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2L. H. Bailey Hortorium, 462 Mann Library, Cornell University, Ithaca, New York 14853-4301 3Department of Plant Biology, Arizona State University, Box 871601, Tempe, Arizona 85287-1601
Received for publication September 26, 1997. Accepted for publication June 23, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONCONCLUSIONSLITERATURE CITED |
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15.6 million years old) chert of the Columbia River Basalt Group in Yakima Canyon, Washington. Quercus hiholensis Borgardt et Pigg sp. nov. is described from anatomical and morphological fruit features, as well as a little recognized anatomical feature, the umbilical complex. Acorns, each comprising a nut and its cupule, are up to 15.3 mm long and 18.8 mm wide with helically arranged, imbricate, tuberculate cupule scales. They show basal aborted ovules, short styles, broad stigmas, and lack grooves in their cotyledons. These characters and the developmental pattern seen in these fossil acorns demonstrate that Q. hiholensis conforms to genus Quercus (Fagaceae), subgenus Quercus, section Quercus (the white oaks). The correspondence of Q. hiholensis to the modern section Quercus reveals that the derived floral and fruit characters that distinguish section Quercus within the genus had evolved by the Middle Miocene.
Key Words: acorn • cupule • Fagaceae • fossil • fruit • Middle Miocene • paleobotany • Quercus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONCONCLUSIONSLITERATURE CITED |
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; Crepet, 1989
). While some authors include the Southern Hemisphere taxon Nothofagus Blume in the Fagaceae, more recent treatments have placed this group of biogeographically important trees in its own monotypic family, Nothofagaceae Kuprianova, based on numerous distinguishing characters (Kuprianova, 1962
; Nixon, 1982
; Jones, 1986
; Romero, 1986
; Hill and Jordan, 1993
; Manos and Steele, 1997
; Rozefelds and Drinnan, 1997
).
An important defining character within the Fagaceae is the presence of a woody subtending structure, called a cupule, around the flower(s). This cupule is sometimes described as an involucre, but cupules are believed to be condensed branching structures rather than involucral bracts (Forman, 1966
; Fey and Endress, 1983
). Flowers in this family have an inferior ovary composed of several fused carpels with two ovules per carpel. After fertilization, the flower develops into a one-seeded, unilocular, indehiscent fruit that is commonly called a nut. Several features, including the mature appearance and dehiscence pattern of the cupule, and the number of fruits enclosed by the cupule are diagnostic for each genus, and numerous authors have addressed the evolutionary significance of these features within Fagaceae (e.g., Berridge, 1914
; Brett, 1963
; Forman, 1966
; Abbe, 1974
; Endress, 1977
; Fey and Endress, 1983
; Nixon, 1989
, 1993
; Crepet and Nixon, 1989a, b
). Two genera of Fagaceae, Quercus and Lithocarpus, form a globose, indehiscent fruit that can be fully to partially enclosed by a non-spiny cupule with a bowl-like shape, and the fruit and cupule described together comprise the acorn. The genus Castanopsis can have globose fruits, but each fruit is typically enclosed within a spiny cupule that opens along sutures to form valves. A few species in Castanopsis develop non-spiny cupules, but suture-like zones are visible, and these cupules also have asymmetric scale-bearing ridges, while almost all acorns have symmetrical scale patterns (Kaul, 1988
). Cupules in Lithocarpus and Quercus are unique in the family as evalvate structures that subtend a single fruit and are thought to be derived within the Fagaceae (Forman, 1966
; Fey and Endress, 1983
; Nixon, 1984
).
Quercus is widely distributed throughout the Northern Hemisphere with its greatest diversity in the southeastern United States, the highlands of Mexico, montane subtropical Eurasia, and east Asia (Nixon, 1993
, 1997
). In contrast, Lithocarpus has its highest diversity in southeast Asia (Soepadmo, 1972
). The leaves, fruits, and growth patterns of Lithocarpus and Quercus are morphologically similar, but the two genera can be distinguished on closer comparison through differences in pollination methods, inflorescences, floral morphology, and number of stamens (see Table 1).
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; Gregor, 1977
), mentioned in a variety of compression floras as Q. sp. acorns and/or cupules (Newberry, 1898
; Knowlton, 1926
; Berry, 1929
, 1931
, 1934
; Brooks, 1935
; Smith, 1939
; Condit, 1944
; Graham, 1963
, 1965
; Smiley and Rember, 1985
; Manchester and Meyer, 1987
), identified to species based on association with leaves of a particular fossil taxon (Fields, 1990
; Axelrod, 1991
), and some acorns have been formally described and assigned a specific epithet (Daghlian and Crepet, 1983
; Manchester, 1994
). However, these fossils show only morphological detail and are limited in the information they provide. The present study documents the first petrified Quercus acorns to be described on the basis of internal anatomical detail. A suite of morphological, anatomical, and developmental features of the fossil acorns are compared to features in extant Lithocarpus and Quercus and provide a unique opportunity to document the evolutionary status of derived features in the family Fagaceae in the Neogene. |
MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONCONCLUSIONSLITERATURE CITED |
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, 1992
; Reidel et al., 1989
; Borgardt and Pigg, 1996
; Pigg, Sophy and Wehr, 1996
). The fossil material occurs in the Museum Flow Package within the interbeds of the Sentinel Bluffs Unit of the central Columbia Plateau N2 Grande Ronde Basalt of the Columbia River Basalt Group (Middle Miocene, Upper Tertiary) dated 15.6 ± 0.2 million years old by 40Ar/39Ar radiometric dating (Swanson et al., 1979
; Long and Duncan, 1982
; Landon and Long, 1989
; Reidel et al., 1989
; Tolan et al., 1989
; Katherine M. Reed, Washington State Department of Natural Resources, Division of Geology and Earth Resources, Olympia, Washington, personal communication, 1994). The specimens were recovered from the "County Line Hole" locality known locally as the "Hi Hole" (R. Foisy, Yakima, Washington, personal communication, 1994). All of the specimens described in this study are part of the T. H. Tuggle-Foisy Collection at the Burke Museum of Natural History and Culture, University of Washington, Seattle (UWBM). Additional material was obtained by the authors during a collecting trip to the locality in June 1994 and is deposited in the Fossil Plant Collections, Arizona State University (ASU).
Previous plants described from these localities include Osmunda wehrii Miller, based on petrified rhizomes and petioles, and Pinus foisyi Miller, based on petrified leaves, pollen, and ovulate cones (Miller, 1982
, 1992
). Additional petrified plant remains at this and adjacent sites include several additional types of filicalian ferns (Rothwell, Arnone, and Pigg, 1996
), members of Taxodiaceae, Hamamelidaceae (Pigg, 1997
), Juglandaceae, Vitaceae, Cornaceae, and Typhaceae and several additional forms (Borgardt and Pigg, 1994
, 1996
; Pigg, Sophy, and Wehr, 1996
). Petrified woods described from Yakima Canyon by Prakash and Barghoorn (1961a,
b
) and nearby sites at Vantage studied by Beck (1945
, 1955)
, Scott, Barghoorn, and Prakash (1962)
, and Prakash (1968)
reveal that Ginkgo, several conifers, and a diverse array of dicots, including three types of Quercus wood, were endemic to the central Washington area in the Middle Miocene. Also nearby and of similar age are the well-known western Latah compression floral outcrops near Grand Coulee and Spokane, Washington (Knowlton, 1926
; Berry, 1929
, 1931
; Brown, 1936
; Fields, 1983
, 1996
).
While some of the specimens in the present study were completely within the chert matrix, others were partially or completely weathered out as individual fruits, cupules, or complete acorns. The chert was initially slabbed with an intermediate-sized diamond blade saw to identify individual specimens. These were then excised out and wafered in serial section on a Buehler Isomet Low-Speed saw. All sections were mounted with a UV-cured adhesive (UV-154, T.H.E. Company, Lakewood, Colorado) onto microscope slides for study and reflected light microphotography. Fragile specimens were stabilized in Bioplastic resin (Ward's, Rochester, New York) and then sectioned with the Buehler Isomet Low-Speed saw and mounted on slides. Anatomical sections of extant Quercus fruits loaned by Mogensen, Northern Arizona University, Flagstaff were studied for comparison (Mogensen, 1965
). Specimens were photographed through a dissecting microscope (SZH, Olympus) and a compound microscope (BH-2, Olympus) with reflected light from fiber optics for anatomical detail. Measurements were made with digital calipers (Digimatic, Mitutoyo Corporation, Japan).
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SYSTEMATICS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONCONCLUSIONSLITERATURE CITED |
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Genus
Quercus L.
Subgenus
Quercus Hickel et Camus.
Section
Quercus Hickel et Camus.
Type species
Quercus hiholensis Borgardt et Pigg sp. nov. (Figs. 5–12, 14, 19–40).
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Holotype
University of Washington, Burke Museum of Natural History and Culture (UWBM) collection B4101; specimen number: 45-I (Figs. 5, 31, 38, 40).
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Type locality
The "Hi Hole", known locally as one of the "County Line Holes" ("Hi Hole", "Lo Hole", and "Ho Hole") is 7.3 km north off the Interstate 82 Firing Center Exit, Yakima County, on Yakima Canyon Road (T14N, R19E, NE 1/4 of NW 1/4 of Sec 3).
Age
15.6 ± 0.2 million years old (Middle Miocene, Upper Tertiary).
Stratigraphy
Museum Flow Package, Sentinel Bluffs Unit, central Columbia Plateau N2, Grande Ronde Basalt member, Columbia River Basalt Group.
Etymology
The specific name, hiholensis, refers to the "Hi Hole" locality in Yakima Canyon, Washington.
Quercus sp.
UWBM collection B4101; specimen numbers (in numerical order): 42 (Fig. 16); 68 (Fig. 18); 55104 (Fig. 15); 55401 (Fig. 13); 56468-19 (Fig. 17).
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DESCRIPTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONCONCLUSIONSLITERATURE CITED |
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In specimens with external scale morphology, the cupule scales measure up to 2.8 x 2.8 mm and are helically arranged. A papery tip on the distal end of the scales can extend the length of the scales by 1.4 mm (Fig. 9) when present. Cupule scales vary in their morphology from base to apex. Basal scales appear highly raised and tuberculate (Fig. 7–10). In contrast, the apical scales assume a more regular morphology and are slightly keeled. The most apical scales appear papery and undulate. In paradermal section, all scales show a regular, helical arrangement under the surface of the cupule (Fig. 14). Lateral scales show a gradual change in morphology from the contorted basal scales to the more regular apical scales. Sclereid patches are scattered throughout the cupule (Figs. 8, 39) and are sometimes organized into white star-shaped clusters associated with the interior appressed edges of the scales (Figs. 6, 11, 36).
Almost all of the specimens are fruits and enclose a single seed in a surviving locule. The ovule enlargement that follows fertilization can be seen in some of the smallest specimens (e.g., Figs. 24–26), but embryos could not be confirmed in all young specimens. One specimen (not illustrated) shows no ovule enlargement and is designated a flower. Because of the rare occurrence of flowers, all specimens in this study will be described as fruits. The wall layers in all of the fruits are intact with no compression or disruption, indicating that none of the fruits were fully mature when they were preserved. The fruits range in sizes from 2.7 x 2.6 mm (Fig. 24) up to 9 x 10.9 mm (Fig. 8), and in shape from ovoid-conical (Fig. 12) to ovoid (Figs. 5–6, 8–11). The youngest fruits show best the features of the perianth and styles with the perianth lobes closely appressed to the styles (Figs. 6, 12, 17–22). The styles, when present, are short, and have a broadly expanding apex (Figs. 18–19).
The fruit wall can be seen in two stages of maturation. In the youngest fruits, the layers are subtle, with only the palisade layer and abscission zone clearly visible (Figs. 12, 18, 23–26, 28–30). In more mature specimens, the fruit wall, except for the abscission zone, can be divided into five distinct layers (Figs. 5, 11, 33, 36), which conform to the layers described by Soepadmo (1968)
. The outer epidermis is uniseriate and is composed of tabular cells. Just inside the outer epidermis, the palisade layer is up to 0.3 mm thick with cells radially elongate and densely packed, forming a visible band. The central layer, the outer parenchymatous layer, comprises the bulk of the fruit wall and is up to 2.5 mm. In this layer, discrete patches of isodiametric sclereids are visible and scattered within parenchymatous tissue. The fourth layer, the inner parenchymatous layer, is up to 0.1 mm thick with cells longitudinally elongate and lacks sclereids. The inner epidermis is uniseriate and composed of tabular cells and unicellular trichomes (Figs. 23, 40). The apical portion of the palisade and outer parenchymatous layers in the fruit wall becomes sclerified early in development (Fig. 19). Basally, a disc-shaped abscission zone is present even in the most immature specimens (Figs. 24, 27–28, 30, 35), and was useful in orienting the specimens. In more mature fruits, this layer becomes quite prominent because of the large, discrete, rectangular sclereid patches (Figs. 33–34).
Some of the youngest fruits show the septa and associated basal aborted ovules that characteristically occur in section Quercus (Figs. 25–26, 30). The umbilical complex, which is a compound structure that includes the funiculus of the ovule, the surviving septum, and the placenta, is present at the base of all the specimens, and can be seen as either a post-like structure (Fig. 29), or as a longer, recurved structure (Figs. 33–34). The abortive ovules are basal (Figs. 31, 34), found between the seed coat and fruit wall, and are either filled with tissue (Figs. 31, 34) or appear hollow (Fig. 32).
The seeds in the very youngest fruits are irregularly-shaped and do not completely fill the surviving locule (Figs. 12, 29–30). The seeds in more mature fruits completely fill the locular cavity and become appressed to the fruit wall (Figs. 5–6, 8, 11, 19–20, 33, 35–36). The seed coat consists of rectangular cells that are compressed along their longitudinal plane (Fig. 32).
Embryos are present in many of the specimens with varying degrees of preservation. Embryos are up to 6 x 6.9 mm in size. The specimens that are the best preserved show two cotyledons (Fig. 11), and even the radicle of the embryo axis (Figs. 37–38). Embryos that have undamaged cotyledons (Figs. 5, 11, 33, 36) are smooth externally with no grooves, are not fused to each other, and assume a symmetrical axial orientation in the seed. In a few specimens, the tissue of the cotyledons became partially transparent while being preserved (Figs. 5, 31, 36) and can be studied with transmitted light. One exceptionally preserved specimen shows a young embryo suspended within endosperm (Fig. 27).
Some of the embryos contain foreign materials that either appear as solid masses or loosely associated clumps (Figs. 5, 36). These acorns are similar in appearance to modern forms damaged by beetle larvae and may represent a similar situation for Q. hiholensis.
Several specimens that show a contrasting cupule scale morphology are designated as Quercus sp. (e.g., Figs. 13, 16). Unlike Q. hiholensis, these forms have cupule scales that are regular in their helical arrangement on the cupule and are distinctly keeled with little surface ornamentation in either the apical or distal portions of the cupule.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONCONCLUSIONSLITERATURE CITED |
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; Tillson and Muller, 1942
; Tucker, 1980
), and in one extreme case, as multiple genera (Schwarz, 1936
). Camus (1934–1954)
recognized two subgenera, Cyclobalanopsis and Quercus, within the genus Quercus, and recent phylogenetic analyses indicate that these two form monophyletic groups, supporting her classification over other treatments (Nixon, 1984,
1993
). This study will follow a modification of the classification system of Camus for the subgenus Quercus (Nixon, 1993,
1997
) (Table 3). The subgenus and section name Euquercus is updated to Quercus in accordance with the International Code of Botanical Nomenclature (Voss et al., 1983
), and the section name Lobatae has priority over the better known name Erythrobalanus for the black or red oaks. The floral and fruit taxonomic differences among the three North American sections (Lobatae, Protobalanus, and Quercus) have been investigated thoroughly in the literature and are well established (Table 2). The black or red oaks of section Lobatae are native only to the New World, present in North America and Central America, with one species extending its range into Colombia, South America. The oaks in the smallest section, Protobalanus, are currently restricted to western North America and possess a mosaic of features in leaves, wood, flowers, and fruit found in the sections Lobatae and Quercus. Section Quercus, the white oaks, has the greatest number of species and has the widest distribution with occurrences in Asia, Europe, North Africa, and North and Central America. Information about floral and fruit anatomy and development of the other sections established by Camus (Cerris, Macrobalanus, and Mesobalanus) is less well known. They are included in section Quercus according to Nixon (1984,
1993
), and further phylogenetic studies are needed to reveal intra-sectional relationships within this group of oaks.
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; Muller, 1942a
; Soepadmo, 1972
; Nixon, 1997
) (Table 3). Acorns within genus Quercus are morphologically distinct between the two subgenera. The cupule scales of subgenus Cyclobalanopsis are concentric, fused lamellae and are easily differentiated from the cupule scales of subgenus Quercus and Q. hiholensis which are imbricate and helically arranged (Soepadmo, 1972
; Kaul, 1985
).
In addition to their presence in the genus Quercus, acorns are also found in one other member of the Fagaceae, the genus Lithocarpus. Because the fossil fruits of Q. hiholensis are not found in association with leaves, it is necessary to compare them to both genera. Lithocarpus is almost entirely restricted to Asia, but one species, L. densiflora (Hook & Arn.) Rehder, is present today in California, hinting at a once wider distribution for members of this genus. Lithocarpus leaves that have been compared favorably with extant L. densiflora have been reported from at least five Tertiary localities from the American Northwest (Knowlton, 1926
; MacGinitie, 1933
; Brooks, 1935
; LaMotte, 1952
; Graham, 1963,
1965,
Smiley and Rember, 1985
; Axelrod, 1964
; Fields, 1990,
1996
; Axelrod and Schorn, 1994
), as well as acorns (Axelrod, 1966
; Smiley and Rember, 1985
). Although the extant American Lithocarpus species has a strikingly different cupule morphology (recurved, thin, papery scales) when compared to Quercus cupules, some extant Asian Lithocarpus species have cupules similar to Quercus, and two Northwest localities have leaves assigned to Lithocarpus that are comparable to forms found today in Asia. Axelrod (1966)
compared L. coatsi from the Eocene Copper Basin flora to Asian forms. In the Miocene Oviatt Creek flora, Boyd (1985)
described L. oviattensis leaves with entire margins and drip tips, a morphology found today only in Asia. Comparison of Q. hiholensis to extant American and Asian Lithocarpus acorns reveals several differences such as the basal position of the abortive ovules and umbilical complex, the absence of staminodia in the persistent perianth of the young fruits, cotyledons that are not grooved, and the solitary nature of the fossil fruit (see Table 1) that demonstrate stronger affinities of the fossil acorns to Quercus.
A few fossil acorns have been illustrated in Tertiary fruit and seed or leaf compression floras (Eocene: Manchester, 1994
; Oligocene: Daghlian and Crepet, 1983
; Manchester and Meyer, 1987
; Miocene: Newberry, 1898
; Knowlton, 1926
; Smith, 1939
; Axelrod, 1991
; Taylor and Taylor, 1993
; Pliocene/Miocene: Condit, 1944
). The best-documented occurrences include those from the Eocene of Oregon (Manchester, 1994
), the Oligocene of Tennessee (Daghlian and Crepet, 1983
) and the Miocene of Idaho (Fields, 1990,
1996
; written communication, 1994; Taylor and Taylor, 1993
).
Quercus paleocarpa Manchester was described from the Middle Eocene Clarno Nut Beds of Oregon based on several specimens including a complete acorn with fruit and cupule. The complete acorns are prolate, 22 x 15.5–21.8 mm, with a bowl-shaped cupule covering one-third to one-half the length of the fruit. The cupule is characterized by seven conspicuous concentric transverse ribs, which decrease in size from base to apex. Although cupules with symmetrical concentric rings can be found in both Quercus and Lithocarpus (Kaul, 1985
), Manchester (1994)
interpreted this specimen as assignable to Quercus, on the basis of the relatively coarse appearance of the cupular lamellae. Additional specimens of this type, particularly those showing such features as persistent perianth parts and style morphology, would aid greatly in determining the affinities of these interesting forms.
Quercus huntsvillensis Daghlian et Crepet was described from the Oligocene Catahoula Formation near Huntsville, Tennessee, based on a single, crushed fruit 17 x 15 mm with a peduncle 17 mm long (Daghlian and Crepet, 1983
). They placed this specimen in the subgenus Lepidobalanus (Endl.) Oersted sensu Trelease (=subgenus Quercus, section Quercus), on the criteria of basally thickened cupule scales. Although basally thickened cupule scales are characteristic of section Quercus, they are also found in section Protobalanus, which, although currently restricted to several areas of western North America, was more widespread in the Tertiary (Chaney and Axelrod, 1959
; Wolfe, 1980
; Axelrod, 1983
).
A third interesting occurrence is that of two different species of compressed acorns found in attachment to corresponding leaf-bearing stems from the Middle Miocene Succor Creek flora of the Sucker Creek Formation of southwestern Idaho (Fields, 1990
, 1996,
written communication, 1994; Taylor and Taylor, 1993
). The first specimen with acorns attached to a stem bears leaves of the fossil leaf taxon Q. hannibili Dorf (Fields, Michigan State, East Lansing, written communication, 1994), which has been compared to the modern oak Q. chrysolepis of section Protobalanus (Axelrod, 1964
; Graham, 1965
). The acorns found associated with this leaf taxon have helically arranged cupule scales that are keeled and are morphologically similar to the acorns found associated with the second leaf taxon in such a way that these acorns cannot be distinguished from one another without corresponding attached leaves (Fields, Michigan State University, East Lansing, written communication, 1994). The second specimen has three cupules, 15–21 mm wide, attached to the stem and two of them envelop fruits, which measure 20 x 18 mm, by half their length. The leaf attached to the stem is assignable to Quercus simulata Knowlton, a fossil taxon that, interestingly, has been compared to every taxonomic group in the genus Quercus. Knowlton (1898)
described and compared it to an extant black oak (section Lobatae) with entire leaf margins, and many authors have also compared Q. simulata to black oaks (Chaney, 1920
; Berry, 1929,
1931,
1934
; Axelrod, 1964
, 1992
). This fossil leaf has also been compared to leaves from Lithocarpus densiflora (Chaney, 1927
), oaks in section Protobalanus (Berry, 1929
; Wolfe, 1960,
1964
), and oaks of subgenus Cyclobalanopsis (MacGinitie, 1933
; Chaney and Axelrod, 1959
; Axelrod, 1964
). A dramatic shift in comparison occurred when Niklas and Giannasi (1978)
published a paleobiochemistry study that found leaves of Q. simulata had the most affinity with white oaks (section Quercus) from Japan and Korea. Wolfe (1980)
commented on this study and also recommended comparing Q. simulata to an extant western North American white oak, Q. sadleriana, which appears to be a relictual species with strong similarities to eastern North American and Asian species of Quercus (Nixon, 1997)
.
Quercus hiholensis differs from these previously described acorns in several ways. Cupule scales of Q. hiholensis are helically arranged, in contrast to the concentric lamellae of Q. paleocarpa from the Eocene Clarno Nut Beds. While both Q. hiholensis and the Oligocene Q. huntsvillensis have helically arranged cupule scales, scales of Q. hiholensis vary in morphology from base to apex, and scales of Q. huntsvillensis have basally thickened scales that are keeled and of uniform shape throughout. The acorns attached to leaf-bearing twigs from the Sucker Creek Formation also have a regular keeled morphology from the base to the apex of the fruit.
Morphological features of fossil acorns are usually the only information available, but anatomical characters such as those found inside the fruits of Q. hiholensis are necessary for unambiguous descriptions and taxonomic assignment. Features such as the final position of the abortive ovules and the umbilical complex, cupule scale ornamentation, and details of the persistent perianth and styles are the result of developmental processes that occur in extant Quercus fruits as they mature (Figs. 1–4). The resulting combination of character states are unique to each section of subgenus Quercus (Table 2). The fossil Q. hiholensis acorns documented in this study show several stages of maturation and development that conform most closely to those of section Quercus, and will be used to illustrate the development of the fruits and cupules for this extant section.
Development of reproductive structures in subgenus Quercus
In subgenus Quercus, inflorescence primordia are initiated at the end of the growing season, remaining quiescent in terminal buds until the next flush of growth. The continued development and enlargement of these primordia in temperate climates occur in response to ambient conditions, such as warm temperatures in the spring or adequate rainfall in desert areas (Turkel, 1950
; Rebuck, 1952
; Sharp and Sprague, 1967
). The staminate flowers are borne on catkins and emerge first, sometimes maturing weeks before the pistillate flowers are fully formed and receptive for pollination (Sharp and Chisman, 1961
; Stairs, 1964
). The pistillate inflorescences emerge from leaf axils distal to the leaf axils that produce staminate inflorescences (Trelease, 1924
; Muller, 1942a
). Each pistillate inflorescence has only a few floral primordia produced, and the cupules in subgenus Quercus do not become deformed from crowding. In contrast, the cupules from some species of Lithocarpus and rarely of Quercus subgenus Cyclobalanopsis can show compressed scales or lamellae from the pressure of adjoining cupules (Kaul, 1985,
1987
). The inflorescence axis, which bears the acorns until they are fully mature, can remain short (subsessile) or elongate to form a peduncle up to several centimetres long (Trelease, 1924
; Soepadmo, 1968,
1972
; Kaul, 1985
; Nixon, 1997
). The cupules of Q. hiholensis that show external morphological detail are not deformed from crowding (Figs. 9–10). Several specimens are pedunculate, but of indeterminate length (Fig. 9).
With development of the pistillate flower, individual scale primordia of the cupule emerge at the base of the ovary (Sattler, 1973
). These primordia fuse into a ring and continue to produce cupule scales in an extremely complex helical pattern (Forman, 1966
), enveloping the ovary. The fusion of the primordia results in cupule scales that are accrescent at their base, but loosely or closely imbricate at their apex. The scales can appear papery and flattened (common in section Lobatae), or are basally thickened and appear keeled in profile and/or tuberculate with a variously thickened surface (common in sections Quercus, Protobalanus) (Trelease, 1924
; Muller, 1942a
; Kaul, 1985
). The cupules of Q. hiholensis have a morphology that conforms more closely to sections Quercus or Protobalanus. All scales are thickened basally. Basal scales of the cupule are tuberculate and irregular, and apical scales are smaller with a keeled morphology and less ornamentation (Figs. 5–6, 9–10).
In extant Quercus, the floral apex of the pistillate flower flattens and usually produces two cycles of three perianth members in a ring on the margin of the floral apex. Gynoecial primordia (usually three) appear to the inside of this perianth ring in the ovary wall. As the flower elongates, the gynoecial primordia grow inward to form the septa and delimit locular spaces. Simultaneously, these gynoecial primordia also elongate acropetally through the perianth to form the styles (Sattler, 1973
). These styles are glabrous distal to the perianth with a spreading stigmatic surface on the adaxial groove in subgenus Quercus (Trelease, 1924
; Muller, 1942a;
Nixon, 1984
). In section Lobatae, each style is elongate and gradually spreading (see Fig. 2). In comparison, those of sections Protobalanus and Quercus are shorter and can appear blunt (see Fig. 1), and are most similar to the morphology seen in Q. hiholensis (Figs. 6, 18–20).
The perianth of the pistillate oak flower is urceolate, usually with six lobes. In the sections Quercus and Protobalanus and in Q. hiholensis, these lobes are reduced and are often tightly appressed to the styles (Figs. 6, 17–22), while in section Lobatae, most of the species have enlarged perianth lobes, and the apical cupule scales become entrapped under them early in development (see Fig. 2) (Ørsted, 1871
; Trelease, 1924
; Muller, 1942a
; Nixon, 1984
).
Previous authors have described the region of the extant Quercus flower and fruit that is located between the base of the perianth to the apex of the ovary as a stylopodium (Trelease, 1924
; Muller, 1942a
). The term stylopodium actually refers to fused styles that form an enlarged structure above the perianth at the apex of the ovary, such as in the family Apiaceae (Lawrence, 1951
; Kearney, Peebles, and Collaborators, 1960). In subgenus Quercus, the perianth is raised above the cupule scales after fertilization by a column formed by the elongation of the apex of the ovary. This pattern can be recognized clearly in Q. hiholensis (Figs. 17, 19–20), where the perianth is above the cupule at even the earliest stages of fruit development. In extant oaks, this column of ovary tissue is persistent on the mature fruit as an apical mucronate umbo, and may be subtended in some species by either a hemispherical swelling or depression in the apical end of the fruit wall (Trelease, 1924
; Muller, 1942a
; Kaul, 1985
). Quercus hiholensis has a depression around the umbo in more developmentally mature fruits (Fig. 5).
Styles, stigmas, perianth, carpels, immature ovules, and the first few rows of cupule scales are all present at the time of pollination in extant subgenus Quercus (Hjelmqvist, 1953
; Stairs, 1964
; Mogensen, 1965
, 1972
). Because the development of the gynoecial primordia is delayed for a time after the floral apex begins to elongate, the locules are incomplete with an open space at the base of the ovary (Figs. 25–26). Prior to fertilization, the collateral, anatropous ovules finish development of the double integument and embryo sac, and most of the ovules are similar in size. Although all six ovules have an equal possibility of developing into the single seed, only one survives. The other five ovules undergo either embryo sac disintegration, zygote or embryo abortion, or fail to develop a coherent embryo sac prior to pollen tube penetration (Brown, 1971
; Brown and Mogensen, 1972
; Mogensen, 1975a
, b
). After fertilization, which can be delayed one year in oaks with the biennial fruiting habit (Mogensen, 1965
), the seed enlarges quickly and fills the locular spaces. Septa in some members of section Lobatae may persist and become imbedded in and/or create grooves in the seed coat and cotyledons (Ørsted, 1871
; Trelease, 1924
; Muller, 1942a
; Nixon, 1984
, 1993
). In sections Protobalanus and Quercus, the cotyledons and seed coat appear smooth. All examined specimens of Q. hiholensis have smooth cotyledons (Figs. 5–6, 8, 11, 20, 33, 36), suggesting greater affinity to sections Protobalanus or Quercus.
Several distinct fruit wall layers are visible in even the youngest fruits (Figs. 5, 8, 11–12, 24–26, 30, 33, 36). Regions of the fruits that are heavily sclerified at maturity, such as the abscission zone, the palisade layer, and the outer parenchymatous layer at the fruit apex, are especially prominent. In extant oaks, the wall layers become compressed and/or break down at the time of fruit maturity, forming the hard "shell" of the acorn. Mogensen (1965)
noted that the fruit wall sclerenchyma in section Lobatae remained thick at maturity, while the fruit wall layers in section Quercus became thin and undefined. All of the Q. hiholensis fruits examined in this study have coherent wall layers with no compression, so this character cannot be resolved.
Historically, a character that has been used to distinguish between extant sections Lobatae and Quercus is the pubescence of the inner epidermis of the fruit wall, or endocarp (Trelease, 1924
; Tillson and Muller, 1942
; Mogensen, 1965
; Tucker, 1980
; Nixon, 1984
). However, presence or absence of trichomes is only relevant in the mature fruit. The endocarp of all three sections of subgenus Quercus is pubescent until the final stages of acorn maturation. The endocarp pubescence of section Lobatae ranges from usually very pubescent to rarely patchy at maturity (see Fig. 4), and can be patchy in section Protobalanus (Nixon, 1984
, 1997
; Landrum, 1993
). The endocarp pubescence of section Quercus can also appear patchy when the seed is removed, but in most cases, the "glabrous" endocarp of section Quercus is actually the inside of the seed coat that detaches from the embryo at maturity and becomes attached to the fruit wall (see Fig. 3). If the seed coat is peeled from the endocarp, trichomes are still visible on the endocarp (Nixon, 1984
, 1997
). Although Q. hiholensis has trichomes present with no fusion of seed coat and fruit wall in all specimens studied, none of the fruits are completely mature, so the significance of these characters in the fossils is unclear.
The embryo axis can also be used as a taxonomic character. In sections Lobatae and Protobalanus, the shoot apex lacks leaf primordia until after germination, but the embryo of section Quercus can possess one to a few whorls of leaf primordia while still in the seed (Mogensen, 1965
; Sutton and Mogensen, 1970
). In the few fossil specimens of Q. hiholensis that contain suitably preserved embryo axes, the shoot apex is out of the plane of section (Figs. 37–38), so this character could not be verified.
The final position of the aborted ovules in acorns is a very important taxonomic character in subgenus Quercus. In almost all of section Lobatae, these aborted ovules occur above the seed, attached at the apex of the fruit (see Fig. 4). A few black oak species have been described with what appears to be basal or lateral abortive ovules (Muller, 1942a
), but whether these are developmentally similar to the white oaks is unknown at this time (Nixon, Cornell University, Ithaca, New York, personal communication, 1998). In section Protobalanus, the aborted ovules are found laterally, while section Quercus has basal abortive ovules (see Fig. 3). These patterns hold in general for the sections of subgenus Quercus, but exceptions are well known (Trelease, 1924
; Muller, 1942a
, b
). The apical abortive ovules in section Lobatae can shift down and appear lateral, while the basal abortive ovules in section Quercus can shift upward and also appear lateral. Although this abortive ovule position character can be misleading if examined superficially, the underlying basis for this character is a more consistent feature that has received little or no attention in the past.
The final position of the aborted ovules for each section of subgenus Quercus is due to fruit wall and umbilical complex development. The term "umbilical complex" is used in this study to indicate the compound structure that vascularizes the developing seed and includes the placenta, the funiculus of the ovule, and the surviving septum (referred to as the columella in Nixon, 1984
, 1993
, 1997
). Because all the ovules begin development on septa that are extensions of the fruit wall, changes that occur in the fruit wall also affect the final position of the abortive ovules and the umbilical complex. In section Lobatae, attachment of the seed occurs at the apex of the fruit, near the apical abortive ovules. The umbilical complex is either part of one of the sclerified septal ridges that groove the cotyledons or one of the thin, papery "strings" along the inside wall of the fruit, and is therefore present from the apex to the base of the fruit. In section Protobalanus, the surviving septum is present on the inside of the fruit wall from the base to the apex of the fruit as a papery string. The position of the umbilical complex and the attachment of the seed to the fruit have yet to be determined for section Protobalanus, but the lateral abortive ovules may provide a clue. In contrast to the previous two sections and all other ge
