Bisexual Flowers from the Coniacian (Late Cretaceous) of Vancouver
Island, Canada: Ambiplatanus washingtonensis gen. et sp. nov.
(Platanaceae)
Mindell, R. A., Karafit, S. J., & Stockey, R. A. (2014). Bisexual Flowers from the
Coniacian (Late Cretaceous) of Vancouver Island, Canada: Ambiplatanus
washingtonensis gen. et sp. nov. (Platanaceae). International Journal of Plant
Sciences, 175(6), 651-662. doi:10.1086/676306
10.1086/676306
University of Chicago Press
Version of Record
http://cdss.library.oregonstate.edu/sa-termsofuse
Int. J. Plant Sci. 175(6):651–662. 2014.
䉷 2014 by The University of Chicago. All rights reserved.
1058-5893/2014/17506-0002$15.00 DOI: 10.1086/676306
BISEXUAL FLOWERS FROM THE CONIACIAN (LATE CRETACEOUS) OF VANCOUVER
ISLAND, CANADA: AMBIPLATANUS WASHINGTONENSIS
GEN. ET SP. NOV. (PLATANACEAE)
Randal A. Mindell,* Steven J. Karafit,† and Ruth A. Stockey1,‡
*Department of Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada;
†Department of Biology, University of Central Arkansas, Conway, Arkansas 72035, USA; and ‡Department of Botany and
Plant Pathology, 2082 Cordley Hall, Oregon State University, Corvallis, Oregon 97331, USA
Editor: Patrick S. Herendeen
Premise of research. Anatomically preserved platanaceous inflorescences have been found in rocks of Late
Cretaceous (Coniacian) age at the Eden Main locality on Vancouver Island, British Columbia, Canada.
Methodology. Specimens occur in calcium carbonate concretions near the base of the Nanaimo Group
(Comox Formation) and are studied with the cellulose acetate peel technique and SEM.
Pivotal results. Inflorescences are small (!5 mm), pedunculate heads, with fewer than 30 flowers per
inflorescence. Both bisexual and unisexual flowers are present on the same inflorescence. Elongate and overarching tepals enclose the developing gynoecium and androecium at early stages of development. More mature
specimens have individual flowers with at least two whorls of tepals fused toward the base. Flowers have a
whorl of five free stamens, each with short filaments and a sclerotic apical connective extension. In situ pollen
is small, prolate, tricolpate, 11–13 mm in polar diameter, and 8–9 mm in equatorial diameter. In flowers where
it can be observed, the staminal whorl encloses five short, conduplicate, glabrous carpels lacking differentiated
styles but bearing broad, flat apices. The ovary of these carpels is occupied by a single ovule.
Conclusions. While the Eden Main specimens display a five-parted gynoecial arrangement common in
the pistillate flowers of other Cretaceous platanoids, Ambiplatanus washingtonensis gen. et sp. nov. provides
the first fossil evidence of functional bisexuality in fossil Platanaceae.
Keywords: Cretaceous, Platanaceae, Platanus, Proteales.
Introduction
2010; Carpenter et al. 2005), these three families remain disparate when interpreted only through extant representatives.
Fortunately, Platanaceae are an ancient, diverse group with
an extensive fossil record reaching well into the Early Cretaceous (Hickey and Doyle 1977; Crane 1989; Friis et al. 2011).
Significant differences in leaf morphology (Hickey and Doyle
1977; Maslova 2010), pollination biology (Crane 1989), floral
arrangement (Manchester 1986; Magallón-Puebla et al. 1997),
pollen ultrastructure (Pedersen et al. 1994; Denk and Tekleva
2006), and dispersal mechanisms (Manchester 1986) have all
been revealed by these fossils. Specifically of interest for this
work are numerous specimens from the Cretaceous and Tertiary that suggest the presence of bisexual flowers and inflorescences (Friis 1985; Magallón-Puebla et al. 1997; Mindell
et al. 2006). Living Platanaceae have predominantly unisexual
inflorescences and flowers (Nixon and Poole 2003), although
rare bisexual inflorescences and flowers are observed (Boothroyd 1930; Schwarzwalder and Dilcher 1981; Floyd et al.
1999). Thus, it is expected that the stratigraphic record would
preserve bisexual flowers and inflorescences (Mindell et al.
2006).
The Late Cretaceous (Coniacian) Eden Main Quarry locality
provides three-dimensional, cellular preservation for the study
of fossil plants. Anatomically preserved fossils from this age
The 10 living species of Platanaceae show relatively little
morphological variation when compared to the extensive and
diverse fossil record for the family (Crane 1989; MagallónPuebla et al. 1997; Maslova 2003, 2010; Nixon and Poole
2003; Friis et al. 2011; Denk et al. 2012). Living representatives are generally united by palinactinodromous leaves, globose, unisexual inflorescences that are typically borne on a
peduncle, and mature achenes with a persistent style (Nixon
and Poole 2003). This anomalous suite of characters has lent
itself to various interpretations as to the systematic placement
of the family (Griggs 1909; Schwarzwalder and Dilcher 1991;
Hoot et al. 1999; Hilu et al. 2003). Current interpretations of
the higher-order phylogeny based on molecular data place the
family as sister to Proteaceae and Nelumbonaceae in the order
Proteales (APG III 2009; Barniske et al. 2012). Although some
efforts have been made to find morphological synapomorphies
within this group (Hoot et al. 1999; Doyle and Endress 2000,
1
Author for correspondence; e-mail: stockeyr@science.oregonstate
.edu.
Manuscript received October 2013; revised manuscript received February 2014;
electronically published May 28, 2014.
651
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INTERNATIONAL JOURNAL OF PLANT SCIENCES
are uncommon (Nishida 1991; Takahashi et al. 1999; Karafit
and Stockey 2008; Jud et al. 2010), and thus Eden Main offers
a rare view into the reproductive biology of Mesozoic Platanaceae. This article provides the first formal description of an
angiosperm from the locality and reveals a new genus with
implications for the evolution of the family.
in two alternating cycles overarching gynoecium and androecium. Stamens five, in single whorl; filaments short; thecae
elongate, apical connective dome shaped. Pollen prolate, tricolpate; exine semitectate, reticulate. Carpels five, conduplicate, wedge shaped in cross section; styles undifferentiated;
apical platform broad. Ovules one per carpel.
Material and Methods
Type Species—Ambiplatanus washingtonensis Mindell,
Karafit & Stockey sp. nov. (Figs. 1–5)
Floral features are preserved in four inflorescences. Three
other inflorescences show only abraded receptacular tissues.
These specimens are anatomically preserved in three dimensions in calcareous marine concretions collected from the
quarry at the Eden Main fossil locality (Karafit and Stockey
2008). The Eden Main quarry (lat. 49⬚4952.04N, long.
125⬚2510.41W, universal transverse Mercator 10N 325999
5522663) is 35 km northwest of Courtenay, British Columbia,
Canada (Karafit and Stockey 2008). The concretions are embedded in a sandstone matrix in the Dunsmuir Member of the
Comox Formation (Nanaimo Group) and are thought to have
formed in a nearshore marine environment (Mustard 1994;
Haggart et al. 2003; Johnstone et al. 2006). The locality is
dated as early Coniacian on the basis of bivalves, gastropods,
and ammonites (Haggart et al. 2003, 2005). Fossil decapods
have been described from material found at the locality
(Schweitzer et al. 2003, 2009). Mosses, lycophytes, ferns, cycadeoids, conifers, and angiosperms are abundant in the concretions (Karafit 2008; Jud et al. 2010). One species of fertile
schizaeaceous fern has been recognized and described (Karafit
and Stockey 2008).
Concretions bearing plant fossils were cut into serial sections
and studied with the cellulose acetate peel technique (Joy et
al. 1956). Peels were mounted on slides with Eukitt mounting
medium (O. Kindler, Freiberg, Germany). Images were captured with a Powerphase One scanner (Phase One, Copenhagen) mounted to a compound microscope and were processed with Adobe Photoshop 10.0 (Adobe Systems). Pollen
samples were prepared with a modified Daghlian and Taylor
(1979) technique by dissolving an acetate peel bearing anthers
with acetone on filter paper over a 1-mm mesh filter in a Millipore filter, then mounting the filter paper to an SEM stub.
The pollen grains obtained were coated with 15 nm of gold
by a Nanotek sputter-coater and examined with a Japan Electronics Optics (JEOL 6301F) scanning electron microscope
(SEM) at 5 kV. All specimens are housed in the University of
Alberta Paleobotanical Collections, Edmonton, Alberta
(UAPC-ALTA).
Specific diagnosis.
Receptacular head parenchymatous.
Peduncle at least 1.5 mm long. Flowers fewer than 30 per
inflorescence; up to 0.5 mm in diameter; unisexual and bisexual flowers present in same inflorescence. Tepal whorls two or
more, extending above gynoecium and androecium. Flowers
pentamerous; stamens bithecate; thecae elongate; anther with
sclerenchymatous apical connective. Pollen prolate, tricolpate,
11–13 mm polar diameter, 8–9 mm equatorial diameter. Carpels
glabrous, apices broad, flattened. Styles glabrous. Fruits one
seeded.
Holotype, hic designatus.
UAPC-ALTA P14460 Ebot,
Ftop (figs.1A–1C, 2, 3, 4, 5A–5C).
Locality. Eden Main Quarry, Vancouver Island, British
Columbia, Canada.
Stratigraphic position and age. Comox Formation, Coniacian, Late Cretaceous.
Etymology. The genus Ambiplatanus is named in recognition of the bisexuality of the flowers and inflorescences. The
specific epithet washingtonensis is given in reference to the
proximity of Mount Washington to the fossil sites on Vancouver Island, British Columbia, Canada.
Systematic Description
Order—Proteales de Jussieu ex Berchtold & J. Presl
Family—Platanaceae T. Lestiboudois
Genus—Ambiplatanus Mindell, Karafit & Stockey gen.
nov. (Figs. 1–5)
Generic diagnosis. Inflorescences bisexual, globose, pedunculate heads, containing fewer than 30 flowers. Flowers
bisexual, some appearing unisexual; perianth well developed
Description
Seven inflorescences, three containing flowers and four
showing only an abraded receptacle, are present in the Eden
Main collection. Inflorescences are globose heads, 4–5 mm in
diameter and borne on short peduncles (fig. 1A–1C). Since
peduncles are abraded, the length of 1.5 mm is a minimum,
and they might have been longer in life. Fewer than 30 sessile
flowers are attached to a parenchymatous receptacular head
(fig. 1C). Individual flowers, up to 0.5 mm in diameter, are
either unisexual (fig. 2A) or bisexual (fig. 2C, 2D), with both
types of flowers present on a single inflorescence (fig. 1B, 1C).
At least two whorls of overarching tepals enclose the reproductive organs early in development (fig. 1D, 1E). When
the inflorescences are abraded, it is difficult to interpret tepal
length and extent. However, in the best-preserved specimens,
the tepals extend above the level of the stamens and carpels
at maturity and are up to 1.25 mm long (figs. 2A, 2C, 2D,
3A, 3B). Unisexual flowers bear either carpels or stamens in
whorls of five (figs. 1C, 1E, 3C, 5A, 5C).
Five stamens in a single whorl are bithecate and up to 1.0
mm long, with short filaments and large, sclerenchymatous,
dome shaped, apical connective extensions up to 230 mm in
width and 380 mm high (fig. 3A–3C). Thecae are elongate, up
to 400 mm long. Microsporangial walls are 1 cell thick (fig. 3D).
Pollen grains are small, prolate, tricolpate, 11–13 mm in polar
diameter, and 8–9 mm in equatorial diameter (figs. 3D, 4A, 4C).
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Fig. 1 Inflorescence anatomy of Ambiplatanus washingtonensis gen. et sp. nov. A, Longitudinal section showing flowers in inflorescence.
Note central receptacular tissue and flower with both stamens and carpels at bottom. Holotype P14460 Ftop 2. Scale bar p 1 mm. B, Longitudinal
section through inflorescence, showing peduncle (P) attachment and numerous pentamerous flowers. Holotype P14460 Ebot 2. Scale bar p 1
mm. R p receptacle. C, Section showing flowers in longitudinal (top left) and transverse (center right) view. Note receptacular tissue (R).
Holotype P14460 Ebot 3. Scale bar p 0.5 mm. D, Section of immature inflorescence, showing numerous flowers on head. Note overarching
tepals. Paratype P14320 Ebot 2b. Scale bar p 0.5 mm. E, Oblique tangential section through flower of immature inflorescence. Note five carpels
and whorls of tepals. Paratype P14320 Ebot 25b. Scale bar p 250 mm.
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Fig. 2 Anatomy of flowers and inflorescences of Ambiplatanus washingtonensis gen. et sp. nov. A, Longitudinal section through flowers.
Flower at left shows carpels. Note ovules (O), undifferentiated styles, and broad stigmatic platform (S). Flower at right shows stamen with apical
connective tissue (C). Holotype P14460 Ebot 5. Scale bar p 250 mm. B, Oblique longitudinal section of flower, showing stamens with broad,
overarching, apical connectives (C) and light-colored anthers (A) with pollen. Holotype P 14460 Ebot 25. Scale bar p 360 mm. C, Oblique
transverse section through single bisexual flower with central whorl of five carpels enclosed by androecium and at least two whorls of tepals.
Holotype P14460 Ftop 30. Scale bar p 500 mm. D, Oblique transverse section of flower in C, showing carpels (red), tepals (green), and stamens
(blue). P14460 Ebot 5. Scale bar p 500 mm.
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Fig. 3 Androecium anatomy of Ambiplatanus washingtonensis gen. et sp. nov. A, Longitudinal section through single flower, showing stamens
with apical connective tissue and elongate anthers. Note apical connective tissue (C), anthers (A), basal carpels (Ca), and elongate tepals at right.
Holotype P14460 Ftop 2. Scale bar p 250 mm. B, Transverse section of flower in area of stamens, showing stamens (numbers). Note apical
connective tissue (C) and filament (F). Holotype P14460 Ftop 35. Scale bar p 250 mm. C, Slightly oblique transverse section of flower showing
five stamens (numbers), three represented by apical connective tissue and two showing anther tissue. Holotype P14460 Ebot 37. Scale bar p
100 mm. D, Transverse section through anther. Note cluster of tricolpate pollen. P14460 Ebot 62. Scale bar p 50 mm.
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INTERNATIONAL JOURNAL OF PLANT SCIENCES
Fig. 4 SEM images of Ambiplatanus washingtonensis gen. et sp. nov. pollen. Holotype specimen P14460 Ebot 70. A, Cluster of pollen
grains. Scale bar p 2 mm. B, Pollen grain in polar view, showing three colpi and microreticulate ornamentation. Scale bar p 2 mm. C, Pollen
grain in equatorial view, showing elongate colpi, prolate shape, and microreticulum. Scale bar p 2 mm. D, Detail of microreticulate exine
ornamentation, showing high muri. Scale bar p 200 nm. E, Detail of colpus, showing granular ornamentation. Scale bar p 200 nm. F, Section
through pollen wall, showing thick tectum and elongate columellae. Scale bar p 1 nm.
The exine is semitectate and reticulate, and the muri are high
(fig. 4). The nexine is up to 240 nm thick, the columellae up to
550 nm tall, and the tectum up to 340 nm thick (fig. 4F).
Carpels (up to 720 mm long and 240 mm in diameter) are
glabrous, conduplicate, and triangular in transverse section,
with a very prominent median suture (fig. 5). Styles are undifferentiated, and carpel apices are broad and flattened, providing a broad stigmatic platform (fig. 5A). Carpels are ob-
served to contain a single ovule that in some sections appears
to have a wing-like extension (fig. 5C, 5D). It is difficult to
determine ovule attachment because of preservation; therefore,
placentation and ovule curvature are currently unknown.
Discussion
These fossils show pentamerous, sessile flowers with a welldeveloped perianth, arranged on small, pedunculate, globose
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MINDELL ET AL.—CRETACEOUS BISEXUAL PLATANACEAE
657
Fig. 5 Gynoecium anatomy of Ambiplatanus washingtonensis gen. et sp. nov. A, Longitudinal section of carpels (from fig. 2A). Note ovules
(O), undifferentiated styles, and broad stigmatic surfaces at top. Holotype P14460 Ebot 5. Scale bar p 250 mm. B, Oblique transverse section
of flower (from fig. 2C), showing a single whorl of five carpels. Holotype P14460 Ftop 30. Scale bar p 500 mm. C, Mature carpels/fruits in
transverse section. Note deltoid shape and seed coat remnants at arrow. Holotype P14460 Ebot 62. Scale bar p 500 mm. D, Longitudinal section
through mature fruits, showing winged seeds. Paratype P14468 Cbot 74b. Scale bar p 250 mm.
heads. Single-seeded carpels have undifferentiated styles and
are commonly encircled by an overarching whorl of stamens
with short filaments, a large apical connective extension, and
in situ tricolpate, reticulate pollen. These characters clearly
place the fossil within the Platanaceae (Boothroyd 1930; Denk
and Tekleva 2006; APG III 2009; von Balthazar and Schönenberger 2009; Maslova 2010; Friis et al. 2011). However,
the Eden Main heads bear bisexual and possibly unisexual
flowers (see floral diagrams in fig. 6). This is evidenced by
well-developed in situ pollen co-occurring in the flowers alongside mature ovule-bearing carpels and seed-bearing fruits.
Living Plantanaceae are represented by ∼10 species in the
genus Platanus L. (Nixon and Poole 2003). Platanus, like the
fossil described here, has dome-shaped apical connectives
above the anthers and can have stalked inflorescences (Nixon
and Poole 2003). Furthermore, some species of Platanus have
been observed to occasionally bear rudimentary to functionally
bisexual flowers or inflorescences (Boothroyd 1930; Schwarzwalder and Dilcher 1981; Floyd et al. 1999; von Balthazar
and Schönenberger 2009). The two mature Eden Main inflorescences show both bisexual and unisexual flowers and differ
markedly from extant Platanus in having a well-developed
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INTERNATIONAL JOURNAL OF PLANT SCIENCES
Fig. 6 Bisexual flowers and inflorescences of Ambiplatanus washingtonensis gen. et sp. nov. (Platanaceae). A, Floral diagram through basal
portion of an individual flower. Note staminal tube uniting androecium and pentamery of floral whorls. B, Floral diagram showing stamens
opposite carpels. C, Floral diagram representing distal portions of flower, showing five large masses of apical connective tissues above anthers.
D, Inflorescence diagram showing possible arrangement of uniformly bisexual flowers. E, Inflorescence diagram showing alternate possible
arrangement of mixed staminate, carpellate, and perfect flowers.
overarching perianth, fixed pentamerous organ arrangement,
and short carpels with undifferentiated styles that mature into
glabrous fruits (see table 3 in Tschan et al. 2008; table 2 in
Maslova 2010).
Circumscription of the family Platanaceae is best understood
through the fossil record, as extant species represent only a
fraction of the known diversity (Hickey and Doyle 1977; Manchester 1986; Crane 1989; Magallón-Puebla et al. 1997). It is
thought that the genus Platanus had possibly appeared as early
as the Cretaceous, as evidenced by the fossil species Platanus
quedlinburgensis (Pacltová) Tschan, Denk & von Balthazar
(Tschan et al. 2008). This taxon is known from compression
fossils of staminate and pistillate inflorescences from the Upper
Cretaceous (Santonian) of Germany (Pacltová 1982; Knobloch
and Mai 1986). The unisexual inflorescences are stalked but
are ∼10 mm in diameter, making them twice the diameter of
the Eden Main fossils (Tschan et al. 2008). The pistillate inflorescences of P. quedlinburgensis have long styles, and the
staminate inflorescences contain prolate to subovate pollen,
while the Eden Main fossils have only prolate pollen that is
much smaller than that reported in P. quedlinburgensis (Tschan
et al. 2008). Like that of the Eden Main fossils and unlike that
of most extant taxa, the perianth of P. quedlinburgensis is well
developed (Tschan et al. 2008). A second species, Platanus
laevis Velenovský (1882), from the Cretaceous (Cenomanian)
of Germany, is described from impression fossils of pistillate
inflorescences; however, as in specimens of P. quedlinburgensis,
details of floral organization are lacking, and it is uncertain
whether either of these fossil taxa belong to Platanus or one
of the extinct genera (Friis et al. 2011).
While the earliest reports of the genus Platanus may be Late
Cretaceous, the earliest known fossils of the family Platanaceae
are from the lower Cretaceous (Hickey and Doyle 1977; Friis
et al. 1988, 2011; Crane et al. 1993; Wang et al. 2011) of
North America. The oldest platanaceous inflorescences are represented by pistillate heads of Friisicarpus brookensis (Friis,
Crane, Pedersen & Drinnan) Maslova & Herman (Maslova
and Herman 2006) and staminate inflorescences of Aquia
brookensis Friis, Crane, Pedersen & Drinnan (Crane et al.
1993), from mesofossils in sediments of the Patapsco Formation (middle Albian) of Virginia (Friis et al. 1988, 2011).
Species of Friisicarpus Maslova & Herman (pPlatanocarpus
Friis, Crane & Pedersen [Friis et al. 1988]), like the Eden Main
fossils, have five-parted flowers with stout carpels and undifferentiated styles, but they differ in having only unisexual flowers and sessile inflorescences (Friis et al. 1988; Crane et al.
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MINDELL ET AL.—CRETACEOUS BISEXUAL PLATANACEAE
1993; Wang et al. 2011), rather than bisexual, pedunculate
inflorescences, as seen in the Eden Main fossils.
Staminate inflorescences with characters similar to those of
the Eden Main fossil are common in the fossil record. Platananthus synandrus Manchester has staminate inflorescences
with pentamerous flowers; the five stamens are interconnected
by dome-shaped, apical connective extensions (Manchester
1986). While the type specimen was described from the Eocene
of Oregon, the generic diagnosis was amended to accommodate staminate inflorescences of Platananthus hueberi from the
late Albian of Maryland/Santonian-Campanian of North Carolina and Platananthus scanicus Friis, Crane and Pedersen
from the late Santonian-Campanian of Sweden (Friis et al.
1988). Platananthus speirsiae Pigg & Stockey is known from
staminate inflorescences from the Palaeocene of Canada (Pigg
and Stockey 1991). Platananthus synandrus, P. hueberi, and
P. scanicus are similar to the Eden Main specimens in being
five-parted with a well-developed perianth and having domeshaped apical connective extensions and small, prolate pollen
with granular aperture sculpture (Friis et al. 1988). All Platananthus species, like all previously described Platanaceae,
have unisexual flowers and inflorescences, while the Eden
Main specimens have bisexual flowers and inflorescences.
Quadriplatanus georgianus Magallón-Puebla, Herendeen &
Crane was described from the Coniacian-Santonian (Upper
Cretaceous) sediments of Georgia in the United States. The
pistillate flowers in these specimens bear carpels similar to
those found in the flowers at Eden Main. These lack differentiated styles, having broad stigmatic platforms with prominent sutures along the length of the carpel (Magallón-Puebla
et al. 1997). Furthermore, flowers of Quadriplatanus have easily distinguished perianths, fruits that lack trichomes, and prolate pollen (Magallón-Puebla et al. 1997), as in the specimens
described in this article. Ovules in Quadriplatanus were observed to have a median longitudinal flap of tissue that was
interpreted as either a raphe or a wing (Magallón-Puebla et
al. 1997). Ovules/seeds in the Eden Main specimens also have
a crest or wing-like projection in longitudinal section. These
projections could be wings, or they may be a taphonomic artifact. Like other Cretaceous platanoids, Q. georgianus inflorescences had flowers with a fixed merosity, but this species
differs in being tetramerous. Furthermore, unlike those of the
Eden Main specimens, the inflorescences of Quadriplatanus
were unisexual. It should be noted that an inner whorl of
protruding tissues was found within the staminate flowers of
Quadriplatanus. This was interpreted as possibly representing
pistillodes (Magallón-Puebla et al. 1997), as opposed to the
Eden Main flowers, where this whorl is fully developed into
functional carpels.
Gynoplatananthus oysterbayensis Mindell, Stockey &
Beard, based on staminate inflorescences from the Eocene of
British Columbia, is similar to Platananthus but differs in the
presence of rudimentary carpels (Mindell et al. 2006). Like the
Eden Main fossils, Gynoplatananthus is five-parted with a
well-developed perianth and apical connective extensions
(Mindell et al. 2006). Pollen in Gynoplatananthus is larger
(16-mm average polar diameter) and spheroidal, while the pollen from the Eden Main platanoids is prolate and significantly
smaller (11–13 mm in polar diameter). Furthermore, the Eden
Main inflorescences and many of the flowers are bisexual, as
659
opposed to flowers of Gynoplatananthus, which have only
rudimentary carpels.
Manchester (1986) described pistillate infructescences from
the Eocene of Oregon as Macginicarpa glabra Manchester,
which includes flowers with five carpels and elongate, recurved
styles not unlike those observed in extant Platanus. Another
species of Macginicarpa, Macginicarpa manchesteri Pigg et
Stockey, was reported from the Paleocene of Alberta, Canada
(Pigg and Stoockey 1991). Like the Eden Main specimens, all
species of Macginicarpa lack a tuft of trichomes at the base
of the mature fruits, unlike the fruits of extant Platanus (Manchester 1986; Pigg and Stockey 1991). However, Macginicarpa
differs in having persistent, elongate, recurved styles (Manchester 1986; Pigg and Stockey 1991) instead of the undifferentiated stout carpels seen in the Eden Main fossils. Macginicarpa also differs from Ambiplatanus as it has unisexual,
pistillate flowers and wingless seeds dispersed as an elongate
achene, although some specimens seem to show some remnant
staminal tissues.
Bisexuality
Living Platanus represents a very narrow range of the diversity within the family Platanaceae as it has existed through
time. If familial circumscription was based solely on morphology of living species, the Eden Main specimens, along with
many other fossil species, would not fit within the limited
taxonomic boundaries of Platanaceae as deduced using solely
extant taxa. Specifically, their bisexual flowers, fixed merosity,
fruits that lack trichomes, and winged seeds form a unique
combination of characters. However, the fossil record for the
family accommodates, or at least suggests, a wide range of
variation in the family during the Cretaceous and early Paleogene, including bisexuality, pentamerous flowers, and winged
seeds. Many of these characters recur in numerous fossils of
disparate ages (Manchester 1986; Friis et al. 1988; Crane
1989; Magallón-Puebla et al. 1997; Mindell et al. 2006;
Tschan et al. 2008). The clear presence and co-occurrence of
these characters within a single inflorescence does not fit any
known generic diagnosis, extinct or extant. Thus, we have
designated the specimens Ambiplatanus washingtonensis gen.
et sp. nov. to accommodate the novel suite of inflorescence,
floral, and seed morphologies.
Proteales
Platanaceae were historically placed among the wind-pollinated plants artificially grouped together in Hamamelidae
(Thorne 1973; Cronquist 1981; Schwarzwalder and Dilcher
1991). Phylogenetic analyses utilizing molecular data have offered a novel hypothesis placing the family in the order Proteales as sister group to Proteaceae in a clade with Nelumbonaceae (Chase et al. 1993; Hoot et al. 1999; Doyle and
Endress 2000; Hilu et al. 2003; APG III 2009). However, few
morphological synapomorphies have been found in extant representatives to support this grouping; those that have been
described include ovule, endosperm, and embryo characters
(Doyle and Endress 2000), flower merosity (Hoot et al. 1999),
and cuticular features (Carpenter et al. 2005). The fossil record
provides evidence as to the validity of this relationship between
such geographically disjunct and morphologically distinct
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taxa. Carpel and stamen number is highly variable in the unisexual flowers and inflorescences of extant Platanaceae (Nixon
and Poole 2003). This is in contrast to Proteaceae, where bisexual flowers typically feature four stamens and one carpel
(Cronquist 1981). In the fossil record, there is abundant evidence of a fixed pentamerous merosity in Platanaceae (Manchester 1986; Friis et al. 1988, 2011; Crane 1989; Crane et
al. 1993; Pedersen et al. 1994; Maslova 2003, 2010; Mindell
et al. 2006), with the exception of one taxon having tetramerous flowers (Magallón-Puebla et al. 1997). Bisexual flowers
also have been strongly suggested in the fossil record
(Magallón-Puebla et al. 1997; Mindell et al. 2006). Ambiplatanus washingtonensis provides clear evidence of bisexual
flowers in the Late Cretaceous (early Coniacian), perhaps a
relict of ancestral Platanaceae. Living Proteaceae commonly
have winged seeds (Cronquist 1981; Watson and Dallwitz
1992; Schurr et al. 2005), a feature hitherto unknown in fossil
and living Platanaceae. However, Ambiplatanus and Quadriplatanus (Magallón-Puebla et al. 1997) show what may be a
winged seed or possibly a raphe. If these are winged seeds,
this evidence would be consistent with the relationship between
Proteaceae and Platanaceae within the order Proteales (APG
III 2009; Doyle and Endress 2010).
Extant Platanus fruits are achenes, which act as the dispersal
unit. They are subtended by a tuft of trichomes that aid in
wind dispersal (Kubitzki 1993). This tuft of trichomes is absent
from many fossil fruits of Platanaceae (Manchester 1986; Pigg
and Stockey 1991; Mindell et al. 2006) and has resulted in
speculation as to the ancient dispersal of seeds in the family
(Manchester 1986; Tiffney 1986). Macginicarpa fruits from
the Eocene of Oregon share achene morphology with extant
Platanaceae; however, their fruits lack the basal trichome tuft
and, consequently, it was suggested that the fruits were dispersed by water (Manchester 1986). We suggest that the prominent and extensive sutures on the carpels in Cretaceous Platanaceae flowers reported from charcoalified mesofossils (Friis
et al. 1988; Pedersen et al. 1994; Magallón-Puebla et al. 1997)
may represent dehiscence lines for the release of winged seeds.
Fruits of Ambiplatanus lacked trichomes and had clear suture
lines, suggesting that the seed may have been the dispersal unit.
However, empty achenes showing open carpels cannot be demonstrated in these fossils because of abrasion and because of
preservation that is sometimes affected by the presence of pyrite, probably due to bacterial degradation before preservation.
Extant Platanus has a ventral suture (Endress and Igersheim
1999), but the achene functions as the dispersal unit. It appears
that there has been a change in dispersal vectors through time
that can be proposed in light of the structure seen in Ambiplatanus and other presumed wind-dispersed seeds in the Cretaceous, water-dispersed fruits in the Paleogene, and winddispersed fruits in the present, but this requires further
investigation.
Ambiplatanus provides another line of support for the ancient relationship between families of Proteales. The genus is
the first reported instance of functionally bisexual flowers in
the fossil record of Platanaceae. Contemporaneous Cretaceous
localities predominantly feature other modes of preservation
(i.e., compression/impression, charcoalification; Bell 1957;
Crane and Herendeen 1996; Takahashi et al. 1999; Craggs
2005; Maslova 2010; Friis et al. 2011). Only the TuronianSantonian of Hokkaido, Japan, has comparable fossils of an
age and state of preservation similar to those of the Eden Main
locality (Nishida 1991). Permineralized fossils like those found
at Eden Main allow for the identification and description of
fine structural details. The description of A. washingtonensis
should provide an impetus for the reevaluation of platanaceous
compression material, where the mode of preservation could
obscure winged seeds, short carpels within elongate stamens,
and other delicate tissues.
Acknowledgments
We thank Dr. Dirk Meckert (Courtenay, BC) and Rick Ross
(Vancouver Island Paleontological Society) for their help collecting in the field; Dr. Peter R. Crane (Yale University), Dr.
Else-Marie Friis (Swedish Museum of Natural History), Dr.
Kai R. Pedersen (University of Aarhus), and Dr. James A. Doyle
(University of California, Davis) for helpful discussion and
critical review of the manuscript; and Gar W. Rothwell (Ohio
University) for technical assistance and use of laboratory facilities. This work was supported in part by NSERC grant A6908 to R. A. Stockey.
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