4 Revised geochronology, correlation, and dinosaur ... · 4 Revised geochronology, correlation, and...
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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
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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 Outdated 25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
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40Ar / 39Ar 410
411
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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21
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
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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
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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
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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
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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
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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
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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
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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
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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
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(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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
1689
1690
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