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Paleobotany |
2Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9 Canada; 3Missouri Botanical Garden, P.O. Box 299, St. Louis, Missouri 63166-0299 USA
Received for publication October 2, 2003. Accepted for publication March 6, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXLITERATURE CITED |
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Key Words: Decodon • Duabanga • Eocene • leaf histology • Lythraceae • Punicaceae • Sonneratiaceae
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXLITERATURE CITED |
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; Graham et al., 1993
). Punicaceae (Punica L.) and Sonneratiaceae (Sonneratia L.f. and Duabanga Buchanan-Hamilton) have been included in Lythraceae by several authors, sometimes with Trapaceae (Trapa L.) (Dahlgren and Thorne, 1984
; Thorne, 1992a
, b
; Graham et al., 1993
; Graham, 1999
; Judd et al., 1999
). Phylogenetic analyses using morphological (Johnson and Briggs, 1984
; Graham et al., 1993
) and molecular characters (Conti et al., 1996
, 1997
; Shi et al., 2000
; Huang and Shi, 2002
) have resulted in the recognition of a lythraceous clade that also includes Sonneratia, Duabanga, and Punica. Lythraceae sensu lato includes five subfamilies: Lythroideae with 28 genera, and Punicoideae, Sonneratioideae, Duabangoideae, and Trapoideae (Graham et al., 1998
), each with a single genus.
The oldest fossil records of Lythraceae are of seeds from the Campanian (Cretaceous) of Mexico (Rodríguez-de la Rosa et al., 1998
) and from the Paleocene of southern England (Reid and Chandler, 1933
; Chandler, 1961
). Fossils of Lythraceae include fruits, seeds, leaves, and pollen (Graham and Graham, 1971
; Muller, 1981
; Tiffney, 1981
; Friis, 1985
). Leaves assignable to the genus Decodon J. F. Gmelin have been reported from compression/impression fossils from North America (Wolfe and Tanai, 1980
; Wehr and Hopkins, 1994
; Stockey and Wehr, 1996
) and western Europe (Kva
ek and Sakala, 1999
). However, detailed anatomical data are not known for these leaves, and we cannot be certain that they represent this genus. Manchester et al. (1998)
point out that assigning isolated leaves of the order Myrtales to extant taxa in the absence of fruit and flower data is not advised because of the similarities in venation pattern in the order. The mosaic of characters seen in fossil myrtalean taxa such as Syzygioides Manchester Dilcher et Wing (1998)
illustrate that with isolated organs, fossils of Myrtales may be difficult to assess and that whole-plant reconstructions are necessary.
In this paper, we describe a lythraceous leaf type from the Middle Eocene Princeton chert of British Columbia, Canada. These leaves are closely associated in the same chert layers with fruits, seeds, and recently described vegetative axes with lacunate phellem, of Decodon allenbyensis Cevallos-Ferriz et Stockey (Cevallos-Ferriz and Stockey, 1988
; Little and Stockey, 2003
). We histologically compare the fossil leaves to those of Myrtales and in particular to those of Lythraceae sensu lato, using characters of the lythraceous leaves described by Keating (1984)
, and using new characters, to assess their affinities. This comparison, along with original observations of Decodon verticillatus (L.) Ell. leaves, demonstrates newly recognized diversity in Lythraceae of the Middle Eocene of western North America.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXLITERATURE CITED |
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; H. Baadsgaard, University of Alberta, personal communication 1999). Leaves come from layer #43 of the chert and are most often associated with the fruits, seeds, and vegetative axes of Decodon allenbyensis (Little and Stockey, 2003
) as well as the fruits and seeds of Paleomyrtinaea princetonensis Pigg, Stockey et Maxwell (1993)
.
Fossil specimens were prepared with the cellulose acetate peel technique (Joy et al., 1956
) modified for hydrofluoric acid (Basinger and Rothwell, 1977
; Basinger, 1981
). Peel sections were mounted with Eukitt rapid mounting medium (O. Kindler, Freiburg, Germany) for microscopic examination. All fossil specimens are housed in the University of Alberta Paleobotanical Collection (UAPC-ALTA).
Slides of extant Myrtales were examined from the material sectioned by Keating (1984)
. The sample of species of Lythraceae sensu lato were originally illustrated by camera lucida, but are photographed for the first time here, and additional data are provided based on these microscopic sections. Leaves were preserved in FAA or FPA, embedded in paraffin, sectioned at 10 µm and stained with safranin-O/fast green (Keating, 1984
). All leaves were sectioned at midlevel in transverse section (Keating, 1984
). The sections of extant leaves used in this study are currently housed at the University of Alberta.
Leaves of extant Decodon verticillatus, examined here for the first time, were studied using paraffin-embedded sections stained with safranin-O/fast green (Johansen, 1940
). In addition, leaves of extant D. verticillatus were examined by Cryo-SEM (scanning electron microscopy) using an EMITECK (K1250) cryosystem, and the chromium trioxide technique (Alvin and Boulter, 1974
). Samples were coated with 15 nm Au with a Nanotek sputter coater, and viewed with a Japan Electronics Optics (JEOLUSA) scanning electron microscope (JSM 6301) at 5 kV. Images were taken with a Leaf Microlumina System version 1.2 (Leaf Systems, Westborough, Massachusetts, USA) and a Phase One digital studio camera (Frederiksberg, Denmark) using a Leitz Aristophot and processed using Adobe Photoshop (Adobe Systems, Inc., San Jose, California, USA).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXLITERATURE CITED |
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The mesophyll is well differentiated into a double-layered palisade that makes up 50% of the lamina thickness; spongy layers are 5–7 cells thick at the lamina. Midribs are flat to convex adaxially and prominently rounded/convex to V-shaped abaxially (Figs. 2, 3, 4). Midvein xylem and phloem are rarely well preserved, possibly degraded by fungi (Figs. 3, 4). Although preservation makes observation of midvein and surrounding tissues difficult, midveins appear to be C-shaped, incurved adaxially, surrounded by periphloic fibers except for an adaxial gap, and surrounded by ground tissue (Figs. 3, 4). Secondary vein ribs are flat adaxially and abaxially convex or slightly biconvex, with C-shaped secondary veins that are surrounded by fibers (Table 1). Minor veins are also surrounded by fibers (Fig. 5). Crystals were not observed in the leaves.
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Leaves of Decodon verticillatus (L.) Ell
In surface view, anomocytic stomata are level with the epidermis and occur on both leaf surfaces (Figs. 11–14). Epidermal cells appear polygonal when observed with cryo-SEM, with a clear outline and smooth surface (Figs. 13, 14). Isolated, dried cuticle observed with SEM appears striated/wrinkled, and the outline of the epidermal cells is often less clear (Figs. 11, 12). Multicellular, branched trichomes occur on both leaf surfaces (Fig. 14) and are more dense on or near veins. Fine ornamentation is visible on the trichomes (Fig. 14).
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Lythrum alatum var. lanceolatum (Ell.) Rothrock
The lamina is dorsiventral and 120–130 µm thick (Fig. 23). The adaxial and abaxial epidermis is similar in thickness with rectangular to rounded cells and with some enlarged and mucilaginous cells. Some epidermal cells contain spherical clusters of birefringent crystals. The cuticle is thin and appears striated. Leaves are amphistomatic with guard cells level with the epidermis. Trichomes were not observed. The mesophyll has one distinct palisade layer, occasionally with a less well-differentiated layer below, and together the layer(s) make up 30% of the lamina thickness. Spongy mesophyll is three cells thick. Midribs are sunken, grooved adaxially, and slightly V-shaped abaxially (Fig. 23). Midveins are short arcs, bicollateral, and surrounded by ground tissue. Secondary vein ribs are slightly biconvex with small and circular secondary veins (Table 1). Druses are observed in the midrib ground tissue and the mesophyll.
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Ammannia coccinea Rottb
The lamina is dorsiventral and 130–160 µm thick (Fig. 25). Adaxial epidermal cells are larger than cells of the abaxial epidermis. Epidermal cells are rectangular to rounded, with some enlarged mucilaginous cells. Some epidermal cells contain spherical clusters of birefringent crytstals. Cuticle is very thin and smooth. Leaves are amphistomatic with guard cells level with the epidermis. Trichomes were not observed. The mesophyll is well differentiated with a single palisade layer that makes up 50% of the lamina thickness. Spongy mesophyll is 3–5 cells thick at the lamina. Midribs are concave/grooved adaxially and rounded/convex abaxially (Fig. 25). Midveins are weak arcs, bicollateral, and surrounded by ground tissue. Secondary vein ribs are flat adaxially and abaxially, with circular secondary veins (Table 1). Druses are observed in the midrib ground tissue and mesophyll.
Cuphea spectabilis S. Graham
The lamina is dorsiventral and 120–160 µm thick (Fig. 26). Adaxial epidermal cells are slightly larger than cells of the abaxial epidermis. Epidermal cells are rectangular to rounded, with some enlarged mucilaginous cells. Cuticle is very thin and smooth. Leaves are amphistomatic with guard cells level with the epidermis. Trichomes are uniseriate, 1–4 celled, and thin walled. The mesophyll is well differentiated with a single palisade layer that makes up 30% of the lamina thickness. Spongy mesophyll is five cells thick. Midribs are concave/grooved adaxially and convex/square abaxially (Fig. 26). Midveins are weak arcs, bicollateral, and surrounded by ground tissue. Secondary vein ribs are adaxially grooved and abaxially convex, similar to midribs, with weakly C-shaped secondary veins surrounded by ground tissue (Table 1). Druses are observed in the midrib ground tissue and mesophyll.
Heimia salicifolia (H.B.K.) Link
The lamina is dorsiventral, 100–125 µm thick (Fig. 27). Epidermal cells are rectangular to rounded, with some enlarged mucilaginous cells. Adaxial epidermal cells are slightly larger than those of the abaxial epidermis. The cuticle is thin, striated, and ornamented. Stomata are found only on the abaxial surface with guard cells level with the epidermis. Trichomes are not present. The well-differentiated mesophyll is a single palisade layer that makes up 30% of the lamina thickness and a spongy layer 3–5 cells thick. Midribs are concave/grooved adaxially and V-shaped abaxially (Fig. 27). Midveins are weakly C-shaped, bicollateral, and surrounded by ground tissue. Secondary vein ribs are adaxially and abaxially convex with weakly C-shaped secondary veins surrounded by ground tissue (Table 1). Druses are observed in the midrib ground tissue and mesophyll.
Lagerstroemia speciosa (L.) Pers
The lamina is dorsiventral and 180–210 µm thick (Fig. 28). Epidermal cells are rectangular to rounded, with periclinally divided cells occurring regularly. Some epidermal cells contain spherical clusters of birefringent crystals, and some cells are enlarged and mucilaginous. The mucilage cells tend to protrude into the mesophyll and sometimes appear to be below the epidermis. Cells of the adaxial epidermis are about twice as large as those of the abaxial epidermis. The cuticle is thin, smooth, and slightly ornamented. Stomata are found only on the abaxial surface with guard cells level with the epidermis. Trichomes are uniseriate, 1–4 celled, and thin walled. The mesophyll is well differentiated, composed of a double palisade layer that makes up 40% of the lamina thickness and spongy layers 4–6 cells thick. Midribs are convex/rounded adaxially and convex/ square abaxially (Fig. 28). Bicollateral midveins form a cylinder surrounded by periphloic fibers and ground tissue. Secondary vein ribs are flat adaxially and abaxially convex, with weakly C-shaped secondary veins that have an abaxial band of periphloic fibers, abaxial ground tissue, and a parenchymatous adaxial extension (Table 1). Prismatic crystals and druses are abundant in midrib and mesophyll tissues.
Lafoensia speciosa (H.B.K.) DC
The lamina is dorsiventral and 130–185 µm thick (Fig. 29). Adaxial and abaxial epidermis is similar in thickness, with rectangular to rounded cells and with some enlarged mucilaginous cells. Some epidermal cells contain spherical clusters of birefringent crystals. Cuticle is thin and smooth. Stomata are found only on the abaxial surface with guard cells level with the epidermis. Trichomes were not observed. The mesophyll is well differentiated, with a single palisade layer that makes up 25% of the lamina thickness and spongy layers 6–8 cells thick. Midribs are grooved adaxially and convex/rounded abaxially (Fig. 29). Midveins are C-shaped, bicollateral with an abaxial band of periphloic fibers, and surrounded by ground tissue. Secondary vein ribs are flat adaxially and abaxially convex, with weakly C-shaped secondary veins that have an abaxial band of periphloic fibers, abaxial ground tissue, and a parenchymatous adaxial extension (Table 1). Prismatic crystals and druses are abundant in midrib and mesophyll tissues.
Lawsonia inermis L
The lamina is dorsiventral and 180– 210 µm thick (Fig. 30). Adaxial and abaxial epidermis is similar in thickness, with rectangular to rounded cells, with some enlarged mucilaginous cells. The cuticle is thin, smooth, or slightly ornamented. Leaves are amphistomatic with guard cells level with the epidermis. Trichomes are not present. The mesophyll cells are dense, with a single palisade layer, occasionally double, that makes up 30–40% of the lamina thickness; spongy layers are 4–6 cells thick. Midribs are convex/ rounded adaxially and convex/rounded abaxially (Fig. 30). Bicollateral midveins are C-shaped and surrounded by ground tissue. Secondary vein ribs are flat adaxially and abaxially, with circular secondary veins (Table 1). Druses are observed in midrib and mesophyll tissues.
Punica protopunica Balf. f
The lamina is dorsiventral, tending to isobilateral, and 230–290 µm thick (Fig. 31). Epidermal cells are rectangular to rounded, with some enlarged mucilagious cells. Adaxial epidermal cells are about twice as large as abaxial epidermal cells. The cuticle is thick and ornamented. Stomata are only found on the abaxial surface with guard cells level with the epidermis. Trichomes were not observed. The mesophyll is well differentiated into a palisade that is two or three cells thick that makes up 30–40% of the lamina thickness and spongy mesophyll 5–8 cells thick. The three most abaxial spongy mesophyll layers are densely packed and rectangular, imparting a partially isobilateral appearance to the lamina. Midribs are slightly convex to flat adaxially and convex/round abaxially (Fig. 31). Midveins are weak arcs, bicollateral, and surrounded by ground tissue. Secondary vein ribs are slightly convex adaxially and abaxially, similar to midribs, with weakly C-shaped secondary veins that have a parenchymatous adaxial extension (Table 1). Prismatics are observed in mesophyll tissues.
Punica granatum L
The lamina is dorsiventral and 345– 380 µm thick (Fig. 32). Epidermal cells are rectangular to rounded, and no enlarged mucilaginous cells were observed. Cells of the adaxial epidermis are larger than those of the abaxial epidermis. Cuticle is thin, smooth to slightly ornamented. Stomata are found only on the abaxial surface, with guard cells level with the epidermis. Trichomes were not observed. The well-differentiated mesophyll is a single palisade layer, about 40–50% of the lamina thickness, and spongy layers 3–4 cells thick. Midribs are slightly concave to flat adaxially and convex/round abaxially (Fig. 32). Midveins are C-shaped, bicollateral, and surrounded by ground tissue. Secondary vein ribs are flat adaxially and slightly convex/rounded abaxially with weakly C-shaped to circular secondary veins (Table 1). Prismatics and druses are observed in midrib and mesophyll tissues.
Sonneratia sp
The lamina is dorsiventral, tends toward isobilateral, and is 325–415 µm thick (Fig. 33). Epidermal cells are rectangular to rounded, with some enlarged mucilaginous cells that intrude into the mesophyll appearing to be just internal to the epidermis. Adaxial and abaxial epidermal cells are similar in size. The cuticle is thick and ornamented. Leaves are amphistomatic, and guard cells are slightly sunken with slightly overarching subsidiary cells. Trichomes were not observed. Mesophyll is well differentiated into a palisade 3–4 cells thick, that makes up 25–30% of the lamina thickness, and spongy layers 10–12 cells thick. The lowest 3–4 mesophyll layers are densely packed and rectangular, imparting a partially isobilateral appearance to the lamina. Midribs are slightly flat to concave adaxially and convex/round to V-shaped abaxially (Fig. 33). Midveins are C-shaped, bicollateral, and surrounded by ground tissue. Secondary vein ribs are not apparent with the C-shaped secondary veins embedded in the mesophyll (Table 1). Sclereids are observed in the mesophyll, and druses in the midrib and mesophyll tissues.
Sonneratia apetala Buch.-Ham
The lamina is isobilateral and 244–510 µm thick (Fig. 34). Epidermal cells are rectangular to rounded, with enlarged mucilaginous cells intruding into the mesophyll appearing just internal to the epidermis. Abaxial and adaxial epidermal cells are similar in size. Cuticle is thick and ornamented. Leaves are amphistomatic, and guard cells are slighly sunken, with slightly overarched subsidiary cells. Trichomes are not present. Well-developed adaxial and abaxial palisade, both three cells thick, together make up 50– 60% of the lamina thickness. Spongy mesophyll layers are 5– 8 cells thick. Midribs are convex/round adaxially and abaxially (Fig. 34). Bicollateral midveins form a cylinder, surrounded by ground tissue. Secondary vein ribs are not apparent with the C-shaped secondary veins embedded in the mesophyll (Table 1). Sclereids are observed in the mesophyll and druses in the midrib and mesophyll tissues.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXLITERATURE CITED |
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; Smith and Stockey, 2003
); Allenbya, Nymphaeaceae (Cevallos-Ferriz and Stockey, 1989
); Eorhiza arnoldii, incertae sedis (Robinson and Person, 1973
; Stockey and Pigg, 1994
); Paleomyrtinaea, Myrtaceae (Pigg et al., 1993
); and Decodon allenbyensis (Cevallos-Ferriz and Stockey, 1988
; Little and Stockey, 2003
). Of these taxa, the leaves are hypothesized to belong to Decodon on the basis of their lythraceous features and close association to abundant fruits, seeds, stems, and aquatic stems and roots of this taxon (Little and Stockey, 2003
). Other plants of layer #43 either have leaves that are already known (i.e., Eorhiza arnoldii) or would likely have dissimilar leaves (i.e., Nymphaeaceae and monocots) to the fossil leaves described here. Finally, the most closely related taxon to Decodon, Paleomyrtinaea (Myrtales), a guava-like plant, is unlikely to have this leaf type because the fossil leaves lack secretory canals and hypodermis, features that are typically found in the mesophyll of many Myrtaceae (Keating, 1984
). For these reasons we believe the leaves most likely belong to Decodon allenbyensis, if not some other unknown taxon.
The fossil Decodon allenbyensis has wood and periderm anatomy in its stems and roots that is very similar to that seen in extant D. verticillatus (Little and Stockey, 2003
). We, therefore, investigated leaf anatomy of D. verticillatus, the only living species in the genus, to assess if there are any diagnostic characters of the leaves that may serve to link the fossil leaves with the D. allenbyensis axes. However, leaves of D. verticillatus have distinctive leaf anatomy in Lythraceae and differ from the fossil leaves (Table 1). The fossils have adaxially convex/round midribs and adaxially flat secondary vein ribs. The adaxially ridged midribs and secondary vein ribs of D. verticillatus are distinctive in shape and prominence, unlike those found in other Lythraceae. The trapezoidal abaxial midrib in D. verticillatus is most similar to the square shaped midrib of Cuphea spectabilis or Lafoensia speciosa and is unlike midribs of the fossil leaves that are abaxially convex/ round to V-shaped (Table 1). Secondary vein ribs with a trapezoidal abaxial shape also appear to be diagnostic for D. verticillatus. Decodon verticillatus has a single palisade layer and no extraxylary fibers around its veins, traits common in Lythraceae (Table 1), and the large multicellular hairs are similar to those seen in Lagerstroemia (Solereder, 1908
; Gin, 1909
; Metcalfe and Chalk, 1950
). However, the fossil leaves have fibers around veins, a double palisade, and no multicellular trichomes have been observed.
Isolated Decodon verticillatus cuticle viewed with SEM has striations, but fresh leaf surfaces with cryo-SEM have no striations. These striations, an artifact of dessication due to the thin and delicate cuticle, may be misinterpreted as cuticle ornamentation in Lythraceae, where true ornamentation also occurs. Kvaek and Sakala (1999)
observed striated cuticle from preparations of Miocene Decodon from Northern Bohemia. These leaves were borne on axes bearing fruits the striations were used to identify other isolated foliage with similar venation to Decodon gibbosus (Reid) Reid in Nitikin. One cannot debate that the leaves attached to fruits with seeds, from the Miocene of Northern Bohemia, are those of Decodon. However, many leaves in order Myrtales tend to have similar venation (Manchester et al., 1998
), and cuticle thickness may vary depending on environmental conditions. Therefore, striations alone cannot distinguish Decodon from other Myrtales in the fossil record. We have avoided using striations as a taxonomic character, and we suggest that using them to identify and link isolated fossil leaves of Myrtales may prove spurious without further evidence such as anatomy or attachments of leaves to reproductive structures.
The fossil leaves fit well into the general range of characters seen in Myrtales (Keating, 1984
) and appear to be most similar to those of Lythraceae. Lythraceae typically have adaxially grooved to convex and abaxially convex/rounded midribs, with C-shaped midveins, rounded to rectangular epidermal cells, and 1–3 palisade layers (typically one or two in a given leaf) (Solereder, 1908
; Gin, 1909
; Metcalfe and Chalk, 1950
; Keating, 1984
). Within this family several anatomical features, such as extraxylary fiber distribution, presence of leaf sclereids, and trichomes are diagnostic (Table 1). Enlarged mucilage-filled cells, often in the epidermis, are considered diagnostic for Lythraceae within order Myrtales (Keating, 1984
). These have not been observed in the fossil leaves; however, well-preserved epidermis is rare.
The combination of characters seen in the fossils is most similar to Duabanga grandiflora, sharing 16 of 22 characters (Table 1). Leaves of D. grandiflora and the fossils have the same number of palisade layers, type of midrib and midvein shape, midvein fibers, and the same secondary vein and secondary vein rib shape. Fossil leaves and leaves of D. grandiflora have overlapping ranges in midrib shape and in the number of spongy mesophyll layers (Table 1). Variability of observed fossil midrib shapes may be due, in part, to the placement of the section, as the midrib shape may change from the petiole to the apex, and the exact level of sectioning is unknown for the fossil leaves. Punica protopunica shares 10 characters and Duabanga moluccana shares 13 characters with the fossil leaves. If one includes overlapping ranges of features such as number of mesophyll layers and midrib shape, along with shared qualitative features, such as presence or absence of druses, the fossils share 6–8 characters with most of the extant taxa surveyed here (Table 1).
Fossil leaves and leaves of Duabanga grandiflora share a distinctive papillate abaxial epidermis among all the leaves described. These unique papillae were called "long mamilliform papillae" by Solereder (1908)
and "mamilliforme" by Gin (1909)
. Solereder (1908)
stated that this type of epidermis is diagnostic for Duabanga. In this study, we observed it only in one species, D. grandiflora, but not in D. moluccana (Table 1). Abaxial epidermal papillae are also reported in other Myrtales, including species of Olinia (Oliniaceae) (Mujica and Cutler, 1974
) and in Crypteronia paniculata (Crypteroniaceae) (Gin, 1909
). However, when present in Olinia, they are more irregular and less elongate than in the fossil leaves. Olinia leaves further differ from the fossils in having a hypodermis over the veins and at the laminar margins, sclereids in the mesophyll, distinct angular epidermal cells, and thick cuticles (Mujica and Cutler, 1974
). Crypteronia paniculata abaxial papillae resemble small, uniseriate trichomes with attenuate tips and occur occasionally (Gin, 1909
), in contrast to the distinctive "mamilliform" papillate abaxial epidermal cells present throughout the lamina of the fossil leaves. This taxon also differs from the fossil in having a circular midvein and regular rectangular epidermal cells (our observation from Myrtales leaf slide collection).
Diffuse mesophyll sclereids are mentioned as characters for Sonneratiaceae (Rao and Das, 1979
) (comprised by Duabanga and Sonneratia). In contrast, we observed diffuse sclereids only in Sonneratia in the current study (Table 1).
Types of crystals are taxonomically important for families of Myrtales (Mujica and Cutler, 1974
; Keating, 1984
). For example, raphides in Myrtales are found only in Onagraceae (Keating, 1982
). Druses occur in all lythraceous taxa, but prismatic crystals were restricted to a few taxa (Table 1). Epidermal cells containing birefringent crystals were also observed for certain lythraceous taxa (Table 1). Unfortunately, the silicification that formed the chert, as well as the HF etching in the cellulose acetate peel technique, makes observation of crystals in Princeton chert fossils unlikely.
Trichomes are diverse in Lythraceae and are used to identify and describe taxa (Koehne, 1903
). Amarasinghe et al. (1991)
systematically surveyed trichome types in the genus Cuphea and found them to be valuable taxonomic characters. Although our data does not at first indicate the importance of trichomes (Table 1), certain types of trichomes such as globose multicellular trichomes in Woodfordia, Lourtella, Adenaria, Koehneria, Pehria, and possibly Cuphea are likely a synapomorphy for the clade containing these taxa (Graham et al., 1993
; S. A. Graham, Missouri Botanical Garden, personal communication, 2003). Therefore, further systematic studies of leaf and floral indument may yield more taxonomically useful characters, as well as characters for phylogenetic analyses.
The large number of anatomical similarities between the fossil leaves and those of Duabanga grandiflora, especially the diagnostic papillate abaxial epidermis, may indicate that a Duabanga-like plant was present at the Princeton chert locality during the Middle Eocene. Because extant species of Duabanga range from the rainforest region of southeastern Himalaya to New Guinea (Jayaweera, 1967
), the presence of these fossil leaves supports the interpretation that Princeton was more tropical in the Middle Eocene (Pigg and Stockey, 1996
). If our fossil leaves are Duabanga then this is, to our knowledge, the only macrofossil record for the genus. However, we believe that these leaves are those of Decodon allenbyensis. This idea is supported by the mutual abundance and close association of these leaves and in situ roots, isolated stems, and fruits of Decodon in layer #43 (Little and Stockey, 2003
). Because the chert represents a near-shore lacustrine environment (Cevallos-Ferriz et al., 1991
), we would expect the leaves to have been deposited parautochthonously and preserved along with the other aboveground organs (Little and Stockey, 2003
). If this hypothesis is supported by future work, then Decodon allenbyensis possesses a combination of characters not known for any living Lythraceae: it is similar in growth habit and wood, fruit, and seed anatomy to Decodon, but has Duabanga-like leaves.
The reconstruction of Decodon allenbyensis will become increasingly valuable for cladistic analyses. As demonstrated by Huelsenbeck (1991)
, a fossil taxon, complete in its character set and near the common ancestor, can improve resolution of a phylogeny. This is caused by improved knowledge of ancestral characters, constraining the possible inferred states at internal nodes of the cladogram, thereby reducing the number of most parsimonious trees in phylogenetic analysis. A fully reconstructed 48 million-year-old fossil Decodon is clearly closer to the common ancestor of the clade than any living species, thereby fulfilling both of Huelsenbeck's criteria for when fossils are more valuable than extant taxa in phylogenetic analysis. As not all intrafamilial relationships in Lythraceae are well supported in both morphological and molecular cladistic analyses (Graham et al., 1993
; Conti et al., 1997
; Shi et al., 2000
; Huang and Shi, 2002
), the importance of a complete character set for a fully reconstructed and anatomically preserved fossil plant is highlighted. Therefore, the Princeton chert has great value, not only providing in depth knowledge of an ancient biota (Cevallos-Ferriz et al., 1991
; Pigg and Stockey, 1996
), but also towards elucidating uncertain phylogenetic hypotheses through detailed anatomical character sets, including those from reconstructed fossil plants.
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APPENDIX |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXLITERATURE CITED |
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FOOTNOTES |
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4 E-mail: ruth.stockey{at}ualberta.ca
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXLITERATURE CITED |
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Amarasinghe V. S. A. Graham A. Graham 1991 Trichome morphology in the genus Cuphea (Lythraceae). Botanical Gazette 152: 77-90[CrossRef]
Basinger J. F. 1981 The vegetative body of Metasequoia milleri from the Middle Eocene of southern British Columbia. Canadian Journal of Botany 59: 2379-2410[Medline]
Basinger J. F. G. W. Rothwell 1977 Anatomically preserved plants from the Middle Eocene (Allenby Formation) of British Columbia. Canadian Journal of Botany 55: 1984-1990[ISI][Medline]
Cevallos-Ferriz S. R. S. R. A. Stockey 1988 Permineralized fruits and seeds from the Princeton chert (Middle Eocene) of British Columbia: Lythraceae. Canadian Journal of Botany 66: 303-312[ISI][Medline]
Cevallos-Ferriz S. R. S. R. A. Stockey 1989 Permineralized fruits and seeds from the Princeton Chert (Middle Eocene) of British Columbia—Nymphacaeceae. Botanical Gazette 150: 207-217[CrossRef]
Cevallos-Ferriz S. R. S. R. A. Stockey K. B. Pigg 1991 The Princeton chert: evidence for in situ aquatic plants. Review of Palaeobotany and Palynology 70: 173-185[CrossRef]
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