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Fort Union Formation Fossil Leaves (Paleocene, Williston Basin,
North Dakota, USA) Indicate Evolutionary Relationships
between Paleocene and Eocene Plant Species
Daniel J. Peppe1and Leo J. Hickey2*
1Corresponding author: Department of Geology,
Baylor University, One Bear Place #97354, Waco, TX 76798 USA
—email: daniel_peppe@baylor.edu
2Department of Geology and Geophysics, Yale University,
P.O. Box 208109, New Haven, CT 06520-8109 USA
*Deceased
ABSTRACT
Six fossil leaf species are described from impression fossils collected from the Paleocene Fort
Union Formation in the Williston Basin in southwestern North Dakota, USA. They are Meliosma
vandaelium sp. nov., Meliosma thriviensis sp. nov., Ternstromites paucimissouriensis sp. nov.,
Macginitiea nascens sp. nov., Dicotylophyllum horsecreekium sp. nov. and Dicotylophyllum han-
sonium sp. nov. These species represent some of the elements of the Fort Union Formation that
are biostratigraphically important megafloral zone taxa or are species that demonstrate an evolu-
tionary relationship to floras from the Eocene Golden Valley Formation. Some of the species de-
scribed here suggest that new species endemic to the Williston Basin evolved through the
Paleocene and into the Eocene. If the pattern of the origination of endemic daughter species seen
in the Williston Basin is consistent across the Western Interior basins of North America, it might
have driven up gamma diversity through the Paleogene. This provides a possible explanation for
the relatively steep vegetation diversity gradients seen in mid- and high-latitude pollen floras in
North America during the late Paleocene and early Eocene.
KEYWORDS
Angiosperms, megafloral paleobotany, evolution, endemism, Paleocene, Eocene, Fort Union
Formation, Ludlow Member, Tongue River Member, Golden Valley Formation
Bulletin of the Peabody Museum of Natural History 55(2):171–189, October 2014.
© 2014 Peabody Museum of Natural History, Yale University. All rights reserved. • http://peabody.yale.edu
Introduction
The North American Paleogene floras have been
studied for more than 100 years, and many of the
species and species associations are well under-
stood (e.g., Newberry 1863, 1868; Lesquereux
1883; Newberry 1898; Knowlton 1930; Brown
1962; Hickey 1977, 1980; Johnson 1989; Crane
et al. 1990; Wing et al. 1995; Manchester 1999;
Johnson 2002; Johnson and Ellis 2002; Manches-
ter et al. 2002; Dunn 2003; Wilf and Johnson
2004; Manchester and Hickey 2007; Pigg et al.
2008; Manchester et al. 2009). However, few
plant fossil collections have been made from
geochronologically calibrated stratigraphic sec-
tions. Furthermore, many common species have
not been formally described or are in need of revi-
sion. This lack of age control and the taxonomic
uncertainty of many fossil plant species make it
difficult to assess evolutionary change within
plant groups through time.
Plant fossil collections from the Fort Union
Formation in the Williston Basin represent one
of the few Paleocene floras from the Western
Interior of North America for which the relative
and absolute ages of a succession of collections are
known (Peppe 2009, 2010). Furthermore, these
early Paleocene Fort Union floras can be com-
pared with floras from the Eocene Golden Valley
Formation, which were sampled within a detailed
lithostratigraphic framework (Hickey 1977). The
chronostratigraphic and lithostratigraphic frame-
work for the Fort Union and Golden Valley col-
lections in the Williston Basin makes is possible to
assess evolution of plant lineages from the early
Paleocene to the Eocene.
We describe six new species from the Ludlow
and Tongue River Members of the Fort Union
Formation. These species are biostratigraphically
important or have clear taxonomic affinities to
species from the Golden Valley Formation. The
species indicate that in at least two genera, there
seems to be origination of new species from ances-
tral species through the Paleocene into the Eocene
in the Williston Basin. These species persist for
millions of years, suggesting that in addition to
plant species that are widespread (i.e., occurring
in multiple basins across the Western Interior of
North America) and long lived, there might have
also been long-lived endemic lineages within the
Western Interior terrestrial basins during the Pale-
ogene. This pattern of evolutionary change in
megafossils mirrors that seen in the pollen record
of Juglandaceae across North America (e.g.,
Nichols and Ott 1978; Nichols 2003). Addition-
ally, this evidence for within-basin evolution of
plant lineages through time might also help
explain the relatively steep vegetation diversity
gradients in mid- and high-latitude pollen floras
in North America during the late Paleocene and
early Eocene (Harrington 2004).
Localities and Geological Setting
Across the Williston Basin, the Paleocene Fort
Union Formation is widely exposed (Figure 1)
and has been the focus of many studies assessing
the sedimentology, lithostratigraphy, geochronol-
ogy and paleontology of the Paleocene and the
response of terrestrial ecosystems to the Creta-
ceous-Paleogene extinctions (e.g., Lloyd 1914;
Lloyd and Hares 1915; Thom and Dobbin 1924;
Johnson 1989; Johnson et al. 1989; Hunter et al.
1997; Hunter 1999; Hicks et al. 2002; Johnson
2002; Hunter and Hartman 2003; Nichols 2003;
Peppe 2009; Peppe et al. 2009; Peppe 2010; Rook
et al. 2010; Chin et al. 2013). In western North
Dakota, the Fort Union Formation conformably
overlies the Cretaceous Hell Creek Formation and
conformably underlies the Golden Valley Forma-
tion (Figure 1D). The Ludlow, Tongue River and
Sentinel Butte Members of the Fort Union For-
mation and the Golden Valley Formation are pre-
dominately terrestrial, except for two intervals of
marine deposition: the Cannonball Member,
which interfingers with the Ludlow Member, and
an unnamed marine incursion in the Tongue
River Member (Fox and Ross 1942; Fox and Ols-
son 1969; Cvancara 1972, 1976; Hartman 1993;
Kroeger and Hartman 1997; Belt et al. 2005).
With the exception of these two marine deposits,
the Fort Union and Golden Valley Formation are
composed of fluvial deposits and paleosols
interbedded with abundant, thick, laterally con-
tinuous lignite deposits.
Fossils in this study were collected from local-
ities in the Ludlow and Tongue River Members
of the Fort Union Formation along the Little Mis-
souri River in southwestern North Dakota (Figure
1) and then compared with species from the
Golden Valley Formation in western and south-
central North Dakota (Hickey 1977). None of the
fossil leaves in this study were collected from
marine or marginal marine strata.
Recent geochronological work indicates that
in the Little Missouri River valley, the Ludlow and
Tongue River strata were deposited from approx-
imately 66.0 to 58.2 Ma and there is an approxi-
mately 2 mya disconformity at the Ludlow-
Tongue River formational contact (Belt et al.
2004; Bowring et al. 2008; Peppe 2009; Peppe et al.
2009; Peppe 2010; Peppe, Johnson et al. 2011).
Peppe (2009, 2010) interpolated the ages of each
collection within the Fort Union Formation based
on stratigraphic position, magnetostratigraphy
and sedimentation rates. Peppe (2010) also used
the floral composition and stratigraphic ranges of
taxa to develop a megafloral biozonation for the
Ludlow and Tongue River Members. From the
base to the top, the megafloral biozones are as fol-
lows: zone Williston Basin I (WBI), Williston
Basin II (WBII) and Williston Basin III (WBIII).
WBI and WBII occur in the Ludlow Member,
and WBIII occurs in the Tongue River Member.
Materials and Methods
The plant fossils described here are preserved as
impressions, and their identifications are based
on their leaf-architectural characteristics. Sys-
temic descriptions follow the format of Johnson
(1996) and Peppe et al. (2008), which is a modifi-
cation of Hickey (1977). The terms used are based
on Ash et al. (1999) and Hickey (1973). The syn-
onymy follows rules of the International Code of
Botanical Nomenclature (Grueter 2000). The
material was initially sorted into morphotypes
and given a morphotype number (see Johnson
Bulletin of the Peabody Museum of Natural History 55(2) • October 2014
172
Fort Union Formation Fossil Leaves • Peppe and Hickey 173
1989; Ash et al. 1999; Johnson 2002; Peppe et al.
2008 for detailed discussions of the morphotype
system) (Appendix 1). The morphotype number
for each species described here is also included.
The morphological characteristics of each
morphotype were used to place them into a Com-
pendium Index Category (CIC) and a Denver
Museum of Nature and Science (DMNS) bin.
The CIC is a categorical system based on leaf mor-
phology that is used to sort morphotypes into mor-
phological groups (Ash et al. 1999). The DMNS
bins are a categorical system designed to sort fossils
into groups based on shared leaf-architectural
characteristics (DMNS 2005). Additional informa-
tion in each description includes the Morphotype
Quality Index (MQI) for each species, which
expresses the number of specimens collected and
the preservation of the specimens (Ash et al. 1999);
the Holomorphotype Quality Index (HQI) for each
holotype, which expresses the preservation of the
FIGURE 1. Location map of fossil leaf localities in the Fort Union Formation and Cretaceous-Eocene stratigra-
phy in the Williston Basin. A, Location map of the United States of America. B, Map of major Paleogene basins
in the Western Interior Basins in North America. Abbreviations: BB, Bighorn Basin; BH, Black Hills; BM, Bull
Mountain; CB, Carbon Basin; CCA, Cedar Creek Anticline; CFB, Clark’s Fork Basin; CM, Crazy Mountains; DB,
Denver Basin; GRB, Green River Basin; HB, Hanna Basin; PRB, Powder River Basin; RB, Raton Basin; SJB, San
Juan Basin; UB, Uinta Basin; WB, Williston Basin; WRB, Wind River Basin. C, Location of fossil leaf localities
in the Little Missouri River valley in southwestern North Dakota. D, Generalized stratigraphy for the latest Cre-
taceous-Eocene deposits in the Williston Basin.
holotype (DMNS 2005); and the stratigraphic
occurrence and the megafloral biostratigraphic
zone of each species (Peppe 2009, 2010).
All type and illustrated specimens are depo-
sited in the paleobotany collections at the Peabody
Museum of Natural History, Yale University (YPM
PB). Each specimen is labeled with a catalog num-
ber and a locality number. Locality information for
all specimens is in Peppe (2009, 2010).
Systematic Paleontology
ORDER Sabiales
FAMILY Sabiaceae
GENUS Meliosma Blume
Meliosma vandaelium
Peppe and Hickey, sp. nov.
Morphotype LM113
Figures 2 and 3
Quercus sullyi auct. non Newberry, 1883; Brown, 1962: pl. 23,
fig. 4, pl. 27, fig. 9.
Diagnosis. Eight to 14 secondary vein pairs (average 10); sec-
ondary vein angle 25° to 60° (average 44°); length-to-width ratio
3:1; secondary vein angle decreases basally; areolation moder-
ately well developed, four sided, uniformly shaped; size and ori-
entation of areoles variable.
Description. Simple, ovate to elliptic to lanceolate to slightly obo-
vate, notophyll- to mesophyll-sized leaf. Leaf symmetrical to
slightly basally asymmetrical; length-to-width ratio 3:1; apex
straight, apical angle acute; base cuneate to slightly decurrent; peti-
ole attachment marginal, petiole 2 cm long or greater, petiole base
swollen; margin serrate to dentate, margin often appears rolled.
Pinnate primary venation; secondary venation semicraspedodro-
mous, veins very thin, vein spacing uniform to occasionally irreg-
ular, vein angle slightly decreasing toward the base, vein angle 25°
to 60° (average 44°), 8 to 14 vein pairs (average 10); intersecondary
veins weak to absent; tertiary venation opposite or alternate per-
current, veins often hard to distinguish from higher order vein
network, vein course sinuous, vein angle obtuse to primary veins;
fourth-order venation opposite percurrent; fifth-order venation
regular polygonal reticulate; areolation moderately well devel-
oped and four sided, uniform areolation shape, size and orienta-
tion somewhat variable; freely ending veinlets absent. One order
of teeth; three teeth per centimeter; tooth spacing irregular to
occasionally uniform; tooth shape concave/convex (CC/CV),
straight/convex (ST/CV), concave/concave (CC/CC), concave/
flexuous (CC/FL); sinus rounded; apex simple.
Occurrence. DP0732, DP0746.
Zone. WBIII.
Categories. CIC: 108, 109, 112, 113, 114, 120, 121, 123; DMNS
bin: 16; MQI: 1; HQI: 6.
Holotype. YPM PB 175122.
Paratypes. YPM PB 175116, YPM PB 175117, YPM PB 175118.
Etymology. This species is named for its type locality on the Van
Daele Ranch and is in honor of Gary Van Daele and the Van
Daele family for support of paleontological research on their
land.
Discussion. The semicraspedodromous secondary venation,
very thin secondary veins, the opposite percurrent quaternary
veins, the regular polygonal reticulate fifth-order venation and
the four-sided, moderately well-developed areolation of this
morphotype are diagnostic of the genus Meliosma (see discus-
sion in Hickey 1977). M. vandaelium (LM113) is morphologi-
cally similar to the Golden Valley Formation species M.
longifolia and the Ludlow Member of the Fort Union Formation
species M. thriviensis. M. vandaelium can be distinguished from
both other species based on the number of secondary vein pairs.
Hickey (1977) demonstrated that the average number and range
of number of secondary vein pairs is a diagnostic species char-
acteristic in Meliosma. M. vandaelium (LM113) has an average
of 10 vein pairs and a range of 8 to 14, which is considerably less
than M. longifolia (average 16, range 13–20) and much larger
than M. thriviensis (range 9–10). M. vandaelium also has less
organized areolation, the secondary veins are at a lower angle,
the secondary vein angle decreases basally and the length-to-
width ratio is larger than in M. longifolia. M. vandaelium can
be distinguished from M. thriviensis by its considerably higher
secondary vein angle, the shape of its teeth, the irregular spac-
ing of its teeth and the greater number of teeth per centimeter.
Meliosma vandaelium was misidentified as Quercus sullyi
by Brown (1962). Q. sullyi can be distinguished from M. van-
daelium by is brochidodromous secondary venation. Addition-
ally, the type material of Q. sullyi is fragmentary, which makes
it impossible to determine the number of secondary veins and
angle of the secondary veins. Therefore, it is not possible to
relate M. vandaelium to Q. sullyi.
Meliosma vandaelium is common in the Tongue River
Member of the Fort Union Formation and is a biostratigraphic
zone taxon for megafloral zone WBIII (Peppe 2010).
Meliosma thriviensis Peppe and Hickey, sp. nov.
Morphotype LM118
Figure 4
Diagnosis. Nine to 10 secondary vein pairs; secondary vein
angle 45° to 65°; uniform secondary vein spacing; fourth-order
venation alternate percurrent; tooth spacing uniform; one to
two teeth per centimeter.
Description. Simple, elliptic, notophyll-sized leaf. Leaf symmet-
rical; length-to-width ratio 3:1; apex straight, apical angle acute;
base cuneate to slightly decurrent, base angle acute; petiolar
attachment marginal, petiole 0.5 m long and 0.02 cm wide; mar-
gin dentate. Pinnate primary venation; secondary venation sem-
icraspedodromous, vein spacing and angle uniform, vein angle
45° to 60°, 9 to 10 vein pairs; tertiary venation opposite percur-
rent; vein course sinuous; fourth-order venation opposite per-
current. One order of teeth; one to two teeth per centimeter;
Bulletin of the Peabody Museum of Natural History 55(2) • October 2014
174
Fort Union Formation Fossil Leaves • Peppe and Hickey 175
FIGURE 2. Meliosma vandaelium Peppe and Hickey sp. nov. A, YPM PB 175117, paratype, leaf with cuneate base.
B, YPM PB 175112a, holotype, leaf with nine secondary veins pairs. C, YPM PB 175112b, holotype, cuneate to
slightly decurrent leaf base. D, YPM PB 175118, paratype, leaves with thin secondary veins.
tooth spacing uniform; tooth shape CC/CV, flexuous/flexuous
(FL/FL), flexuous/concave (FL/CC); sinus rounded; apex pos-
sibly glandular.
Occurrence. DP0515.
Zone. WBII.
Categories. CIC: 108; DMNS bin: 16; MQI: 2; HQI: 5.
Holotype. YPM PB 175100.
Etymology. The species is named in honor of the Three V
Ranch, which is owned by George Weinreiss and the Weinreiss
family, for the ranch’s support of paleobotanical research in the
area since the days of Roland Brown in the 1940s and 1950s.
Discussion. The cuneate to slightly decurrent base, the strongly
impressed tertiary and quaternary venation and the tooth shape
of LM118 allow its assignment into the genus Meliosma. The
number of secondary vein pairs, the angle of the secondary
veins, the fourth-order venation and the uniform tooth spacing
preclude M. thriviensis from M. longifolia and M. vandaelium.
Meliosma thriviensis occurs at only one locality in the Ludlow
Member of the Fort Union Formation and is relatively rare. Most of
the zone taxa for megafloral zone WBII occur at the same locality as
M. thriviensis (Peppe 2010). This indicates that M. thriviensis was
likely a rare member of the WBII plant community.
In stratigraphic order from oldest to youngest, Meliosma
thriviensis was collected from the early Paleocene Ludlow Mem-
ber of the Fort Union Formation, M. vandaelium from the mid-
dle Paleocene Tongue River Member of the Fort Union
Formation and M. longifolia from the early Eocene Golden Val-
ley Formation. The three species do not have overlapping strati-
graphic ranges and show an increasing trend in the range of the
number of secondary vein pairs in different specimens (M. van-
daelium: range 9–10; M. thriviensis: average 10, range 8–14; M.
longifolia: average 16, range 13–20). The lack of overlapping age
ranges for each species and their morphological similarities with
a trend toward increasing variability in secondary vein pairs sug-
gest that M. thriviensis, M. vandaelium and M. longifolia might
represent a pattern of evolutionary change within Meliosma such
that M. thriviensis could represent an evolutionary precursor to
M. vandaelium, and M. vandaelium to M. longifolia.
ORDER Ericales
FAMILY Theaceae
GENUS Ternstromites Berry
Ternstromites paucimissouriensis
Peppe and Hickey, sp. nov.
Morphotype LM38
Figure 5
Diagnosis. Leaf ovate to elliptic with 8 to 10 secondary veins; sec-
ondary vein angle 40° to 60° (average 52°); serrulate margin with
three to six teeth per centimeter (average four per centimeter).
Description. Simple, ovate to elliptic, mesophyll- to megaphyll-
sized leaf. Base slightly asymmetrical; average length 14 cm;
Bulletin of the Peabody Museum of Natural History 55(2) • October 2014
176
FIGURE 3. Meliosma vandaelium Peppe and Hickey sp.
nov. A, M. vandaelium, LM113, YPM PB 175116,
paratype, leaf with eight or more secondary vein pairs.
B, M. vandaelium, LM113, YPM PB 175116, paratype,
higher order venation. Scale bar equals 1 cm.
average width 5 cm; length-to-width ratio 3:1; apex straight, api-
cal angle acute; base cuneate, base angle acute; petiolar attach-
ment marginal, petiole base swollen, petiole approximately 3
cm long; margin serrulate. Pinnate primary venation; second-
ary venation weakly brochidodromous to eucamptodromous,
vein spacing and angle uniform, vein angle 40° to 60°, average
vein angle 52°, 8 to 10 secondary veins, veins alternately
arranged, veins gradually curved upward toward margin; inter-
secondary veins absent; tertiary venation irregularly opposite
percurrent, vein course curved; fourth-order venation irregu-
larly percurrent to dichotomizing; marginal ultimate venation
looped with teeth. One order of teeth; teeth not present on lower
third of lamina, tooth spacing regular near apex, occasionally
teeth uniformly spaced along entire lamina; three to six teeth
per centimeter; tooth shape CC/CV; sinus rounded; apex seta-
ceous; teeth very small. Leaf subcoriaceous.
Occurrence. DP0721, DP0716, DP0739, DP0503, DP0515,
DP0513, DP0724.
Zone. WBI, WBII, WBIII.
Categories. CIC: 120, 114, 112, 110; DMNS bin: 16; MQI: 2; HQI:
5.
Holotype. YPM PB 078654.
Paratypes. YPM PB 175158, PB 175159.
Discussion. The serrulate margin with setaceous teeth and the
higher order venation of LM38 allow the classification of this
morphotype to the genus Ternstromites. T. paucimissouriensis
(LM38) compares most closely to T. aureavallis Hickey (1977)
from the Eocene Camel’s Butte Member of the Golden Valley
Formation in the Williston Basin and the latest Paleocene Fort
Union Formation in the Bighorn Basin based on its teeth shape
and glands and similar higher order venation. T. paucimis-
souriensis can be distinguished by its lower number of second-
ary veins, lower angle of secondary vein departure and wider
spacing of the serrulations on its margin. The similarities
between T. paucimissouriensis and T. aureavallis suggest that
the Paleocene T. paucimissouriensis could be an evolutionary
predecessor to the Eocene T. aureavallis.
The shape, venation and glandular teeth of the species of
Ternstromites allow for its classification into the family Theaceae
(Hickey 1977). A more complete discussion of the species of
Ternstromites and their relationship to modern Theaceae can
be found in Hickey (1977).
Theaceae are often found in swamps and black-water
rivers in warm temperate to subtropical regions (Cronquist
1981). Thus, the occurrence of Ternstromites paucimissourien-
sis suggests a warm climate with only rare or occasional frost.
Further, climate estimates for WBIII (Peppe 2010; Peppe, Royer
et al. 2011) and the sedimentology of the collected floral locali-
ties suggest a similar environment for T. paucimissouriensis and
its modern relative in the Theaceae.
Ternstromites paucimissouriensis is found in the Ludlow
and Tongue River Members of the Fort Union Formation. It is
particularly common in the upper Ludlow Member and is a
biostratigraphic zone taxon for megafloral zone WBII (Peppe
2010).
Fort Union Formation Fossil Leaves • Peppe and Hickey 177
FIGURE 4. Meliosma thriviensis Peppe and Hickey sp.
nov. A, YPM PB 175100a, holotype, leaf with nine sec-
ondary vein pairs. B, M. thriviensis, LM118, YPM PB
175100a, holotype, leaf margin, arrows indicating loca-
tion of teeth with possible glandular apices.
ORDER Proteales
FAMILY Platanaceae
GENUS Macginitiea Wolfe and Wehr
Emended generic diagnosis. Leaves simple; venation basally
palinactinodromous; major veins extending to margin, each
entering a lobe; intersecondary veins occasionally present; sec-
ondary veins smoothly and shallowly curving; tertiary veins per-
current; quaternary veins form a coarse reticulum; axillary
bracing between primary veins formed by either secondary or
tertiary veins that diverge from the primary veins at acute angles,
curve and merge to form a concave up chevron pattern pointed
toward to sinus of the lobe; laminae lobed; sinuses between lobes
curved.
Discussion. Wolfe and Wehr (1987) defined Macginitiea for
species of “Platanophyllum”-type foliage that had a distinct,
chevron-patterned axillary bracing between primary veins
and that were typically symmetrical and had uniform loba-
tion and venation patterns across the leaf. In their original
diagnosis, Wolfe and Wehr (1987) stated that the axillary
bracing between primary veins was formed by secondary
veins that curved upward and merged to form chevrons
pointing toward the sinus of the leaf lobe. We have emended
the diagnosis to also note that in some species (M. nascens)
the axillary bracing between primate veins is made by tertiary
veins that form the same chevron pattern pointing toward
the lobe sinus.
Macginitiea nascens Peppe and Hickey, sp. nov.
Morphotype LM12
Figures 6 and 7
Species diagnosis. Primary venation palinactinodromous with
three primary veins; three lobed; tertiary veins connecting
between primary veins to form chevrons concave upward
Bulletin of the Peabody Museum of Natural History 55(2) • October 2014
178
FIGURE 5. Ternstromites paucimissouriensis Peppe and Hickey sp. nov. A, YPM PB 078654, holotype, leaf with
straight apex, cuneate base and serrulate margin. B, YPM PB 175179, paratype, leaf with slightly asymmetrical
base. C, YPM PB 175178a, paratype, leaf with slightly asymmetrical base and petiole with swollen base.
FIGURE 6. Macginitiea nascens Peppe and Hickey sp.
nov. YPM PB 078671, holotype, leaf with palinactin-
odromous primary venation and apically arched per-
current tertiary venation.
Fort Union Formation Fossil Leaves • Peppe and Hickey 179
FIGURE 7. Macginitiea nascens Peppe and Hickey sp. nov. A, YPM PB 078668, margin of leaf. B, YPM PB 175111,
paratype, palmately lobed leaf. C, YPM PB 080190, leaf with palinactinodromous venation and apically arched
tertiary veins. D, YPM PB 078674, paratype, palmately lobed leaf with apically arched percurrent tertiary veins.
E, YPM PB 078675, paratype, leaf with apically arched percurrent tertiary venation.
toward the sinus that is often reinforced by a fourth-order
vein.
Description. Elliptic to ovate, microphyll- to mesophyll- to
megaphyll-sized leaf. Lamina symmetrical; base cuneate,
basal angle acute; apex convex, when lobed, lobes are convex,
apical angle obtuse to odd-lobed obtuse; petiolar attachment
marginal, margin serrate; three lobed to rarely unlobed. Pri-
mary venation palinactinodromous, primary veins diverge
at 30° to 35°; when unlobed, primary venation occasionally
pinnate; agrophic veins absent in lobed leaves, simple in
unlobed leaves; three basal veins; secondary venation craspe-
dodromous, vein spacing and angle uniform; typically five
to seven vein pairs along midvein, vein angle 20° to 30°;
intersecondary veins rare; tertiary veins thin, slightly irreg-
ular, mixed opposite/alternate percurrent, veins diverge
from primary veins acutely and merge to form concave up
chevrons that point toward the sinus, the chevron is typically
reinforced by a fourth-order vein; fourth- and fifth-order
venation regular polygonal reticulate; areolation moderately
developed, five or more sided; freely ending veinlets absent.
One to two orders of teeth; one to two teeth per centimeter;
tooth spacing regular; tooth shape CC/CC; sinus rounded;
apex simple.
Occurrence. DP0527, DP0518, DP0513, DP0535.
Categories. CIC: 140; DMNS bin: 14; MQI: 1; HQI: 6.
Holotype. YPM PB 078671.
Paratypes. YPM PB 175111, PB 078675, PB 078674.
Discussion.Macginitiea nascens (LM12) can be placed in the
genus Macginitiea Wolfe and Wehr (1987) by the presence of
the diagnostic chevron pattern that is formed by tertiary veins
of adjacent primaries connecting in an inverted V pattern
between the lobes. It can be distinguished from the other species
in the genus (e.g., M. gracilis,M. angustiloba,M. whitneyi and
M. wyomingensis) by having only three lobes and three basal
veins and by having the chevron pattern between primary veins
formed by a tertiary vein that is often reinforced by a fourth-
order vein. In all other species, the chevron pattern is formed by
adjacent inner-secondary veins.
Macginitiea nascens is morphologically similar to Platanus
raynoldsii (LM4) and P. nobilis (LM124). M. nascens can be dis-
tinguished from P. raynoldsii because it does not have strongly
impressed equant quadrangular higher order venation; its ter-
tiary veins are thinner, more irregular and merge to form a con-
cave-up (e.g., distally) chevron pattern; and its teeth are doubly
concave and have a folded margin. M. nascens can be distin-
guished from P. nobilis because it has three primary veins; the
outer primary vein has a series of secondary veins that become
stronger toward the base but are never promoted to primary
veins as in P. nobilis. Furthermore, in P. nobilis the secondary
veins merge to form chevrons that often disappear toward the
lobe sinus, the spacing of the secondary vein pairs is uniform
and closely spaced and there are approximately 16 vein pairs
along the midvein; this is distinct from M. nascens, which has
five to seven vein pairs along the midvein that are somewhat
irregularly spaced.
Macginitiea nascens is the earliest species of the genus
Macginitiea yet described. M. nascens is relatively common in
the Ludlow Member of the Fort Union Formation and is a zone
taxon for megaflora zone WBII (Peppe 2010).
ORDER Incertae sedis
FAMILY Incertae sedis
GENUS Dicotylophyllum
Dicotylophyllum horsecreekium
Peppe and Hickey, sp. nov.
Morphotype LM29
Figure 8
Diagnosis. Stout, slightly sinuous primary vein; secondary vein
angle abruptly increasing to base; opposite and alternate sec-
ondary vein arrangement with alternate arrangement common
at the base; slightly decurrent attachment of basal secondary
veins.
Description. Simple, ovate to elliptic, microphyll to mesophyll
leaf. Leaf symmetrical; apex rounded to convex, apical angle
acute; base rounded to slightly decurrent, base angle obtuse;
petiolar attachment marginal; petiole thick and up to 5 cm
long; margin entire. Pinnate primary venation, midvein stout
and often slightly sinuous; secondary venation brochidodro-
mous to weakly brochidodromous, secondary vein spacing
decreasing to base, secondary vein angle abruptly increasing to
base, vein arrangement both opposite and alternate, alternate
arrangement common at base, veins occasionally bifurcate
near the margin; intersecondary veins weak; tertiary venation
random reticulate.
Occurrence. DP0717, DP0716, DP0515.
Zone. WBI, WBII.
Categories. CIC: 111, 117; DMNS bin: 30, 23, 24; MQI: 2; HQI:
4.
Holotype. YPM PB 175005.
Paratypes. YPM PB 175006, PB 175007, PB 175015.
Etymology. This species is named in honor of the Horse Creek
Grazing Association and its members for their support of pale-
obotanical research in the Cretaceous and Paleocene deposits
on their ranchland.
Discussion. The diagnostic characteristics of this morphotype,
LM29, and its well-rounded base and apparent thin texture
allow Dicotylophyllum horsecreekium to be distinguished from
all morphotypes in the Williston Basin Fort Union Formation.
The order and family assignment of D. horsecreekium is uncer-
tain because no modern family examined had the same combi-
nation of characters.
Dicotylophyllum horsecreekium is found throughout the
Ludlow Member of the Fort Union Formation. In particular, it
is most common in the basal Ludlow Member. D. horsecreek-
ium is a zone taxon for megafloral zone WBI (Peppe 2010).
Bulletin of the Peabody Museum of Natural History 55(2) • October 2014
180
Fort Union Formation Fossil Leaves • Peppe and Hickey 181
FIGURE 8. Dicotylophyllum horsecreekium Peppe and Hickey sp. nov. A, YPM PB 175015a, paratype, leaf with
slightly decurrent base, a long petiole and with secondary veins that are both alternately and oppositely arranged.
B, YPM PB 175007, paratype, leaf with secondary veins that bifurcate near the margin. C, YPM PB 175006a,
paratype, leaf with rounded base, with secondary veins that increase vein angle to the leaf base and with basal
secondary veins that are decurrently attached to the midvein. D, YPM PB 175005a, holotype, leaf with stout
midvein and petiole.
Dicotylophyllum hansonium
Peppe and Hickey, sp. nov.
Morphotype LM97
Figure 9
Vitis olriki auct. non Heer, 1868; Brown, 1962: pl. 27, fig. 10.
Populus richardsonii auct. non Heer, 1868; Knowlton, 1930: pl.
23, fig. 6.
Diagnosis. Setaceous, glandular teeth; acrodromous primary
venation, thin, random reticulate tertiary venation, five basal
veins, a cordate base and punctations, likely representing resin
dots, covering the lamina.
Description. Simple, ovate, notophyll-sized leaf. Leaf symmetri-
cal; length-to-width ratio 1.5:1 to 1.25:1; base cordate; apex
straight; petiolar attachment marginal; margin crenulated. Actin-
odromous primary venation; simple agrophic veins; secondary
venation semicraspedodromous, very few inner secondary veins,
inner-secondary veins at low angle to midvein; tertiary venation
random reticulate; fourth-order venation regular polygonal retic-
ulate. One order of teeth; seven teeth per centimeter; tooth spac-
ing regular; shape convex/convex (CV/CV); sinus rounded; apex
glandular and possibly setaceous. Leaf texture coriaceous; leaf has
punctations across the lamina that might represent resin dots.
Occurrence. DP0725.
Zone. WBIII.
Categories. CIC: 136; DMNS bin: 31; MQI: 2; HQI: 5.
Holotype. YPM PB 175058.
Paratype. YPM PB 175057.
Etymology. This species is named in honor of John Hanson and
the Hanson family, owners of the Logging Camp Ranch, for
their fervent support and interest of geology and paleontology
on their ranch and the surrounding region.
Discussion. The diagnostic characteristics for Dicotylophyllum
hansonium (LM97) allow it to be easily distinguished from all
other Williston Basin Fort Union Formation morphotypes. It can
be distinguished from the morphologically similar Cercidiphyl-
lum genesevianum (LM21) by its teeth, which are more closely
spaced and have a setaceous apex, the presence of punctations
across the lamina and its random reticulate Tertiary venation. D.
hansonium (LM97) can also be distinguished from “Populus”cor-
data (LM106), “Populus”acerfolia (LM31) and “Populus”nebras-
censis (LM5) by its setaceous teeth, acrodromous primary
venation and higher order venation. The order and family assign-
ment of D. hansonium is uncertain, and no modern family exam-
ined had a similar suite of characteristics to D. hansonium.
Dicotylophyllum hansonium is found in the Tongue River
Member of the Fort Union Formation and is relatively
rare and found only at a single locality. The co-occurrence of
D. hansonium with the zone taxa for megafloral zone WBIII
(Peppe 2010) suggests that it was likely a rare species within
the WBIII plant community.
Bulletin of the Peabody Museum of Natural History 55(2) • October 2014
182
FIGURE 9. Dicotylophyllum hansonium Peppe and
Hickey sp. nov. A, YPM PB 175057a, paratype, leaf
with cordate base and actinodromous primary vena-
tion. B, YPM PB 175058, holotype, leaf with cordate
base and five basal veins.
Discussion
The morphological similarity between the species
described herein and their Eocene relatives sug-
gests some interesting relationships between early
and middle Paleocene floras of Ludlow and
Tongue River Member and early Eocene floras
from the Golden Valley Formation. The Pale-
ocene species Meliosma thriviensis,M. van-
daelium and Ternstromites paucimissouriensis are
morphologically similar to the Eocene species M.
longifolia and T. aureavallis from the Golden Val-
ley Formation. These Eocene species either immi-
grated into the Williston Basin or represent
descendants of the earlier Paleocene species.
Given the morphological similarities between the
Paleocene and Eocene forms, we suggest that it is
more likely that the Eocene species M. longifolia
and T. aureavallis represent the evolution of
daughter species from ancestral species endemic
to the Williston Basin.
In either case (i.e., immigration or “in situ”
evolution), the newly described species Meliosma
thriviensis, M. vandaelium and Ternstromites
paucimissouriensis demonstrate that new species
of Meliosma and Ternstromites appeared over
time in the Paleocene and Eocene. The evolution
of new species through the Paleocene in the Sabi-
aceae and Theaceae families is similar to that
noted in the Juglandaceae pollen record (Nichols
and Ott 1978; Nichols 2003). Together, these
studies indicate that evidence for within-lineage
evolution of new species might be found in other
plant groups that span the early Paleogene in the
Western Interior of North America.
In addition to these evolutionary relationships
within Meliosma and Ternstromites, there are sev-
eral species in common between the Fort Union
and Golden Valley Formations, including Aver-
hoites affinis,Cercidiphyllum genetrix,Ziziphoides
flabella,Browniea serrata,Penosphyllum corda-
tum,Querexia angulata,Aesculus hickeyii,Davidia
antiqua,Beringiaphyllum cupanoides and Platanus
nobilis. These shared taxa indicate that many of
the Paleocene-Eocene plant species and species
assemblages that existed within the Williston
Basin were long lived, and in some cases, such as
A. affinis and B. serrata, species likely persisted
within the Williston Basin for 10 million years or
longer. Most of these species are commonly found
in Paleocene deposits across North America (e.g.,
Brown 1962), which indicates that species associ-
ations during the Paleocene were remarkably long
lived. This long-lived associate might be related to
ecosystem reorganization and recovery after the
Cretaceous-Paleogene extinctions or to a combi-
nation of broad environmental tolerances of many
of these long-lived plant species relative to the
magnitude of environmental change they experi-
enced during the early Paleogene.
Finally, the new species described here have
only been found in the Williston Basin and the
geographically adjacent Powder River Basin
(Peppe 2009). This suggests that they might rep-
resent the evolution of species that are endemic
to the Williston Basin and possibly a few of the
surrounding basins. If this evolution of endemic
species through the Paleocene and Eocene is a
common pattern across North America, it sug-
gests that there might have been a great deal of
heterogeneity in plant communities across the
Western Interior of North America. The “in situ”
evolution of plant species through the Paleocene
might have led to relatively high gamma diversity
across North America, which might help explain
the relatively steep vegetation diversity gradients
seen in mid- and high-latitude pollen floras dur-
ing the late Paleocene and early Eocene relative to
the early Paleocene (Harrington 2004).
Acknowledgments
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, American Association of Petroleum
Geologists Grants-in-Aid, the Explorers Club
Exploration Fund, the Yale University Depart-
ment of Geology and Geophysics, the Yale
Peabody Museum Division of Paleobotany and
Baylor University. Thanks to Dean Pearson, Dar-
ren Larson, Karew Shumaker, Georgia Knaus,
Antoine Bercovici, Terry and Blaine Schaeffer,
Don and Kathy Wilkening, Merle Clark, Ashley
Rose Gould, Bradley Broadhead, Amy Shapiro,
Jesse Self, Nick Cuba, Steve Manchester and
David Evans for assistance in the field. Special
thanks to Linda Klise, formerly of the Yale
Peabody Museum Division of Paleobotany, and
countless museum volunteers for assistance with
Fort Union Formation Fossil Leaves • Peppe and Hickey 183
Appendix:
Systematic List of Megafloral Morphotypes
from the Williston Basin
Taxa are listed by affinity in larger taxonomic category. Margin is the dicotyledon angiosperm leaf margin.
Abbreviations: E, entire; T, toothed. New species described here indicated by bold typeface.
Taxonomic
name
(morphotype
number) Order Family Organ Margin
Algae
LM86 Incertae sedis — Leaf —
Equisetopsida
Equisetum sp. (LM19) Equisetales Equisetaceae Root/rhizomorph —
Pteridophtye
LM52 Polypodiales Incertae sedis Leaf —
LM73 Polypodiales Incertae sedis Fertile leaf —
LM81 Polypodiales Incertae sedis Leaf —
Onoclea sensibilis (LM54) Polypodiales Dryopteridaceae Leaf —
Woodwardia gravida (LM53) Polypodiales Blechnaceae Leaf —
Conifer
Cupressinocladus interuptus Pinales Cupressaceae Leaf —
(LM16)
Glyptostrobus europeaus Pinales Taxodiaceae Leaf —
(LM9)
LM98 Pinales Cupressaceae Cone —
Metasequoia occidentalis Pinales Cupressaceae Leaf —
(LM8)
Monocotyledon
Amesoneuron sp. (LM11) Pinales Taxodiaceae Leaf —
LM102 Incertae sedis — Leaf —
LM96 Incertae sedis — Leaf —
Sabalites sp. (LM126) Incertae sedis — Leaf —
unpacking, curating and loaning fossils; to Kirk
Johnson for many discussions, comments and
constructive criticism on this work; and to an
anonymous reviewer for comments that greatly
improved this manuscript. We would like to
especially acknowledge the Brown, Clark, Davis,
Hanson, Krutzfeld, Van Daele, Walser and
Weinreiss families; the Horse Creek Grazing
Association; and the US Forest Service for land
access. Finally, this work is the result of remark-
ably fun and always interesting fieldwork, lab
analyses and discussions between the coauthors.
I (D.J.P.) want to specifically recognize and thank
Leo Hickey for all of his contributions to this and
many other projects. Leo was an amazing collab-
orator, mentor and friend. As both a scientist and
a person, I am better for having known him.
Received 31 March 2014; revised and accepted 9
June 2014.
Bulletin of the Peabody Museum of Natural History 55(2) • October 2014
184
Continued
Fort Union Formation Fossil Leaves • Peppe and Hickey 185
APPENDIX continued.
Taxonomic
name
(morphotype
number) Order Family Organ Margin
Dicotyledon
Beringiaphyllum Cornales Cornaceae Leaf T
cupanioides (LM24)
Davidia antiqua (LM25, LM74) Cornales Cornaceae Leaf, fruit/flower T
aff. Browniea serrata (LM64) Cornales Nyssaceae Leaf T
B. serrata (LM10) Cornales Nyssaceae Leaf T
Ternstromites paucimissouriensis Ericales Theaceae Leaf T
sp. nov. (LM38)
Corylus macquarii (LM59) Fagales Betulaceae Leaf T
Fagopsiphyllum groenlandicum Fagales Fagaceae Leaf T
(LM15)
cf. “Castanea” intermedia (LM51) Fagales Incertae sedis Leaf T
Juglandiphyllites glabra (LM14) Fagales Juglandaceae Leaf T
Melastomatites montanensis Laurales Lauraceae Leaf E
(LM117)
Penosphyllum cordatum (LM57) Malvales Malvaceae Leaf E
Nelumbago montanum (LM2) Nymphales Nelumbonaceae Leaf —
Averhoites affinus (LM17) Oxidales Oxidalaceae? Leaf E
Macginicarpa sp. (LM46) Proteales Platanaceae Fruit/flower —
Macginitiea nascens sp. nov. Proteales Platanaceae Leaf T
(LM12)
“Platanus” nobilis (LM124) Proteales Platanaceae Leaf T
P. raynoldsii (LM4) Proteales Platanaceae Leaf T
Protophyllum semotum (LM60) Proteales Platanaceae Leaf T
LM100 Proteales Platanaceae Leaf T
LM80 Proteales Platanaceae Leaf T
LM90 Proteales Platanaceae Leaf T
Celtis aspera (LM22) Rosales Canabaceae Leaf, fruit/flower T
Meliosma vandaelium sp. nov. Sabiales Sabiaceae Leaf T
(LM118)
M. thriviensis sp. nov. (LM113) Sabiales Sabiaceae Leaf T
Cercidiphyllum genetrix (LM7) Saxifrigales Cercidiphyllaceae Leaf T
C. genesevianum (LM21) Saxifrigales Cercidiphyllaceae Leaf T
“Populus” acerifolia (LM31) Saxifrigales Cercidiphyllaceae? Leaf T
“P.” cordata (LM31) Saxifrigales Cercidiphyllaceae? Leaf T
“P.” nebrascensis (LM5) Saxifrigales Cercidiphyllaceae? Leaf T
Nyssidium arcticum (LM65) Saxifrigales Cercidiphyllaceae? Fruit/flower —
N. eckmanii (LM101) Saxifrigales Cercidiphyllaceae? Fruit/flower —
Aesculus hickeyi (LM103) Sapindales Hippocastanaceae Leaf T
Zizyphoides flabella (LM6) Trochodendrales Trochodendraceae Leaf E/T
Paleonelumbo macroloba Incertae sedis — Leaf —
(LM95)
Paranymphaea crassifolia Incertae sedis — Leaf —
(LM1)
Quereuxia angulata (LM3) Incertae sedis — Leaf —
LM79 Incertae sedis — Leaf —
LM82 Incertae sedis — Leaf —
“Cornus” nebrascensis (LM39) Incertae sedis — Leaf E/T
Continued
APPENDIX continued.
Taxonomic
name
(morphotype
number) Order Family Organ Margin
Dicotylophyllum Incertae sedis — Leaf E
hansonium sp. nov. (LM97)
D. horsecreekium sp. nov. Incertae sedis — Leaf E
(LM29)
LM104 Incertae sedis — Leaf E
LM105 Incertae sedis — Leaf E
LM107 Incertae sedis — Leaf E
LM112 Incertae sedis — Leaf E
LM114 Incertae sedis — Leaf E
LM119 Incertae sedis — Leaf E
LM120 Incertae sedis — Leaf E
LM125 Incertae sedis — Leaf E
LM131 Incertae sedis — Leaf E
LM133 Incertae sedis — Leaf E
LM134 Incertae sedis — Leaf E
LM135 Incertae sedis — Leaf E
LM136 Incertae sedis — Leaf E
LM30 Incertae sedis — Leaf E
LM41 Incertae sedis — Leaf E
LM48 Incertae sedis — Leaf E
LM62 Incertae sedis — Leaf E
LM87 Incertae sedis — Leaf E
LM88 Incertae sedis — Leaf E
Myrtophyllum torreyi (LM69) Incertae sedis — Leaf T
“Ficus” artocarpoides (LM26) Incertae sedis — Leaf T
“Planera” crenata (LM13) Incertae sedis — Leaf T
Dicotylophyllum hebronensis Incertae sedis — Leaf T
(LM63)
LM108 Incertae sedis — Leaf T
LM109 Incertae sedis — Leaf T
LM110 Incertae sedis — Leaf T
LM111 Incertae sedis — Leaf T
LM115 Incertae sedis — Leaf T
LM116 Incertae sedis — Leaf T
LM121 Incertae sedis — Leaf T
LM122 Incertae sedis — Leaf T
LM123 Incertae sedis — Leaf T
LM127 Incertae sedis — Leaf T
LM128 Incertae sedis — Leaf T
LM129 Incertae sedis — Leaf T
LM130 Incertae sedis — Leaf T
LM132 Incertae sedis — Leaf T
LM137 Incertae sedis — Leaf T
LM32 Incertae sedis — Leaf T
LM35 Incertae sedis — Leaf T
LM36 Incertae sedis — Leaf T
LM37 Incertae sedis — Leaf T
Bulletin of the Peabody Museum of Natural History 55(2) • October 2014
186
Continued
Fort Union Formation Fossil Leaves • Peppe and Hickey 187
APPENDIX continued.
Taxonomic
name
(morphotype
number) Order Family Organ Margin
LM44 Incertae sedis — Leaf T
LM47 Incertae sedis — Leaf T
LM55 Incertae sedis — Leaf T
LM67 Incertae sedis — Leaf T
LM71 Incertae sedis — Leaf T
LM72 Incertae sedis — Leaf T
LM75 Incertae sedis — Leaf T
LM76 Incertae sedis — Leaf T
LM77 Incertae sedis — Leaf T
LM78 Incertae sedis — Leaf T
LM83 Incertae sedis — Leaf T
LM84 Incertae sedis — Leaf T
LM85 Incertae sedis — Leaf T
LM89 Incertae sedis — Leaf T
LM91 Incertae sedis — Leaf T
LM92 Incertae sedis — Leaf T
LM93 Incertae sedis — Leaf T
LM94 Incertae sedis — Leaf T
LM99 Incertae sedis — Leaf T
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