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Int. J. Plant Sci. 178(9):000–000. 2017.
q 2017 by The University of Chicago. All rights reserved.
1058-5893/2017/17809-00XX$15.00 DOI: 10.1086/694186
Sarah E. Allen1,*
*Department of Biology, University of Florida, and Florida Museum of Natural History, Gainesville, Florida 32611, USA
Editor: Michael T. Dunn
Premise of research. Permineralized wood along with fossil leaves, reproductive structures, and dispersed
pollen and spores are preserved in the uppermost Lower Eocene (∼49.0 Ma) Blue Rim escarpment of the Bridger
Formation in southwestern Wyoming. Each component of the flora provides a slightly different view of the diversity and environment, but the wood assemblage provides additional information on the taxonomic composition,
forest structure, and local seasonality and climate.
Methodology. Fossil wood specimens, including occasional in situ stumps, were collected from Bridger
strata exposed at Blue Rim, north of Green River, Wyoming. Thin sections in the three standard orientations
were prepared of each specimen and examined by light microscopy. Specific gravity and vulnerability index values were calculated for well-preserved specimens. Tree height was estimated from diameter measurements obtained from 10 specimens.
Pivotal results. Seven wood types are recognized in the assemblage, including a single specimen of Pinaceae,
the only conifer macrofossil recovered at Blue Rim to date, and six angiosperms. Angiosperm woods have affinities to Canellaceae, Fabaceae, and Anacardiaceae. The specimen of Canellaceae is the only Blue Rim wood with
scalariform perforation plates, predominately uniseriate rays, and mostly solitary vessels. A new Caesalpinioid
species, Peltophoroxylon diversiradii, lacks storied structure and has conspicuous paratracheal axial parenchyma.
The most abundant wood type has radial canals and prismatic crystals in the ray cells and is assigned to Edenoxylon parviareolatum (Anacardiaceae).
Conclusions. While the wood assemblage has significantly lower taxonomic diversity compared to the
leaves, reproductive structures, and dispersed palynoflora, it includes some taxa not otherwise recognized from
Blue Rim. These trees, reaching ∼16–28 m tall, would have supported co-occurring climbers in the same flora.
Diffuse porosity, generally absent or indistinct growth rings, rare scalariform perforation plates, and the high vulnerability indices of the Blue Rim woods suggest that climate conditions were warm with limited seasonality.
Keywords: Anacardiaceae, Canellaceae, Eocene Bridger Formation (Wyoming), Fabaceae, fossil wood anatomy,
yielded numerous well-preserved impressions of leaves and reproductive structures, dispersed pollen and spores, and silicified
woods. The woods, preserved as stumps and logs, add to the understanding of the local diversity and environment of this area
during the latest Early Eocene. A radiometric age of ∼49 Ma
has been estimated from just above the most productive fossil
layer at BR (M. E. Smith, personal communication, 2016).
In this article, the anatomy of the BR woods is described and
used to determine taxonomic affinities. In addition to the taxonomic treatment, wood characters are evaluated to interpret
broad paleoclimate and paleoecological parameters. Specific
gravity and vulnerability to freezing conditions are estimated
from well-preserved woods, and specimens with complete diameters are used to estimate tree heights. Finally, the taxonomic
affinities of the BR wood assemblage are compared with other
fossil wood assemblages from the Eocene of western North
Fossils have been collected in the Eocene Bridger Formation of
Wyoming for more than a century, but most attention has been
devoted to the fauna (Murphey and Evanoff 2011). The plant
remains are known only from a few scattered reports of individual taxa, including Lygodium kaulfussi, Landeenia aralioides,
Populus cinnamomoides, Phoenix windmillis, and fruits and
leaves of Icacinaceae (Manchester and Zavada 1987; Manchester and Hermsen 2000; Manchester et al. 2006; Allen 2015a;
Allen et al. 2015). The Blue Rim (BR) escarpment, exposing the
lower Bridger Formation in southwestern Wyoming (fig. 1), has
Manuscript received December 2016; revised manuscript received April 2017;
electronically published October 23, 2017.
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Material and Methods
The Bridger Formation (upper Lower Eocene to lower Middle
Eocene) is composed of mostly fluvial and reworked volcanic
sediments (Murphey and Evanoff 2011). This formation is well
known for its vertebrate fauna and is the stratotype for the
Bridgerian North American Land Mammal Age (Krishtalka et al.
1987; Murphey et al. 2001; Robinson et al. 2004). The local stratigraphy of BR was documented by Kistner (1973), who noted
the occurrence of various fossils, including permineralized wood.
Wood was subsequently collected at BR in the mid-1980s by
Jane Landeen (who discovered one of the most productive quarry
sites), S. R. Manchester, and others. A larger collection was made
in 1995 by S. R. Manchester and S. Hack, with additional specimens acquired in 2003, 2009, 2010, and 2014. These wood
specimens (more than 30 in total) come from eight different sites
(University of Florida [UF] localities 00341S, 15761, 18289,
18591, 19031, 19225, 19338W, and 19406) along the BR escarpment. Other macrofossils have been collected from or very
near to each of these sites with the exception of 18591 and
19406, which yielded only fossil wood. Specimens (and slides)
are housed in the paleobotanical collection of the Florida Museum of Natural History in Gainesville. The BR wood specimens
were mostly allochthonous, but occasional in situ stumps were
encountered. Seven of the eight wood sites were within a ∼2.6-km
span of the BR escarpment, but locality UF 18591 was much farther south, and when included, the distance between the northernmost and southernmost wood sites expanded to ∼4 km. Some
of the BR wood specimens were the subject of a preliminary study
by S. R. Manchester and E. A. Wheeler presented at the 2006
Botanical Society of America annual meeting. They recognized
four dicotyledonous, diffuse-porous xylotypes.
Hand specimens were cut in transverse, radial, and tangential
orientations with a Lortone slab saw with a 10-in diamond
blade. Resulting wafers were trimmed and mounted to glass
slides with epoxy. Sections were ground to ∼30 mm thick or less
(if very dark) using a Buehler PetroThin thin-sectioning machine, and coverslips were mounted with Canada balsam and
CitriSolv. Specimens were observed using a Nikon Eclipse E600
microscope and photographed with a Canon EOS Rebel Xsi camera. Images were further edited using Adobe Photoshop CS5.
Descriptions follow the terminology and definitions outlined in
the International Association of Wood Anatomists (IAWA) list
of microscopic features for softwood and hardwood identification (IAWA Committee 1989, 2004). Quantitative measurements are indicated by the average first, followed by the standard
deviation in parentheses, and then the range (except where specified otherwise). Unless stated in the text, the number of measurements (n) for the following features is as follows: intervessel pit
size: n p 10; mean tangential diameter of vessel lumina: n p 30;
vessels per square millimeter: n p 6; ray height and ray width:
n p 30; and rays per millimeter: n p 10. When multiple specimens of a xylotype were included in a description, quantitative
characters were reported as a range of the calculated lowest
and highest mean and standard deviation of all the samples
followed by the total range (of raw measurements) observed.
Measurements of wood anatomical features were completed on
photos using ImageJ (Rasband 1997–). Means, standard deviations, and other relevant calculations were completed using Microsoft Excel. Presence (p) or absence (a) of IAWA hardwood
features (from IAWA Committee 1989) were entered into the
multiple-entry key of the InsideWood website (http://insidewood to obtain lists of taxa with the combinations of
features seen in the fossil woods (InsideWood 2004–; Wheeler
2011). Multiple searches with different combinations of features were conducted on InsideWood for each fossil xylotype.
The list of features entered for the most useful search is provided
in the discussion of each taxon. The results of each InsideWood
search were reviewed to try to narrow down the possible affinities of a particular fossil xylotype. Once a few potential families
were identified using InsideWood, primary literature references
were consulted for additional information. Angiosperm xylotypes were ordered using the list provided by the Linear Angiosperm Phylogeny Group (Haston et al. 2009).
Two approaches were used to estimate the specific gravity
(SG) of seven different well-preserved wood specimens. The first
Fig. 1 Location of the Blue Rim site (blue dot) in Sweetwater County in southwestern Wyoming. Thick lines demarcate state boundaries,
whereas thin gray lines demarcate county boundaries. Major highways are indicated in red for reference.
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followed the methods of Wheeler et al. (2007b). Using ImageJ
(Rasband 1997–), 500 points (100 points repeated five times)
were randomly placed on an image of a transverse section taken
at 40# total magnification. The number of times a point fell on
lumen (vs. cell wall) was recorded. This allowed the percentage
of wall material to be calculated, which was then multiplied by
1.05 (specific gravity estimate of swollen cell walls, thought to
be similar in both dicotyledons and conifers) as indicated by
Wheeler et al. (2007b). The second approach used the work of
Martínez-Cabrera et al. (2012) and required vessel diameter, fiber
wall thickness, and fiber lumen diameter measurements, which
were also completed in ImageJ (Rasband 1997–). Twenty-five
measurements of fiber wall thickness and corresponding fiber
lumen diameter in the tangential direction were measured in addition to average vessel diameter (n p 30, unless otherwise indicated). These were entered into the first equation in table 1 of
Martínez-Cabrera et al. (2012): SG p 0.802 1 1.346 (WLR) 2
0.22 (LOGvdiam), where WLR is the average fiber wall thickness
to lumen diameter ratio and LOGvdiam is the log of the average
vessel diameter (Martínez-Cabrera et al. 2012). It is important
to note that the fibers and/or vessels were frequently compressed
and distorted in the BR specimens during or prior to fossilization,
reducing the chance that a point would fall in the lumen space and
likely leading to inflated specific gravity estimates. Compression
also affects the ability to obtain accurate measurements of the
original vessel diameter and the fiber lumen diameter. MartínezCabrera et al. (2012) note that their values are consistently greater
than those one would obtain from a basic specific gravity measurement (dry mass/green volume) because the specific gravity estimates they used from extant vegetation were based on an ovendry specific gravity measurement (oven-dry mass/oven-dry volume;
Barajas-Morales 1987; Williamson and Wiemann 2010). However, the samples used in the source data of Barajas-Morales
(1987) were oven dried for 24 h at 1057C, which eliminates
one source of error (samples oven dried at !1007C) noted by
Williamson and Wiemann (2010).
The vulnerability index (V) was calculated for 16 BR wood
specimens. This index, which has been used to infer a plant’s
ability to withstand water stress, is calculated by simply dividing the mean tangential vessel diameter (in micrometers) by the
vessel frequency (Carlquist 1975a, 1975b, 1977; Wheeler 1991).
Finally, five methods (detailed below; McMahon 1973; McMahon and Kronauer 1976; Rich et al. 1986; Brown et al.
1989; Niklas 1994; Feldpausch et al. 2011) were used to estimate
tree height from complete stem diameters.
Systematics and Results
Division—Pinophyta Cronquist, Takht. & W. Zimm. ex Reveal
Family—Pinaceae Spreng. ex Rudolphi
Genus—cf. Pinus L.
Specimen. UF 19406-61964 (fig. 2).
Description. Growth rings intermediate between distinct and
indistinct (but difficult to determine; variation may be due to
preservational differences). Normal axial resin canals present
(fig. 2A, 2B), scattered, distorted, averaging 105 mm in diameter
(21 mm, n p 9), 82–148 mm. Uniseriate intertracheary pits on radial walls of longitudinal tracheids; pits large, averaging 18.7
mm in diameter (1.9 mm, n p 5), 16.2–20.9 mm (fig. 2F, 2G).
The presence or absence of organic deposits in heartwood tracheids and tracheid length not determined due to poor preservation. Mean tangential diameter of longitudinal tracheids 43 mm
(7 mm), 26–55 mm (fig. 2C). Some intercellular spaces present,
but these areas distorted due to compression during fossilization. Torus poorly preserved but appears smooth in outline. Helical thickenings and callitroid thickenings not observed. Axial
parenchyma not observed in either transverse or longitudinal
sections. Ray tracheids present with small circular pits, but poorly
preserved (fig. 2E). Cross-field pitting not well enough preserved
to determine. Ray height averages 198 mm (91 mm), 97–464 mm
(fig. 2H). Rays average eight cells high (four cells, n p 30), four
to 22 cells. Rays mostly uniseriate but occasionally two to three
cells wide (fig. 2H). Ray frequency averages to 10 per millimeter (range: 6–16). Radial canals present (diameter p ∼30 mm,
n p 1) but sparse and generally obscured in tangential section
by the compressed nature of the specimen (fig. 2I, 2J). No minerals or crystals observed.
Remarks. Wood is poorly preserved, compressed, and distorted. Intertracheary pits rarely visible, and cross-field pitting
type indeterminable. The absence of vessels and presence of large
circular bordered pits on the tracheids indicates that this wood is
coniferous and thus the only nonangiosperm wood specimen
from BR.
Discussion of affinities. Among the conifers, only a few genera in Pinaceae (Cathaya, Keteleeria, Larix, Picea, Pinus, and
Pseudotsuga) have normal intercellular canals (Phillips 1941;
Wu and Hu 1997; Heinz 2004; IAWA Committee 2004). Keteleeria has only axial canals, but Cathaya, Pseudotsuga, Pinus,
Picea, and Larix have both normal radial and axial canals as
was observed in this specimen (Phillips 1941; Hu and Wang
1984; Heinz 2004; IAWA Committee 2004). The BR specimen
has some axial canals in pairs, which is common in Pseudotsuga
and Larix. Axial canals in Pinus can generally be distinguished
from those of other genera because of their thin-walled epithelial cells and larger diameters (Phillips 1941; Wu and Hu 1997;
Heinz 2004; IAWA Committee 2004). However, Hilton et al.
(2016) note that thin-walled epithelial cells occur occasionally
in Larix, Picea, and Pseudotsuga. It is difficult to distinguish epithelial cells from the surrounding tracheids in the BR specimen;
however, their absence may be a consequence of being originally
thin-walled, making them less likely to preserve and to be compressed beyond recognition. The diameter of axial resin canals in
extant species of Larix, Pseudotsuga, and Picea is small (∼40–
100 mm), whereas species of Pinus often have medium-sized
canals (∼100–170 mm; IAWA Committee 2004). The axial resin
canals in the BR specimen range from 82 to 148 mm in diameter
with an average of 105 mm. These measurements may slightly
underestimate the diameter of the canals in life because of distortion in the fossil. In addition, Larix often has biseriate (rather
than uniseriate) tracheid pitting in its radial walls (Jagels et al.
2001; Heinz 2004; IAWA Committee 2004), and Cathaya has
prominent growth rings and a lower ray frequency (four to eight
per millimeter; Hu and Wang 1984) compared to the BR specimen (six to 16 per millimeter; average p 10).
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Fig. 2 Pinaceae, UF 19406-61964. A, Overview of the transverse section with scattered axial resin canals. B, Close-up of axial resin canal,
TS. C, Close-up of tracheids, TS. D, Overview of radial section. E, Close-up of ray with ray tracheids along the top of the ray, RLS. F, G, Longitudinal tracheids with large uniseriate pits, RLS. H, Rays mostly uniseriate, TLS. I, Two larger rays, possibly with remnant resin canals, TLS.
J, Ray with resin canal, TLS. RLS p radial longitudinal section; TLS p tangential longitudinal section; TS p transverse section.
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Given the uniseriate intertracheary pitting, resin canal sizes,
and probable thin-walled epithelial cells, this specimen is most
likely a representative of Pinus. Wood of Pinaceae cf. Pinus is
present in the Eocene Clarno Formation of Oregon, but that
taxon has well-defined distinct growth rings and larger axial
canals than the BR specimen (Wheeler and Manchester 2002).
Two species of Pityoxylon, Pityoxylon aldersoni Knowlton
and Pityoxylon amethystinum Knowlton, have been recovered
from the Specimen Ridge site of the Eocene Lamar River Formation in Yellowstone National Park (Knowlton 1899; Conard
1930). Both P. aldersoni and P. amethystinum have numerous
large axial resin canals, but they also have well-defined growth
rings (Knowlton 1899; Conard 1930), unlike the BR specimen.
Fusiform rays are rare in P. aldersoni and the BR specimen, but
they are abundant in P. amethystinum (Knowlton 1899; Conard
1930). The rays average eight cells high in the BR specimen
(range: four to 22 cells); this is intermediate between P. aldersoni, whose rays range from two to 30 cells high with an average of eight to 15, and P. amethystinum, whose rays range from
one to 15 cells high with an average of six to eight (Knowlton
1899; Conard 1930). Knowlton (1899) and Conard (1930) also
note that P. aldersoni has three to 10 rows of tracheids between
each ray in transverse section, whereas P. amethystinum has
three to 25 cells between rays in transverse section; the lower
number of P. aldersoni aligns with observations on the BR specimen. Based on this information, P. aldersoni has more features
in common with the BR specimen, but the presence of welldefined growth rings is an important difference between the
Yellowstone and the BR material.
Division—Magnoliophyta Cronquist, Takht. & W. Zimm. ex
Family—Canellaceae Mart.
Blue Rim Canellaceae Xylotype
Specimen. UF 18591-33036 (fig. 3).
Description. Growth boundaries indistinct with occasional
tangential lines where cells are more crushed (possible thinwalled earlywood; fig. 3A). Wood diffuse-porous with predominantly solitary vessels (92%) and occasional multiples of two
(likely where vessel ends overlap; fig. 3A, 3B). Vessels circular
to oval in outline. Perforation plates scalariform with 10–20 bars
(counted 13–20 bars on seven different plates; fig. 3C, 3E, 3H,
3I). Intervessel pits not observed (probably due to the high proportion of solitary vessels). Vessel-ray pitting with distinct borders (poorly preserved). Mean tangential diameter of vessel lumina 57 mm (14 mm), 28–87 mm. Mean vessel frequency 12 mm2
(range: 4–18 mm2). Some vasicentric tracheids present (alternately, fibers with distinctly bordered pits on both radial and
tangential walls; fig. 3C, 3E, 3F, 3H). Fiber pits visible in radial
section (possibly in tangential section as well, but preservation is
not as good). Fibers thin- to thick-walled. Axial parenchyma likely
rare to absent, as it was not observed in any section. Rays exclu-
sively uniseriate (tangential section poorly preserved; fig. 3G).
Ray height averages 200 mm (85 mm, n p 23), 91–388 mm.
Ray cellular composition poorly preserved, appears to have
procumbent body cells and one or two rows of upright and/or
square marginal cells. Ray frequency ranges from five to 20 per
millimeter (average p 10). No storied structure, intercellular
canals, or crystals observed.
Vulnerability index. Specimen UF 18591-33036 has an estimated V of 4.8.
Discussion of affinities. This specimen is type III as mentioned by Manchester and Wheeler (2006). They noted that the
combination of characters suggested affinities to Ericaceae or
Myrtaceae. However, further investigation suggested affinities
with Canellaceae.
The multiple-entry key of InsideWood was searched for these
features: indistinct growth rings (2p), diffuse-porous wood (5p),
mostly solitary vessels (9p), scalariform perforation plates with
10–20 bars (14p, 16p), vessel-ray parenchyma pits bordered
(30p), and exclusively uniseriate rays (96p). This search yielded
six matches: Balanops australiana (Balanopaceae), Canella winterana, Cinnamosma sp., Pleodendron sp. (Canellaceae), Columellia sp. (Columelliaceae), and Tepuia venusta (Ericaceae; InsideWood 2004–). Tepuia venusta, the only species of Ericaceae
recovered, has solitary vessels with an angular outline, a feature
not observed in the BR material. The character of vasicentric
tracheids (60p) was not used, because it is possible that the
vasicentric tracheids are instead fibers with distinctly bordered
pits. If vasicentric tracheids (60p) is added to the same search list
as above in InsideWood, Myrtaceae (the second family suggested by Manchester and Wheeler [2006]) is only recovered if
two mismatches are allowed (InsideWood 2004–). However,
with the addition of two characters (vessel diameter 50–100 mm
[41p] and five to 20 vessels per square millimeter [47p]) to the
original list of features with no mismatches allowed, the results
were narrowed to C. winterana and Cinnamosma sp. (InsideWood 2004–). Based on this result, a detailed comparison to
both the modern genera in Canellaceae and the fossil species
Wilsonoxylon edenense N. Boonchai et Manchester was undertaken.
Wilson (1960) examined the wood anatomy of Canellaceae.
Comparing his descriptions to the BR Canellaceae Xylotype specimen, two (Cinnamodendron, Warburgia) of the five currently
accepted extant genera can be eliminated from consideration.
All extant genera have scalariform perforation plates, but Cinnamodendron has significantly more bars (50–100) than the
BR specimen (13–20). In addition, the BR specimen has uniseriate rays, yet Wilson (1960) noted that Warburgia has rays ranging from one to four, typically two or three, cells wide. Vessel diameter and frequency is similar between the remaining three
genera, Canella, Cinnamosma, and Pleodendron, and the BR
specimen (Wilson 1960; InsideWood 2004–). Vessels in Pleodendron are usually solitary with a few pairs (similar to the BR
Canellaceae Xylotype), whereas they are usually solitary or occasionally in small clusters in Canella and Cinnamosma (Wilson
1960). However, vessels in Pleodendron tend to be arranged
in a diagonal/radial pattern (InsideWood 2004 –), which was
not observed in the fossil specimen. Canella (five to 28), Cinnamosma (11–49), and Pleodendron (15–40) have a larger range
of bars in their scalariform perforation plates (which can vary to
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Fig. 3 Canellaceae, UF 18591-33036. A, Mostly solitary vessels with tangential lines of crushed cells (darker), TS. B, Close-up view of transverse section showing rare pairs of vessels, likely where vessel element ends overlap. C, Part of a scalariform perforation plate and pits of vasicentric
tracheids and/or fibers, RLS. D, Radial section overview. E, Scalariform perforation plate and pits of vasicentric tracheids and/or fibers, RLS.
F, Close-up of pits of vasicentric tracheids and/or fibers, RLS. G, Uniseriate rays, TLS. H, Mostly intact scalariform perforation plate, TLS.
I, Close-up of scalariform perforation plate, TLS. RLS p radial longitudinal section; TLS p tangential longitudinal section; TS p transverse section.
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reticulate; Wilson 1960) than the BR specimen (13–20), but bars
in the fossil material may be undercounted due to preservational
bias and the difficulties in observing the ends of vessel elements
in sections as opposed to macerations. The BR Canellaceae Xylotype has exclusively uniseriate rays, similar to the predominately
uniseriate rays found in Canella, Cinnamosma, and Pleodendron
(Wilson 1960). There are a few other differences between the BR
Canellaceae Xylotype and extant Canellaceae. The BR Canellaceae Xylotype has thin- to thick-walled fibers, but Canella, Cinnamosma, and Pleodendron have thick- to very thick-walled
fibers. Paratracheal axial parenchyma is present in all extant genera, Cinnamosma can have oil cells, and Canella and Cinnamosma have rhombohedral crystals (Wilson 1960), but these
features were not observed in the fossil. Based on these comparisons, it is not possible to determine which modern genus
of Canellaceae might be the closest to the BR specimen, but it is
probably not Cinnamodendron or Warburgia.
Wilsonoxylon edenense (Canellaceae), from the nearby Eocene Big Sandy Reservoir (BSR) fossil flora (Boonchai and Manchester 2012), shares with the BR Canellaceae Xylotype the
features of mostly solitary vessels, scalariform perforation plates
with 10–20 bars, and indistinct growth rings. The BR Canellaceae Xylotype is diffuse-porous, and the BSR taxon ranges
from semi-ring to diffuse-porous. The BR specimen and W. edenense have mostly solitary vessels with occasional pairs. These
pairs may represent where the section crossed overlapping end
walls of two vessel elements. However, vessel density is significantly different between W. edenense (40–60 mm2; lowest was
22 mm2 in latewood) and the BR specimen (average p 12 mm2).
The mean tangential diameter of vessels in the BR Canellaceae
Xylotype is ∼57 mm, whereas vessel diameters average 35 and
40 mm in the BSR specimens (Boonchai and Manchester 2012).
It is possible that these variations in vessel diameter and density
are due to sampling different parts of the plant (e.g., trunk vs.
branch) or varying wood maturities. Ray cellular composition
is similar between the BR and BSR material. However, W. edenense is clearly distinguished from the BR specimen because it
has two- or three-seriate rays rather than uniseriate. This character, ray width, also precludes the BR specimen from conforming to the generic diagnosis of Wilsonoxylon. It does not seem
advisable to emend the diagnosis of Wilsonoxylon to include
this specimen, because many of the other diagnostic features of
that fossil genus are not preserved in the BR specimen. Furthermore, the BR Canellaceae Xylotype is represented by only a single specimen. Mean ray height is also shorter in the BR specimen
(200 mm) than in the BSR specimens (1300 mm; Boonchai and
Manchester 2012).
This BR wood specimen is not well enough preserved to identify to the generic level, but it does fit the broad characters of the
family Canellaceae. Extant Canellaceae are generally small to
medium-sized evergreen trees. The five genera are disjunct and
found in tropical and subtropical east Africa (Warburgia), Madagascar (Cinnamosma), and the Americas (Canella, Cinnamodendron, and Pleodendron) including South America, the Caribbean, and south Florida (Salazar 2006; Mabberley 2008; Salazar
and Nixon 2008; Müller et al. 2015). However, the range of
Canellaceae extended to at least southwestern Wyoming in the
Eocene based on the presence of W. edenense and this specimen
from BR.
Family—Fabaceae Lindl.
Subfamily—Caesalpinioideae DC.
Genus—Peltophoroxylon Müller-Stoll et Mädel
Species—Peltophoroxylon diversiradii S.E. Allen, sp. nov.
Diagnosis. Diffuse-porous. Vessels in radial multiples of
two or three and solitary; perforation plates simple. Intervessel
pits generally alternate, small to medium in size. Vessel-ray parenchyma pitting similar to intervessel pitting. Nonseptate fibers
present. Prominent paratracheal axial parenchyma: vasicentric,
occasional lozenge-aliform, and confluent. Occasional marginal
parenchyma bands present or bands more than three cells wide.
Rays mostly biseriate, occasionally uniseriate, rarely wider. Ray
cellular composition variable from homocellular procumbent
to heterocellular with procumbent body cells and one to three
rows of upright/square marginal cells. No storied structure.
Holotype, hic designatus. UF 19338W-61966 (fig. 4B, 4D–
4F, 4J).
Repository. Florida Museum of Natural History (FLMNH),
Gainesville, Florida.
Type locality. Blue Rim, Sweetwater County, Wyoming.
Stratigraphic position and age. Lower Bridger Formation,
latest Early Eocene.
Paratypes. UF 15761-56323; 18591-33042; 19338W-61965
(fig. 4A, 4C, 4G–4I).
Etymology. The specific epithet is Latin for “diverse rays.”
Extant species of Peltophorum and many fossil species of Peltophoroxylon have homocellular rays, with all cells procumbent. However, this taxon has predominately biseriate rays that
are both homocellular (all cells procumbent) and heterocellular
(procumbent body cells and one to three rows of upright/square
marginal cells).
Description. Growth ring boundaries variable from weakly
defined by irregular and discontinuous bands of marginal parenchyma to indistinct; wood diffuse-porous (fig. 4A– 4C). Vessel
arrangement scattered with a weak radial pattern. Vessels solitary (23%) or grouped in radial multiples of two or three, occasional clusters or radial multiples of four or more (fig. 4A– 4C).
Infrequent, short tangential bands of vessels present. Solitary
vessel outline rounded; perforation plates simple (fig. 4E). Intervessel pits usually alternate, very rarely opposite with a mean
diameter across the samples of 5.3 mm (0.3 mm, UF 19338W61965) to 7.1 mm (0.8 mm, UF 19338W-61966) and a total
range of 4.8–8.8 mm (fig. 4J). Intervessel pits usually rounded
or with four or fewer sides but occasionally polygonal in outline;
pit aperture possibly vestured (fig. 4J). Vessel-ray pits are small,
inconspicuous, and similar to intervessel pits (fig. 4I). Helical
thickenings not observed. Mean tangential diameter of vessel lumina 45 mm (17 mm, UF 19338W-61966) to 74 mm (20 mm, UF
18591-33042) with a total range of 13–108 mm. Vessel frequency
variable (range: 0 – 46 mm2, n p 3). Mean vessel element length
173 mm (40 mm, n p 11, UF 15761-56323) to 213 mm (88 mm,
n p 30, UF 19338W-61966) with a total range across all specimens of 79–517 mm. Tyloses and ground tissue pits not observed. Nonseptate fibers present; no septate fibers observed.
Fibers vary from thin- to very thick-walled. Paratracheal axial
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Fig. 4 Peltophoroxylon diversiradii (Caesalpinioideae s.l., Fabaceae). A, UF 19338W-61965. Vasicentric and banded axial parenchyma, TS.
B, UF 19338W-61966. Vessels grouped in short radial multiples, occasional clusters or solitary, TS. C, UF 19338W-61965. Rays and vessels
in radial multiples surrounded by axial parenchyma, TS. D–F, UF 19338W-61966. D, Parenchyma clearly visible adjacent to the vessel,
RLS. E, Simple perforation plate and heterocellular rays, RLS. F, Ray with procumbent body cells and a row of upright/square marginal cells,
RLS. G, H, UF 19338W-61965. G, Overview of tangential section. H, Close-up of mostly biseriate rays and strands of axial parenchyma, TLS.
I, UF 15761-56323. Vessel-ray pits, RLS. J, UF 19338W-61966. Alternate intervessel pits, possibly with vestured apertures, TLS. RLS p radial longitudinal section; TLS p tangential longitudinal section; TS p transverse section.
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parenchyma present, with patterns including vasicentric, occasional lozenge-aliform, and confluent (fig. 4A– 4C). Occasional
banded parenchyma more than three cells wide, and marginal
parenchyma bands present. Axial parenchyma strand length varies from three to more than eight cells. Rays generally one or two
cells wide, very rarely wider, with two-seriate the most common
condition (fig. 4G, 4H). Rays average 189 mm (77 mm, UF
19338W-61966) to 263 mm (93 mm, UF 15761-56323) high
with a total range of 64–522 mm. Ray cellular composition variable. Rays homocellular procumbent or heterocellular with procumbent body cells and one, two, or three rows of upright and/
or square marginal cells (fig. 4D– 4F). Ray frequency ranges
from six to 12 per millimeter (n p 3). No storied structure or intercellular canals observed. Rare prismatic crystals in chambered axial parenchyma cells.
Vulnerability index. The value of V averages 5.2 (4.4, n p 3);
however, the range (1.9–10.3) is large due to variations in vessel
frequency among specimens.
Remarks. The prominent vasicentric to lozenge-aliform to
confluent axial parenchyma differentiates this wood xylotype
from others at BR. The absence of radial canals readily distinguishes this from the co-occurring and most common BR wood
species, Edenoxylon parviareolatum Kruse. Peltophoroxylon
diversiradii corresponds to wood type IV as identified by Manchester and Wheeler (2006), who noted that these characters are
found in Fabaceae, Sapindaceae, and Oleaceae. Further investigations (discussed below) indicate that these wood specimens
correspond to Fabaceae.
Discussion of affinities. When the most conspicuous and
well-preserved features of this xylotype were entered into InsideWood (5p, diffuse-porous; 13p, simple perforation plates; 22p,
24a, and 27a, intervessel pits alternate and not minute or large;
66p, nonseptate fibers present; 69p, fibers thin- to thick-walled;
79p, 80p, 81p, 83p, 85p, and 89p, axial parenchyma vasicentric, aliform, confluent, in bands more than three cells wide
and in marginal bands; 97p, rays one to three cells wide; 104p
and 106p, all ray cells procumbent and with one row of upright
cells; 118a and 120a, rays, axial parenchyma, and vessel elements not storied) with one mismatch allowed, the results were
focused on taxa in Caesalpinioideae and the genus Terminalia
in Combretaceae (InsideWood 2004–; Wheeler 2011). Terminalia does not match with the fossil material based on its common tyloses, frequent traumatic intercellular canals, and lack
of axial parenchyma bands more than three cells wide (InsideWood 2004–). The InsideWood search returned only two members of Sapindaceae and none of Oleaceae—the other two families considered by Manchester and Wheeler (2006). However,
if vestured pits (29p, possible in the fossil material) are added
to the same feature list as above, Sapindaceae is no longer recovered. None of the 12 Caesalpinioideae genera suggested on
InsideWood match the BR specimens based on the presence
of one or more characters. However, further comparison within
the legumes, especially to Caesalpinioideae, is warranted.
The characters of the other two traditional subfamilies of Fabaceae, Mimosoideae and Papilionoideae, were reviewed first to
ensure that the fossil material fit best with the Caesalpiniodeae.
Although the differences in wood anatomy between the traditional subfamilies of Fabaceae are subtle, they can be helpful
(Baretta-Kuipers 1981; Wheeler and Baas 1992; Ogata et al.
2008). Members of the Mimosoideae have exclusively homo-
cellular rays, whereas ∼30% of Caesalpinioid genera and ∼20%
of genera in Papilionoideae have heterocellular rays (BarettaKuipers 1981; Wheeler and Baas 1992; Ogata et al. 2008). Small,
low rays with exclusively procumbent cells are rare in Caesalpinioideae and Papilionoideae, but when present in either of these
subfamilies, the rays are storied (Baretta-Kuipers 1981). Approximately 75% of Papilionoid genera have all elements storied,
whereas this is absent in Mimosoideae and found in less than
25% of Caesalpinioid genera (Baretta-Kuipers 1981; Wheeler
and Baas 1992; Ogata et al. 2008). In addition, members of Caesalpinioideae and Mimosoideae rarely have helical thickenings
or ring porosity, but both of these characters are common in
Papilionoideae (Wheeler and Baas 1992). Furthermore, Caesalpinioid genera are the least likely of the three subfamilies to
have one- or two-celled parenchyma strands; ∼35% of taxa
have strands with more than four cells, similar to P. diversiradii
(Baretta-Kuipers 1981; Wheeler and Baas 1992). As neither storied structure nor small, low, homocellular rays are present in
P. diversiradii, both Papilionoideae and Mimosoideae are less
likely to accommodate this fossil. Two of the fossil specimens
have prismatic crystals in chambered axial parenchyma cells, a
common feature in Caesalpinioideae (Melandri and Espinoza
de Pernía 2009). This process of elimination suggests that the
fossil specimens represent Caesalpinioideae sensu lato.
The traditional Caesalpinioideae subfamily has approximately 2250 species in over 170 genera and four tribes (Lewis
et al. 2005–; LPWG 2013a, 2013b). Recent phylogenetic work
on Fabaceae has found Caesalpinioideae to be paraphyletic;
Papilionoideae and Mimosoideae are monophyletic clades that
arose from within Caesalpinioideae (LPWG 2013a, 2013b). The
Legume Phylogeny Working Group recently released a new
subfamily-level classification for Fabaceae (LPWG 2017). In this,
the legumes are now composed of six subfamilies; the former
Mimosoideae is now a clade in a recircumscribed Caesalpinioideae, Papilionoideae is still a subfamily, and Cercidoideae, Detarioideae, Dialioideae, and Duparquetioideae are newly recognized at the rank of subfamily (LPWG 2017). Here, the use of
Caesalpinioideae is in the broader, traditional circumscription
(one of three subfamilies within Fabaceae). A brief commentary
about the new classification and its potential effect on the placement of P. diversiradii is provided at the end of this discussion.
Taxonomic affinities within extant Caesalpinioideae. BarettaKuipers (1981) divided the Caesalpinioideae into two groups
based on wood anatomy. The first group, composed of the tribes
Caesalpineae, Cassieae, and Cercideae, is characterized by homocellular rays averaging 450 mm in height. The wood anatomy
of eight genera within Caesalpinieae has been explored further,
and of these, Dimorphandra shares the most characters with the
BR specimens (Espinoza de Pernía and Melandri 2006). Mora
is also similar but generally has homocellular rays, whereas the
fossils have both homocellular and heterocellular rays. Characters that exclude the other genera studied in Caesalpinieae include regular, very large parenchyma bands; intervessel pits that
are too large; mostly or exclusively uniseriate rays; or having storied structure (Espinoza de Pernía and Melandri 2006). The second group within Caesalpinioideae includes the tribes Detarieae
and Amherstieae (Amherstieae is not currently recognized as a
tribe; Lewis et al. 2005–; LPWG 2013a) and has mostly heterocellular rays with an average height of 650 mm (Baretta-Kuipers
1981). The wood anatomy of 10 genera in the tribe Detarieae
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has been explored in detail, and four (Copaifera, Elizabetha,
Eperua, and Heterostemon) can be eliminated from consideration due to either the presence of axial intercellular canals or
mostly/exclusively uniseriate rays, as neither of these characters
is present in P. diversiradii (Melandri and Espinoza de Pernía
2009). Many of the genera in both Caesalpinieae and Detarieae
have large intervessel pits, but Brownea, Cynometra, Dimorphandra, and Mora tend to have smaller pits that are more consistent with P. diversiradii (Melandri and Espinoza de Pernía
2009). However, these genera have other characters that do not
align with the fossil material (InsideWood 2004 –). The similarity of wood characters across Caesalpinioideae s.l. and the inability to observe important features in the fossil specimens that
could help narrow down the possible affinities precludes a taxonomic assignment to an extant genus in Caesalpinioideae.
Comparison to other fossil wood of Fabaceae. Three fossil
legume woods from the middle Eocene Clarno Formation in
Oregon were also compared with P. diversiradii (table 1; Wheeler
and Manchester 2002). All three Clarno taxa, Dichrostachyoxylon herendeenii, cf. Euacacioxylon, and cf. Mimosoxylon,
have a larger average tangential diameter of vessels and differing ray width and axial parenchyma patterns as compared to
P. diversiradii (table 1; Wheeler and Manchester 2002). Intervessel pits are larger in two of the Clarno taxa, and average ray
height in D. herendeenii is similar to the BR specimens, but it is
greater in both cf. Euacacioxylon and cf. Mimosoxylon (table 1;
Wheeler and Manchester 2002). Although there is some overlap
in characters between each of the Clarno legumes and P. diversiradii, there are enough differences to conclude that they are distinct.
Fritz and Fisk (1978) noted a possible fabaceous wood from
Yellowstone National Park. Recent examination of “Laurinoxylon pulchrum” Knowlton indicates that it is a legume rather
than Lauraceae (InsideWood 2004–; E. Wheeler, personal communication, 2016). However, “L. pulchrum” differs from the
BR material in having locally storied parenchyma and semi-ring
porosity (Wheeler and Baas 1992; InsideWood 2004–). Legume
wood has also been documented from Eocene strata in the
Willwood Formation in Park County, Wyoming. Whereas
the Willwood material has features similar to P. diversiradii
in transverse section, it differs in having well-defined storied
structure in longitudinal section (Wheeler and Baas 1992).
Assignment to Peltophoroxylon. Müller-Stoll and Mädel
(1967) established multiple genera for fossil legume wood. The
BR legume is not a perfect match to any of these, but it is most
closely aligned with Peltophoroxylon, in part because the rays
range to having one row of upright marginal cells rather than
being exclusively homocellular procumbent. The main differences are that the fibers are generally septate (rather than nonseptate) and the parenchyma is occasionally in zig-zag-shaped
bands in Peltophoroxylon (Müller-Stoll and Mädel 1967), neither of which was observed in the fossil specimens. If “fibers
not septate” is selected in Müller-Stoll and Mädel’s (1967) key
to the fossil legume genera and the characters matching the fossil are followed (as opposed to following the key to Peltophoroxylon), it terminates at the genera of Berlinioxylon, which usually has uniseriate homocellular rays, or Pahudioxylon, which
also has homocellular rays and distinct growth rings with large
vessels, unlike the fossil material.
Peltophoroxylon includes fossil woods with features similar
to extant Cassia, Peltophorum, and Xylia (Müller-Stoll and
Mädel 1967; Bande and Prakash 1980; Awasthi 1992). Prakash
(1975) later excluded fossil woods from Peltophoroxylon with
similarities to extant Cassia and created a new fossil genus,
Cassinium. However, the broader diagnosis of Peltophoroxylon
is retained here, as these two genera are very similar, and minor
differences are challenging to recognize in fossils.
Multiple species of Peltophoroxylon have been documented,
primarily from the Miocene of southern Asia (Bande and Prakash 1980; Awasthi 1992; InsideWood 2004–; Gregory et al.
2009). Most of these species have significantly larger vessel diameters than the BR taxon, and many have exclusively homocellular (all cells procumbent), wider, or storied rays (Awasthi
1992; InsideWood 2004–). In the Western Hemisphere, Peltophoroxylon uruguayensis was recently documented from the
late Pleistocene of eastern Argentina (Ramos et al. 2014). Peltophoroxylon uruguayensis shares the feature of predominately
biseriate rays with P. diversiradii, but its rays are exclusively
homocellular procumbent (Ramos et al. 2014). The taxon most
similar to the BR specimens is Peltophoroxylon sp. from the late
Eocene marine sands in Helmstedt, Lower Saxony, Germany
(Gottwald 1992). This species has many features in common
with P. diversiradii, including a lack of clear growth rings,
small-diameter vessels, similar-sized intervessel pits, and the presence of crystals (Gottwald 1992). However, the Helmstedt taxon
has predominately uniseriate rays that are homocellular and only
weakly heterocellular, with some marginal ray cells tending to be
Extant Peltophorum remains within Caesalpinioideae even
with the new subfamily classification of legumes (LPWG 2017).
A wood anatomy feature that separates the new subfamilies is
the presence or absence of vestured pits. Vestured pits are absent
in the newly recognized subfamilies of Duparquetioideae, Cercidoideae, and most genera of Dialioideae, whereas they are
present in the other three subfamilies (Quirk and Miller 1985;
Herendeen 2000; Gasson et al. 2003; Herendeen et al. 2003;
Bruneau et al. 2008; LPWG 2017). It is difficult to observe vestured pits with light microscopy in modern woods and even
more challenging in fossil woods that are not always well preserved. Even at high magnification, it was not possible to confidently say whether the pit apertures on the BR specimens are
vestured or are simply obscured by debris.
Family—Anacardiaceae R. Br.
Genus—Edenoxylon Kruse
Species—Edenoxylon parviareolatum Kruse emend. N. Boonchai
et Manchester
Specimens. UF 00341S-61961; 15761-56320; 18289-56305,
56306, 56308; 18591-33008, 33021, 33023, 33030, 33034,
33041 (figs. 5, 6).
Description. Growth ring boundaries indistinct (occasional
changes in fiber preservation but discontinuous) to absent.
Wood diffuse-porous with vessels solitary (27%), in radial multiples of two to four, occasionally in radial multiples of five or
six, or rarely clusters (figs. 5A, 6A, 6B). Solitary vessel outline
rounded. Perforation plates simple. Intervessel pits alternate
to rarely opposite, minute to small with an average across specimens of 3.8 mm (0.4 mm, UF 00341S-61961) to 4.8 mm (0.7 mm,
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Paratracheal: vasicentric, aliform,
occasionally lozenge-aliform,
confluent, occasionally banded/
marginal bands
1–2-seriate, rarely larger
Homocellular procumbent or
heterocellular with procumbent
body cells and one to three
rows of upright/square
marginal cells
Rare, in chambered axial parenchyma
Peltophoroxylon diversiradii
1– 4-seriate
Homocellular procumbent or
heterocellular with procumbent
body cells and one or occasionally
two rows of upright/square
marginal cells
Present, in chambered axial parenchyma
Paratracheal: vasicentric, aliform,
occasionally confluent
Dichrostachyoxylon herendeenii
Not observed
Mostly 3– 4-seriate
Homocellular procumbent or
heterocellular with procumbent
body cells and one row of
upright/square marginal cells
Paratracheal: vasicentric, aliform,
cf. Euacacioxylon
Clarno Formation
Not observed
1– 4-seriate
Heterocellular with procumbent
body cells and one or occasionally
two rows of upright/square
marginal cells
Scanty paratracheal to narrow
cf. Mimosoxylon
Clarno Formation data from Wheeler and Manchester (2002). IP p intervessel pit size; RH p mean ray height; VD p mean tangential diameter of vessel lumina.
Storied structure
RH (µm)
Ray width
Ray cellular composition
VD (µm)
IP (µm)
Axial parenchyma
Bridger Formation
Comparison of the Blue Rim (Bridger Formation, Wyoming) and Clarno Formation (Oregon) Legumes
Table 1
Fig. 5 Edenoxylon parviareolatum (Anacardiaceae), UF 00341S-61961 (all except B). A, Vessels grouped in radial multiples and solitary,
TS. B, UF 18289-56306. Radial section showing ray with procumbent body cells and upright/square marginal cells. C, Close-up of vessels and
fibers, RLS. D, Crystals common in square/upright ray cells, RLS. E, Alternate intervessel pits and crystals in the adjacent ray cells, TLS. F, Vessel
elements with alternate intervessel pits interspersed between rays with prismatic crystals, TLS. G, Ray with well-defined radial canal, TLS. H, Broader
view of tangential section. RLS p radial longitudinal section; TLS p tangential longitudinal section; TS p transverse section.
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Fig. 6 Edenoxylon parviareolatum. A, UF 18591-33008. Transverse section overview. B, UF 18289-56308. Tyloses present in the vessels,
TS. C, UF 18591-33008. Close-up of fiber cells, TS. D, UF 18289-56308. Tangential section overview with radial canals. E, UF 18591-33008.
Multiseriate ray with radial canal. Ray to the lower right has a prismatic crystal, TLS. F, UF 18591-33030. Double radial canal and vessel
elements, TLS. G, UF 18591-33023. Ray with two radial canals, TLS. H, I, UF 18289-56305. H, Ray cells with prismatic crystals, RLS. I, Oblong
rounded to horizontal (gash-like) vessel-ray pitting, RLS. J, UF 18289-56306. Septate fibers, RLS. RLS p radial longitudinal section; TLS p tangential longitudinal section; TS p transverse section.
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UF 18591-33030), with a total range of 3.2–6.6 mm (fig. 5E).
Vessel-ray pitting not preserved in many specimens, but pits
oblong rounded or horizontal with much-reduced borders to
apparently simple when observed (fig. 6I). Vestured pits and helical thickenings not observed. Mean tangential diameter of vessel lumina across all specimens 47 mm (15 mm, UF 18591-33030)
to 78 mm (17 mm, UF 18289-56306), with a total range of 14–
113 mm. Vessels per square millimeter ranges from eight to 54.
Tyloses present (occasionally appearing sclerotic), often extensive and obscuring vessel element end walls (figs. 5C, 5H, 6B).
Fibers with simple to minutely bordered pits. Helical thickenings
in ground tissue fibers not observed. Both septate and nonseptate fibers present (fig. 6J). Fibers occasionally very thin- or very
thick-walled, more frequently thin- to thick-walled (fig. 6C).
Axial parenchyma present but uncommon, diffuse apotracheal,
scanty paratracheal, and in thin, discontinuous marginal bands.
Axial parenchyma with approximately three to more than eight
cells per strand.
Rays mostly one or two cells wide (figs. 5E, 5F, 5H, 6D), occasionally three (wider in rays with radial canals). When single
cells present in center of multiseriate ray, width of uniseriate
portion equals width of neighboring multiseriate portion. Ray
width (without canals) averages 17 mm (5 mm, UF 1859133008) to 21 mm (6 mm, UF 18591-33030), with a total range
of 7–36 mm. Ray height averages 167 mm (48 mm, UF 1828956308) to 253 mm (98 mm, UF 18591-33008), with a total range
of 94– 487 mm. Ray cellular composition variable ranging from
body cells procumbent with one to more than four rows of upright/square cells to procumbent, square, upright mixed throughout ray (figs. 5B, 5D, 6H). Rays per millimeter ranges from six
to 20. No storied structure. Radial canals present (figs. 5G,
6D–6G) but variable in frequency among specimens.
Across specimens, for rays with a single canal, the average
heights are 345 mm (129 mm, n p 6, UF 18289-56308) to 509 mm
(288 mm, n p 2, UF 18289-56305), with a total range of 194–
712 mm; their average widths are 60 mm (6 mm, n p 3, UF
18591-33030) to 98 mm (18 mm, n p 14, UF 18591-33023),
with a total range of 47–136 mm; the canal openings have average lengths of 50 mm (16 mm, n p 6, UF 18289-56308) to 92 mm
(42 mm, n p 2, UF 18289-56305), with a total range of 26–
121 mm, and average widths of 28 mm (4 mm, n p 3, UF 1859133030) to 61 mm (17 mm, n p 14, UF 18591-33023), with a total
range of 21–97 mm.
Rays with two canals (three examples measured from all specimens; fig. 6F, 6G) range from 437 to 499 mm high by 71 to
123 mm wide. Within these rays, canal size ranges from 35 to
81 mm high by 21 to 70 mm wide. Radial canals generally oblong
in shape but occasionally more rounded. Well-preserved canals
with a ring of well-defined epithelial cells around the inside of
the canal opening.
Prismatic crystals present, common in upright/square and
procumbent ray cells (figs. 5D, 6H). Rarely more than one crystal of the same size per cell. Crystals in the ray cells average
16 mm (2 mm, n p 5, UF 18591-33030) to 20 mm (3 mm, n p 13,
UF 18591-33021) in the longest dimension, with a total range
of 12–29 mm. Occasionally prismatic crystals in axial parenchyma and/or fibers.
Specific gravity. Two approaches were used to estimate the
specific gravity of six specimens of this wood xylotype. Specific
gravity averages 0.75 (0.10, range: 0.58–0.87) using the meth-
ods of Wheeler et al. (2007b), whereas using equation (1) of
Martínez-Cabrera et al. (2012), the average specific gravity is
1.05 (0.20, range: 0.82–1.35).
Vulnerability index. Values of V were calculated for 10 of
the 11 E. parviareolatum specimens (all except UF 1859133041) for an average of 3.0 (1.9, range: 1.4–5.7).
Remarks. This is the most common wood type at BR, and
it is represented by stems of varying diameters from different
sites across the escarpment. Most samples are only small fragments of larger logs or branches (e.g., UF 18591-33008 is
∼3.5 cm in diameter), but they are occasionally much larger
(e.g., UF 18591-33023 is ∼30.5 cm in diameter). Although all
BR specimens with numerous tyloses, radial canals, and prismatic crystals are considered to be a single xylotype, there are
differences. A few specimens (e.g., UF 18289-56306, UF 1859133021, and UF 18591-33023) have significantly lower vessel frequency (∼8–20 mm2) as compared to the other specimens (∼24–
44 mm2). This variation might be due to differences in wood
maturity or origin in the plant (e.g., trunk vs. branch wood).
Fichtler and Worbes (2012) found large differences in vessel frequency within a species and between sites. The frequency of radial canals is also variable among specimens. A few specimens
(e.g., UF 18289-56308 and UF 18591-33023) have numerous
radial canals, but they are rare in other specimens (e.g., UF
00341S-61961 and UF 18591-33021). The frequency of radial
canals has been shown to vary greatly between individuals, the
location in the stem (e.g., close to the pith), or the maturity of
the wood (Chattaway 1951; Terrazas and Wendt 1995).
Discussion of affinities. The most distinguishing feature
of this xylotype, radial intercellular canals, is found in several
families of the Sapindales, including Anacardiaceae and Burseraceae in addition to some species of Fabaceae, Apiaceae, Araliaceae, Moraceae, Clusiaceae, Dipterocarpaceae, and Euphorbiaceae (InsideWood 2004–; Carlquist 2012). The rhomboidal
prismatic calcium oxalate crystals found in many of the rays of
these specimens can also be of systematic value (Carlquist 2012).
When the characters 2p (growth rings indistinct/absent); 5p
(diffuse-porous); 13p (simple perforation plates); 22p, 26a,
and 27a (intervessel pits alternate, not medium or large); 30a
(vessel-ray pitting different from intervessel pits); 56p (tyloses
common); 97p (rays one to three cells wide); 130p (radial canals);
and 136p (prismatic crystals present) were entered into InsideWood
with 0 mismatches allowed, there were five results: Melanochyla sp.,
Myracrodruon urundeuva, Pistacia terebinthus (Anacardiaceae),
Canarium indicum (Burseraceae), and Baloghia sp. (Euphorbiaceae; IAWA Committee 1989; InsideWood 2004–; Wheeler
2011). Although none of these taxa were a match to the BR material, Anacardiaceae was the best fit overall, which matches the
findings of Manchester and Wheeler’s (2006) wood type I,
which they also identified as Anacardiaceae.
Taxonomic affinities within extant Anacardiaceae. The
minute to small (!7-mm) intervessel pits in the BR specimens
were used to determine which extant genera of Anacardiaceae
to compare with the fossils. This character is usually consistent
even if the wood is of variable maturity or from a different part
of the plant. Faguetia, Haplorhus, Lithraea, Loxostylis, Micronychia, Rhus, and Trichoscyphya have intervessel pits under
7 mm (Terrazas 1994). However, Faguetia, Lithraea, and Micronychia do not have radial canals, which is a significant contrast
with the BR specimens, so they were not considered further.
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The remaining genera, Haplorhus, Loxostylis, Rhus, and
Trichoscypha, are all members of the tribe Rhoeae (Pell 2004).
Haplorhus, an evergreen shrub or small tree found in Peru and
Chile, has small intervessel pits (Terrazas 1994), but none of
the other quantitative characters examined are in the same range
as the BR specimens. In addition, Haplorhus has homocellular rays with very few prismatic crystals (Terrazas 1994), which
differs from the BR material. Loxostylis, a small evergreen tree
or shrub found in South Africa, is one of a few genera in Anacardiaceae with toothed leaf margins (Terrazas 1994). Vessel
frequency and diameter in Loxostylis matches the ranges observed on the BR specimens. Furthermore, ray height in Loxostylis is on the shorter side for Anacardiaceae with an average
of 318 mm (Terrazas 1994). Rhus can be evergreen or deciduous
and is found in temperate to tropical regions of Asia and North
America (Terrazas 1994). Some species of Rhus have rays !300 mm
high, and vessel frequency and diameter are also similar to the
BR specimens. Trichoscypha, an evergreen tree from tropical
West Africa, has silica bodies and significantly higher rays (Terrazas 1994) than the BR Edenoxylon specimens.
Additional characters that are well preserved in the BR Edenoxylon (tyloses, fibers, axial parenchyma, ray cellular composition, and crystals) were compared to the three remaining
genera (Loxostylis, Rhus, and Trichoscypha). Although none
of these have characters that specifically exclude it, Rhus is the
best match (of extant genera) with the fossil specimens because
it has frequent prismatic crystals in both marginal and procumbent ray cells and similar vessel-ray pits (characters 31 and/or
32; InsideWood 2004 –). Furthermore, Rhus is the most geographically widespread taxon today, and both Loxostylis and
Trichoscypha are confined to Africa (Terrazas 1994). Wood
of temperate Northern Hemisphere Rhus is typically semi-ringporous to ring-porous (Boonchai and Manchester 2012); however, 10 of the 24 Rhus species in InsideWood are diffuse-porous
(InsideWood 2004–). Most of the diffuse-porous species have
wider vessels and much larger intervessel pits than observed
in the BR specimens (Boonchai and Manchester 2012). Leaves
assigned to Rhus nigricans (Lesquereux) Knowlton in the Green
River flora have also been found at BR (MacGinitie 1969).
However, this wood xylotype might not represent a living genus.
At least one extinct anacardiaceous genus, Pentoperculum,
which is known from fruits of tribe Spondioideae (Manchester
1994; Collinson et al. 2012), is also found at BR.
Comparison to E. parviareolatum. Edenoxylon parviareolatum, established by Kruse in 1954, is an anacardiaceous fossil
wood that was first described from the Green River Formation
Hays’ Ranch site (∼16 mi to the east of Farson) in southwestern
Wyoming and later was found to be represented by numerous
specimens from the BSR flora, also from southwestern Wyoming
(Kruse 1954; Boonchai and Manchester 2012).
Edenoxylon parviareolatum from BSR has growth ring boundaries weakly defined by a change in fiber radial diameter and
marginal parenchyma. The BR specimens have absent to indistinct growth rings, although specimen UF 18591-33034 did
have variations in fiber preservation across the transverse section. The distinctness of growth rings can vary within an individual tree, as rings tend to be more obvious in trunks than in
branches (Tarelkin et al. 2016). Furthermore, coding of growth
ring distinctness can vary between researchers, and some of the
features that define growth rings such as changes in fiber wall
thickness or radial diameter (IAWA Committee 1989; Fichtler
and Worbes 2012; Tarelkin et al. 2016) are difficult to see on
fossils and may be masked by poor preservation. The BR specimens are diffuse-porous, whereas some of the BSR specimens are
semi-ring-porous. All BR specimens have significantly lower vessel frequency (∼8–54 mm2) than E. parviareolatum from other
sites (56–70 mm2 at BSR and ∼100 mm2 at Hays’ Ranch; Kruse
1954; Boonchai and Manchester 2012). However, vessel frequency was recounted using the images of the BSR E. parviareolatum and the corresponding scale bar (Boonchai 2012;
Boonchai and Manchester 2012) with the same ImageJ approach as the BR specimens, and lower numbers that overlapped
with the BR material were obtained. Moreover, there can be considerable variation in wood anatomical characters both within
species and between sites (Fichtler and Worbes 2012). Vessel
arrangement and diameter are comparable in both the BR and
the BSR specimens, and both have simple perforation plates.
Due to the numerous tyloses obscuring end walls, mean vessel
element length could be determined in only two BR specimens
(average: 80 and 108 mm; range: 30–195 mm), and they are much
shorter than in the BSR specimens (average: 224–362 mm; Boonchai and Manchester 2012) but within the range of the Hays’
Ranch material (100–300 mm; Kruse 1954). It is probable that
a few of the measurements for vessel element length in the BR
specimens are less than the actual length of the vessel element
if a tylose mimicked an end wall. The BSR, Hays’ Ranch, and
BR specimens all have numerous tyloses and alternate intervessel
pits that are comparable in size (Kruse 1954; Boonchai and
Manchester 2012). The BR and BSR specimens have similar axial parenchyma patterns and ray width, and black deposits fill
the lumina of some of the vessels in E. parviareolatum from both
sites (Boonchai and Manchester 2012). In general, rays without
radial canals are shorter in the specimens from BR. However,
rays with radial canals are similar or larger in size in the BR specimens as compared to E. parviareolatum from BSR. The BR
specimens also have fewer rays per millimeter than the BSR specimens. The BR, BSR, and Hays’ Ranch specimens all have septate fibers and prismatic crystals in the rays and share similar
ray cellular composition and vessel-ray pitting type (Kruse
1954; Boonchai and Manchester 2012).
Kruse’s (1954) stem of E. parviareolatum from Hays’ Ranch
is slightly different from the BSR specimens in that axial parenchyma is rarer and the rays are almost exclusively uniseriate
(Boonchai and Manchester 2012). The mean tangential vessel
diameter of 50 mm in Kruse’s specimen is closer to the averages
of the BR specimens than the BSR material (Kruse 1954; Boonchai and Manchester 2012). However, the differences in vessel
diameter could relate to plant maturity.
Comparison to other fossil wood of Anacardiaceae. Wood
of three species of Anacardiaceae, Maureroxylon crystalliphorum, Tapirira clarnoensis, and Terrazoxylon ductifera, are documented from the middle Eocene Clarno Nut Beds in Oregon and
warrant comparison to the BR material (Wheeler and Manchester 2002). All three Clarno species have larger-diameter vessels,
larger intervessel pits, and higher rays as compared to the BR
specimens. Maureroxylon crystalliphorum further differs from
the BR E. parviareolatum specimens in that it has clearly defined growth rings, wider rays, and radial canals are absent
(Wheeler and Manchester 2002). Tapirira clarnoensis and Terrazoxylon ductifera have radial canals. All three Clarno taxa
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have tyloses and crystals, as does the BR material. However,
none of the Clarno taxa are similar enough to the BR xylotype
to suggest that it is the same taxon.
The fossil species, Rhus crystallifera (Anacardiaceae), has
been documented from the Eocene Amethyst Mountain locality
in Yellowstone National Park (Wheeler et al. 1978). It differs
from the BR E. parviareolatum in having distinct growth rings;
semi-ring porosity; vessels mostly solitary with occasional clusters, especially in the latewood; larger intervessel pits; and no
radial canals (Wheeler et al. 1978).
Boonchai and Manchester (2012) also compared the BSR specimens to other Eocene anacardiaceous woods, including Edenoxylon aemulum found in Kent, England (Brett 1966; Wheeler
and Manchester 2002). Brett (1966) provided a generic diagnosis for Edenoxylon (unlike Kruse who had a combined genus
and species description), which matches the BR material. Furthermore, Brett (1966) noted that E. parviareolatum, so named
for the small intervessel pits, matches a few extant genera in the
tribe Rhoeae, including Rhus, Faguetia, and Trichoscyphya, to
which the similarities with the BR material have already been
discussed. Most of these fossil anacardiaceous wood species, including the BR specimens, are diffuse-porous (R. crystallifera is
semi-ring-porous), with similar vessel arrangements, heterocellular rays, and simple perforation plates. However, of the Eocene woods assigned to Anacardiaceae, the BR material is most
closely aligned with E. parviareolatum.
Indet. Family
Incertae Sedis
Blue Rim Xylotype 1
Specimen. UF 19225-54694 (fig. 7A–7C).
Description. Likely diffuse-porous wood with growth rings
indistinct or absent. Vessels solitary (40%), in radial multiples
of two or occasionally three or four (fig. 7A). Rare clusters of
vessels (often three) tangentially arranged. Solitary vessel outline circular to oval. Perforation plates simple. Intervessel pits
poorly preserved and challenging to measure but appear alternate, averaging to 5.8 mm (1.0 mm), 4.4–7.3 mm. Vessel-ray pitting not observed (rays filled with dark deposits). Mean tangential diameter of vessel lumina 57 mm (15 mm, n p 24), 23–86 mm.
Transverse view poorly preserved, but vessel frequency averages
14 mm2 (range: 0–25 mm2). Some of the vessels in transverse and
radial section are filled with black deposits (fig. 7A); it is unclear
whether these are tyloses or other deposits (e.g., gum residue), as
vessels are not obscured in tangential section. Fibers very thickwalled (fig. 7B). No axial parenchyma observed (but it might
be due to the poor preservation). Rays frequently two-seriate,
occasionally one or three cells wide (fig. 7C). Rays vary from exclusively uniseriate to having alternating uniseriate and multiseriate portions. In some rays, the uniseriate and multiseriate
portions are equal in width. Rays average 227 mm in height
(85 mm), 92– 414 mm. Ray cellular composition not clear; many
procumbent cells, possibly all procumbent. Black deposits present in many of the ray cells. Rays closely spaced in tangential section with an average of 22.8 rays per millimeter (range: 16–28).
Rays not storied. No intercellular canals observed.
Remarks. This specimen, BR Xylotype 1, is poorly preserved
but does not have the diagnostic characters of any of the taxonomically identified wood types at BR. The features that could
be coded with confidence were entered into InsideWood (with
0 mismatches): 2p (growth rings indistinct/absent), 5p (diffuseporous), 9a (vessels not exclusively solitary), 13p (simple perforation plates), 22p (intervessel pits alternate), 41p (vessel diameter 50–100 mm), 70p (fibers very thick-walled), 97p (rays one
to three cells wide), 104p (all ray cells procumbent), 116p (≥12
rays per millimeter), and 118a (rays not storied). This search
yielded 19 results in 10 families (InsideWood 2004–); these were
reviewed, but none was found to be a good match to the fossil
specimen. BR Xylotype 1 will be compared to the other two unidentified specimens, BR Xylotype 2 (UF 18591-33017) and BR
Xylotype 3 (UF 19225-57353), following the discussion of BR
Xylotype 3.
Incertae Sedis
Blue Rim Xylotype 2
Specimen. UF 18591-33017 (fig. 7D–7F).
Description. Growth ring boundaries indistinct to absent
(areas with fewer and smaller vessels, but these are not continuous). Wood diffuse-porous, with vessels tending to be weakly
arranged in short diagonals or discontinuous tangential bands.
Vessels solitary (38%) and in radial multiples of two to three,
rarely four, or occasional clusters (fig. 7D). Solitary vessels oval
to rounded in outline. Perforation plates not observed but likely
simple. Intervessel pits alternate (fig. 7E), apertures frequently
slit-like, small to medium, averaging 6.8 mm across (0.4 mm),
6.3–7.5 mm. Mean tangential diameter of vessel lumina 75 mm
(29 mm), 29–136 mm. Vessel frequency averages 11 mm2 (range:
6–16 mm2). Mean vessel element length 169 mm (62 mm, n p 15),
85–337 mm (fig. 7F). Axial parenchyma could not be distinguished from fibers due to the poor preservation of the specimen
(compressed and distorted). Ray start and end points difficult
to distinguish but average 209 mm in height (62 mm, n p 19),
163–314 mm (fig. 7F). Rays mostly one or two cells wide, occasionally wider, often biseriate. Rays frequent and closely spaced,
averaging 14 per millimeter (range: 10–16). Large black spherical structures present in ray cells. Storied structure, intercellular
canals, and crystals not observed.
Vulnerability index. BR Xylotype 2 has an estimated V of 6.8.
Remarks. This specimen (and possibly others) was described as type II by Manchester and Wheeler (2006). BR Xylotype 2 is poorly preserved, but it lacks the diagnostic features
of the previously described BR woods assigned to Canellaceae,
Fabaceae, and Anacardiaceae. The following characters were
entered into InsideWood: 2p (growth rings indistinct to absent),
5p (wood diffuse-porous), 13p (simple perforation plates), 22p
(intervessel pits alternate), 25p (intervessel pits small), 41p (mean
tangential diameter of vessel lumina 50–100 mm), 47p (five to
20 vessels per square millimeter), 52p (mean vessel element length
≤350 mm), and 97p (ray width one to three cells), and modern
wood was searched with no mismatches allowed. This recovered
282 results spanning 46 families (InsideWood 2004–). The characters of this wood will be compared to the other two unknown
xylotypes at BR following the discussion of BR Xylotype 3.
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Fig. 7 Incertae sedis specimens. A–C, Blue Rim Xylotype 1, UF 19225-54694. A, Vessels generally grouped in short radial multiples, TS.
B, Very thick-walled fibers, TS. C, Biseriate rays, TLS. D–F, Blue Rim Xylotype 2, UF 18591-33017. D, Vessels grouped in radial multiples,
clusters, or solitary, TS. E, Alternate intervessel pits, TLS. F, Rays and vessels, TLS. G–I, Blue Rim Xylotype 3, UF 19225-57353. G, Vessels
solitary, grouped in short radial multiples, or occasionally in clusters, TS. H, Rays often biseriate, TLS. I, Close-up of rays; enlarged white cells
may represent idioblasts, TLS. TLS p tangential longitudinal section; TS p transverse section.
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Incertae Sedis
Blue Rim Xylotype 3
Specimen. UF 19225-57353 (fig. 7G–7I).
Description. Growth ring boundaries indistinct, possibly
defined by irregular areas with no vessels alternating with areas
with irregular rows of vessels. Wood diffuse-porous with vessels
solitary (39%) and in radial multiples of two or three, occasionally four or five, or rarely clusters (fig. 7G). Solitary vessels oval
to circular in outline; perforation plates likely simple, as no scalariform observed. Intervessel pits alternate, averaging 8.7 mm in
diameter (0.4 mm), 8.3–9.3 mm. Mean tangential diameter of vessel lumina 66 mm (17 mm), 28–98 mm. Vessel frequency averages
13 mm2 (range: 8–20 mm2). Tyloses not obvious in transverse
section, but some vessels obscured in longitudinal section. Parenchyma not distinguishable from fibers in transverse section.
Fibers nonseptate, possibly rare septate. Occasional strands of
diffuse apotracheal axial parenchyma observed in tangential section—strand length variable from two? to nine? cells. Rays mostly
two-seriate, occasionally three-seriate or uniseriate (fig. 7H, 7I).
If uniseriate and biseriate in same ray, uniseriate portion usually
narrower than biseriate portion. Rays average 152 mm in height
(70 mm), 60–337 mm. Ray cellular composition heterocellular,
but specific arrangement could not be determined. Rays per millimeter averages to 15 (range: 12–18). Ray cells often with large
open lumens and thin walls (fig. 7I). Storied structure and intercellular canals not observed. Frequent enlarged white cells in
rays—possibly idioblasts. Dark brown to black rounded deposits also present in rays.
Vulnerability index. This specimen has an estimated V of 5.1.
Remarks. Although specimen UF 19225-57353 was found
in vertical position within the UF 19225 leaf quarry (estimated
to be ∼17 m tall; table 3), poor preservation (most areas are compressed and distorted) did not allow its taxonomic affinities to
be determined, because many critical diagnostic features could
not be seen. However, it is unlike any other BR wood. This
specimen differs from the BR Canellaceae Xylotype because it
has vessels in radial multiples, and rays are generally biseriate
rather than uniseriate. BR Xylotype 3 does not have the conspicuous axial parenchyma of P. diversiradii. It also differs from
E. parviareolatum because it lacks radial canals and rhomboidal prismatic crystals.
The characters that could be scored with high confidence (5p,
wood diffuse-porous; 22p, intervessel pits alternate; 26p, intervessel pits medium; 41p, mean tangential diameter of vessel lumina 50–100 mm; 47p, five to 20 vessels per square millimeter;
97p, rays one to three cells wide; 102a, ray height !1 mm;
116p, ≥12 rays per millimeter; 118a, rays not storied) were
entered into InsideWood (2004–; Wheeler 2011) with 0 mismatches, and searching only modern wood revealed 93 results
in 29 families. Sixteen matches were in the Myrtaceae, whereas
18 results were in the Sapotaceae. When additional characters
(2p, growth ring boundaries indistinct or absent; 43a, mean
tangential diameter of vessel lumina not ≥200 mm; 76p, axial
parenchyma diffuse; 92p, four cells per parenchyma strand;
115p, four to 12 rays per millimeter) were included and modern
wood was searched with 0 mismatches, there were eight results:
Guiera senegalensis J.F. Gmel. (Combretaceae), Geissois (Cunoniaceae), Tamarindus indica L. (Fabaceae, Caesalpinioideae),
Henriettella (Melastomataceae), Eucalyptus dealbata Schauer,
Eucalyptus macrorhyncha F. Muell. ex Benth., Syncarpia (Myrtaceae), and Nothofagus subgrp. Brassospora (Nothofagaceae;
InsideWood 2004–). These results were reviewed, and none
were found to be a good match with the fossil.
This wood might not belong to a modern genus, however.
The angiosperm megafossil compressions found in the same
quarry as this wood specimen were almost exclusively fruits of
the extinct sapindalean genus Landeenia (Manchester and Hermsen 2000) and leaflets called “Cedrela” schimperi (Meliaceae;
MacGinitie 1974). The association of “C.” schimperi leaves
with Landeenia fruits and flowers at Kisinger Lakes (MacGinitie
1974) and with the same kind of fruits in Paleocene Eocene
Thermal Maximum (∼56 Ma) quarries in the Bighorn Basin
(S. L. Wing and S. R. Manchester, personal communication,
2016) suggest that the leaves and fruits represent part of the
same extinct plant. Although it cannot be proven because of
the lack of a physical connection, the close proximity of these
organs around the base of the stem described here leads to the
hypothesis that it might represent the wood of Landeenia. If
this hypothesis is correct, better-preserved fossils of this wood
type may provide additional characters to resolve the systematic
position of Landeenia, whose fruit, flower, and pollen characters
are consistent with Sapindales but are not a match to any modern genus (Manchester and Hermsen 2000).
Comparison of incertae sedis xylotypes. Although the three
incertae sedis specimens have some anatomical characters in
common, differences include the very thick-walled fibers in BR
Xylotype 1. The intervessel pits in BR Xylotype 1 are not well
preserved, and the measurements likely have a larger error than
normal, but the average pit size (5.8 mm) is smaller than either
BR Xylotype 2 (6.8 mm) or BR Xylotype 3 (8.7 mm). Whereas
ray width is similar among these specimens, BR Xylotype 1 often
has rays where the uniseriate portion equals the multiseriate
portion, which is not as common in the other two specimens.
In addition, ray frequency is higher in BR Xylotype 1, as compared to the other two incertae sedis specimens. Strands of axial
parenchyma were observed in BR Xylotype 3 (UF 19225-57353)
and not in the other two unknowns, but that may be due to
variations in preservation. BR Xylotype 3 might have idioblasts
in the ray cells, which were not observed in either BR Xylotype 1
(UF 19225-54694) or BR Xylotype 2 (UF 18591-33017). All
three incertae sedis specimens share multiple characters, including absent to indistinct growth rings, diffuse porosity, similar
vessel grouping, simple perforation plates, similar vessel diameters and frequency, ray height, and the presence of black structures in the ray cells. However, many of these characters are
common across angiosperm woods.
Taphonomy and Size of Specimens
Some of the fossilized wood samples were found upright and
in situ (e.g., BR Xylotype 3, UF 19225-57353) or with minimal
transport. However, others appeared to have traveled a more
significant distance (e.g., cf. Pinus, UF 19406-61964). An in situ
stump (not sampled for wood anatomy due to poor preservation) was 25 cm in diameter with a lateral root trace extending
approximately 67 cm from the edge of the trunk. The largest
fossilized wood piece observed in the field at BR measured ap-
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proximately 33 cm in diameter by 42 cm in length. This specimen (not sampled for wood anatomy) was not in situ and appeared to have rolled down one of the badland fingers; it was
resting near a gully at the bottom of the escarpment along with
many other pieces of petrified wood. The majority of the wood
specimens at BR were prostrate and not in situ. However, some
specimens (including UF 18591-33014 through UF 1859133018 from the 1995 field season) were upright. Whereas most
of the wood samples suggest relatively small trees or shrubs, a
tree over 30 cm in diameter is quite large and suggests that larger
and/or more mature trees were present on the landscape. Specimens whose full diameter was measured in the field were used to
estimate tree height (tables 2, 3).
Estimated tree height. Complete trees are almost never preserved in the fossil record; stumps and logs are more common.
The diameter of these tree fragments can be used to estimate tree
height (tables 2, 3). McMahon (1973) and McMahon and Kronauer (1976) used the record trees (largest known individuals)
of over 500 North American species to plot height and diameter.
They also drew a line representing the buckling height of a hypothetical uniform cylinder with a consistent ratio of E/r, with
E representing the elastic modulus and r the mass density. Although McMahon (1973) and McMahon and Kronauer (1976)
did have a large data set, using the record specimen of a species
is not a reasonable representative of the average size of that
taxon. Record specimens are often found in fields or in areas
with optimal growing conditions and little competition. Furthermore, the ratio of E/r is not consistent across species. For
these reasons, the values calculated using the work of McMahon
(1973) and McMahon and Kronauer (1976) were excluded from
the second average calculation in table 3.
Rich et al. (1986) used a forest of dicots and palms from a
tropical wet forest in Costa Rica to measure diameter at breast
height and total height. The relationship between diameter
and height was plotted for all dicot species together and for individual palm and dicot species. Rich et al. (1986) found that diameter varies in relation to height, but they also noted that the
tallest individuals of a few of the species they examined exceeded
the buckling height as determined in McMahon (1973). This provides additional support for being cautious when using McMahon (1973) and McMahon and Kronauer’s (1976) regression to estimate height.
Brown et al. (1989) developed regression equations (table 2)
using height and diameter measurements from ∼4000 trees from
35 sites in Venezuela, Papua New Guinea, and Puerto Rico.
Brown et al. (1989) provided equations for estimating height
in both moist and wet life zone conditions (Holdridge 1967).
Holdridge’s (1967) life zone definitions were examined to determine which of the two conditions (moist or wet) were more
aligned with the climate conditions inferred for the BR locality
in the early Eocene. The tropical wet life zone has a mean annual
precipitation of 400–800 cm, and the tropical moist life zone
is defined by 200– 400 cm of mean annual precipitation (Holdridge 1967). The subtropical wet life zone has a mean annual
precipitation of 200–800 cm, with the subtropical moist life
zone mean annual precipitation ranging from 100 to 200 cm
(Holdridge 1967). Based on this information and the precipitation estimates from leaf area analysis and the Climate Leaf Analysis Multivariate Program for the leaf fossils at BR (S. E. Allen,
unpublished data), the moist life zone is more aligned with the
conditions at BR ~49.0 Ma. Therefore, the regression equation
developed from plants growing in moist conditions, rather than
wet, was used.
Niklas (1994) also developed correlations between stem length
and diameter. Data from 265 self-supporting tree or shrub species from multiple plant clades were used to create equations to
predict the height of fragmented fossil plants (Niklas 1994). A
subset of these specimens were classified as woody, and that
equation (table 2) was used to estimate the height of the BR specimens (table 3).
Feldpausch et al. (2011) compiled an extensive database of
39,955 diameter and height measurements of trees from 283 tropical sites in 22 countries. Feldpausch et al. (2011) noted numerous factors that impact the height/diameter relationship, including forest type and geographic region. Whereas many of the
equations presented by Feldpausch et al. (2011) contained variables that could not be calculated with the limited information
from fossil logs, an equation detailed in the appendix of that
work removed these coefficients and proved useful for estimating tree height (table 2).
The formulas of Brown et al. (1989), Niklas (1994), and
Feldpausch et al. (2011) all give height estimates within 2 m of
each other for an individual stump or log (table 3). Estimates
using the Rich et al. (1986) approach tend to be slightly lower
Table 2
Equations Used to Estimate Tree Height from Diameter
McMahon 1973; McMahon
and Kronauer 1976
log(H) p m(log(D)) 1 log(b)
or H p bDm
m: slope p 2/3; b: value
of H where D p 1 p 25
Rich et al. 1986
log(H) p m(log(D)) 1 log(b)
or H p bDm
H p exp(1.0710 1 .5688 ln
log(H) p 1.59 1 .39(logD) 2
ln(H) p b0 1 b1ln(D*)
m: slope p .58; b: value
of H where D p 1 p 33
m and b calculated from line in
fig. 5 of McMahon and Kronauer
1976 (p. 456)
m and b calculated from line
in fig. 1 (p. 243)
Moist life zone eq. (5) from
table 2 (p. 886)
Woody species equation from
table 2 (p. 1240)
Pantropical equation from
table A2 (p. 1100)
Brown et al. 1989
Niklas 1994
Feldpausch et al. 2011
b0 p 1.2229; b1 p .5320
D p measured diameter in meters; D* p measured diameter in centimeters; H p estimated height in meters.
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Table 3
Estimated Tree Heights of 10 Blue Rim Fossil Wood Specimens Calculated from the Diameter
Estimated height (m) from the five methods in table 2
Blue Rim specimen
FO (7/8/2014)
FO (7/8/2014)
UF 18591-33014
UF 18591-33015
UF 18591-33016
UF 18591-33017
UF 18591-33018
UF 18591-33022
UF 18591-33023
UF 19225-57353
Diameter (m)
Average (SD)
Average (SD) excluding A
Note. A p McMahon 1973; McMahon and Kronauer 1976; B p Rich et al. 1986; C p Brown et al. 1989; D p Niklas 1994; E p Feldpausch
et al. 2011; FO p field observation.
than those three, with the McMahon (1973) and McMahon and
Kronauer (1976) estimates being the shortest (table 3). As previously mentioned, the McMahon (1973) regression is likely biased (it estimates shorter heights using the same diameter) by a
data set based on the largest specimens of a species. Furthermore, both the McMahon and Kronauer (1976) and Rich et al.
(1986) equations were manually calculated from graphs in the
original manuscripts. This likely added a small level of additional error not present in the estimates using Brown et al. (1989),
Niklas (1994), and Feldpausch et al. (2011) because those equations were directly provided in the original manuscripts.
Field observations suggest that trees were not closely spaced
and were generally small, perhaps representing an early successional forest or an environment less favorable to massive trees.
However, this assumption does not account for taphonomic effects that may have preferentially preserved smaller woody
stems. Alternatively, it could be that wood of the dominant tree
type on the landscape, such as Populus, which is well represented in the leaf flora, is not preserved because Populus wood
decays rapidly and is less likely to enter the fossil record (Barrett
1995). Overall, based on the fossils recovered, trees on the BR
landscape ~49.0 Ma were in the vicinity of 28 m or 92 ft tall (table 3).
Density and Specific Gravity
Wood density and wood specific gravity are related but are
not interchangeable terms (even though they are often treated
as the same thing in the literature). Wood density is the mass of
the wood per unit volume—this can be measured at any moisture content (Williamson and Wiemann 2010). By contrast,
wood specific gravity is the density of the wood (using ovendry mass) relative to the density of water (Williamson and Wiemann 2010). Wood density can be thought of as a measure of
how much of a stem is cell wall versus open space; this is directly
connected to plant form and function (Swenson and Enquist
2007). Specific gravity was estimated for a few of the well-
preserved BR woods (six specimens assigned to Edenoxylon
parviareolatum). Although specific gravity estimates could not
be calculated for any of the Peltophoroxylon diversiradii specimens, abundant axial parenchyma (and therefore fewer fibers)
frequently correlates with a lower-density wood (Zheng and
Martínez-Cabrera 2013). Work on modern floras suggests that
leaf size decreases with increasing wood density (Chave et al.
2009). Whereas it is not possible to know for sure whether
any of the leaf morphotypes at BR represent the same species
as a wood type, E. parviareolatum (estimated specific gravity
0.58–0.87 using the methods of Wheeler et al. [2007b]) may
be aligned with leaves assigned to Rhus nigricans, which are
on the smaller side.
The specific gravity estimates from BR (0.58–0.87 using the
methods of Wheeler et al. [2007b] and 0.82–1.35 using the
methods of Martínez-Cabrera et al. [2012]) seem high. The average specific gravity of 156 tree species in the United States
that comprise over 95% of the total volume of trees is 0.48
(SD p 0.11, range: 0.29–0.80; Miles and Smith 2009). The average specific gravity is 0.41 (SD p 0.08, range: 0.29–0.68) for
the 56 species of gymnosperms and 0.52 (SD p 0.10, range:
0.31–0.80) for the 100 angiosperm species (Miles and Smith
2009). These numbers are much lower than specific gravity estimates from the BR woods, but the values obtained from the
fossils are likely inflated due to some of the cells being crushed,
lowering the percentage of lumen visible.
Tropical lowlands, which are dominated by angiosperms, have
more variation in wood density than temperate, high-elevation,
or stressful environments, which are often dominated by gymnosperms (Swenson and Enquist 2007; Chave et al. 2009). Taxa
with low specific gravity wood are likely fast-growing, lightdemanding colonizers or pioneers, whereas taxa with high specific gravity wood are more likely to be shade-tolerant, slowgrowing members of the subcanopy (Wiemann and Williamson
2002; Baker et al. 2004). Species with intermediate specific gravities are typically mature forest species or emergents.
Some studies have found that wood density is positively correlated with climate conditions including mean annual temperature, maximum monthly temperature, and mean annual precipitation (Wiemann and Williamson 2002; Swenson and Enquist
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2007; Chave et al. 2009). By contrast, mean specific gravity has
been shown to be negatively correlated with soil fertility; sites
with higher specific gravity woods had poorer soils (Muller-Landau
2004). Yet, studies examining the correlation between various
environmental conditions and wood specific gravity have often
contradicted each other; these differences might be due in part
to how specific gravity is calculated (Muller-Landau 2004).
Vulnerability Index
Vulnerability index (V) was also calculated for all BR wood
specimens with both average vessel frequency (mm2) and average tangential vessel diameter (mm; Carlquist 1975a, 1975b,
1977; Wheeler 1991). A low V (below 1) suggests that a plant
is capable of withstanding freezing or water stress conditions,
because narrow vessels can withstand lower negative pressures,
and in plants with high vessel frequency, the effect of each embolism on total conductivity is minimized (Carlquist 1977, 2012).
For example, desert and arctic shrubs have an average V of 0.08
and 0.10, respectively (Carlquist 1975a, 1977). By contrast,
vines and lianas have an average V of 8.22 (Carlquist 1975a,
1977). The BR woods have V values ranging from 1.43 to 10.30
with a median of 3.51 and an average of 4.22 (SD p 2.52,
n p 16). Extant tropical trees tend to have high V values, more
than 2 or 3 (Wheeler 1991; Burnham and Johnson 2004). No
BR specimens have V ! 1, whereas nine specimens have V 1 3.
All three of the fossil angiosperm wood types identified to family have specimens with values falling in the modern tropical
range. The nine specimens with V 1 3 include four assigned to
E. parviareolatum, two legumes, the specimen of Canellaceae,
BR Xylotype 2 (UF 18591-33017), and BR Xylotype 3 (UF
19225-57353). High V values suggest that the BR woods did
not experience significant water stress (Wheeler 1991). This observation is supported by the local geology. The Bridger Formation in the BR area (lower Bridger) preserves fluvial, deltaic,
and floodplain environments interbedded with lacustrine sandstones representing transgressions of the Laney Member of the
Green River Formation (M. E. Smith, personal communication,
2014). However, Wheeler (1991) cautions that V does not account for the amount of active, conducting sapwood, which can
vary greatly.
Ecology and Climate
Even though only seven wood types have been recognized, the
BR wood flora is dominated by angiosperms, with only a single
Pinaceae sample present. It is suggested that one of the ways
angiosperms may have been able to diversify and dominate so
many ecosystems, especially in the lowland tropics, is the development of vessels. These structures permit higher maximum
conductivity and therefore higher maximum productivity compared to tracheids alone (Chave et al. 2009). Many wood anatomical features, including vessel characters, have been shown
to be closely correlated with environmental conditions. Woody
species with narrow-diameter vessels (!100 mm), a high vessel
frequency (≥40 per square millimeter), and vessels grouped in
clusters, features that provide safeguards against embolism,
are more common in cool temperate, arctic, tropical montane,
alpine, and very dry environments. By contrast, woody taxa
found in frost-free lowland tropical forests often have characters
that provide less resistance to hydraulic flow, including largediameter vessels (1200 mm), a low vessel frequency (!5 per square
millimeter), and simple perforation plates (Baas 1983; Wheeler
and Baas 1993; Baas et al. 2004; Ewers et al. 2007; Chave
et al. 2009; Carlquist 2012). Within a genus, generally the widest vessels are found in species growing in tropical lowlands, the
narrowest in species at high latitudes, and those of intermediate
diameter in tropical alpine species (Baas et al. 2004). Many BR
specimens are diffuse-porous with narrow (!100-mm) vessels.
Baas (2004) noted that diffuse porosity with narrow vessels is
characteristic of evergreen tropical montane, evergreen temperate, and many deciduous temperate trees and shrubs.
Plant size, in addition to environmental factors, also contributes to variations in vessel diameter. For example, very wide
vessels are rare in small trees and shrubs (Wheeler and Baas
1993; Baas et al. 2004; Olson et al. 2014). Many of the BR wood
specimens are small hand samples, possibly from plants with a
smaller stature, but they could also represent branch rather than
trunk wood. This factor could account for the smaller vessel
diameters in the BR woods. Alternatively, it has been shown that
trees growing in more stressful environments tend to have significantly smaller vessel diameters than trees growing in less stressful sites (Fichtler and Worbes 2012).
Shrubs and branches often have a higher vessel frequency than
mature tree trunk wood (Wheeler and Baas 1993). In general,
the BR woods have an intermediate vessel frequency (averages
range between seven and 40 per square millimeter from 17 specimens), which may indicate that they represent mostly branch
or shrub wood of tropical lowland species. However, multiple
specimens were measured at over 1 ft in diameter, clearly indicating that at least some were trees. One of these specimens
(UF 18591-33023, 31 cm) was assigned to E. parviareolatum,
but most of the other large wood samples documented in the
field were too poorly preserved to describe or identify. One exception is BR Xylotype 2 (UF 18591-33017), which had a stem
diameter of over 55 cm and was described, but the taxonomic
affinity could not be determined.
Conspicuous axial parenchyma patterns including aliform,
confluent, and bands more than three cells wide, such as those
observed in P. diversiradii, are generally found in tropical, nondrought-tolerant taxa (Baas 1983; Wheeler et al. 2007a; Zheng
and Martínez-Cabrera 2013; Morris et al. 2016). Ample axial
parenchyma has been correlated with higher conduction capacity, in part because axial parenchyma may help prevent cavitation or repair vessels when embolisms occur (Zheng and MartínezCabrera 2013; Morris 2016; Morris et al. 2016).
Other wood characters also provide ecological and climate
signals. For example, regular growth rings are a consequence of
periodic dormancy, a strategy many trees use to withstand conditions including drought, cold, or photoperiodic events, and are
therefore less common in plants from wet tropical lowlands
(Carlquist 2012). Even though only seven wood types were recovered from BR, most have indistinct growth rings, indicating
likely tropical conditions (nonseasonal, equable warm, and wet;
Wheeler and Baas 1993). There are no BR woods with ring porosity or helical thickenings, characters more common in temperate environments. Randomly arranged vessels is the most frequently encountered pattern both globally and in the BR woods;
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clusters (rare at BR) are less common in the tropics than in other
environments (Wheeler and Baas 1993). Crystals (observed in
the species of Anacardiaceae and Fabaceae) and silica bodies
(possible in P. diversiradii and a few of the unidentified specimens) are also more common in the tropics than in temperate
zones (Wheeler et al. 2007a).
The taxonomically identified woods also provide clues about
the climate and ecological conditions at BR in the latest Early Eocene. For example, legumes have a worldwide distribution, but
the Caesalpinioideae are mainly tropical and subtropical woody
trees, shrubs, or lianas with species diversity centered in Southeast Asia, Africa, and South America (Klitgård and Lewis 2010;
LPWG 2013a). Edenoxylon parviareolatum is thought to be subtropical, in part because of the taxonomic affinities of the other
woods recovered at BSR, including Palmoxylon macginitiei Tidwell, a species of palm (Boonchai and Manchester 2012).
The nearest living relatives of the BR Canellaceae Xylotype are
from subtropical and tropical environments. However, both extant and fossil taxa of Canellaceae, including the BR specimen,
have scalariform perforation plates. Scalariform perforation
plates provide more resistance to hydraulic flow, as the end walls
of vessel elements account for ∼50% of the total resistance in a
vessel; they are thought to help localize any air embolisms that
might occur (Baas 1983; Baas et al. 2004; Chave et al. 2009;
Carlquist 2012). The percentage of species with scalariform perforation plates is higher in cool temperate to arctic (23%–53%)
and tropical high montane (15%–33%) environments but lower
in tropical lowland and arid environments (0%–8%; Wheeler
and Baas 1993). The taxonomic composition of the BR flora,
in conjunction with temperature estimates from leaf physiognomic data (S. E. Allen, unpublished data), is more suggestive
of tropical montane conditions rather than a cool temperate or
arctic environment.
The specimen of Pinaceae is not very informative, as pines are
distributed from subtropical to temperate areas throughout the
Northern Hemisphere and many species are tolerant of extreme
conditions including high elevation and/or high latitudes (Farjon
and Styles 1997). Overall, the wood characters observed at BR,
including diffuse porosity, the absence of distinct growth rings,
and rarity of plants with scalariform perforation plates, suggest
warm to tropical conditions with limited seasonality in southwestern Wyoming in the latest Early Eocene. This agrees with
paleoclimate estimates from leaf physiognomic methods (S. E.
Allen, unpublished data) and the presence of frost-intolerant
taxa such as Phoenix windmillis (Allen 2015a).
Comparison with Other Western Interior
North American Eocene Wood Floras
Available data from the limited number of Eocene wood localities studied from the Western Interior of North America suggest that there was variation in taxonomy and wood anatomy
across the region (Wheeler and Michalski 2003). Most early
and middle Eocene dicotyledonous woods in the United States
lack growth rings (Wheeler 2001), which is similar to what was
observed at BR. The BR assemblage is distinctive in some respects. For example, most middle Eocene wood assemblages
documented from the United States have a plane tree relative,
Plataninium or Platanoxylon, and Quercinium (cf. evergreen
oaks; Wheeler et al. 1978; Wheeler 2001; Gregory et al. 2009).
Yet, neither of these taxa is represented at BR.
Wheeler and Michalski (2003) documented silicified woods
from Paleocene and Eocene strata in the Denver Basin of Colorado. Among those, the Eocene sites were dominated by wood of
Paraphyllanthoxylon (probable Lauraceae; Wheeler and Michalski 2003). The characters of the Denver Basin woods, including diffuse porosity (which is shared with the BR woods),
are most similar to the wood anatomy of taxa growing in the
frost-free lowland tropics today (Wheeler and Michalski 2003).
However, there is no taxonomic overlap between the identified
BR and Denver Basin woods.
Although leaf floras from the Eocene of Yellowstone National
Park have more angiosperms than conifers, the woods preserve
a higher percentage of conifer taxa (Knowlton 1899; Wheeler
et al. 1977; Fritz and Fisk 1978). The BR leaf flora is similarly
dominated by angiosperms, with a few ferns and no gymnosperms (Allen 2015b); however, the wood assemblage differs
from that of Yellowstone in having only one coniferous specimen. More than 30 dicotyledonous wood types have been found
from the Yellowstone localities of the Lamar River Formation (Wheeler et al. 1977). Families represented by the wood include Betulaceae, Lauraceae, Magnoliaceae, and Platanaceae
(Wheeler et al. 1977, 1978). Growth rings in the Yellowstone
woods range from indistinct to distinct (Wheeler et al. 1977,
1978), whereas none of the BR specimens have well-preserved
distinct growth rings (however, it is difficult to determine in
the Pinaceae specimen). The presence of species in the Yellowstone flora with well-defined growth rings may be due to either
its slightly younger age (and therefore further from the peak of
early Eocene warming) or the higher elevation of Yellowstone
contributing to more seasonality than BR. Chadwick and Yamamoto (1984) examined 119 wood specimens from the Specimen Creek area of the Gallatin Petrified Forest, Yellowstone
National Park, Montana. They found representatives of three
gymnosperm families, including Pinaceae, and 15 genera in 13 angiosperm families, including wood they assigned to Salix (Salicaceae; Yamamoto and Chadwick 1982; Chadwick and Yamamoto
1984). The potential presence of wood assignable to Salicaceae is
notable because there are definitive leaves and fruits of Populus at
BR. Populus wood should be easily recognizable by its medium to
large polygonally shaped alternate intervessel pits and exclusively
uniseriate rays with all ray cells procumbent (Metcalfe and Chalk
1950; InsideWood 2004–), but this combination of characters is
not present in any of the BR woods. Given the abundance of
Populus leaves and fruits at BR and other Eocene sites in Wyoming, Utah, and Colorado (MacGinitie 1969, 1974), it might
seem surprising that wood of Salicaceae has not been recovered.
However, this absence might be due to taphonomic bias and the
ease with which Populus wood degrades due to its low specific
gravity and extractive content (Barrett 1995; E. Wheeler, personal communication, 2016). Furthermore, Elisabeth Wheeler
(personal communication, 2016) has examined Yamamoto and
Chadwick’s original slides from Specimen Creek, in addition to
woods from other areas of Yellowstone, and has not found any
evidence of Salix or Populus.
The middle Eocene (∼44 Ma) wood assemblage from the
Clarno Formation in Oregon is extremely diverse, with 66 genera. Representatives of Anacardiaceae, Fabaceae, and Pinaceae
are present in both the BR and the Clarno floras, but there are
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no shared species (Wheeler and Manchester 2002). The latest
Eocene Florissant Fossil Beds National Monument (∼34 Ma)
in central Colorado preserves ring-porous woods belonging to
the Ulmaceae, Sapindaceae, and Fabaceae (Wheeler 2001). This
suggests distinct seasonality in the later part of the Eocene as
compared to the early Eocene.
probable leaves (R. nigricans) assigned to Anacardiaceae in
the BR flora (MacGinitie 1969; Manchester 1994; Collinson
et al. 2012). Multiple organs assigned to Anacardiaceae provide
the potential for whole-plant reconstruction.
Concluding Remarks
Comparison with Other Blue Rim Floral Elements
The wood assemblage, comprised of seven xylotypes from
over 30 specimens, along with the rest of the macroflora from
BR, is dominated by angiosperms. Some broad taxonomic overlap between the woods and other elements of the macroflora and
microflora is present. The occurrence of a wood assignable to
Pinaceae is significant, as no other conifer macrofossils have
been recovered from BR. Coniferous megafossils are also very
rare in the adjacent Green River Formation (MacGinitie 1969;
Wilf 2000). Yet, pollen assignable to Pinaceae (including Pinus)
occurs at multiple stratigraphic horizons at BR, indicating that
the family was present in the regional flora with possible rare local trees based on the presence of the pinaceous wood specimen.
However, the cf. Pinus specimen was found at the bottom of the
BR escarpment in a gully, with no evidence that it originated
nearby. Furthermore, both the hand specimen and microscopic
views suggest that the cf. Pinus fossil wood was transported from
its original growing location and was subjected to compression
and distortion during or prior to the permineralization process.
No other macrofossils of Canellaceae have been recovered at
BR to date. However, representatives of Canellaceae might be
overlooked in the leaf assemblage because they have evergreen,
entire-margined, pinnately veined leaves lacking special characters, which makes them difficult to distinguish from various
other entire-margined dicotyledonous taxa. By contrast, the
flowers are rather distinctive, with three sepals, five to 12 petals,
two to six connate carpels, and six to many stamens fused into
a tube (Judd et al. 2008). This combination of characters has
not been observed in any of the BR flower morphotypes.
In addition to the wood, a few legume leaflets have been recovered at BR. The leaves of this large family are often pinnately
compound with entire leaflets and pulvini. There are many (often poorly preserved) laminae with entire margins at BR. However, only one specimen had an obvious pulvini (which was
matched to a few other specimens), allowing for a confident assignment to Fabaceae. Additional specimens could easily represent leaves or leaflets of Fabaceae or other families with entire
margins. Legume flowers have also not been recognized from
BR, even though there are still multiple unidentified flower morphotypes. Legumes—both leaves and fruits—are well represented
in the nearby Green River flora (MacGinitie 1969; Johnson and
Plumb 1995; Grande 2013), and fruits have also been recovered
at the Kisinger Lakes site in northwestern Wyoming.
Along with the anacardiaceous wood, there are also fruits of
Pentoperculum minimus (Reid and Chandler) Manchester and
This survey found only seven different wood types from the
BR localities. Hence, BR preserves a low-diversity wood flora
dominated by Edenoxylon parviareolatum (Anacardiaceae). Specimens assigned to Fabaceae, Canellaceae, and Pinaceae are also
present, along with a few unidentifiable but distinct xylotypes.
Other elements of the same fossil flora, including leaves, reproductive structures, and dispersed pollen and spores, suggest that
overall richness is much higher than is represented by the wood.
Stem diameter measurements obtained from some specimens
yield tree height estimates of ∼16–28 m. Self-supporting taxa
would have been essential for climbers such as Iodes, Vitis, and
Lygodium present in the same flora (Manchester and Zavada
1987; Allen et al. 2015). The angiosperm woods are diffuseporous with absent or indistinct growth rings and rare scalariform perforation plates. These characters suggest that climate
conditions were warm with limited seasonality, which agrees
with estimates from leaf physiognomic methods (S. E. Allen, unpublished data) and the presence of frost-intolerant taxa such
as Phoenix windmillis (Allen 2015a).
Thank you to Steven R. Manchester, my advisor, for his valuable comments and support. Work on this article was also assisted by discussions and comments from Nareerat Boonchai,
Nathan Jud, Chris Nelson, and Elisabeth Wheeler. Hongshan
Wang’s curatorial assistance at the FLMNH is greatly appreciated. Recent wood slides were prepared by Chris Nelson. My
participation in an NSF-funded short course on Plant Anatomy
(microMORPH) at the Arnold Arboretum of Harvard University in the summer of 2015 (instructors: Pieter Baas, Pam Diggle,
William [Ned] Friedman, Peter Gasson, Elisabeth Wheeler)
greatly facilitated my knowledge of wood anatomy and allowed
me to complete this aspect of my dissertation project. Thank you
also to those who helped me with fieldwork in 2012 and 2014:
Jim Barkley, Sahale Casebolt, Ellen Currano, Xiaoyan Liu, Terry
Lott, Steve Manchester, Paul Murphey, Mike Smith, and Greg
Stull. Thanks are also extended to two anonymous reviewers
who provided valuable suggestions that greatly improved the article. Funding was provided by the NSF (DEB-1404895), the
Evolving Earth Foundation, the Paleontological Society, and the
UF Department of Biology. Finally, thank you to my friends
and family for their continued support.
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