Onoclea sensibilis in the Paleocene of North America, a dramatic example of structural and ecological stasis

Review of Palaeobotany and Palynology, 70 (1991): 113-124 Elsevier Science Publishers B.V., Amsterdam

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Onoclea sensibilis in the Paleocene of North America, a dramatic example of structural and ecological stasis G a r W. R o t h w e l l a n d R u t h A. S t o c k e y Department of Botany, Ohio University, Athens, Ohio, U.S.A. Department of Botany, University of Alberta, Edmonton, Alta., Canada (Received February 1, 1990, revised manuscript accepted January 19, 1991)

ABSTRACT Rothwell, G.W. and R.A. Stockey, Onoclea sensibilis in the Paleocene of North America, a dramatic incidence of structural and ecological stasis. Rev. Palaeobot. Palynol., 70:113-124. Thousands of specimens of the filicalean fern Onocleasensibilis, including large segments of whole plants in growth position, have been recovered from nonmarine sediments of the Paleocene, Paskapoo Formation in central Alberta, Canada. The fossils are preserved by coalified compression/impressionwithin a community dominated by a betulaceous species of Paleocarpinus and Metasequoia. Four relatively complete and rooted Onoclea plants were exposed at the outcrop. Such deposition indicates that the community was periodically inundated by flooding and that at such times, the fronds were bent over and buried in place. In all recognizable features of habitat, community structure, plant size, growth form, morphology of the vegetative fronds, morphology of the fertile spikes, venation, sporangia and spores, the fossils conform to the living species Onoclea sensibilis. These data demonstrate that essentially modern onocleoid ferns had evolved by the Paleocene and that at least one species has remained virtually unchanged throughout the Cenozoic.

Introduction Onoclea sensibilis L. is a m e m b e r of the onocleoid complex of the dennstaedteoid-asplenioid line, which is widely regarded as the most advanced group of filicalean ferns (Wagner, 1969; Lloyd, 1971). Some authors recognize the onocleoids as a family (Pichi-Sermolli, 1977), while others interpret them to be a subfamily (Crabbe et a l . , 1975) or a tribe of the Dryopteridaceae ( = Aspidaceae, Tryon and Tryon, 1982), or a genus of the Woodsiaceae (Lellinger, 1985). Although the genus Onoclea is monotypic, O. sensibilis is widely distributed in northern Asia and throughout central and eastern N o r t h America, where it is c o m m o n along marshy lake and stream sides and in moist woodland habitats (Lloyd, 1971; Tryon and Tryon, 1982; Lellinger, 1985). Compressed frond remains from Cretaceous and Tertiary deposits, including both vegetative and fertile fragments of fronds, have been interpreted 0034-6667/91/$03.50

as Onoclea for over a hundred years (Brown, 1962; Stewart, 1983). Some authors regard such specimens as O. sensibilis (i.e., Onoclea sensibilis fossilis Newberry, 1868), while others have described them as extinct species [e.g., O.fecunda Knowlton, Brown, 1962; O. hebridica (Forbes) Gardner and Ettingshausen, 1879; O. hesperia Brown, 1962]. Up to the present study, such fossils typically have consisted of only a small number of fragmentary frond specimens within allochthonous assemblages. M a n y of the taxonomically diagnostic characters of the plants they represent as well as the habitat(s) in which they grew are incompletely understood. As a result, the specific relationships of such fossils to each other and to the living O. sensibilis remain conjectural (Brown, 1962). In the past two years, thousands of specimens of Onoclea have been collected from a recently discovered terrestrial biota in Paleocene deposits of central Alberta, Canada (Fox, 1990). Most of the specimens consist of fragments of vegetative

© 1991 Elsevier Science Publishers B.V. All rights reserved

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fronds or fertile spikes, but four large segments of plants that display a rooted rhizome with attached stipes have been exposed in growing position at the collecting site. These demonstrate that deposition of the ferns was largely autochthonous. Together, the newly discovered fossils provide an opportunity to present detailed description of the growth form, plant size, frond and fertile spike morphology, venation, sporangia and spores of this species and to interpret both the plant community and habitat in which it grew. In all of these features, the fossils conform to the living species O. sensibilis and thereby provide one of the most dramatic examples of structural and ecological stasis known for vascular plants. Materials and methods

The fossils described in this study were collected from Munce's Hill, near Red Deer, Alberta. Paleocene strata are exposed along a large roadcut 3 km northeast of Canyon Ski Quarry (NW 1/4 Sec. 34, T 38, R 26, W 4) at approximately 975 m above sea level ("One-jaw gap" of Fox, 1990). Specimens occur as coalified compressions and impressions within nonmarine deposits of the Paskapoo Formation. Sediments at this locality consist of buff to light gray medium-grain sandstones interbedded with highly fractured, finer sandstones and mudstones that are more or less flat lying and less lenticular than many of the fluvial sediments that characterize the Paskapoo Formation in this area (Irish, 1970; Crane and Stockey, 1985). Specimens of Onoclea fronds and fertile spikes almost always extend to the edge of the rock fragments upon which they occur. This led us to hypothesize that the plant fragments were typically larger than the rock fragments available for collection. To test this hypothesis, we exposed a single bedding plane in each of two areas approx. 3 x 4 m and then uncovered the fossils layer by layer. As a result, we were able to expose large segments of intact plants of Onoclea, to determine which species of plants consistently occurred together in which sediments and to formulate preliminary hypotheses about paleoenvironment and plant community structure. Large specimens on the fractured rock matrix

G.W. ROTHWELL AND R.A. STOCKEY

were measured, sketched and photographed in the field. Smaller fragments were collected and later photographed with reflected light at the University of Alberta and at Ohio University. Spores were recovered from coalified sporangial tissues by standard coal maceration techniques, demineralized in 48% hydrofluoric acid, rinsed in distilled water and dehydrated in 95% EtOH. For scanning electron microscopy, drops of EtOH containing the spores were allowed to air dry onto specimen stubs, coated with 150 A AU with a Nanotec sputter coater and photographed with a Cambridge $250 at 20 kV. Fossils are housed in the University of Alberta Paleobotanical Collections (U.A.P.C.), where they bear numbers S17971-S18236, S18238-S19626, $23405-$23523, $23526-$23556, $23968-$23997, $26372-$26273, $26292-$26317, $26319-$26328, $26479-$26480. Living populations of Onoelea sensibilis were examined and specimens collected from a previously studied colony at Dow Lake, near Athens, Ohio (Buckley, 1986). Pressed specimens of O. sensibilis were examined in the Bartley Herbarium, Department of Botany, Ohio University.

Description of material Whole plant morphology

Of the thousands of specimens of vascular plants collected from this locality, nearly all are preserved on rock fragments 15 cm or less in maximum size. However, when a part of an Onoclea frond or fertile spike is exposed on a bedding plane, the remainder of the specimen often can be uncovered, revealing that it extends across the surfaces of several adjacent blocks of the fractured matrix (Plate I, 1). Large specimens exposed at the outcrop consist of several complete fronds (Plate I, 1) and fertile spikes and four relatively complete plants (Plate I, 2, 5; Fig.l). Each plant consists of a segment of horizontal, rooted rhizome located 2-3 cm below the bedding plane upon which numerous attached stipes are preserved (Plate I, 6; Fig.l). As with the living O. sensibilis, the fronds are dimorphic. Two of the fossil plants include a complete vegetative frond (e.g., Plate I, 2, 3; Fig. 1a)

ONOCLEA

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Fig.1. Paleocene Onocleaplants. (a) Plant with a vegetativefrond figured on Plate I, 1 and 2, showing three fronds diverging from a segment of rooted rhizome. (b) Fertile plant figured on Plate I, 4-6, showing several fronds divergingfrom a segment of horizontal, rooted rhizome. Both figs. × 0.3. and another individual has an attached fertile spike (Plate I, 4, 5; Fig.lb). Segments of rhizome are up to 20 cm long, and one branches at an angle of approx. 60 ° . Each rhizome is represented by an impression 1-2 cm wide from which several stipes diverge on the upper surface (Plate I, 2, 6; Fig.l), as they do in extant O. sensibilis (Plate I, 7). Rhizomes of the fossils have an irregular surface (Plate I, 2 and 6, at arrows) that is comparable to that of the living Onoclea rhizome when it is covered with a dense mass of adventitious roots (Plate I, 7).

Vegetative fronds Entire fronds are 34-42 cm long and up to 16.7 cm wide, with an elongated stipe and a terminal blade. The stipe is approximately 5 mm wide

at the base. Each blade is pinnately dissected, with subopposite to alternate, pinnatifid pinnules (Plate I, 3; Plate II, 1). Pinnules have lobed margins, with each lobe terminating in an acute tip (Plate II, 1, 4, 6). Some pinnules have relatively short lobes (Plate I, 1, 3), while others are incised up to approximately one half the distance to the midvein (Plate II, 4). In all of these features the fossils fall within the range of variation of the extant O. sensibilis (Lloyd, 1971). In the latter, there is a great deal of morphological variability associated with frond size, habitat and possibly genetic diversity (Buckley, 1986). For example, the pinnules of small fronds and those near the apex of larger fronds often have smooth margins (Plate II, 2), whereas pinnules near the base of large fronds are incised so deeply that they approach bipinnate morphology (Plate II, 5).

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The fossil Onoclea pinnules display lonchopteroid venation, with a midvein that extends to near the tip (Plate II, i, 4) and secondary veins that anastomose to form a reticulum (Plate II, 1, 3, 4, 6, 7). As in living Onoclea, the areoles are distinctly elongated with prominent costal areoles paralleling the midvein (Plate II, 3, 6) and the remainder elongated toward the margin at an obtuse angle (Plate II, 6, 7). Also as in the fronds of living

plants, veins near the margin frequently bend apically to close an areole (Plate II, 7 at lower right). Fertile spikes Most specimens of fertile spikes consist of the apical region, and display the same bipinnate morphology as living Onoclea (Plate III, 1-4). Two

PLATE I Paleocene and extant Onoclea l. Vegetative frond of Onoclea (at O) and Metasequoia cone and shoot tip (at M) exposed at outcrop. Note the highly fractured nature of the matrix, x 0.3. 2. Onoclea plant diagramed on Fig.la, consisting of rooted rhizome (at arrows), several attached stipes (at S) and one attached vegetative frond. In situ plant #2 x 0.2. 3. Enlargement of vegetative frond in fig.2. In situ plant//2 x 0.4. 4. Apical region of fertile spike attached to whole plant in fig.5. In situ plant #1 × 0.45. 5. Fertile plant diagramed on Fig.lb, consisting of rooted rhizome (at r), several attached stipes and one attached fertile spike (arrows) at upper left. In situ plant #1 x0.2. 6. Enlargement of rhizome with attached fronds in fig.5. Note that the rhizome (at arrows) is 2-3 cm below the bedding plane upon which the fronds are compresed. In situ plant//1 x 0.4. 7. Rhizome of extant Onoclea sensibilis with attached petioles. Note how the dense covering of adventitious roots gives the rhizome an irregular surface similar to that of the rhizome of the fossils (fig.6). PLATE II

(see p. 118)

Vegetative fronds of Paleocene and extant Onoclea 1. Pinnate frond of fossil Onoclea, showing subopposite pinnules with lobed margins at base and more entire margins toward apex. Note also reticulate venation U.A.P.C. #S18138 x 0.75. 2. Frond apex of extant O. sensibilis showing pinnule arrangement and venation comparable to those of fossils. Note entire margin of pinnules comparable to that of small Paleocene specimens (fig.7) O.U. herbarium #22834 × 0.75. 3. Frond of fossil showing pinnatifid pinnules and long costal areoles comparable to those of living Onoclea. U.A.P.C. #S18576 x2.5. 4. Pinnule of fossil with lobed margin comparable to that of the fronds of living Onoclea. U.A.P.C. #S18056 x 1.0. 5. Basal pinnules of extant O. sensibilis showing highly lobed configuration found among living specimens. O.U. Herbarium #17317. 6. Dentate pinnule showing venation of Paleocene specimens. U.A.P.C. gS18585 x 4. 7. Pinnule with entire margin showing venation of Paleocene specimens. U.A.P.C. #18378 x 4.8. PLATE III

(see p. 119)

Fertile spikes of Paleocene and extant Onoclea. 1,2. Apical region showing lanceolate shape of spike and subalternate arrangement of pinnae. U.A.P.C. #S19267 and #$26481 respectively, both × 1. 3,4. Enlargements of apical region showing bipinnate morphology and pinnules rolled into globular divisions. U.A.P.C, #S19305 and #$26482 respectively, × 2 and x 2.3 respectively. 5. Primary pinna of dried spike of extant O. sensibilis collected near Athens, OH, showing subopposite-alternate arrangement of enrolled pinnules with pinnate venation, x 4, 6. Primary pinna of fertile spike of Paleocene Onoclea, showing features comparable to those of living specimens (figs.5 and 7). U.A.P.C. ~/$26479 ×4. 7. Dried fertile spike of extant O. sensibilis collected near Athens, OH, showing size, shape and venation of enrolled fertile pinnules, x .4. 8. Enrolled pinnules of Paleocene Onoclea showing size, shape and venation comparable to those of living species (figs.5 and 7). U.A.P.C. #$26482 x4.

ONOCLEA SENSIBILIS IN PALEOCENE OF N-AMERICA

PLATE I

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118 PLATE

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II

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complete specimens are 38 and 45 cm long, with the fertile region comprising approx. 1/5 1/6 of the length (e.g., Plate I, 5: Fig.lb). Primary pinnae are typically subopposite and terete (Plate III, 3, 4, 6), with the enrolled pinnules forming two rows that are directed to one side of the pinna rachis (Plate I, 4; Plate III, 1, 4). In surface views the rolled pinnules are ellipsoidal or gently lobed (Plate III, 6, 8), 3-6 mm in greatest dimension and in all these features they compare favorably with extant specimens (Plate III, 5, 7; Lloyd, 1971). Also as in living Onoclea (Plate III, 5, 7) the fossil pinnules display a prominent midvein, with equally prominent lateral veins diverging in a subopposite pattern (Plate III, 6, 8). Adaxial epidermal cells that lie adjacent to the veins are rectangular to polygonal (Plate IV, 2) and range 25-50 jam in greatest dimension. In cross sections, the lamina of the rolled fertile pinnules appears as a thin black line ofcoalified tissue (Plate IV, 1 at arrows). Sporangia and spores When a fertile spike is split through one of the enrolled pinnules, the surface of the rock matrix within exhibits a large number of sporangia (Plate IV, 1). Each sporangium is represented by a uniseriate annulus that forms a partial ring (Plate IV, 1, 3, 4). The sporangia of living Onoclea have a capsule that is subspheroidal, approximately 225 jam in diameter, and with a vertical annulus that consists of about 22-41 cells (mean=28, Lloyd, 1971). Most annuli of the fossil are only partly exposed at the rock surface (Plate IV, 3), but one complete example is 250 jam in greatest

G.W. ROTHWELLAND R.A. STOCKEY

dimension and displays 26 cells of the annulus. Although the fossils show no evidence of membranous indusia, sporangial stalks or cells of the capsule other than the annulus, we interpret the sporangia of the specimen figured on Plate IV (1, 3, 4) to have been intact at the time of burial. This is because there are thin areas between the thickened lateral walls of the annulus when seen from the outside (Plate IV, 3, 4). Had the sporangia been senescent, the annuli would have been inverted, with the uniformly thickened inner cell walls oriented toward the outside. When the matrix from inside fertile pinnules is macerated, remnants of the sporangia disintegrate, but the preparations yield large numbers of ellipsoidal, monolete spores (Plate IV, 5, 6). The fossil spores are 48-60 ~tm long and 32-37 lain wide, with a smooth exine in most areas and a prominent monolete suture (Plate IV, 5. 6). Spores of this type compare favorably to those of living O. sensibilis~ when the latter have lost the perispore (Lloyd, 1971). Spores of the living plants are 47-85 jam long and 32-68 jam wide. Although the fossil spores approach the lower end of the size range for the living plants, they were derived from undehisced sporangia. Therefore, they may not have reached their mature size prior to fossilization. The absence of a perispore from the fossils is also understandable, because perispore usually does not withstand acid treatments like those used to macerate fossil spores. However, there is an irregular substance on small areas of some of the fossil spores (Plate IV, 6 at arrows) that may represent remnants of a granulose or echinate perispore like that of living plants (Tryon and Tryon, 1982).

PLATE IV Pinnules, sporangia and spores of Palaeocene Onoclea 1. Primary pinna (at diagonal line) showing enrolled pinnule (at arrows) that is fractured to expose the central area, where numerous sporangia are embedded in the matrix. U.A.P.C. #$36480 x 20. 2. S.E.M. of mold of epidermis, showing cellular pattern adjacent to vein of enroled pinnule. U.A.P.C. ~S19309 x 85. 3. Enlargement of central region of enrolled pinnule in fig.l, showing numerous sporangia, each of which is represented by a uniseriate annulus. U.A.P.C. #$26480 x 55. 4. S.E.M. showing annulus of sporangium. U.A.P.C. #$26480 x 180. 5. Ovate spores macerated from sporangia within enrolled pinnule. U.A.P.C. #S19309 x 330. 6. Proximal view of spore showing monolete suture and psilate surface of exine. Irregulra material on surfaces (at arrows) may represent remnants of loosely attached, granulose perispore. U.A.P.C. #S19309 x 1,250.

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Discussion

Plant associations and depositional setting The several thousands of specimens of plant remains that have been collected at Munce's Hill provide a clear picture of plant associations, and allow us to present a preliminary interpretation of the plant community that produced the fossils. Among the most abundant fossils are betulaceous dicotyledons including over 2500 leaves, seedlings, catkins, Paleocarpinus-like fruits, and seeds. Metasequoia shoots, cones, seeds and seedlings are also exceedingly abundant, as are the Onoclea fossils (over 2000 specimens catalogued thus far). Equisetum shoots and roots with tubers occur in some of the layers. A small number of Joffrea-like stems with short shoots and infructescences, an unidentified fern, and one specimen of a possible bryophyte also have been collected. Betulaceous remains that may represent a single species of Paleocarpinus dominate some of the finer layers, while remains of Metasequoia are most common in others. In still other layers the Equisetum specimens predominate, although at a much lower density than fossils in the betulaceous and Metasequoia layers. Onoclea sometimes occurs with Equisetum. In other layers Onoclea predominates, but Metasequoia is also common. The whole plants of Onoclea occur intermixed with Metasequoia shoots and cones (Plate I, 1). Rhizomes are preserved within fine to medium grained sandstone and the bases of the stipes extend diagonally to the top of the sandstone where they bend over onto the bedding plane. At more distal levels the fronds are compressed within finer sediment just above the sandstone (Plate I, 2, 5, 6).

Paleoecology In central Alberta, the Paleocene Paskapoo Formation is widely regarded to be of fluvial origin (Irish, 1970), with numerous deposits of vertebrate, invertebrate and plant fossils (Bell, 1949; Wilson, 1978, 1980: Mitchell and Wighton, 1979; Wighton, 1980; Stockey and Crane, 1983; Taylor and Stockey, 1984; Crane and Stockey, 1985; Fox,

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1990) providing abundant evidence of a rich terrestrial biota. At Munce's Hill the occurrence of rooted Onoclea and Equisetum plants at different levels demonstrates that at least some of the fossils are autochthonous and that different plants were resident at different times during the depositional history of the sampled areas. The large and well preserved specimens of apparently unsorted betulaceous and Metasequoia plants appear to represent canopy litter from trees that either grew at or near the site of deposition and are therefore also autochthonous or hypoautochthonous (sensu Gastaldo, 1987). Autochthonous deposition is also suggested by the occurrence of Metasequoia remains intermixed among the rooted Onoclea plants. As at the nearby Joffre Bridge Locality (Stockey and Crane, 1983), the sediments at Munce's Hill are oxidized and yield seedlings in growth position, features that indicate that the sediment surface was periodically emergent. We envisage that the fossils represent well established forest communities at or near the marshy edge of oxbow lakes, but at some distance from the active river channel. This would account for the influx of sufficient sediments during periodic flooding to bury some herbaceous plants, but still allow for the development of rather stable forest communities. The Equisetum and seedling layers may represent the lake shore, with Onoclea forming the understory of adjacent taxodiaceous marshes, and the betulaceous dominated forests rooted on slightly higher ground.

Taxonomic and evolutionary interpretations Paleontologists are justifiably reluctant to assign plants of differing geological ages to the same taxon (Dilcher, 1974), and the degree of reluctance intensifies with increasing age disparity. For example, Quaternary remains of angiosperms often are assigned to species that have been established on the basis of extant specimens, but those from Tertiary sediments are more frequently regarded as extinct species or genera (e.g., Wolf and Wehr, 1987; Schorn and Wolf, 1989). Likewise, many flowering plant specimens from Cretaceous and early Tertiary deposits are interpreted as represent-

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ing extinct genera, families or even orders (Wolf, 1972; Doyle and Hickey, 1976; Crane, 1984). In contrast to angiosperms, nearly all the families and many of the genera of extant conifers (Miller, 1982), several other genera of gymnosperms (e.g., Ginkgo) and numerous genera of pteridophytes can be recognized well back into the Mesozoic (Taylor, 1981; Stewart, 1983). These include Equisetum, Isoetes and numerous genera assignable to the more primitive families of filicalean ferns (e.g., Harris, 1961; Skog, 1988) from the Triassic onward and Botrychium from the Paleocene (Rothwell and Stockey, 1989). However, all of these examples represent what are considered to be remnants of exceedingly ancient groups (Niklas et al., 1985) , whereas many angiosperm groups and species of the onocleoid complex (including Ococlea) are not believed to have begun significant evolutionary radiations until the midand Upper Cretaceous respectively (Doyle and Hickey, 1976; Lovis, 1977: Crane and Lidgard, 1990). Even authors that suggest an earlier origin of the onocleoids base their opinion on more primitive genera that are considered to be related to the group. In light of this view of dennstaedtioid evolution, the occurrence of an extant species of Onoclea in basal Tertiary deposits is most unexpected. Assignments of fossil remains to O. sensibilis (viz., O. sensibilis fossilis) by early workers were based on the external morphology and venation pattern of a relatively small number of fragments of either vegetative fronds (Newberry, 1868; Lesquereux, 1978) or vegetative and fertile fronds (Knowlton, 1902) and made as the result of there being no recognizable differences between features of the fossils and comparable features of living O. sensibilis (Knowlton, 1902). Subsequent workers have considered the characters upon which the specific identifications were made to be inadequate to separate putative extinct species from the extant species and established new species for the Tertiary Onoclea remains (Brown, 1962). These include O. hebridicia and O. hesperia (Brown, 1962). The fossil Onoclea specimens examined in this study provide a much broader suite of data upon which to make specific determinations than previously has been available. Structural features that conform to those of O, sensibilis include plant habit

and size, as well as external morphology of the rhizome, vegetative fronds and fertile spikes. Venation of the vegetative fronds and fertile spikes, size and morphology of the sporangia, and size, shape, aperture configuration and surface features of the spores also are consistent with comparable characters of the living species. Associated taxa, role in the community and the environment of growth are additional points of agreement between the Paleocene and living Onoclea plants. Such features indicate that the physiological tolerances and reproductive strategies of the fossils fall within the ranges of variation that characterize extant plants (Buckley, 1986) and strongly imply genetic similarities as well. In the light of all of these data we conclude that the fossils from Munce's Hill do, indeed, represent Onoclea sensibilis and that the species has remained virtually unchanged in both structural features and ecological tolerances throughout the Cenozoic. As such, it provides a dramatic example of evolutionary stasis among terrestrial vascular plants.

Acknowledgements We thank, Dr. Kathleen Pigg, University of Arizona, Rudolf Serbet, Ohio University and Lisa Bahach, University of Alberta for field assistance and especially Mrs. Betty Speirs of Red Deer, Alberta whose extensive collecting provided the majority of the fossils upon which this study is based. The drawings in Fig.l were rendered by Rebecca Sampson. This study was supported in part by grants from the Ohio University Baker Fund (to G.W.R.) and the Natural Sciences and Engineering Research Council of Canada (A-6908 to R.A.S.).

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