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Paleobotany |
Department of Environmental and Plant Biology, Ohio University, Athens, Ohio 45701 USA
Received for publication February 15, 2002. Accepted for publication June 7, 2002.
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ABSTRACT |
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Key Words: Chesterian • Medullosaceae • Mississippian • ovule • Rhynchosperma quinnii • trigonocarpalean
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONLITERATURE CITED |
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for permineralized ovules that resemble compression/impression fossils known as Rhynchogonium Heer in external morphology, retaining the generic name Rhynchogonium for compression/impression specimens. Boroviczia Zalessky is another Lower Carboniferous ovule that resembles Rhynchogonium, with one species that may have preserved anatomy (Zalessky, 1905
); however, this material is so poorly known that it may in fact be a cast or compression fossil (Taylor and Eggert, 1967a
). Nathorst (1914)
suggested that Rhynchogonium can be distinguished from Boroviczia by sclerotestal rib morphology, but Taylor and Eggert (1967a)
suggest these differences are taphonomic. Further study of Boroviczia will be needed to clarify both the structure and affinities of this enigmatic ovule.
Rhynchosperma quinnii was described from four specimens recovered from marine shales of the Fayetteville Formation (Chesterian, upper Mississippian) of northwestern Arkansas (Fig. 1). As originally described, R. quinnii is an ovule of unknown affinity, with a two-parted integument, nucellus attached to the integument for seven-eighths of the nucellus length, and vascularization by discrete bundles preserved only at the chalazal end of the integument. The nucellus is apparently unvascularized (Taylor and Eggert, 1967a
).
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), thereby suggesting medullosan affinity for the genus. |
MATERIALS AND METHODS |
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) and miospores (Owens, Loboziak, and Coquel, 1979
). All specimens of Rhynchosperma were collected by R. H. Mapes from exposures at Town Branch Creek, Cato Springs, and White River by wading the stream channels and recovering specimens exposed by erosion. Two of the three new specimens (M3027, M3032) were prepared for analysis by embedding in commercial bioplastic and slicing on a Buehler Isomet Low Speed Rock Saw (Buehler, Evanston, Illinois, USA). From one specimen, two cross sections were removed (Figs. 9 and 10), acetate peels were made of the blocks, and the specimen was reassembled and sliced longitudinally (Figs. 8 and 11). The second specimen was sectioned transversely at the chalazal and micropylar ends and sliced longitudinally in the center. Serial acetate peels were made of the center (longitudinal) block. The third specimen (M219) was fractured longitudinally when recovered. The larger fragment was mounted in Durhams Water Putty (Donald Durham, Des Moines, Iowa, USA) and analyzed from longitudinal serial acetate peels; the smaller portion was embedded in bioplastic and wafered in cross section. Specimens were mounted on microscope slides: wafers were analyzed by reflected light and acetate peels were analyzed by transmitted light. Type specimens of R. quinnii were also examined. These were degraded by pyrite oxidation, and several wafers required stabilization and re-preparation before reexamination. Photo documentation was accomplished with a Leaf Systems Micro Lumina scanning digital camera. Images were processed and plates were constructed using Adobe Photoshop 5.5 (Adobe Systems, San Jose, California, USA), stored as TIFF and PSD files and printed on a Shinko CH446i dye-sublimation printer (Shinko Electric, Tokyo, Japan). New specimens are reposited in the Ohio University Paleobotanical Herbarium (OUPH 14006–14099). The holotype (specimen 2002) and paratypes (2000, 2001, 2003) are reposited in the Paleobotany Division of the University of Kansas Natural History Museum.
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SYSTEMATICS |
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Emended generic diagnosis
Radially symmetrical ovules with attenuated apex and three-layered integument; upper portion of sclerotesta ribbed. Apically, ribs expand into internally open hollow lobes that form a stellate micropylar canal. Nucellus variably confluent with integument, apically differentiated into a pollen chamber with nucellar beak and pollen chamber floor. Sarcotesta vascularized by bundles at the tips of sclerotestal ribs and by unknown number of discrete bundles in the sarcotesta, nucellus vascularized by anastomosing sheath of tracheids.
Type species
Rhynchosperma quinnii Taylor and Eggert.
Emended species diagnosis
Ovate to oblong ovules 1.2–2.2 cm long (mean = 1.8 cm), 0.9–1.4 cm (mean = 1.2 cm) in diameter, with rounded chalaza and apiculate apex. Integument delimited into three layers bounded by peripheral epidermis; outer sarcotesta of isodiametric to polygonal thin walled cells with apparent secretory cells. Middle sclerotesta of approximately 6–15 longitudinally directed thick-walled cells; sclerotesta sculptured by 8–10 ribs. Endotesta one cell thick and cutinized. Base broadly rounded except for centrally located stalk.
Holotype
Specimen 2002 in Paleobotanical Collection, Department of Biological Sciences, University of Kansas.
Paratypes
Specimens 2000, 2001, and 2003 in above collection.
Additional new specimens
Thirty-eight peels, one block, and 13 wafers of specimen M219, OUPH 14011, 14016–14056, 14096, and 14097; 13 wafers and four peels of M3027, OUPH 14006–14009, 14012–14014, 14057–14062, and 14098; and 13 wafers, one block, and 21 peels of M3032, 14010, 14063–14095 and 14099 in the Ohio University Paleobotanical Herbarium.
Collection localities
M219, M3032: Town Branch Creek (NW1/4, Sec. 20, T. 16 N., R. 30 W. Fayetteville 71/2' Quadrangle). M3027: White River (N/W1/4, Sec. 29, T14N, R28W, Durham 71/2' Quadrangle): 2000, 2001, 2002, 2003: White River or Cato Springs Branch Creek (SW1/4, Sec. 21, T. 16 N., R. 30 W. Fayetteville 71/2' Quadrangle), Washington County Arkansas.
Age and stratigraphy
Chesterian (Upper Mississippian), lower Namurian A or Lower Serpukhovian equivalent. In the lower shale unit Fayetteville Shale, Fayetteville Formation, Chester Series.
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DESCRIPTION |
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(Figs. 9 and 13). Parenchyma cells are isodiametric to polygonal, 120–360 µm in diameter (mean = 185 µm), with cell walls less than 3 µm thick (Figs. 17, 19 and 21). The sarcotesta and sclerotesta are often separated by a tissue discontinuity (Figs. 10 and 17).
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report for one of the type specimens. Eight to ten ribs are formed by the sclerotesta. These ribs originate near the chalaza (Fig. 10) and become more prominent toward the apex (Fig. 9). Above the level of the pollen chamber, the ribs open internally into eight to ten bulbous hollow lobes that form a stellate micropylar canal (Figs. 11, 12 and 14). Lobes are up to 1.2 mm wide, 1.5 mm deep, and 1.7 mm tall. Sclerotesta of one specimen appears to be two parted; however, this may be a function of sequential permineralization during diagenesis since the walls of some cells are well preserved in some areas and poorly preserved in others (Fig. 17, box). Endotesta is uniseriate (Figs. 17 and 18) with an internal cuticle. Cells are isodiametric to slightly oval and 60–70 µm in diameter (mean = 62 µm); cell walls are 12–20 µm thick (mean = 16 µm).
Nucellus
In the original description, the nucellus of R. quinnii is described as being confluent with the integument for approximately seven-eighths of its length; however, the connection of the nucellus to the integument is variable in the new specimens (Figs. 8 and 10). In a single longitudinal plane of one specimen the nucellus is connected for approximately one-fourth of its length on one side (Fig. 8, left arrow) and one-half of its length on the other side (Fig. 8, right arrow). Therefore, the nucellus may be attached to the integument on one side of a cross section and separated on the other side (Fig. 10). Overall, the available specimens reveal that the degree of fusion of integument to nucellus may vary from 85% to less than 25% of the length of the seed cavity. Two of the new specimens show features of the pollen chamber that is 1.2–1.4 mm high (mean = 1.25 mm) and approximately 2.0 mm in diameter with an apical nucellar beak and a pollen chamber floor (Fig. 15). The opening of the nucellar beak has a maximum diameter of 860 µm. At the base of the pollen chamber, directly below the opening, a poorly preserved structure, suggesting a central column in location and size, is preserved in two specimens (e.g., Fig. 15). This column-like structure is approximately the same diameter as the pollen chamber opening.
Vascularization
A single vascular bundle enters the base of the ovule (Fig. 22) and branches to vascularize integument and nucellus. Where the primary bundle enters the base of the ovule it is terete and up to 15 tracheids in diameter (Fig. 22). Individual tracheids are approximately 35 µm wide.
Integumentary vascularization originates from discrete bundles branching from the primary bundle (Fig. 22). Irregularly shaped bundles vascularize the integument at the tips of each rib (Figs. 10, 16) and extend approximately one-third of the length of the ovule. These bundles are represented by tightly packed tracheids (Fig. 16), diffuse tracheids (Fig. 28), or as lacunae. Tracheids are approximately 40 µm in diameter. Additional integumentary vascularization is preserved as discrete bundles in the outer parenchyma of the sarcotesta. The number and disposition of these latter bundles are unknown, but each bundle is up to 290 µm wide (Figs. 19 and 27) and extends two-thirds to three-fourths of the length of the ovule. These outer bundles are up to seven tracheids wide: individual tracheids are approximately 40 µm in diameter. The helical to scalariform wall thickening pattern described by Taylor and Eggert (1967a)
are not as distinct in the new material as in the type specimens.
The chalazal bundle extends into the base of the nucellus and distally forms a cup-shaped sheath of anastomosing tracheids (Figs. 20, 21, and 23–25). This sheath is up to five tracheids thick and extends for approximately one-third of the length of the ovule. Nucellar tracheids range from 16 to 30 µm in diameter. In longitudinal section these cells can be recognized by position, relatively thick cell walls, and long length (Fig. 26).
Gametophytes
The megaspore membrane is present in all specimens and is approximately 15 µm thick (Fig. 17). The holotype (2002), one paratype (2001), and one of the new specimens (M219) contain well-preserved cellular megagametophyte tissue. The new specimen also exhibits radial orientation of peripheral cells (Fig. 29) that reflect apparent alveolar cell development. Gametophyte cell size is highly variable, ranging from 80–145 µm (mean 115 µm) near center to radial peripheral cells that are up to 200 µm long. Over 28 spheroidal structures (Fig. 15) occur in the pollen chamber of one specimen (M-219). These structures are 90–100 µm in diameter (mean = 91 µm) with no sculpture or haptotypic marks preserved. Whether these structures represent pre-pollen grains, fungal spores, or acritarchs could not be determined.
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DISCUSSION |
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The distinct cutinization of the endotesta is clear in the new specimens (Fig. 30) and the types (Fig. 31). Likewise, new data reveal that the range of variation of the integument-nucellus connection is greater than for most previously described Paleozoic ovules. In Rhynchosperma the integument-nucellus connection ranges from a maximum of seven-eighths to a minimum one-fourth of the length of the nucellus.
An interesting feature of Rhynchosperma is the elaborate micropylar structure formed by the ribs of the sclerotesta. This structure was not reported by Taylor and Eggert (1967a)
, but further preparation and reanalysis of the one of the paratypes (2003A) reveals the presence of this feature. As shown in Fig. 13 only sclerotestal ribs can be observed in the original publication, however the re-prepared paratype clearly reveals that the ribs open into bulbous hollow lobes (Fig. 14) like those of the new specimens. As discussed below, elaborate micropylar structures are also known from a number of medullosan ovules including Stephanospermum and Polylophospermum Brongniart 1874.
Upper Paleozoic ovules were originally characterized by a two-fold classification system based on bilateral vs. radial symmetry by Brongniart (1881)
. Subsequently, additional data suggested the two-fold system was inadequate, and a three-fold classification system that added internal morphology was proposed; these classifications are the form orders Lagenostomales, for small radiospermic ovules with the nucellus and integument adnate and vascularization restricted to the inner integument; the Trigonocarpales, for large radiospermic ovules with the nucellus attached to the integument only at the base and both the integument and nucellus vascularized; and the Cardiocarpales for platyspermic ovules with simple pollen chambers and only integumentary vascularization (Seward, 1917
). With reduced emphasis on symmetry (Rothwell, 1986
), these form orders are still relatively useful (Taylor and Taylor, 1993
).
A number of newly interpreted features of Rhynchosperma (three-parted integument, basal integument-nucellus connection) along with certain previously recognized characters (exterior shape and symmetry, integumentary vascular bundles) suggest these specimens are trigonocarpalean ovules (e.g., Hoskins and Cross, 1946
; Stewart, 1951
). In particular, the presence of an anastomosing sheath of tracheids in the nucellus suggests a close relationship with the trigonocarpalean genus Stephanospermum (e.g., Drinnan, Schramke, and Crane, 1990
). Stephanospermum is the only other trigonocarpalean ovule that also possesses an anastomosing sheath of tracheids in the nucellus. Of the seven currently recognized Stephanospermum species (Serbet and Rothwell, 1995
), the presence of this character is the only synapomorphy uniting the genus within the trigonocarps (Drinnan, Schramke, and Crane, 1990
).
In addition to Stephanospermum, two other trigonocarpalean genera, Polylophospermum and Codonospermum Brongniart (Renault), have characters that link Rhynchosperma to the Trigonocarpales. Stephanospermum is known from Europe, North America (Brongniart, 1881
; Oliver, 1903
; Combourieu and Galtier, 1985
), and southern China (Li, 1991
); in most species the sclerotesta is modified into an apical crown with a variable number of teeth (see Table 1 in Serbet and Rothwell, 1995
) and a micropylar beak. Between the crown and the beak is a "moat" or "perimicropylar trough" (Leisman and Roth, 1963
): Hall (1954)
reported that these moats are filled with parenchymatous sarcotesta cells. In Rhynchosperma, the parenchymatous sarcotesta is well preserved between the hollow lobes of the sclerotesta (Figs. 8, 11, 12, and 14), and this may be homologous with the parenchyma-filled "perimicropylar troughs" of Stephanospermum.
The sclerotesta of Polylophospermum forms an apical chamber that is comparable to the chamber formed by the apical structures of Stephanospermum (Seward, 1917
) and of Rhynchosperma. Polylophospermum is known only from Europe (Combourieu and Galtier, 1985
) and has six sclerotestal ribs that internally open at the micropyle (Combourieu and Galtier, 1985
, Plate 3, Fig. 5). Although much reduced on Polylophospermum, the sclerotesta ribs open in a similar manner to those of Rhynchosperma, and both genera form a stellate micropylar canal.
Codonospermum is known from two species, and like Polylophospermum, is known only from Europe (Combourieu and Galtier, 1985
). One of these species (C. anomalum) is characterized in part by the presence of eight sclerotestal ribs. This eight-angled symmetry may be correlatable with the eight to ten sclerotestal ribs of Rhynchosperma.
Trigonocarpalean ovules are widely regarded as the ovules of medullosan seed ferns based on reports of ovules attached to medullosan vegetative organs (Taylor and Taylor, 1993
), their frequency of co-occurrence with medullosan vegetative organs in many coal ball assemblages (Ramanujam, Rothwell, and Stewart, 1974
; Stidd, 1981
), and a similarity of morphological and histological features (Good, Rothwell, and Taylor, 1982
). The most widespread and diverse medullosan ovule is Pachytesta, which is widely believed to have a close phylogenetic relationship to Stephanospermum (Taylor, 1962
). These two taxa are suggested to have had a common origin (Leisman and Roth, 1963
) with both lineages diverging in the late Mississippian or early Pennsylvanian (Good, Rothwell, and Taylor, 1982
). The presence of an anastomosing sheath of tracheids in the nucelli of both Rhynchosperma and Stephanospermum suggests a close relationship between these two taxa. Therefore Rhynchosperma may be a basal morphotaxon of the medullosans or possibly a closely related sister taxon to the group (Fig. 32), and an anastomosing sheath of tracheids in the nucellus may be a symplesiomorphic character for the Medullosaceae.
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; Serbet and Rothwell, 1995
): an assemblage that represents the oldest unequivocal evidence for the Medullosaceae (Mapes et al., 2000
). Co-occurring medullosan morphotaxa from the Fayetteville Formation include an undescribed species of Medullosa (Taylor and Eggert, 1967b
) that has been interpreted as a vine-like medullosan stem (Mapes et al., 2000
) and the primitive medullosan stem Quaestora amplecta (Mapes and Rothwell, 1980
). The data presented in this paper further establish the Rhynchosperma-Medullosaceae connection based primarily on new morphological data with co-occurrence data as supporting evidence.
The pollen chamber of Rhynchosperma recalls both the hydrasperman and the medullosan pattern of pollen chamber sealing and suggests a transition in the mode of post-pollination sealing between these two patterns. The poorly preserved structure at the base of the pollen chamber directly below the opening of the nucellar beak (Fig. 15) occupies the same position as the central column, which is characteristic of the hydrasperman type of reproduction and reflects the reproductive biology of the most primitive seed plants (Rothwell, 1986
; Rothwell and Scheckler, 1988
; Rothwell and Serbet, 1992
). In this type of reproductive biology, the integument remains open at the micropyle and postpollination sealing is accomplished by the growing megagametophyte pushing the central column into the lagenostome (Serbet and Rothwell, 1995
).
Although a distinct pollen chamber floor with a thickened central column is widely regarded as characterizing hydrasperman reproduction, a similar morphology also occurs in a number of previously described trigonocarpalean ovules including Hexapterospermum delevoryii Taylor and Stephanospermum tridentatum Serbet and Rothwell. Hexapterospermum delevoryii has a nucellar beak and a dome of tissue that was originally described as a central column (Taylor, 1966
): this tissue has subsequently been reinterpreted as megagametophyte (T. N. Taylor, University of Kansas, personal communication in Serbet and Rothwell, 1995
). In one specimen of Rhynchosperma, the central column could also be megagametophyte (Fig. 15), however, in the other, this structure is external to the megaspore membrane.
Stephanospermum tridentatum also has a mass of tissue that resembles the central column in ovules with hydrasperman reproduction, but the prominent cell walls and distinctive wall thickenings that characterize functional central columns are absent (Serbet and Rothwell, 1995
). Prominent cell walls and distinctive wall thickenings are also absent from the central column of Rhynchosperma.
As noted above, the presence of an elaborately developed pollen chamber floor (Fig. 15) is a character that has been used to distinguish ovules with hydrasperman reproduction from other ovules (Rothwell, 1986
), but more recent data reveal that a pollen chamber floor also occurs in some trigonocarps (Serbet and Rothwell, 1995
). Of the 36 currently recognized species of trigonocarpalean ovules (Serbet and Rothwell, 1995
; Dunn, Mapes, and Rothwell, 2002
), 36% have a pollen chamber floor, in 28% the presence of a pollen chamber floor is unknown, in 22% its presence is ambiguous, and in 11% a pollen chamber floor is absent. Therefore the presence of a pollen chamber floor allies Rhynchosperma with the plurality of known trigonocarpalean ovules.
These new data add to the range of variation of R. quinnii, suggest that an anastomosing sheath of tracheids vascularized the nucellus in the oldest known trigonocarpalean ovule, and establish these ovules as the products of medullosan seed ferns. Identifying Rhynchosperma as a medullosan ovule, but with certain hydrasperman reproductive characters, strengthens the hypothesized link between the ancestral Calamopityaceae (Mapes and Rothwell, 1980
) and/or Lyginopteridaceae and the Medullosaceae. This current paper adds to the growing list of discoveries from the Fayetteville Formation (e.g., White, 1936
; Mapes, 1966
; Eggert and Taylor, 1971
; Mapes and Rothwell, 1980
; Mapes, 1985
; Tomescu, Rothwell, and Mapes, 2001
) that are greatly expanding our understanding of the morphology, distribution, and habits of plants at Upper Mississippian time, thus reemphasizing Taylor and Eggert's (1967b)
hypothesis that the Fayetteville Formation may be the "most paleobotanically valuable Upper Mississippian strata in North America" (p. 415).
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FOOTNOTES |
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2 Author for reprint requests (telephone: 593-740-1118; FAX: 740-593-1130; md369590{at}ohiou.edu
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONLITERATURE CITED |
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Combourieu N. J. Galtier 1985 Nouvelles observations sur Polypterospermum, Polylophospermum, Colpospermum, et Codonospermum, ovules de Pteridospermales du Carbonifère Supérieur Français. Palaeontographica B 196: 1-29
Drinnan A. N. J. M. Schramke P. R. Crane 1990 Stephanospermum konopeonus (Langford) comb. nov.: a medullosan ovule from the middle Pennsylvanian Mazon Creek flora of northeastern Illinois, U.S.A. Botanical Gazette 151: 358-401
Dunn M. T. G. Mapes G. W. Rothwell 2002 On Paleozoic plants from marine strata: Hexaloba finisensia n. gen. et., sp., a trigonocarpalean ovule from the Virgilian (Upper Pennsylvanian: Gzhelian) Finis Shale of Texas. Journal of Paleontology 76: 173-180
Eggert D. A. T. N. Taylor 1971 Telangiopsis gen. nov., an upper Mississippian pollen organ from Arkansas. Botanical Gazette 132: 30-37[CrossRef]
Good C. W. G. W. Rothwell T. N. Taylor 1982 A new Stephanospermum from the Appalachian Basin of North America. Review of Palaeobotany and Palynology 36: 231-240[CrossRef][ISI]
Hall J. W. 1954 The genus Stephanospermum in American coal balls. Botanical Gazette 115: 346-360[CrossRef]
Hoskins J. H. A. T. Cross 1946 Studies in the Trigonocarpales. Part II. Taxonomic problems and a revision of the genus Pachytesta. American Midland Naturalist 36: 331-361[CrossRef][ISI]
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Li Z.-M. 1991 Stephanospermum c.f. akenioides Brongniart and Sarcospermum petiolulatum Z. M. Li sp. nov. in Chinese coal balls. Cathaya 3: 115-133
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Meeks L. K. A. L. Titus W. L. Manger 1997 Taphonomy and biostratigraphy of ammonoid cephalopods, Fayetteville Shale (middle Chesterian, Mississippian), Northern Arkansas. Proceedings of the XIII International Congress on the Carboniferous and Permian, Part 2. Prace Pa
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Nathorst A. G. 1914 Zur fossilen flora der Polarländer. Sur Paläozoischen Flora der Artinschen Zone. Kongrel Svenska Vetenskapakademi Handlingar 26: 8-80
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Rothwell G. W. S. E. Scheckler 1988 Biology of ancestral gymnosperms. In C. B. Beck [ed.], Origin and evolution of gymnosperms, 85–134. Columbia University Press, New York, New York, USA
Rothwell G. W. R. Serbet 1992 Pollination biology of Elkinsia polymorpha, implications for the origin of Gymnosperms. In F. Schaarschmidt [ed.], International symposium on palaeobotany 147:225–231. Courier Forschungsinstitut, Senckenberg, Frankfurt, Germany
Serbet R. G. W. Rothwell 1995 Functional morphology and homologies of gymnospermous ovules: evidence from a new species of Stephanospermum (Medullosales). Canadian Journal of Botany 73: 650-661
Seward A. C. 1917 Fossil plants, III, 650. Hafner Publishing, New York, New York, USA
Stewart W. N. 1951 A new species of Pachytesta from the Berryville locality of southwestern Illinois. American Midland Naturalist 46: 717-742
Stidd B. M. 1981 The current status of medullosan seed ferns. Review of Palaeobotany and Palynology 32: 63-101
Taylor T. N. 1962 Additional observations on Stephanospermum ovoides, a middle Pennsylvanian seed. American Journal of Botany 49: 794-800[CrossRef][ISI]
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Taylor T. N. E. L. Taylor 1993 Biology and evolution of fossil plants. Prentice Hall, Englewood Cliffs, New Jersey, USA
Tomescu A. M. F. G. W. Rothwell G. Mapes 2001 Lyginopteris royalii sp. nov., from the Upper Mississippian of North America. Review of Palaeobotany and Palynology 116: 159-173
White D. W. 1936 Fossil flora of the Wedington Sandstone Member of the Fayetteville Shale. U.S. Geological Survey Professional Paper 186-B
Zalessky M. 1905 Über Früchte aus den Unter-Carbon-Ablagerungen des Mstabeckens in Nord-Russland. Bulletin de Academie Impériale des Sciences de St. Petersbourg 22: 113-120
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