2010) 1051

Kaiparowits dates are indeed recalibrated is confirmed by Roberts et al. (2013; p.85) who 1052

state, "recalibration of Kaiparowits Formation ash beds demonstrates that the formation is 1053

approximately half a million years older than previously suggested, deposited ~76.6–74.5 1054

Ma.", i.e., exactly the same age duration as given by Sampson et al. (2010). 1055


48

1056

This demonstrates unequivocally that Sampson et al. (2010) used a mixture of 1057

40Ar / 39Ar dates calibrated to different standards to plot the stratigraphic occurrence of 1058

chasmosaurine taxa, creating the illusion that certain ataxa overlapped. Utahceratops and 1059

Kosmoceratops from the Kaiparowits Formation were the only taxa that were plotted 1060

based on radiometric dates recalibrated to the current standard (Kuiper et al., 2008). 1061

Other taxa from different units (Dinosaur Park Formation, Alberta; Fruitland and Kirtland 1062

formations, New Mexico) were plotted based on unrecalibrated dates which used 1063

previous standards, mostly that of Samson and Alexander (1987). This results in taxa 1064

from the Kaiparowits Formation being shown ~0.5 m.y. relatively older (Roberts et al., 1065

2013) than they would have been if they had been plotted to the same standard as the taxa 1066

from the other units. 1067

1068

When all the available dates are recalibrated to the same standards (as in the current 1069

work), the stratigraphic overlap between key taxa is no longer recovered. Only the lower 1070

part of the Kaiparowits Formation stratigraphically overlaps with the fossiliferous portion 1071

of the Dinosaur Park Formation (see S1 Table). This is important as the lower 1072

Kaiparowits Formation does not yield the taxa purportedly endemic to southern Utah, and 1073

fragmentary specimens suggest that taxa are shared between the upper part of the 1074

Dinosaur Park and lower Kaiparowits Formations (Gates et al., 2013). Here it is 1075

considered more likely that differences between dinosaur species found in the Dinosaur 1076

Park Formation and middle Kaiparowits Formation are mostly an artifact of sampling 1077

different stratigraphic levels, rather than biogeographic segregation (also see Lucas et al., 1078


49

2016). Similarly, differences between the middle Kaiparowits taxa and those of the 1079

Fruitland and Kirtland formations, New Mexico, are also more parsimoniously explained 1080

by the slight difference in age of the units, with the Fruitland and Kirtland formations 1081

being slightly younger than the middle Kaiparowits Formation (Sullivan and Lucas, 2006; 1082

Lucas et al., 2016). Moreover, the recent identification of purportedly southern 1083

Pentaceratops-lineage chasmosaurines within the Dinosaur Park Formation, Alberta 1084

(Longrich, 2014; Fowler, 2016), demonstrates that this lineage was able to move between 1085

northern and southern regions in the middle Campanian. 1086

1087

Biostratigraphy 1088

1089

Cobban and Reeside (1952) used the ceratopsid dinosaur Triceratops as an index 1090

taxon of the latest Maastrichtian. Similarly, dinosaurs were part of the original Land 1091

Vertebrate Ages (LVA; Aquilian; Judithian; Edmontonian; Lancian) described by Russell 1092

(1964) before revision into North American Land Mammal Ages (NALMA; Russell, 1093

1975; Lillegraven & McKenna 1986; Cifelli et al., 2004). More recently, dinosaurs have 1094

been used to stratigraphically correlate Campanian and Maastrichtian units of the United 1095

States (Lawson, 1976; Lehman 1987, 2001; Sullivan, 2003), and were utilized by 1096

Sullivan and Lucas (2003, 2006) in their definition of the “Kirtlandian”: an additional 1097

LVA roughly equivalent to the early deposition of the Bearpaw Shale and positioned in 1098

the gap between the Judithian and Edmontonian identified by Russell (1964; 1975). 1099

Dinosaurs were also strongly utilised for biostratigraphy in the definition or redefinition 1100

of 10 vertebrate biochrons for the Cretaceous of the Western Interior (Lucas et al., 2012). 1101


50

1102

The demonstration that individual dinosaur species form stratigraphically stacked 1103

sequences of non-overlapping taxa could make them useful for biostratigraphy. This 1104

might be seen as controversial, since generally dinosaur taxa are known from relatively 1105

few specimens and are arguably less abundant than mammals or other groups typically 1106

used in terrestrial biostratigraphy. However, at least some clades of dinosaurs would 1107

seem ideal for biostratigraphic correlation, especially if current hypotheses of rapid 1108

evolution are correct (e.g. Horner et al., 1992; Holmes et al., 2001; Scannella et al., 2014; 1109

Fowler, 2016). For example, the chasmosaurine dinosaur Triceratops has been 1110

demonstrated to evolve at least three different metaspecies through the duration of the 1111

Hell Creek Formation in Montana (Scannella et al., 2014). Although the duration of the 1112

Hell Creek Formation is not precisely known, two stratigraphically separated metaspecies 1113

of Triceratops (T. prorsus and T. sp.; Scannella et al., 2014) are recorded from the 1114

uppermost 30m, which has been recently demonstrated by Ar / Ar dates as representing 1115

~300 k.y. of deposition (Sprain et al., 2014; see S1 Table). If we are able to understand 1116

the stratigraphic distribution and ontogenetic variation of dinosaurs well enough, then 1117

conceivably many more clades may be biostratigraphically informative at resolutions of 1118

~300Ka (or less; see S1 Table). 1119

1120

Conclusions 1121

1122


51

Understanding the paleobiology of extinct organisms requires explicit knowledge 1123

of their relative positions in time. In turn, this depends upon the accurate correlation of 1124

the geological formations from which fossil remains are recovered. 1125

1126

Here, recalibrated radiometric dates are combined with existing stratigraphic data 1127

to create a comprehensive stratigraphic correlation chart for terrestrial units of the U.S 1128

Western Interior. This revised stratigraphic framework is intended to be a tool for use by 1129

other researchers to investigate dinosaur evolution. Recalibration of radiometric dates to 1130

the same standard should remove artifacts of miscorrelation, permitting a clearer search 1131

for evolutionary patterns. Conflicts between different kinds of stratigraphic data are 1132

highlighted, particularly where they may affect paleontological understanding. 1133

1134

Future expansions of the chart will increase the geographic scope of formations covered, 1135

and include additional taxa. 1136

1137

Acknowledgements 1138

1139

Special thanks to my supervisor John Horner, David Eberth, Liz Freedman Fowler, Jack 1140

Wilson, Paul Renne, and Robert Sullivan. Thanks to David Bowen, Dennis Braman, Ray 1141

Butler, Peter Dodson, Federico Fanti, Jim Fassett, Joe Hartman, Rebecca Hunt-Foster, 1142

Neil Landman, Spencer Lucas, Jay Nair, Jason Noble, Ray Rogers, Julia Sankey, John 1143

Scannella, Courtney Sprain, David Varricchio, Anton Wroblewski, and everyone at the 1144


52

library project for discussion and sending me many essential papers. This manuscript was 1145

improved by comments from David Evans, Jim Kirkland, Andrew McDonald, and 1146

reviews from Spencer Lucas, Robert Sullivan, and two anonymous referees. Thanks to 1147

various people at SVP2006 for their helpful comments and suggestions. 1148

1149

References 1150

1151

1. 1152

Archibald JD. Metataxon concepts and assessing possible ancestry using phylogenetic 1153

systematics. Syst Biol. 1994;43: 27–40. doi:10.1093/sysbio/43.1.27 1154

2. 1155

Atchley SC, Nordt LC, Dworkin SI. Eustatic control on alluvial sequence stratigraphy: a 1156

possible example from the Cretaceous-Tertiary transition of the Tornillo Basin, Big Bend 1157

National Park, west Texas, U.S.A. Journal of Sedimentary Research. 2004;74: 391–404. 1158

doi:10.1306/102203740391 1159

3. 1160

Baksi AK, Archibald DA, Farrar E. Intercalibration of 40Ar39Ar dating standards. 1161

Chemical Geology. 1996;129: 307–324. 1162

4. 1163

Beckinsale RD, Gale NH. A reappraisal of the decay constants and branching ratio of 1164

40K. Earth and Planetary Science Letters. 1969;6: 289–294. 1165

5. 1166


53

Befus KS, Hanson RE, Lehman TM, Griffin WR. Cretaceous basaltic phreatomagmatic 1167

volcanism in west Texas: maar complex at Pena Mountain, Big Bend National Park. 1168

Journal of Volcanology and Geothermal Research. 2008;173: 245–264. 1169

6. 1170

Bowen CF. The Stratigraphy of the Montana Group: With Special Reference to the 1171

Position and Age of the Judith River Formation in North-central Montana. US Geological 1172

Survey Professional Paper. 1915;90: 95–153. 1173

7. 1174

Breyer JA, Busbey III AB, Hanson RE, Befus KE, Griffin WR, Hargrove US, et al. 1175

Evidence for Late Cretaceous volcanism in Trans-Pecos Texas. The Journal of Geology. 1176

2007;115: 243–251. doi:10.1086/510640 1177

8. 1178

Brinkman DB, Russell AP, Eberth DA, Peng J. Vertebrate palaeocommunities of the 1179

lower Judith River Group (Campanian) of southeastern Alberta, Canada, as interpreted 1180

from vertebrate microfossil assemblages. Palaeogeography, Palaeoclimatology, 1181

Palaeoecology. 2004;213: 295–313. 1182

9. 1183

Brinkman DB. A review of nonmarine turtles from the Late Cretaceous of Alberta. 1184

Canadian Journal of Earth Sciences. 2003;40: 557–571. 1185

10. 1186

Campione NE, Evans DC. Cranial growth and variation in edmontosaurs (Dinosauria: 1187

Hadrosauridae): implications for Latest Cretaceous megaherbivore diversity in North 1188

America. PLoS ONE. 2011;6: e25186. doi:10.1371/journal.pone.0025186 1189


54

11. 1190

Cebula GT, Kunk MJ, Mehnert HH, Naeser CW, Obradovich JD, Sutter JF. The Fish 1191

Canyon Tuff, a potential standard for the 40Ar-39Ar and fission-track dating methods. 1192

Terra Cognita. 1986;6: 139–140. 1193

12. 1194

Cifelli RL, Eberle JJ, Lofgren DL, Lillegraven JA, Clemens WA. Mammalian 1195

biochronology of the latest Cretaceous. In: Woodburne MO, editor. Late Cretaceous and 1196

Cenozoic Mammals of North America. New York: Columbia University Press; 2004. pp. 1197

21–42. 1198

13. 1199

Cobban WA, Reeside JB. Correlation of the Cretaceous formations of the Western 1200

Interior of the United States. Geological Society of America Bulletin. 1952;63: 1011–1201

1044. 1202

14. 1203

Cook OF. Factors of species-formation. Science. 1906;23: 506–507. 1204

doi:10.1126/science.23.587.506 1205

15. 1206

Duffield WA, Dalrymple GB. The Taylor Creek Rhyolite of New Mexico: a rapidly 1207

emplaced field of lava domes and flows. Bull Volcanol. 1990;52: 475–487. 1208

doi:10.1007/BF00268927 1209

16. 1210


55

Eaton JG. Biostratigraphic framework for the Upper Cretaceous rocks of the Kaiparowits 1211

Plateau, southern Utah. Geological Society of America Special Papers. 1991;260: 47–64. 1212

doi:10.1130/SPE260-p47 1213

17. 1214

Eberth DA, Deino AL. A geochronology of the non-marine Judith River Formation of 1215

southern Alberta. SEPM Theme Meeting, Mesozoic of the Western Interior. 1992. pp. 1216

24–25. 1217

18. 1218

Eberth DA, Braman DR. A revised stratigraphy and depositional history for the 1219

Horseshoe Canyon Formation (Upper Cretaceous), southern Alberta plains. Can J Earth 1220

Sci. 2012;49: 1053–1086. doi:10.1139/e2012-035 1221

19. 1222

Eberth DA, Hamblin AP. Tectonic, stratigraphic, and sedimentologic significance of a 1223

regional discontinuity in the upper Judith River Group (Belly River wedge) of southern 1224

Alberta, Saskatchewan, and northern Montana. Canadian Journal of Earth Sciences. 1225

1993;30: 174–200. doi:10.1139/e93-016 1226

20. 1227

Eberth DA. The geology. In: Currie PJ, Koppelhus EB, editors. Dinosaur Provincial Park: 1228

A spectacular ancient ecosystem revealed. Bloomington, Indiana: Indiana University 1229

Press; 2005. pp. 54–82. 1230

21. 1231


56

Eldredge N, Gould SJ. Punctuated equilibria: an alternative to phyletic gradualism. In: 1232

Schopf TJM, editor. Models in Paleobiology. San Francisco: Freeman Cooper; 1972. pp. 1233

82–115. 1234

22. 1235

Evans DC, Reisz RR. Anatomy and relationships of Lambeosaurus magnicristatus, a 1236

crested hadrosaurid dinosaur (Ornithischia) from the Dinosaur Park Formation, Alberta. 1237

Journal of Vertebrate Paleontology. 2007;27: 373–393. 1238

23. 1239

Evans DC, Ryan MJ. Cranial anatomy of Wendiceratops pinhornensis gen. et sp. nov., a 1240

centrosaurine ceratopsid (Dinosauria: Ornithischia) from the Oldman Formation 1241

(Campanian), Alberta, Canada, and the evolution of ceratopsid nasal ornamentation. 1242

PLOS ONE. 2015;10: e0130007. doi:10.1371/journal.pone.0130007 1243

24. 1244

Evans DC, Schott RK, Larson DW, Brown CM, Ryan MJ. The oldest North American 1245

pachycephalosaurid and the hidden diversity of small-bodied ornithischian dinosaurs. 1246

Nature Communications. 2013;4: 1828. doi:10.1038/ncomms2749 1247

25. 1248

Evans DC, Witmer LM, Horner JR. A new low-crested lambeosaurine hadrosaurid from 1249

the Dinosaur Park Formation of Sandy Point, eastern Alberta. International Hadrosaur 1250

Symposium abstracts volume. Drumheller, Alberta: Royal Tyrell Museum; 2011. pp. 48–1251

49. 1252

26. 1253


57

Fowler D. Terrestrial Late Cretaceous stratigraphy of North America and the utility of 1254

ceratopsids in biostratigraphy. Society of Vertebrate Paleontology Annual Meeting. 1255

2006;26: 63A. 1256

27. 1257

Fowler DW. Dinosaurs and Time: Chronostratigraphic frameworks and their utility in 1258

analysis of dinosaur paleobiology. Ph.D. dissertation, Montana State University. 2016. 1259

28. 1260

Freedman Fowler EA, Horner JR. A new brachylophosaurin hadrosaur (Dinosauria: 1261

Ornithischia) with an intermediate nasal crest from the Campanian Judith River 1262

Formation of northcentral Montana. PLOS ONE. 2015;10: e0141304. 1263

doi:10.1371/journal.pone.0141304 1264

29. 1265

Gates TA, Lund EK, Boyd CA, DeBlieux DD, Titus AL, Evans DC, et al. Ornithopod 1266

dinosaurs from the Grand Staircase-Escalante National Monument region, Utah and their 1267

role in paleobiogeographic and macroevolutionary studies. Advances in Late Cretaceous 1268

Western Interior Basin Paleontology and Geology. In: Titus AL, Loewen MA, editors. At 1269

the top of the Grand Staircase: The Late Cretaceous of southern Utah. Bloomington, 1270

Indiana: Indiana University Press; 2013. pp. 463–481. 1271

30. 1272

Gates TA, Sampson SD. A new species of Gryposaurus (Dinosauria: Hadrosauridae) 1273

from the late Campanian Kaiparowits Formation, southern Utah, USA. Zoological 1274

Journal of the Linnean Society. 2007;151: 351–376. doi:10.1111/j.1096-1275

3642.2007.00349.x 1276


58

31. 1277

Gill JR, Cobban WA. Stratigraphy and geologic history of the Montana Group and 1278

equivalent rocks, Montana, Wyoming, and North and South Dakota. US Geological 1279

Survey Professional Paper. 1973;776: 37. 1280

32. 1281

Goodwin MB, Deino AL. The first radiometric ages from the Judith River Formation 1282

(Upper Cretaceous), Hill County, Montana. Can J Earth Sci. 1989;26: 1384–1391. 1283

doi:10.1139/e89-118 1284

33. 1285

Gradstein FM, Agterberg FP, Ogg JG, Hardenbol J, Van Veen P, Thierry J, et al. A 1286

Triassic, Jurassic and Cretaceous time scale. In: Berggren WA, Kent DV, Aubry MP, 1287

Hardenbol J, editors. Geochronology Time Scales and Global Stratigraphic Correlation 1288

SEPM Special Publication No 54. Tulsa, Oklahoma: Society for Sedimentary Geology; 1289

1995. pp. 95–126. 1290

34. 1291

Gradstein FM, Ogg JG, Smith AG. A Geologic Time Scale 2004. Cambridge, UK: 1292

Cambridge University Press; 2004. 1293

35. 1294

Gradstein FM, Ogg JG, Schmitz M, Ogg G. The Geologic Time Scale 2012. Oxford, UK: 1295

Elsevier; 2012. 1296

36. 1297


59

Hartman JH, Butler RD, Weiler MW, Schumaker KK. Context, naming, and formal 1298

designation of the Cretaceous Hell Creek Formation lectostratotype, Garfield County, 1299

Montana. Geological Society of America Special Papers. 2014;503: 89–121. 1300

37. 1301

Hicks JF, Johnson KR, Obradovich JD, Miggins DP, Tauxe L. Magnetostratigraphy of 1302

Upper Cretaceous (Maastrichtian) to lower Eocene strata of the Denver Basin, Colorado. 1303

Rocky Mountain Geology. 2003;38: 1–27. 1304

38. 1305

Hicks JF, Johnson KR, Obradovich JD, Tauxe L, Clark D. Magnetostratigraphy and 1306

geochronology of the Hell Creek and basal Fort Union Formations of southwestern North 1307

Dakota and a recalibration of the age of the Cretaceous-Tertiary boundary. Geological 1308

Society of America Special Papers. 2002;361: 35–55. 1309

39. 1310

Hicks JF, Obradovich JD, Tauxe L. A new calibration point for the Late Cretaceous time 1311

scale: The (40Ar/39Ar isotopic age of the C33r/C33n geomagnetic reversal from the 1312

Judith River Formation (Upper Creataceous), Elk Basin, Wyoming, USA. Journal of 1313

Geology. 1995;103: 243–256. 1314

40. 1315

Hicks JF, Watabe M, Fastovsky DE, Johnson KR, Nichols DJ. Magnetostratigraphic 1316

correlation of Late Cretaceous dinosaur-bearing localities, Nemegt Basin, Gobi Desert, 1317

Mongolia. Abstracts with Programs - Geological Society of America. 1999;31: 234. 1318

41. 1319


60

Holmes RB, Forster CA, Ryan M, Shepherd KM. A new species of Chasmosaurus 1320

(Dinosauria: Ceratopsia) from the Dinosaur Park Formation of southern Alberta. 1321

Canadian Journal of Earth Sciences. 2001;38: 1423–1438. 1322

42. 1323

Horner JR, Varricchio DJ, Goodwin MB. Marine transgressions and the evolution of 1324

Cretaceous dinosaurs. Nature. 1992;358: 59–61. doi:10.1038/358059a0 1325

43. 1326

Izett GA, Obradovich JD. 40Ar/39Ar age constraints for the Jaramillo normal subchron. 1327

Journal of Geophysical research. 1994;99: 2925–2934. 1328

44. 1329

Jinnah ZA. Tectonic and Sedimentary Controls, Age, and Correlation of the Upper 1330

Cretaceous Wahweap Formation, Southern Utah. In: Titus AL, Loewen MA, editors. At 1331

the Top of the Grand Staircase: The Late Cretaceous of Southern Utah. Bloomington, 1332


45. 1334

Kirkland JI, Eaton JG, Brinkman DB. Elasmobranchs from Upper Cretaceous freshwater 1335

facies in southern Utah. At the top of the grand staircase: The Late Cretaceous of 1336

Southern Utah. Bloomington, Indiana: Indiana University Press; 2013. pp. 153–194. 1337

46. 1338

Krystinik LF, DeJarnett BB. Sequence stratigraphy of foreland basin deposits: outcrop 1339

and subsurface examples from the Cretaceous of North America. American Association 1340

of Petroleum Geologists Memoir. 1995;64: 11–25. 1341

47. 1342


61

Kuiper KF, Deino A, Hilgen FJ, Krijgsman W, Renne PR, Wijbrans JR. Synchronizing 1343

rock clocks of earth history. Science. 2008;320: 500–504. doi:10.1126/science.1154339 1344

48. 1345

Lanphere MA, Dalrymple GB, Fleck RJ, Pringle MS. Intercalibration of mineral 1346

standards for K-Ar and 40Ar/39Ar age measurements. EOS, Transactions of the 1347

American Geophysical Union. 1990;71: 1–658. 1348

49. 1349

LaRock JW. Sedimentology and taphonomy of a dinosaur bonebed from the Upper 1350

Cretaceous (Campanian) Judith River Formation of north central Montana [Internet]. 1351

Montana State University. 2000. 1352

50. 1353

Lawson DA. Tyrannosaurus and Torosaurus, Maastrichtian dinosaurs from Trans-Pecos, 1354

Texas. Journal of Paleontology. 1976;50: 158–164. 1355

51. 1356

LeCain R, Clyde WC, Wilson GP, Riedel J. Magnetostratigraphy of the Hell Creek and 1357

lower Fort Union formations in northeastern Montana. Geological Society of America 1358

Special Papers. 2014;503: 137–147. doi:10.1130/2014.2503(04) 1359

52. 1360

Lee MSY. Species concepts and the recognition of ancestors. Historical Biology. 1361

1995;10: 329–339. doi:10.1080/10292389509380528 1362

53. 1363


62

Lehman TM, Coulson AB. A juvenile specimen of the sauropod dinosaur Alamosaurus 1364

sanjuanensis from the Upper Cretaceous of Big Bend National Park, Texas. Journal of 1365

Paleontology. 2002;76: 156–172. doi:10.1017/S0022336000017431 1366

54. 1367

Lehman TM, Mcdowell FW, Connelly JN. First isotopic (U-Pb) age for the Late 1368

Cretaceous Alamosaurus vertebrate fauna of west Texas, and its significance as a link 1369

between two faunal provinces. Journal of Vertebrate Paleontology. 2006;26: 922–928. 1370

doi:10.1671/0272-4634(2006)26[922:FIUAFT]2.0.CO;2 1371

55. 1372

Lehman TM, Tomlinson SL. Terlinguachelys fischbecki, a new genus and species of sea 1373

turtle (Chelonioidea: Protostegidae) from the Upper Cretaceous of Texas. Journal of 1374

Paleontology. 2004;78: 1163–1178. doi:10.1017/S0022336000043973 1375

56. 1376

Lehman TM. Late Maastrichtian paleoenvironments and dinosaur biogeography in the 1377

Western Interior of North America. Palaeogeography, Palaeoclimatology, Palaeoecology. 1378

1987;60: 189–217. doi:10.1016/0031-0182(87)90032-0 1379

57. 1380

Lehman TM. Paleosols and the Cretaceous/Tertiary transition in the Big Bend region of 1381

Texas. Geology. 1990;18: 362–364. doi:10.1130/0091-1382

7613(1990)018<0362:PATCTT>2.3.CO;2 1383

58. 1384


63

Lehman TM. Late Cretaceous dinosaur provinciality. In: Tanke DH, Carpenter K, editors. 1385

Mesozoic Vertebrate Life. Bloomington, Indiana: Indiana University Press; 2001. pp. 1386

310–328. 1387

59. 1388

Lerbekmo JF, Braman DR. Magnetostratigraphic and biostratigraphic correlation of late 1389

Campanian and Maastrichtian marine and continental strata from the Red Deer Valley to 1390

the Cypress Hills, Alberta, Canada. Can J Earth Sci. 2002;39: 539–557. doi:10.1139/e01-1391

085 1392

60. 1393

Lerbekmo JF. The Dorothy bentonite: an extraordinary case of secondary thickening in a 1394

late Campanian volcanic ash fall in central Alberta. Can J Earth Sci. 2002;39: 1745–1395

1754. doi:10.1139/e02-079 1396

61. 1397

Lillegraven JA, McKenna MC. Fossil mammals from the “Mesaverde” Formation (Late 1398

Cretaceous, Judithian) of the Bighorn and Wind River basins, Wyoming : with definitions 1399

of Late Cretaceous North American land-mammal “ages.” Mesaverde mammals. 1400

1986;2840: 1–68. 1401

62. 1402

Longrich NR, Sankey J, Tanke D. Texacephale langstoni, a new genus of 1403

pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja 1404

Formation, southern Texas, USA. Cretaceous Research. 2010;31: 274–284. 1405

63. 1406


64

Longrich NR. The horned dinosaurs Pentaceratops and Kosmoceratops from the upper 1407

Campanian of Alberta and implications for dinosaur biogeography. Cretaceous Research. 1408

2014;51: 292–308. doi:10.1016/j.cretres.2014.06.011 1409

64. 1410

Lucas SG, Sullivan RM, Lichtig AJ, Dalman SG, Jasinski SE. Late Cretaceous dinosaur 1411

biogeography and endemism in the Western Interior Basin, North America: a critical re-1412

evaluation. New Mexico Museum of Natural History and Science Bulletin. 2016;71: 1413

195–213. 1414

65. 1415

Lucas SG, Sullivan RM, Spielmann JA. Cretaceous vertebrate biochronology, North 1416

American Western Interior. Journal of Stratigraphy. 2012;36: 436–461. 1417

66. 1418

Malik AME. Sedimentology of the Judith River Formation in the Milk River Valley and 1419

the Little Rocky Mountains, Montana [Internet]. MS. 1990. Available: 1420

http://ecommons.usask.ca/handle/10388/etd-07112012-152552 1421

67. 1422

Mallon JC, Evans DC, Ryan MJ, Anderson JS. Megaherbivorous dinosaur turnover in the 1423

Dinosaur Park Formation (upper Campanian) of Alberta, Canada. Palaeogeography, 1424

Palaeoclimatology, Palaeoecology. 2012;350–352: 124–138. 1425

doi:10.1016/j.palaeo.2012.06.024 1426

68. 1427

McDougall I, Harrison TM. Geochronology and thermochronology by the 40Ar/39Ar 1428

method [Internet]. 2nd ed. Oxford, UK: Oxford University Press; 1999. 1429


65

69. 1430

Meyers SR, Siewert SE, Singer BS, Sageman BB, Condon DJ, Obradovich JD, et al. 1431

Intercalibration of radioisotopic and astrochronologic time scales for the Cenomanian-1432

Turonian boundary interval, Western Interior Basin, USA. Geology. 2012;40: 7–10. 1433

doi:10.1130/G32261.1 1434

70. 1435

Miall AD, Catuneanu O, Vakarelov BK, Post R. The Western Interior Basin. In: Andrew 1436

D. Miall, editor. Sedimentary Basins of the World. Oxford, UK: Elsevier; 2008. pp. 329–1437

362. Available: http://www.sciencedirect.com/science/article/pii/S1874599708000099 1438

71. 1439

Min K, Mundil R, Renne PR, Ludwig KR. A test for systematic errors in 40 Ar/39 Ar 1440

geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite. Geochimica 1441

et Cosmochimica Acta. 2000;64: 73–98. 1442

72. 1443

Montgomery P, Hailwood EA, Gale AS, Burnett JA. The magnetostratigraphy of 1444

Coniacian-late Campanian chalk sequences in southern England. Earth and Planetary 1445

Science Letters. 1998;156: 209–224. doi:10.1016/S0012-821X(98)00008-9 1446

73. 1447

Nordt L, Atchley S, Dworkin S. Terrestrial evidence for two greenhouse events in the 1448

latest Cretaceous. GSA today. 2003;13: 4–9. 1449

74. 1450

Obradovich JD. A Cretaceous time scale. Geological Association of Canada Special 1451

Paper. 1993;39: 379–396. 1452


66

75. 1453

Obradovich JD, Cobban WA. A time-scale for the Late Cretaceous of the Western 1454

Interior of North America. Geological Association of Canada Special Paper. 1975;13: 1455

31–54. 1456

76. 1457

Obradovich JD. Geochronology of Laramide synorogenic strata in the Denver Basin, 1458

Colorado. Rocky Mountain Geology. 2002;37: 165–171. doi:10.2113/gsrocky.37.2.165 1459

77. 1460

Ogg JG, Hinnov LA. Cretaceous. In: Gradstein FM, Ogg JG, Schmitz MD, Ogg G, 1461

editors. The Geologic Time Scale. Oxford, UK: Elsevier; 2012. pp. 793–853. 1462

78. 1463

Ogg JG. Geomagnetic polarity time scale. In: Gradstein FM, Ogg JG, Schmitz MD, Ogg 1464

G, editors. The Geologic Time Scale. Oxford, UK: Elsevier; 2012. pp. 85–113. 1465

79. 1466

Peng J, Russell AP (Anthony P, Brinkman D. Vertebrate microsite assemblages 1467

(exclusive of mammals) from the Foremost and Oldman formations of the Judith River 1468

Group (Campanian) of southeastern Alberta: an illustrated guide. Provincial Museum of 1469

Alberta Natural History Occasional Paper. 2001;25: 1–54. 1470

80. 1471

Prieto-Marquez A. New information on the cranium of Brachylophosaurus canadensis 1472

(Dinosauria, Hadrosauridae), with a revision of its phylogenetic position. Journal of 1473

Vertebrate Paleontology. 2005;25: 144–156. 1474

81. 1475


67

Renne PR, Balco G, Ludwig KR, Mundil R, Min K. Response to the comment by WH 1476

Schwarz et al. on “Joint determination of 40 K decay constants and 40 Ar∗/40 K for the 1477

Fish Canyon sanidine standard, and improved accuracy for 40 Ar/39 Ar geochronology” 1478

by PR Renne et al.(2010). Geochimica et Cosmochimica Acta. 2011;75: 5097–5100. 1479

82. 1480

Renne PR, Deino AL, Walter RC, Turrin BD, Swisher CC, Becker TA, et al. 1481

Intercalibration of astronomical and radioisotopic time. Geology. 1994;22: 783–786. 1482

doi:10.1130/0091-7613(1994)022<0783:IOAART>2.3.CO;2 1483

83. 1484

Renne PR, Mundil R, Balco G, Min K, Ludwig KR. Joint determination of 40K decay 1485

constants and 40Ar*/40K for the Fish Canyon sanidine standard, and improved accuracy 1486

for 40Ar/39Ar geochronology. Geochimica et Cosmochimica Acta. 2010;74: 5349–5367. 1487

doi:10.1016/j.gca.2010.06.017 1488

84. 1489

Renne PR, Swisher CC, Deino AL, Karner DB, Owens TL, DePaolo DJ. Intercalibration 1490

of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical Geology. 1491

1998;145: 117–152. doi:10.1016/S0009-2541(97)00159-9 1492

85. 1493

Rivera TA, Storey M, Zeeden C, Hilgen FJ, Kuiper K. A refined astronomically 1494

calibrated 40 Ar/39 Ar age for Fish Canyon sanidine. Earth and Planetary Science 1495

Letters. 2011;311: 420–426. 1496

86. 1497


68

Roberts EM, Deino AL, Chan MA. 40Ar/39Ar age of the Kaiparowits Formation, 1498

southern Utah, and correlation of contemporaneous Campanian strata and vertebrate 1499

faunas along the margin of the Western Interior Basin. Cretaceous Research. 2005;26: 1500

307–318. doi:10.1016/j.cretres.2005.01.002 1501

87. 1502

Roberts EM, Sampson SD, Deino AL, Bowring S, Buchwaldt R. The Kaiparowits 1503

Formation: a remarkable record of Late Cretaceous terrestrial environments, ecosystems 1504

and evolution in western North America. In: Titus AL, Loewen MA, editors. At the Top 1505

of the Grand Staircase: The Late Cretaceous of Southern Utah. Bloomington, Indiana: 1506

Indiana University Press; 2013. pp. 85–106. 1507

88. 1508

Roberts EM. Facies architecture and depositional environments of the Upper Cretaceous 1509

Kaiparowits Formation, southern Utah. Sedimentary Geology. 2007;197: 207–233. 1510

doi:10.1016/j.sedgeo.2006.10.001 1511

89. 1512

Rogers RR, Kidwell SM, Deino AL, Mitchell JP, Nelson K, Thole JT. Age, correlation, 1513

and lithostratigraphic revision of the Upper Cretaceous (Campanian) Judith River 1514

Formation in its type area (north-central Montana), with a comparison of low-and high-1515

accommodation alluvial records. The Journal of Geology. 2016;124: 99–135. 1516

90. 1517

Rogers RR, Kidwell SM. Associations of vertebrate skeletal concentrations and 1518

discontinuity surfaces in terrestrial and shallow marine records: a test in the Cretaceous 1519

of Montana. J Geol. 2000;108: 131–154. 1520


69

91. 1521

Rogers RR, Swisher III CC, Horner JR. 40Ar/39Ar age and correlation of the nonmarine 1522

Two Medicine Formation (Upper Cretaceous), northwestern Montana, USA. Canadian 1523

Journal of Earth Sciences. 1993;30: 1066–1075. 1524

92. 1525

Rogers RR. Marine facies of the Judith River Formation (Campanian) in the type area, 1526

north-central Montana. Montana Geological Society: 1993 Field Conference Guidebook: 1527

Old Timers’ Rendezvous Edition: Energy and Mineral Resources of Central Montana. 1528

1993; 61–69. 1529

93. 1530

Rogers RR. Nature and origin of through-going discontinuities in nonmarine foreland 1531

basin strata, Upper Cretaceous, Montana: implications for sequence analysis. Geology. 1532

1994;22: 1119–1122. doi:10.1130/0091-7613(1994)022<1119:NAOOTG>2.3.CO;2 1533

94. 1534

Rogers RR. Sequence analysis of the Upper Cretaceous Two Medicine and Judith River 1535

formations, Montana; nonmarine response to the Claggett and Bearpaw marine cycles. 1536

Journal of Sedimentary Research. 1998;68: 615–631. doi:10.2110/jsr.68.604 1537

95. 1538

Rowe T, Cifelli RL, Lehman TM, Weil A. The Campanian Terlingua local fauna, with a 1539

summary of other vertebrates from the Aguja Formation, Trans-Pecos Texas. Journal of 1540

Vertebrate Paleontology. 1992;12: 472–493. doi:10.1080/02724634.1992.10011475 1541

96. 1542


70

Russell LS. Mammalian faunal succession in the Cretaceous System of western North 1543

America. Geological Association of Canada Special Paper. 1975;13: 137–161. 1544

97. 1545

Russell LS. Cretaceous non-marine faunas of northwestern North America. Life Sciences 1546

Contributions, Royal Ontario Museum. 1964;61: 1–21. 1547

98. 1548

Ryan MJ, Russell AP, Hartman S. A new chasmosaurine ceratopsid from the Judith River 1549

Formation, Montana. In: Ryan MJ, Chinnery-Allgeier BJ, Eberth DA, editors. New 1550

Perspectives on Horned Dinosaurs The Royal Tyrrell Museum Ceratopsian Symposium. 1551

Bloomington, Indiana: Indiana University Press; 2010. pp. 181–188. 1552

99. 1553

Sageman BB, Singer BS, Meyers SR, Siewert SE, Walaszczyk I, Condon DJ, et al. 1554

Integrating 40Ar/39Ar, U-Pb, and astronomical clocks in the Cretaceous Niobrara 1555

Formation, Western Interior Basin, USA. Geological Society of America Bulletin. 1556

2014;126: 956–973. doi:10.1130/B30929.1 1557

100. 1558

Sahni A. The vertebrate fauna of the Judith River Formation, Montana. Bulletin of the 1559

American Museum of Natural History. 1972;147: 325–412. 1560

101. 1561

Sampson SD, Loewen M. Unraveling a radiation: a review of the diversity, stratigraphic 1562

distribution, biogeography, and evolution of horned dinosaurs. (Ornithischia : 1563

Ceratopsidae). In: Ryan MJ, Chinnery BJ, Eberth DA, editors. New Perspectives on 1564


71

Horned Dinosaurs The Royal Tyrrell Museum Ceratopsian Symposium. Bloomington, 1565


102. 1567

Sampson SD, Loewen MA, Farke AA, Roberts EM, Forster CA, Smith JA, et al. New 1568

horned dinosaurs from Utah provide evidence for intracontinental dinosaur endemism. 1569

PLoS ONE. 2010;5: e12292. doi:10.1371/journal.pone.0012292 1570

103. 1571

Sampson SD, Loewen MA, Roberts EM, Getty MA. A new macrovertebrate assemblage 1572

from the Late Cretaceous (Campanian) of southern Utah. In: Titus AL, Loewen MA, 1573

editors. At the top of the Grand Staircase: the Late Cretaceous of Southern Utah. 1574

Bloomington, Indiana: Indiana University Press; 2013. pp. 599–620. Available: 1575

http://researchonline.jcu.edu.au/31093 1576

104. 1577

Sampson SD, Lund EK, Loewen MA, Farke AA, Clayton KE. A remarkable short-1578

snouted horned dinosaur from the Late Cretaceous (late Campanian) of southern 1579

Laramidia. Proceedings of the Royal Society of London B: Biological Sciences. 1580

2013;280: 20131186. doi:10.1098/rspb.2013.1186 1581

105. 1582

Samson SD, Alexander Jr EC. Calibration of the interlaboratory 40Ar/39Ar dating 1583

standard, MMhb-1. Chemical Geology: Isotope Geoscience section. 1987;66: 27–34. 1584

doi:10.1016/0168-9622(87)90025-X 1585

106. 1586


72

Sankey JT. Faunal composition and significance of high-diversity, mixed bonebeds 1587

containing Agujaceratops mariscalensis and other dinosaurs, Aguja Formation (Upper 1588

Cretaceous), Big Bend, Texas. New Perspectives on Horned Dinosaurs The Royal Tyrrell 1589

Museum Ceratopsian Symposium. 2010; 520–37. 1590

107. 1591

Sarna-Wojcicki AM, Pringle Jr MS. Laser-fusion 40 Ar/39 Ar ages of the Tuff of Taylor 1592

Canyon and Bishop Tuff, E. California-W. Nevada. Eos Trans AGU. 1992;73: 43. 1593

108. 1594

Scannella JB, Fowler DW, Goodwin MB, Horner JR. Evolutionary trends in Triceratops 1595

from the Hell Creek Formation, Montana. Proceedings of the National Academy of 1596

Sciences. 2014;111: 10245–10250. 1597

109. 1598

Schmitz MD, Bowring SA. U-Pb zircon and titanite systematics of the Fish Canyon Tuff: 1599

an assessment of high-precision U-Pb geochronology and its application to young 1600

volcanic rocks. Geochimica et Cosmochimica Acta. 2001;65: 2571–2587. 1601

doi:10.1016/S0016-7037(01)00616-0 1602

110. 1603

Schmitz MD. Radiometric Ages used in GTS2012. In: Gradstein FM, Ogg JG, Schmitz 1604

MD, Ogg G, editors. The Geological Time Scale. Oxford, UK: Elsevier; 2012. pp. 1045–1605

1082. 1606

111. 1607


73

Schoene B, Crowley JL, Condon DJ, Schmitz MD, Bowring SA. Reassessing the 1608

uranium decay constants for geochronology using ID-TIMS U–Pb data. Geochimica et 1609

Cosmochimica Acta. 2006;70: 426–445. doi:10.1016/j.gca.2005.09.007 1610

112. 1611

Schott RK, Evans DC, Williamson TE, Carr TD, Goodwin MB. The anatomy and 1612

systematics of Colepiocephale lambei (Dinosauria: Pachycephalosauridae). Journal of 1613

Vertebrate Paleontology. 2009;29: 771–786. 1614

113. 1615

Simpson GG. Tempo and mode in evolution. New York: Columbia University Press; 1616

1944. pp. 237. 1617

114. 1618

Singer BS, Jicha BR, Coe RS, Mochizuki N. An Earthtime chronology for the 1619

Matuyama-Brunhes geomagnetic field reversal. AGU Fall Meeting Abstracts. 2012;21. 1620

Available: http://adsabs.harvard.edu/abs/2012AGUFM.V21E..06S 1621

115. 1622

Sprain CJ, Renne PR, Wilson GP, Clemens WA. High-resolution chronostratigraphy of 1623

the terrestrial Cretaceous-Paleogene transition and recovery interval in the Hell Creek 1624

region, Montana. Geological Society of America Bulletin. 2015;127: 393–409. 1625

doi:10.1130/B31076.1 1626

116. 1627

Stanton TW, Hatcher JB. Geology and paleontology of the Judith River beds. US 1628

Geological Survey Bulletin. 1905;257: 1–128. 1629

117. 1630


74

Steiger R, Jäger E. Subcommission on geochronology: convention on the use of decay 1631

constants in geo-and cosmochronology. Earth and Planetary Science Letters. 1977;36: 1632

359–362. 1633

118. 1634

Sullivan RM, Lucas SG. The Kirtlandian, a new land-vertebrate “age” for the Late 1635

Cretaceous of western North America. New Mexico Geological Society Guidebook. 1636

2003;54: 369–377. 1637

119. 1638

Sullivan RM, Lucas SG. The Kirtlandian land-vertebrate “age”–faunal composition, 1639

temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of 1640

western North America. New Mexico Museum of Natural History and Science Bulletin. 1641

2006;35: 7–29. 1642

120. 1643

Sullivan RM. Revision of the dinosaur Stegoceras Lambe (Ornithischia, 1644

Pachycephalosauridae). Journal of Vertebrate Paleontology. 2003;23: 181–207. 1645

121. 1646

Villeneuve M. Radiogenic Isotope Geochronology. In: Gradstein FM, Ogg G, editors. A 1647

Geologic Time Scale 2004. Cambridge, UK: Cambridge University Press; 2004. pp. 87–1648

95. 1649

122. 1650

Wagner JR, Lehman TM. An enigmatic new lambeosaurine hadrosaur (Reptilia: 1651

Dinosauria) from the Upper Shale member of the Campanian Aguja Formation of Trans-1652


75

Pecos Texas. Journal of Vertebrate Paleontology. 2009;29: 605–611. 1653

doi:10.1671/039.029.0208 1654

123. 1655

Wick SL, Lehman TM. A new ceratopsian dinosaur from the Javelina Formation 1656

(Maastrichtian) of west Texas and implications for chasmosaurine phylogeny. 1657

Naturwissenschaften. 2013;100: 667–682. doi:10.1007/s00114-013-1063-0 1658

124. 1659

Zanno LE, Varricchio DJ, O’Connor PM, Titus AL, Knell MJ. A new troodontid 1660

theropod, Talos sampsoni gen. et sp. nov., from the Upper Cretaceous Western Interior 1661

Basin of North America. PloS one. 2011;6: e24487. 1662

1663

Supporting Information 1664

1665

S1 Table. Stratigraphic Chart. Stratigraphic correlation of Upper Cretaceous terrestrial 1666

strata of the North American Western Interior from the Santonian through to the K-Pg 1667

boundary. Dinosaur taxon ranges plotted on to correlated geological units. 1668

1669

S2 Table. Recalibration Sheet. This sheet shows recalibration calculations for over 200 1670

published Ar / Ar radiometric dates. These are recalibrated to the two current standards 1671

(Kuiper et al., 2008; Renne et al., 2011), shown on separate tabs. References are given 1672

within pop up notes for the respective recalibrated date(s). 1673

1674


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S1 Text. References for Stratigraphic Chart. This text file lists all the references used 1675

in construction of the stratigraphic chart (Table S1). 1676

1677

S2 Text. Comment Boxes for Stratigraphic Chart. This text file provides transcripts of 1678

all the pop-up comment boxes featured in the stratigraphic chart (Table S1). This file 1679

should be of use to readers who prefer the text in this larger format. 1680

1681

S1 Figure. Stratigraphic Chart, graphic version. This is a image file version of the 1682

stratigraphic chart (S1 Table). It is an image only, provided for quick reference, and does 1683

not have embedded pop-up comments. 1684

1685

S2 Figure. Geographic Location of Stratigraphic Sections featured on Stratigraphic 1686

Chart Table S1. This map shows the geographic location of the different stratigraphic 1687

sections shown in stratigraphic chart Table S1. 1688

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4 Revised geochronology, correlation, and dinosaur ... · 4 Revised geochronology, correlation, and dinosaur stratigraphic 5 ranges of the Santonian-Maastrichtian (Late Cretaceous)