Megafloral change in the early and middle Paleocene in the Williston Basin, North Dakota, USA

Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 224–234

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

Megafloral change in the early and middle Paleocene in the Williston Basin, North Dakota, USA Daniel J. Peppe ⁎ Department of Geology, Baylor University, Waco, TX 76798-7354 USA

a r t i c l e

i n f o

Article history: Received 27 January 2010 Received in revised form 14 September 2010 Accepted 29 September 2010 Available online 8 October 2010 Keywords: Paleocene Megafloral paleobotany Williston Basin Fort Union Formation Biostratigraphy K/Pg boundary Paleoenvironment Climate change Species richness

a b s t r a c t This paper presents a quantitative analysis of megafloral changes in composition and diversity using collections of early and middle Paleocene floras (65.51 to ~ 58 Ma) in the Williston Basin of North Dakota, USA. Based on the floral composition and stratigraphic ranges of taxa, the Williston Basin floral record can be subdivided into three megafloral zones (WBI, WBII, and WBIII), each representing ≥ 1 myr. The floral record of the basin implies that local and regional paleoenvironmental and climatic changes contributed to transitions in the early and middle Paleocene plant communities. The Williston Basin floral record documents a decrease in species richness that mirrors a decrease in mean annual temperatures from the latest Cretaceous to middle Paleocene. These results, combined with previous work from the Hanna and Bighorn Basins, suggest that climate may have played an important role in patterns of floral diversity and plant community composition. Further, these data indicate that it took Paleocene plant communities in the Northern Great Plains millions of years to reach diversity levels common in the Cretaceous. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The North American mid-continental, coal-bearing successions are arguably the best terrestrial Paleocene records in the world (Fig. 1), and are the ideal place to assess the terrestrial ecosystem's response to mass extinction and long term climatic change. The megafloral record of these sediments has been well documented for over 100 years (e.g., Newberry, 1868; Lesquereux, 1878; Knowlton, 1930; Brown, 1962; Hickey, 1977; Nichols and Ott, 1978; Hickey, 1980; Johnson, 1989; Wing et al., 1995; Manchester, 1999; Johnson, 2002; Dunn, 2003). In particular, studies focused on the CretaceousPaleogene (K/Pg) boundary intervals have documented major extinctions in the megafloral and pollen records (e.g., Tschudy et al., 1984; Johnson, 1989; Johnson et al., 1989; Johnson and Hickey, 1990; Hotton, 2002; Nichols and Johnson, 2002; Wilf and Johnson, 2004), found demonstrable changes in species composition and diversity across the boundary (e.g., Wolfe and Upchurch, 1986; Johnson, 2002; Wilf and Johnson, 2004), and suggested a correspondence between climatic and floral change (e.g., Wilf et al., 2003; Wilf and Johnson, 2004). However, the response of Paleocene plant communities to the K/Pg boundary extinctions has not yet been characterized fully.

⁎ Tel.: + 1 2547102629; fax: +1 2547102673. E-mail address: [email protected]. 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.09.027

Assessing patterns of floral change through the Paleocene is difficult because most studies have been focused on floras from restricted geographic areas or time intervals (e.g., Johnson and Ellis, 2002) or were conducted without stratigraphic control or independent age constraints (e.g., Brown, 1962). To date, three studies have examined changes in plant assemblages and floral diversity through most of the Paleocene in the Bighorn and Hanna Basins of Wyoming (Hickey, 1980; Wing et al., 1995; Dunn, 2003). These studies suggest that changes in the Paleocene floral record roughly correspond to transitions in the North American Land Mammal Age (NALMA) boundaries. Two of the studies (Hickey, 1980; Dunn, 2003) documented a decrease in species richness from the early to middle Paleocene and a general cooling trend through the sampled interval. These results led Hickey (1980) to hypothesize that the decrease in species richness was linked to cooling. The third study by Wing et al. (1995) showed a gradual increase in plant diversity through the Paleocene. Contrary to Hickey (1980), Wing et al. (1995) suggested that there was no congruence between plant species richness levels and changes in mean annual temperatures. Though the three aforementioned studies characterized much of the Paleocene floral record, none correlated the plant fossil record to either isotopic age determinations or to the geomagnetic polarity time scale. This lack of precise age control makes it difficult to assess if the patterns of plant community change happened at similar times in both basins. Furthermore, none assessed the Cretaceous floral record, preventing comparisons of Cretaceous and Paleocene floral composition and

D.J. Peppe / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 224–234 S as katchewan

Alberta

225

Manitoba

WB A CC

MT

ND

BM

CM

PRB

CFB

inset "C"

MCA

BB

BH

WY ID

PRB

SD

WRB HB GRB

NE

CB

A UB

DB

KS

CO UT RB

B

-103 30'

BILLINGS

GOLDEN VALLEY DP0732 DP0754 DP0728, DP0538, DP0537, DP0517, DP0518 DP0723 DP0511 DP0713

TX

NM

-103 45'

NORTH DAKOTA

MONTANA

-104 00'

OK

SJB

AZ

DP0746

DP0743

DP0545 DP0724 DP0543

DP0536 DP0535 DP0734 DP0516 DP0733 DP0515

DP0741 DP0730, DP0731, DP0729

DP0527, DP0714, DP0711, DP0503, DP0501

46 30'

DP0524

DP0739

Cannonball Creek

46 30'

DP0716, DP0717, DP0718 DP0505 DP0715, DP0532, DP0521, DP0428

DP0722, DP0721 DP0720

SLOPE

DP0525

Marmarth

DP0553

BOWMAN

46 15'

46 15'

12

C -104 00'

Little Missouri River -103 45'

-103 30'

Fig. 1. Locality map of the study area. B. Major Paleogene basins in the Rocky Mountain Region of North America. WB = Williston Basin, BM = Bull Mountain, CM = Crazy Mountains, CFB = Clark's Fork Basin, BB = Bighorn Basin, PRB = Powder River Basin, WRB = Wind River Basin, HB = Hanna Basin, CB = Carbon Basin, GRB = Green River Basin, UB = Uinta Basin, DB = Denver Basin, RB = Raton Basin, SJB = San Juan Basin, BH = Black Hills, CCA = Cedar Creek Anticline, MCA = Miles City Arch. C. Locality map of floral localities in the Little Missouri River Valley in southwestern North Dakota.

diversity. Finally, the interpretations of the relationship between the patterns and mean annual temperature are somewhat conflicting. Thus, the patterns of floral diversity and their relationship to climate in the early and middle Paleocene are unresolved. Herein, I characterize floral composition and diversity of early and middle Paleocene plant communities from the Williston Basin of

North Dakota. I examine species richness, floral composition, environments of deposition, and mean annual temperatures to assess Paleocene plant communities’ temporal response to the K/Pg extinctions, and to explore patterns of change in floral composition and diversity. Finally, I assess the relationship of floral change to local and regional paleoenvironmental and climatic change.

D.J. Peppe / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 224–234

4. Methodology 4.1. Stratigraphy

59

60

C26r

3. Geological setting

Floral Pollen Zone Zone

P5

WBIII

P4

H-

Paleocene

61

62

C27n

C27r

63

F-

C28n

Ludlow Member

64

65

Three V Tongue

E+

C28r

Boyce Tongue

D-

C29n

"T-Cross" coal

C+

Fort Union C29r Formation Cretaceous

Exposures of the Fort Union Formation in the Williston Basin of North Dakota are ideal for assessing questions of plant community change, floral diversity, and the relationship of plant assemblages to paleoenvironmental changes in the Paleocene. Early and Middle Paleocene strata are known to be fossiliferous and laterally continuous (e.g., Brown, 1962; Shoemaker, 1966; Williams, 1988; Johnson, 1989, 2002; Wilf et al., 2006; Peppe, 2009; Peppe et al., 2009). The Paleocene Fort Union Formation is widely exposed in the Williston Basin extending from southeastern Saskatchewan through western North Dakota to eastern Montana (Fig. 1). In the Little Missouri River Valley of the Williston Basin in North Dakota, the Paleocene Fort Union Formation conformably overlies the Cretaceous Hell Creek Formation. These sediments contain a rich fossil record that has been the focus of many lithostratigraphic, magnetostratigraphic, and paleontological studies relating to the biota before and after the K/Pg boundary extinctions (e.g., Lloyd, 1914; Lloyd and Hares, 1915; Thom and Dobbin, 1924; Hares, 1928; Fastovsky, 1987; Fastovsky and McSweeney, 1987; Johnson, 1989; Johnson et al., 1989; Swisher et al., 1993; Hunter, 1999; Arens and Jahren, 2002; Hicks et al., 2002; Hotton, 2002; Hunter and Archibald, 2002; Johnson, 2002; Kroeger, 2002; Murphy et al., 2002; Nichols, 2002; Pearson et al., 2002; Hunter and Hartman, 2003; Warwick et al., 2004; Peppe et al., 2009). The Ludlow, Tongue River, and Sentinel Butte Members of the Fort Union Formation are almost entirely terrestrial except for two intervals of marine deposition: the Cannonball Member that interfingers with the Ludlow Member and an unnamed marine incursion in the Tongue River Member (Fox and Ross, 1942; Fox and Olsson, 1969; Cvancara, 1972,1976; Kroeger and Hartman, 1997; Belt et al., 2005). Recent geochronologic and paleomagnetic work on the Ludlow and Tongue River Members argues for a ~ 2 myr long unconformity at the formational contact between the two members (Belt et al., 2004; Bowring et al., 2008; Peppe, 2009; Peppe et al., 2009). The chronostratigraphic section resulting from this work

Marker Beds

Tongue River Member

C26n

Formation

Polarity

A series of lithostratigraphic sections using the Hell Creek-Fort Union formational contact as the basal reference datum were measured in 2004, 2005, 2006, and 2007. Using these sections, a ~325 m composite lithostratigraphic section of the upper Hell Creek Formation and the Ludlow and Tongue River Members of the Fort Union Formation was constructed (Peppe et al., 2009). Paleomagnetic analyses of nine stratigraphic sections (Peppe et al., 2009) were coupled with previously published radiometric age determinations (Belt et al., 2004; Warwick

GPTS

The K/Pg boundary, and the latest Cretaceous and earliest Paleocene floras have been studied extensively in the Williston Basin (e.g., Berry, 1934; Brown, 1939,1962; Shoemaker, 1966; Williams, 1988; Johnson, 1989; Johnson et al., 1989; Johnson and Hickey, 1990; Johnson, 1992,1996, 2002; Peppe, 2003; Wilf et al., 2003; Wilf and Johnson, 2004; Smrecak, 2006; Wilf et al., 2006; Peppe et al., 2007). These studies have shown a marked difference between Cretaceous and Paleocene floras. Johnson (1989) systematically documented morphotypes and their stratigraphic ranges, and the facies and stratigraphic position of Cretaceous and Paleocene localities. The stratigraphic ranges of morphotypes were then used to designate three floral zones for the Cretaceous (HCI, HCII, and HCIII) and one floral zone for the lowermost 60 m of the Paleocene (FUI) (Johnson, 1989; Johnson and Hickey, 1990). Work in southeastern Montana found that the early Paleocene floras were similar in taxonomic composition to the floras in North Dakota (Williams, 1988; Wilf et al., 2006), which implied that the FUI biozone could be applied across the Williston Basin. Additional collections, refinement of taxonomy and stratigraphic range of morphotypes, and characterization of the facies at all localities led to a modification of the floral biozones and the designation of an additional Cretaceous floral zone (FU0) (Johnson, 2002). Wilf and Johnson (2004) documented a 57% species extinction at the K/Pg boundary, suggesting a significant decrease in species richness from the Cretaceous to the Paleocene.

suggests that the Ludlow and Tongue River rocks were deposited from ~ 65.5 to ~ 58.5 Ma (Fig. 2).

Ma

2. Previous work

Epoch

226

66

P3

P2 WBI

P1

B-

Hell Creek C30n Formation

WBII

A+

Hell Creek Zones

Wodehouseia spinata zone

Fig. 2. Chronostratigraphy of the studied Paleocene sections in the Williston and Powder River Basins. GPTS = Geomagnetic polarity time scale from Ogg and Smith (2004). Grey sections indicate the unconformity in the stratigraphic and polarity sections. Formation = Stratigraphic formation and member. Stars indicate the stratigraphic position of the two ash beds that were isotopically dated. The lower star is dated to 64.1± 1.8 Ma (Warwick, et al., 2004), and the upper star is dated to 61.06 ± 0.33 Ma (Belt, et al., 2004). Marker beds= key stratigraphic beds in the Fort Union Formation in the Williston Basin. Polarity stratigraphy for the Williston Basin is after Peppe et al. (2009). In the Williston Basin, the polarity stratigraphy can be related to the following chrons of the GPTS: A+= C30n, B-= C29r, C+= C29n, D-= C28r, E+= C28n, F-= C27r, H- = C26r. The top of C27r, all of C27n, and the base of C26r not present in the Little Missouri Section. Floral zone = megafloral biostratigraphic zones as defined in this report. Pollen zone= palynostratigraphic zonation defined by Nichols and Ott (1978) and refined by Nichols (2003). The stratigraphic placement of the pollen zone boundaries are based on descriptions in Nichols (2003), Belt et al. (2004), and Warwick et al. (2004).

D.J. Peppe / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 224–234

et al., 2004) to develop a chronostratigraphic section for the Ludlow and Tongue River Members of the Fort Union Formation (Fig. 2). Each fossil locality was directly related to the chronostratigraphic section, either by placing the locality within a measured section or by physically tracing the fossiliferous bed or key marker beds to the closest stratigraphic section. Thus, it was possible to determine the stratigraphic position of each locality relative to the Hell Creek-Fort Union contact in the Williston Basin (Table 1). The localities were related to the formational contact with a precision of a few centimeters to a few

227

meters. Using the stratigraphic position, it was possible to estimate the age of each fossil locality (Table 1). 4.2. Paleobotanical sampling and analysis Fossil leaf deposits are relatively common in the Fort Union Formation and typically collected from fine-grained sandstone to siltstone to mudstone deposits, which represent channel, overbank, and paludal facies (Belt et al., 1984). Plant fossils were collected by

Table 1 Stratigraphic position, age, stratigraphic member, biostratigraphic zone, lithology, facies interpretation, stratigraphic section, and location of the Williston Basin floral localities. Lithology: ss = sandstone, zs = siltstone, ms = mudstone, zss = silty sandstone, vf = very fine, f = fine, med = medium. Facies interpretations are after Belt et al. (1984). Facies: pond sh = shelly pond, pond vb = varigated pond, pond ls = limestone pond. Formation contacts and geomagnetic polarity chron boundaries indicated in bold. Age estimate for chron boundaries from Ogg and Smith (2004). All age estimates are based on sedimentation rates calculated in Peppe et al. (2009). Locality

Stratigraphic position Age above Hell Creek-Fort (Ma) Union Formation Contact

Member

Megafloral Lithology Zone

Facies Stratigraphic Location (Latitude, Interpretation section Longitude, NAD27)

DP0746 DP0743 DP0732 DP0725 DP0545 DP0536 DP0741 DP0735/DP0534 DP0543 DP0724/DP0541 DP0754/DP0551 Base of Tongue River Member Duration of unconformity Top of Ludlow Member DP0516 DP0729 DP0730 C28n-C27r boundary DP0731 DP0733/DP0535 DP0728/DP0539 DP0538 DP0723/DP0513 DP0734 DP0537 DP0518 DP0515 DP0527/DP0712 DP0713 DP0714 DP0503 DP0501 DP0711/DP0526 C28r-C28n boundary DP0413 C29n-C28r boundary DP0511 DP0739 DP0718 DP0505 DP0517 DP0716/DP0504 DP0722 DP0721 C29r-C28n boundary DP0532 DP0521 DP0715/DP0520 DP0720 DP0428 DP0553 WI-04/DP0525 DP0717/DP0425 DP0524 Hell Creek-Fort Union contact

325 314 241 230 227 225 225 224 223 220 210 206.0

58.40 58.61 60.13 60.36 60.42 60.46 60.46 60.48 60.50 60.56 60.77 60.79

Tongue River Tongue River Tongue River Tongue River Tongue River Tongue River Tongue River Tongue River Tongue River Tongue River Tongue River -

WBIII WBIII WBIII WBIII WBIII WBIII WBIII WBIII WBIII WBIII WBIII -

vf ss zs vfss/ms with shells ms ss zms ms/zs zms ms ms calcareous ss -

channel 2 splay pond sh splay channel 1 splay splay splay splay splay pond ls -

DP0742 DP0742 DP0552 DP0542 DP0744 DP0519 DP0742 DP0519 DP0542 DP0542 DP0544 -

46.61818, 46.61605, 46.64151, 46.59237, 46.63532, 46.57832, 46.61337, 46.57621, 46.59244, 46.59191, 46.63979, -

206.0 198.0 198.0 197.0 196.0 196.0 185.0 181.0 180.0 180.0 180.0 178.0 175.0 165.0 130.0 130.0 130.0 128.0 125.0 125.0 124.0 80.0 74.0 68.0 42.0 38.0 33.0 30.0 29.5 27.0 26.0 24.0 12.0 6.0 5.2 5.0 4.0 3.0 2.5 2.0 2.0 0.0

2.20 myr 62.99 63.09 Ludlow 63.09 Ludlow 63.09 Ludlow 63.11 Ludlow 63.11 Ludlow 63.26 Ludlow 63.31 Ludlow 63.33 Ludlow 63.33 Ludlow 63.33 Ludlow 63.36 Ludlow 63.40 Ludlow 63.54 Ludlow 64.04 Ludlow 64.04 Ludlow 64.04 Ludlow 64.07 Ludlow 64.11 Ludlow 64.11 Ludlow 64.13 Ludlow 64.41 Ludlow 64.44 Ludlow 64.52 Ludlow 64.87 Ludlow 64.93 Ludlow 64.99 Ludlow 65.04 Ludlow 65.04 Ludlow 65.08 Ludlow 65.09 Ludlow 65.12 Ludlow 65.30 Ludlow 65.40 Ludlow 65.42 Ludlow 65.42 Ludlow 65.43 Ludlow 65.45 Ludlow 65.46 Ludlow 65.47 Ludlow 65.47 Ludlow 65.51 -

WBII WBII WBII WBII WBII WBII WBII WBII WBII WBII WBII WBII WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI WBI -

ss zs zs/ss zs/ss zs/carb shale zs zs zs/ms ss concretion zs zs ms/zs/ss zs with shells zs with shells zs with shells zs with shells zs zs zss, bioturbated zs zs med ss variegated zs variegated zs variegated zs med ss med ss variegated zs variegated zs variegated zs ms ss zss zs zs ss -

channel 1 splay splay splay swamp splay splay channel 3 channel 1 splay splay splay pond sh pond sh pond sh pond sh splay splay pond sh pond vb splay channel 1 pond vb pond vb pond vb channel 1 channel 1 pond vb pond vb pond vb splay channel 1 splay splay splay channel 1 -

DP0514 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a DP0514 DP0430 DP0430 DP0430 DP0430 DP0430 DP0430 DP0413 DP0506 DP0431 n/a n/a n/a n/a DP0401 DP0401 n/a n/a n/a DP0401 DP0515 n/a n/a DP0517 n/a -

46.54729, 46.62434, 46.62340, 46.62325, 46.56491, 46.58031, 46.58038, 46.55449, 46.56812, 46.58012, 46.57910, 46.54218, 46.51270, 46.51283, 46.51515, 46.51343, 46.51316, 46.51270, 46.46048, 46.51651, 46.47781, 46.46289, 46.46420, 46.57902, 46.46410, 46.43603, 46.43599, 46.45370, 46.45262, 46.45256, 46.43296, 46.45946, 46.31821, 46.34564, 46.46107, 46.46230, -

-103.53568 -103.53537 -103.70775 -103,73209 -103.70756 -103.75305 -103.52045 -103.75339 -103.73212 -103.73199 -103.1087

-103.79961 -103.61594 -103.61756 -103.61780 -103.76246 -103.78089 -103.78224 -103.78954 -103.75820 -103.78307 -103.78197 -103.79922 -103.85899 -103.85918 -103.86102 -103.85969 -103.85932 -103.85899 -103.85151 -103.85419 -103.95673 -103.97012 -103.96900 -103.77997 -103.96904 -103.92752 -103.92747 -103.99152 -103.99316 -103.99298 -103.92538 -103.98376 -103.85232 -103.86914 -103.96812 -103.97662

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removing large blocks of matrix split along bedding planes to expose leaf bearing horizons (sensu Johnson, 1989, 2002). Typical fossil leaf quarries covered a few meters of surface area because fossiliferous layers tended to be relatively thin (~1 - 25 cm). At all quarries, sedimentological features were noted, and the sites were each assigned a sedimentary facies category sensu Belt et al. (1984) and Johnson (1989) (Table 1). Fossil plant specimens were collected quantitatively (census collections) and selectively (voucher collections). During census collections all specimens found in the field were tallied, and some voucher specimens were collected. Because taphonomic studies of modern forest leaf litter have demonstrated≥ 200 specimens are needed to accurately reflect forest composition (e.g., Burnham, 1989; Johnson, 1989; Burnham et al., 1992, 2001), at least 200 identifiable specimens were tallied during the census collections. Due to the variable nature of preservation, many localities yielded less than 200 specimens. In these cases only selective, voucher collections were made. Plant fossil were collected from forty-four localities from the Ludlow and Tongue River Members in the Little Missouri River Badlands (Fig. 1, Table 1). Localities were assigned a field locality number using a twodigit annual number and a sequential site number (e.g., DP0725 is the twenty-fifth locality collected in 2007). In some cases, localities were collected in more than one year, and the localities were assigned a new field number. If a locality has more than one field number, specimens were labeled with both field numbers. All collected specimens are curated at the Yale Peabody Museum. In all cases, the field locality number(s) remains the only locality number(s) assigned to the specimens. In the lab, all identifiable voucher specimens were assigned to a morphotype (Supplementary Table 1). Morphotypes are morphologically distinct entities within a flora with no formal taxonomic status that often represent a biological species (Johnson, 1989, 2002; see Peppe et al., 2008 for a detailed discussion of the system and its development). If possible, the morphotypes were assigned to a Linnaean taxon. Systematic floral lists of the identified taxa are presented in Supplementary Table 1, and systematic floral descriptions and illustrations of the morphotypes are in Peppe (2009). The voucher specimens from each locality were tallied, and these tallied specimens are referred to as the lab census. For selectively collected localities, the lab census represents the total number of specimens at the locality. For quantitatively collected localities, the total number of specimens at the locality is the lab census tally plus the field census tally. In this study, the primary data are field census tallies, lab census tallies, and the museum voucher specimens. The biostratigraphic and rarefaction analyses were conducted using the census data from 17 localities. To combine the lab and field census data and to determine standing richness and the minimum abundance of morphotypes in each bin, all of specimens were placed in 5 m stratigraphic bins, which represent b 100 kyr. Although binning inherently causes some temporal mixing of floras, most bins were dominated by a single locality or by more than one locality from the same stratigraphic level, so any temporal mixing is probably minimal. The voucher and census data were used to construct a presence-absence matrix and stratigraphic range charts of each morphotype and to determine species richness (also referred to as diversity). Analyses were conducted using the statistical program PAST (Hammer et al., 2001). Leaf margin analysis, using the linear regression model in Miller et al. (2006), was used to estimate mean annual temperatures of all localities with more than 10 morphotypes (Supplementary Table 2). Mean annual temperatures were also calculated for each megafloral zone and each geomagnetic polarity chron using the taxa from the Williston Basin that occurred within that zone or chron (Supplementary Table 2). Additionally, mean annual temperatures were calculated for Hell Creek and Fort Union Formation floras collected from southwestern North Dakota using data from Wilf et al. (2003) and Wilf and Johnson (2004).

5. Results 5.1. Megafloral Records In the Williston Basin, 44 localities yielded 6961 specimens, from which 77 morphotypes were identified (Supplementary Table 1). The distribution of species in the floras is as follows: 79% dicotyledonous angiosperm (dicot) leaf species, 3.9% dicot reproductive structures, 5.2% monocotyledonous angiosperms, 5.2% conifer foliage and reproductive structures, 5.2% ferns and fern allies, and 1.3% algae (Table 2). 5.2. Stratigraphic ranges of morphotypes Forty seven of the morphotypes were found at more than one stratigraphic level and are plotted on a stratigraphic range chart in Fig. 3A. Many of the collected taxa have long stratigraphic ranges, with some persisting from near the K/Pg boundary to the top of the section (Fig. 3A). No collections were made from the Hell Creek Formation, and the large number of first occurrences at the base of the section is due in part to this collection bias. Additionally, the step-wise pattern of last occurrences in the Tongue River Member is likely because of the limited number of localities above 250 m (Table 1). Despite these minor gaps, the pattern of first and last occurrences suggests that the megafloral record can be sub-divided into three floral zones (Fig. 3). 5.3. Cluster Analysis Cluster analysis using the census data from the 17 Williston Basin census localities was conducted to assess whether the first and last occurrence patterns were statistically significant. The cluster analysis of census localities using the Raup-Crick index (Raup and Crick, 1979) shows three major groups (Fig. 3B). These groups reflect the age and stratigraphic position of the localities. The Tongue River Member localities form one distinct cluster, and the localities from the Ludlow Member form two clusters. The Ludlow Member clusters are separated by stratigraphic position (e.g. the bottom representing localities from 0 – 130 m and the middle cluster representing localities from 165 – 205 m). 5.4. Facies Analysis To examine the importance of depositional environment on floral composition, I compared the facies of each floral locality. I used the lithology and sedimentary features of each locality to interpret its facies (Table 1). Within the stratigraphic sequence, floral localities were found in four different facies: (1) channels (channel 1, 2, 3), (2) fresh water ponds (pond ls, pond sh, pond vb), (3) swamps (carb shale), and (4) levees or crevasse splays (splay) (e.g., Belt et al., 1984; Johnson, 1989). Table 2 Morphotype sampling by taxonomic category. See Supplementary Table 1 and (Peppe (2009) for detailed taxonomic information. Higher taxon or organ

Morphotypes

Algae 1 Fern and fern allies 4 Conifers 4 Monocotyledonous 4 angiosperms Dicotyledonous angopsperms Leaves 61 Reproductive 3 structures Total 77

Specimens

% Morphotypes

% Specimens

1 137 1119 31

1.3 5.2 5.2 5.2

b 0.1 2.0 16.1 0.4

5458 215

79.2 3.9

78.4 3.1

6961

D.J. Peppe / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 224–234

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environment of deposition on the morphotype composition of a locality. Generally, the DCA shows a similar pattern to the cluster analysis, and the census localities cluster loosely by age and stratigraphic position rather than by facies (Fig. 4B). Localities from the Tongue River Member (210 – 325 m) cluster near the upper right hand corner of the scatter plot, localities from the Ludlow at stratigraphic positions from 0 – 130 m plot near the lower left hand corner in the scatter plot, and the localities from the Ludlow in stratigraphic positions from 165 – 205 m plot in the middle of the scatter plot (Fig. 4B). 5.5. Megafloral biostratigraphy

Fig. 3. A. Stratigraphic range chart for all megafloral taxa that occur at more than one locality (n = 47) based on first occurrence. The Hell Creek-Fort Union contact is at 0 m and the Ludlow-Tongue River contact is at 206 m, which is indicated on the right side of the figure. Dashed lines indicate stratigraphic position of megafloral zone boundaries. WBI = Williston Basin floral zone I, WBII = Williston Basin floral zone II, WBIII = Williston Basin floral zone III, Mbr. = stratigraphic member of the Fort Union Formation. B. Dendograms of Williston Basin census floral localities showing three major clusters of localities. Dendograms were generated from cluster analysis using the Raup-Crick index (Raup and Crick, 1979). Localities cluster together based on the average distance between all members in the two groups. The localities within each major cluster are from restricted stratigraphic intervals. The stratigraphic position and age of each locality in the dendogram are in Table 1. Localities from 0-130 m are in the bottom cluster (WBI). Localities from 165-205 m are in the middle cluster (WBII). Localities from 210-325 m are in the upper cluster (WBIII).

There is a distinct change from the dominance of pond localities (50%) in the first 130 m (cluster I, Fig. 3B) of the stratigraphic section to dominance of splay localities from 165 – 205 m (cluster II, Fig. 3B) (67%) and from 206 – 325 m (cluster III, Fig. 3B) (64%) (Fig. 6A, Table 1). There are no pond localities from 165 – 205 m (cluster II, Fig. 3B). Although, 18% of the localities between 206 and 325 m (cluster III, Fig. 3B) were found in pond facies, there are no variegated bed (pond vb) localities in that stratigraphic interval (Fig. 4A, Table 1). Detrended correspondence analysis (DCA) was conducted using the Williston Basin census data to examine the influence of

The cluster analysis, the DCA, and the stratigraphic ranges of the morphotypes (Figs. 3, 4), suggest that the Williston Basin Paleocene megafloral record can be divided into three stratigraphically restricted zones. Each zone has a distinct floral assemblage that can be recognized by common, and in many cases dominant, taxa. These three zones are herein defined as megafloral biozones Williston Basin I (WBI), Williston Basin II (WBII), and Williston Basin III (WBIII). WBI and WBII occur within the Ludlow Member and WBIII occurs within the Tongue River Member (Fig. 2, Table 1). WBI occurs from 0 – 130 m (~65.51 – 64 Ma). This zone can be recognized by the presence of Paranymphaea crassifolia (LM1) (15.3% of the total flora in that zone), “Cornus” nebrascensis (LM39) (4.2%), “Populus” nebrascensis (LM5) (18.3%), Quereuxia angulata (LM3) (1.4%), Nyssidium eckmanii (LM101) (6.7%), and LM29 (1.5%) (Supplementary Fig. 1). The WBI floral zone is similar to the Fort Union I (FUI) floral zone defined by Johnson (1989) and Johnson and Hickey (1990) from collections in southwestern North Dakota. Research by Williams (1988) and Wilf et al. (2006) on early Paleocene strata in southeastern Montana expanded the geographic range of the FUI floral zone to the southwestern-most part of the Williston Basin in Montana (Fig. 1). The majority of the floral collections in these studies were restricted to the lower 60 m of the Paleocene, and the full temporal range of the zone was not determined. Additionally, some of the taxa Johnson (1989) and Johnson and Hickey (1990) used to characterize FUI, such as Platanus raynoldsii, Browniea serrata, and Zizyphoides flabella, persist through the entire early and middle Paleocene sequence and are therefore not biostratigraphically informative taxa. WBII occurs from 165 – 205 m (~64 – 63 Ma). This floral zone can be recognized by the presence of LM12 (10.8%), “Populus” cordata (LM106) (1.5%), Averhoites affinus (LM17) (0.75%), LM38 (4.3%), Fagopsiphyllum groenlandica (LM15) (1.5%), and “Planera” crenata (LM13) (0.50%) (Supplementary Fig. 2). WBIII occurs from 206 – 325 m (~ 61 – 58.5 Ma). The full temporal range of this floral zone was not determined. This zone can be recognized by the presence of Nyssidium arcticum (LM65) (0.55%), “Populus” acerifolia (LM31) (3.1%), LM113 (2.2%), Aesculus hickeyi (LM103) (2.6%), “Platanus” nobilis (LM124) (4.5%), and Davidia antiqua (LM25) (9.6%) (Supplementary Fig. 3). 5.6. Diversity analyses All analyses indicate that species richness is relatively low through the early and middle Paleocene in the Williston Basin (Fig. 5), and that there was a decrease in diversity into the middle Paleocene. The relative abundance of morphotypes collected per locality was very similar through the stratigraphic section, and at census localities the number of morphotypes ranged from a minimum of seven to a maximum of twenty (Fig. 5 C). Rarified richness of census localities through the stratigraphic section also did not vary considerably. The average estimated species richness of all census localities is 12.11 ± 1.32 morphotypes (down sampled to 300 specimens per locality,

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A Channel 1 Channel 2

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B DP 0724 LM25 LM31 LM53 LM 76

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5

A xis 1 Fig. 4. A. Histogram showing the number of megafloral localities from different facies in the three Williston Basin megafloral zones. B. Detrended correspondence analysis (DCA) scatter-plot of Williston Basin floral localities and morphotypes. Morphotypes are indicated by open circles. In general, the localities do not cluster by facies and more closely clustered by age (e.g. WBI localities plot in the lower left corner of the scatter-plot, WBII localities plot in the middle, and WBIII localities plot in the upper right corner). A close relationship between localities and the morphotype composition is also suggested by the patterns.

n = 15; 95% confidence interval [CI], for this and all other rarefaction estimates) (Fig. 6A). Standing richness, which is the total number of morphotypes in a 5 m bin plus those morphotypes that occurred above and below the bin level (e.g. Johnson, 2002; Wilf and Johnson, 2004), is similar through zones WBI and WBII, then declines at the beginning of WBIII (Fig. 5E). Because there were fewer sites and fewer specimens collected near the top of the sampled interval (Table 1), the full heterogeneity of the flora may not have been sampled, artificially causing the decrease in standing richness (Fig. 5E). This seems unlikely for two reasons: first, there were a similar number of specimens tallied through the stratigraphic section from zone WBI through the

beginning of zone WBIII (Fig. 5B – D); and second, several localities were quantitatively and selectively collected from the beginning of zone WBIII. Both of these lines of evidence suggest that at least the first ~660 kyr of the WBIII zone was as well sampled as both of the two preceding zones. However, it is important to note that the pronounced drop seen at the top of the standing richness curve (beginning at ~60.1 Ma, Fig. 5D) most likely reflects the gap in sampling of ~ 70 m near the top of the stratigraphic section (Fig. 5B – C, Table 1). Thus, the drop is probably not reflective of the true middle Paleocene standing richness in the Williston Basin. Similar to the standing richness data, the species richness of zones WBI and WBII are similar, and then drop markedly in zone WBIII. WBI

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Fig. 5. Estimated mean annual temperatures (A) and megafloral richness (B-D). Age estimates are based on sedimentation rates in Peppe et al. (2009) using the geomagnetic polarity time scale of Ogg and Smith (2004) (Table 1). Dashed lines indicate stratigraphic position of megafloral zone boundaries. White area from ~ 63 ~ 61 Ma represent depositional hiatus in the Williston Basin at the Ludlow-Tongue River formational contact. WBI = Williston Basin megafloral zone I; WBII = Williston Basin megafloral zone II; WBIII = Williston Basin megafloral zone III. A. Estimate mean annual temperatures from leaf margin analysis (Miller, et al., 2006). Grey area around estimate indicates 1σ error for each temperature estimate. Mean annual temperature data are in Supplementary Table 2. B. Number of specimens per 5 m bin level. C. Raw richness, which is equal to the total number of morphotypes per locality, is plotted for each collected floral locality. D. Rarified number of dicot specimens at 300 specimens for each census locality. E. Standing richness based on the number of morphotypes in each 5 m bin.

is comprised of 43 morphotypes, WBII of 42, and WBIII of 28. Thus, the total species richness decreases by ~ 35% from zones WBI and WBII to zone WBIII. Rarefaction analyses of the three zones corroborate this result. At 1000 specimens, the estimated species richness 33.9 ± 2.03 and 35.8 ± 1.91 morphotypes for WBI and WBII, respectively, while it is only 26.4 ± 0.99 morphotypes for WBIII (Fig. 6B). Margalef's Index, which provides a measure of species richness roughly normalized by sample size (Magurran, 1988), is very similar in zones WBI and WBII (5.243, and 5.333, respectively), but drops considerably in zone WBIII (3.588). Using floral data from the Cretaceous Hell Creek Formation (Wilf and Johnson, 2004), I compared the species richness of Hell Creek and Fort Union megafloral records. The results of these analyses are consistent with the hypothesis of Wilf and Johnson (2004) that site based species richness is significantly higher in the Cretaceous than in the Paleocene (Fig. 6A). Only the most diverse Paleocene site (DP0513 at 180 m, ~63.3 Ma) has a similar level of diversity to the least diverse Cretaceous site (87100). Although the CIs of the sites overlap, the trajectory of the rarefaction curve for the Cretaceous site (87100), suggest that the locality has not been fully collected and is likely considerably more diverse than the Paleocene locality (DP0513). A comparison of rarefaction analyses of zone-level species richness between the Cretaceous and Paleocene floral zones are striking. The Cretaceous floral zones are significantly more diverse than any of the Paleocene zones (Fig. 6B). At 1000 specimens, the estimated richness of Cretaceous megafloral zone HCI is 71.23 ± 3.68 morphotypes, zone HCII is: 88.2 ± 4.55, and zone HCIII is 80.8 ± 4.35 morphotypes. This means that the least diverse Cretaceous floral zone (HCI) is almost twice as diverse as the most diverse Paleocene floral zone (WBII). Notably, the middle Paleocene zone WBIII has a significantly lower species richness than all of the other Cretaceous or Paleocene floral zones. 5.7. Climate Change All mean annual temperature (MAT) estimates for the Paleocene are relatively consistent, with a slight decrease in MAT into the middle Paleocene (Fig. 5A, Supplementary Table 2). For example, mean annual

temperatures estimates from floral zones decrease from 10.24 ± 2.97 °C (1σ error for this and all other temperature estimates) in zone WBI to 9.26 ± 2.66 °C in zone WBII to 8.18 ± 2.66 °C in zone WBIII. The temperature estimates for the Paleocene floras are significantly cooler than those in the latest Cretaceous (average mean annual temperature for the last 1 myr of the Cretaceous was 13.90 ± 3.05 °C, Wilf and Johnson, 2004) (Fig. 5A). 6. Discussion 6.1. Floral, environmental, and climate change The series of floral changes within the early and middle Paleocene in the Williston Basin appears to be related to changes in paleoenvironment. The WBI-WBII transition is coincident with a considerable shift in depositional environments (Fig. 4A, Table 1) and the regression of the Cannonball Seaway (Three V Tongue in Fig. 2). The regression of the Cannonball Seaway undoubtedly affected local depositional environments. Thus, it is probable that the regression's influence on the local paleoenvironment was a major contributor to the WBI-WBII floral change. Interestingly, floral diversity is very similar between the WBI and WBII floral zones. This indicates that whatever caused the marked change in the floral composition between the WBI and WBII floras at ~64 Ma did not also affect floral diversity in the Williston Basin. The WBII-WBIII boundary is at the Ludlow-Tongue River contact. There is a distinct sedimentological change between the Ludlow and Tongue River Members, suggesting different depositional environments. In the Tongue River Member there are fewer medium and coarse grained sandstone deposits, the sediment is consistently much finer grained, and there are thicker and more laterally continuous lignite deposits than in the Ludlow Member. This sedimentological change is also partially reflected by a change in the proportional abundance of facies of localities in WBII and WBIII (Fig. 4A, Table 1). Furthermore, MAT estimates of the floral zones, chrons, and floral localities are lower in the WBIII floral zone than in the two preceding zones (Fig. 5A, Supplementary Table 2). The coincidence of the floral zone boundary and the formational contacts, and the general cooling trend from the early to

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9322

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86105

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DP0536

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late Torrejonian and Tiffanian). It is possible that changes in the composition of plant communities in the Bighorn, Hanna, and Williston Basins occurred coincidentally. However, due to the relatively limited age control in the Hanna Basin, the patchy early Paleocene fossil record in the Bighorn Basin, and the lack of good mammalian biostratigraphy in the Williston Basin, it is difficult to determine if the changes were contemporaneous. Future work on the geochronology of the Hanna and Bighorn Basin floras and on the mammalian stratigraphy in the Williston Basin would allow the basins to be related to one another chronostratigraphically. This would make it possible to test if the floral changes occurred at the same time and were regionally significant.

5000

7000

Number of Fossil Specimens Fig. 6. Rarefaction curves for Cretaceous and Paleocene localities in the Williston Basin. A. Rarefaction curves for census data for quarries with at least 300 specimens. Comparison of Paleocene Fort Union census localities (shaded) with Cretaceous Hell Creek census localities (white) demonstrating that Cretaceous localities are significantly more diverse than all Paleocene localities. These curves for the Paleocene localities also indicate that all Paleocene localities through the sequence have similar levels of species richness. White or shaded area indicates 95% confidence interval around rarefaction curve for Cretaceous and Paleocene localities, respectively. All Hell Creek data from Wilf and Johnson (2004). B. Bulk rarefaction of all Paleocene sites in each Williston Basin zone and all Cretaceous sites in each Hell Creek zone (Johnson, 1989; Johnson and Hickey, 1990; Johnson, 2002). These curves show that WBI and WBII have similar levels of species richness and WBIII has significantly lower species richness levels. They also show that the Cretaceous zones are significantly more diverse than the Paleocene floral zones. White or shaded area indicates 95% confidence interval around rarefaction curve for Cretaceous and Paleocene floral zones, respectively. All Hell Creek data from Wilf and Johnson (2004).

middle Paleocene suggests that changes in the environment of deposition and changes in temperature were important factors in the WBII-WBIII transition in the Williston Basin. However, due to the ~2 myr disconformity at the contact, it is difficult to discern the abruptness of these changes in environments of deposition and climate and how the changes affected plant communities. In the Bighorn Basin, Hickey (1980) recognized three floral biostratigraphic zones, which could be related to the Puercan, Torrejonian, and Tiffanian NALMAs. Dunn (2003) noted that it was possible to recognize early Paleocene floras (related to the Puercan and early Torrejonian) from middle Paleocene floras (related to the

Rarefaction analyses indicate that Cretaceous floras were significantly more diverse that Paleocene floras (Fig. 6) supporting the hypothesis that the K/Pg extinctions had a major effect on the species richness of early Paleocene plant communities in the Williston Basin (Wilf and Johnson, 2004). The raw richness data and rarefaction analyses of the census data from the Paleocene Williston Basin floras documents relatively constant site-level species richness through the Paleocene sequence in the Williston Basin (Figs. 5 and 6). Interestingly, standing richness declines from the early to the middle Paleocene, suggesting that the total species richness may have decreased from the early to middle Paleocene (Fig. 5E). Comparisons of species richness between the Cretaceous and Paleocene floral zones demonstrate that all of the Cretaceous zones had significantly higher levels of species richness than any of the Paleocene zones (Fig. 6B). Furthermore, these results, coupled with all diversity analyses of the Paleocene floral zones, indicate that the species richness of the floral zone WBIII, which occurred from ~ 61 – 58.5 Ma, was significantly lower than any other time in the latest Cretaceous ( ~ 66.5 – 65.51 Ma) or the early Paleocene (65.51 ~ 63 Ma) in the Williston Basin (Fig. 6B). All of these analyses suggest that in the Williston Basin Paleocene floral diversity remained significantly lower than what was typical for the latest Cretaceous for at least seven million years after the K/Pg boundary. This pattern of decreasing species richness from the early to middle Paleocene has also been noted in the Bighorn and Hanna Basins (Hickey, 1980; Dunn, 2003), indicating that there may have been a decrease in the species richness of plant communities from the early to middle Paleocene across the Northern Great Plains. The trend of decreasing diversity in the Williston, Bighorn, and Hanna Basin is mirrored by a modest cooling trend through the Paleocene sequence (Hickey, 1980; Dunn, 2003; this study). This coincidence of declining diversity and a cooling climate suggests that decreasing temperatures from the early to middle Paleocene may have affected both floral composition and floral diversity in the Northern Great Plains, consistent with the hypothesis of Hickey (1980, p. 46) that the midPaleocene “nadir in diversity was caused by climate deterioration.” Furthermore, it suggests that climate may have played a role in preventing Paleocene plant communities in the Northern Great Plains from reaching diversity levels that were common in the latest Cretaceous (e.g., Wilf and Johnson, 2004). Interestingly, this pattern of decreasing floral diversity in the Northern Great Plains is dramatically different from the patterns in parts of the more southern Denver Basin (e.g., Johnson and Ellis, 2002; Ellis et al., 2003; Johnson et al., 2003), where there is evidence for a very diverse tropical rainforest within the first few million years of the Paleocene. This indicates that plant communities exhibited considerably varied responses to the K/Pg extinctions across the Western Interior of North America, perhaps related the differences in local climates or paleoenvironments. This suggests that local factors, such as paleogeography, paleoenvironment, and/or paleoclimate, may be important factors in plant communities’ responses to mass extinctions.

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7. Conclusions The early and middle Paleocene floral record in the Williston Basin can be subdivided into three distinct megafloral zones (WBI, WBII, and WBIII), which each represent ≥ 1 myr. Analyses of the floral changes in the Williston Basin suggest that early and middle Paleocene plant communities were affected by both local and regional paleoenvironmental and climatic conditions. The Williston Basin floral records document a general trend of decreasing diversity that mirrors a slight decrease in MAT from the early to middle Paleocene. This record is very similar to ones in the Bighorn and Hanna Basins (Hickey, 1980; Dunn, 2003), indicating that the decrease in floral diversity is probably regionally important. Thus, it is possible that the changing climate played an important role in the composition and diversity of Paleocene plant communities in the northern Great Plains of North America at a local and a regional scale. Furthermore, the results of this study, coupled with the work of Hickey (1980) and Dunn (2003), strongly suggest that it took several million years for Paleocene floral diversity to reach levels common in the latest Cretaceous (e.g., Wilf and Johnson, 2004) in the Northern Great Plains. These data are all consistent with the hypothesis that floral diversity levels in the Northern Great Plains did not return to levels common in the latest Cretaceous until temperatures increased in the latest Paleocene and Early Eocene (e.g., Hickey, 1980; Wing et al., 1995; Wilf, 2000). It is also possible that temperature changes in the Paleocene may simply be coincident with changes in floral diversity seen in the Bighorn, Hanna, and Williston Basins. Plant communities in some parts of North America may have needed millions of years to recover following the K/Pg mass extinction (i.e., floras of the Northern Great Plains), while others were able to quickly rebound and diversify (i.e., the Denver Basin floras), suggesting that difference in local paleoenvironments may have had an important affect on plant diversity. Future research should examine these possibilities. Supplementary materials related to this article can be found online at doi:10.1016/j.palaeo.2010.09.027. Acknowledgements This work was supported by the David and Lucile Packard Foundation, the Geological Society of America, the Evolving Earth Foundation, Yale Institute for Biospheric Studies, Sigma Xi, the Colorado Scientific Society, The Paleontological Society, AAPG Grants-in-Aid, the Explorers Club Exploration Fund, the Yale University Department of Geology and Geophysics, and the Yale Peabody Museum Division of Paleobotany. Thanks to Antoine Bercovici, Bradley Broadhead, Merle Clark, Nick Cuba, David Evans, Ashley Rose Gould, Georgia Knaus, Darren Larson, Steve Manchester, Dean Pearson, Terry and Blaine Schaeffer, Jesse Self, Amy Shapiro, Karew Shumaker, and Don and Kathy Wilkening for assistance in the field. Special thanks are given to Leo Hickey for help in the field, invaluable assistance during the morphotyping process, comments on earlier version of this manuscript, and for many useful discussions, and to Kirk Johnson for help in the field and in the lab, useful comments on this manuscript, and for discussions about this project. Thanks to Scott Wing and an anonymous reviewer for comments that greatly improved this manuscript. I would like to acknowledge the Brown, Clark, Davis, Hanson, Krutzfeld, Van Daele, Walser, and Weinreiss families, the Horse Creek Grazing Association, and the United States Forest Service for land access. References Arens, N.C., Jahren, A.H., 2002. Chemostratigraphic correlation of four fossil-bearing sections in southwestern North Dakota. In: Hartman, J.H., Johnson, K.R., Nichols, D.J. (Eds.), The Hell Creek Formation and the Cretaceous-Tertiary boundary in the Northern Great Plains: An integrated continental record of the end of the Cretaceous: Geological Society of America Special Paper, 361, pp. 75–93.

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