Anatomically Preserved Woodwardia virginica (Blechnaceae) and a New Filicalean Fern from the Middle Miocene Yakima Canyon Flora of Central Washington, USA

by Kathleen B. Pigg, Gar W. Rothwell
Citation
Title:
Anatomically Preserved Woodwardia virginica (Blechnaceae) and a New Filicalean Fern from the Middle Miocene Yakima Canyon Flora of Central Washington, USA
Author:
Kathleen B. Pigg, Gar W. Rothwell
Year: 
2001
Publication: 
American Journal of Botany
Volume: 
88
Issue: 
5
Start Page: 
777
End Page: 
787
Publisher: 
Language: 
English
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Abstract:

American Journal of Botany 88(5): 777-787. 2001.

KATHLEENB. PIGG~

AND GARW. ROTHWELL"~

*Department of Plant Biology. Arizona State University. Box 871601. Tempe. Arizona 85287-1601 USA: and 'Department of Environmental and Plant Biology. Ohio University. Athens. Ohio 45701 USA

Anatomically preserved Woodwnrrlia virgitlica (Blechnaceae) and a newly recognized onocleoid fern are described from the middle Miocene Yakima Canyon flora of central Washington State. USA. Identification of the W. virginicn fossils is based on a combination of vegetative pinnules, rhizome and stipe anatomy, and fertile pinnules with indusiate sori and sporangia like those of extant W. virginica. Fronds are isomorphic. Vegetative pinnae are elongated and pinnatifid. with a secondary vein paralleling the midvein. Secondary veins of the pinnule lobe anastomose to form primary areoles and are either simple or dichotomize toward the margin. Rhizomes have a simple dictyostele with 3-5 cauline vascular bundles and often a sclerotic hypodermis. Leaf traces contain two large adaxial vascular bundles that occur laterally and adaxially, flanking an arc of 4-6 s~naller bundles. Fertile pinnules have linear sori that are somewhat embedded in the laminae and are ellclosed by a thin indusiuni. Leptosporangia display a vertical annulus and an elongated stalk. A second fern, Wessietr );akitmerisis gen. et sp. nov., is represented by anatomically preserved branching rhizolnes and attached frond bases that conform to the Otioclea-type pattern of rhizome and frond-base vasculature. Rhizomes have a simple dictyostele of 4-5 cauline meristeles. Leaf divergence is helical, with paired hippocampiform rachial traces. These two ferns occur in the same matrix with specimens of Osrnunda wehrii. They demonstrate that filicalean fern assemblages similar to those of extant temperate floras were well established in western North America by the middle Miocene and further emphasize the exceptional species longevity of some homosporous pteridophytes.

Key words: Blechnaceae: fossil Filicales; Miocene; silicified: Wessien: Woorlwc~rdici iiigir~icn

Filicalean ferns are well known as components of Creta- ceous and Tertiary vegetation, where they usually have been included in floristic treatments of compressed angiosperm- dominated assemblages (Tidwell and Ash, 1994; Collinson, 1996, in press; Skog, in press). Much less commonly, species have been described from particularly pteridophyte-rich de- posits (e.g., Smith, 1938; Andrews and Pearsall, 1941; Pabst, 1968; Crabtree, 1988). Although numerous occurrences have been documented, relatively few Cretaceous and Tertiary ferns have been studied in the detail necessary to understand their precise taxonomic relationships to living species. Some of the most completely understood filicalean ferns come from com- pressed specimens collected in western North America and include Lygodium kaulfussi (Manchester and Zavada, 1987) and Oizoclea serzsibilis (Rothwell and Stockey, 1991).

Other fossil ferns are represented by anatomically preserved remains (Ogura, 1972). In the Cretaceous, for example, there

' Manuscript received 20 April 2000: revision accepted 18 July 2000.

The authors thank Wesley C. Wehr. Burke Museum of Natural History and Culture, University of Washington, Seattle, and Raymond P. Foisy. Yakima. WA, for providing specimens for study; Sandra J. Borgardt. Raymond I? Foisy. Donald Q. Hopkins. and Ruth A. Stockey for field assistance; Elyssa

A. Arnone. Robert Bloxham. Stefanie M. Ickert-Bond. and Maria Tcherepova for technical assistance and translations; Patrick E Fields for providing fossil Woodwardia foliage; Jackie M. Adalns, Finley A. Bryan, James D. ~ ~ gomery. and James C. Parks for their help in collecting extant material of Woorl\vardia vir%inicri and providing information on fern ecology: Kathleen

M. Pryer. Field Museum, Chicago, and Donald J. Pinkava, ASU Vascular Plant Herbarium. for providing additional material of extant Woorlwnrdiri for study: and Raymond B. Cranfill for information on extant Woorlwardia. This study was funded by a Faculty Grant-in.Aid, Arizona State University and NSF EAR-9980388 to KBP and DEB-9527920 to GWR.

Vuthor for correspondence (e-mail: rothwell@ohio.edu).

are cyatheaceous, schizaeaceous, thyrsopterid, and osmunda- ceous ferns from Hokkaido, Japan (Nishida and Nishida, 1979; Nishida, 1981, 1982; Yoshida, Nishida, and Nishida, 1996a, b) and California (Lantz, Rothwell, and Stockey, 1999), glei- cheniaceous ferns from eastern North America (Gandolfo et al., 1997), a variety of filicalean ferns from south-central Alberta, Canada (Serbet, 1996; Serbet and Rothwell, 1999), and the famous Tenzpskya ferns of Idaho and many other localities (e.g., Tidwell and Hebbert, 1992). In the Paleogene, permi- neralized forms include osmundaceous ferns from the Paleo- cene and Eocene of Wyoming and New Mexico (e.g., Tidwell and Parkel; 1987; Tidwell and Medlyn, 1991), middle Eocene ferns from the Clarno Formation of Oregon (Arnold and Daugherty, 1963, 1964), and coeval Princeton chert of British Columbia (Basinger, 1976; Basinger and Rothwell, 1977; Cev- allos-Ferriz, Stockey, and Pigg, 1991; Rothwell, Stockey, and Nishida, 1994; Pigg and Stockey, 1996; Stockey, Nishida, and Rothwell, 1999). In the middle Miocene of central Washington State, silicified Osnzurzrln wehrii occurs at several localities (Miller, 19g2; including the same locality as described in the current study), and 0. cinrzamonzea has been described from an unspecified locality (Millel; 1967, 1971; Serbet and ~~th~~ll,

1999). Anatomically preserved material often provides an oppor- ~tunity to study internal structural detail .

t as well as external form and therefore to characterize a broad spectrum of system- atically informative characters, particularly if fertile remains are present, When enough material is available to reconstruct fossil taxa as whole plants, recent studies have documented that Cretaceous and filicalean ferns may be remarkably similar to their extant counterparts, often being assignable to modern genera, sections, or sometimes even species (e.g.,

777 778 AMERICANJOURNALOF BOTANY [Vol. 88

Rothwell and Stockey, 1991; Serbet and Rothwell, 1999; Stockey, Nishida, and Rothwell, 1999).

In the present study we describe two filicalean ferns from the middle Miocene Yakima Canyon flora of central Washing- ton State, USA. Among ferns represented at this locality are specimens of the previously described Osnzuizrla wehrii Miller (1982) and two additional smaller rhizomatous species. The first of these is identified as Woodwardia virgirziccr (L.) J. E. Smith (Smith, 1793). This fern is represented by specimens that are structurally identical with the modern W. virgirziccr of eastern North America in all features of pinnule morphology, venation, rhizome and stipe anatomy, soral configuration, and sporangial characters, thus adding additional data to our grow- ing realization that some homosporous pteridophytes display exceptional species longevity. A second, smaller species, Wessiea jakii?zaeizsis Pigg et Rothwell gen. et sp. nov. is repre- sented by branching rhizomes with diverging stipes and ad- ventitious roots. Woodwardia virginica and the smaller fern Wessiea yakimnensis are found within the same matrix as Osmunda wehrii and taxodiaceous conifers, pine, and dicot re- mains. Floristic comparisons of this fossil assemblage to mod- ern plant communities reveal habitat similarities of the fossil ferns to their modern fern counterparts, suggesting that plant community associations of this type were well established by the middle Miocene in western North America.

MATERIALS AND METHODS

Fossils occur in silicified deposits in outcrops of the Yakima Canyon area in central Washington State. USA. between the cities of Ellensburg and Yak- irna (Pigg, Sophy, and Wehr. 1996: Rothwell. Arnone. and Pigg, 1996; Bor- gardt and Pigg. 1999: Pigg and Tcherepova. 2000). Additional remains from this same locality include Osn~zrr~rla

1.ce17rii (Miller. 1982). Pirzlts,foisvi (Miller, 1992), Quercrrs iziiio1rrz.vis (Borgardt and Pigg. 1999). Liqrrirlambar. infruc- tescences (Pigg. 1997), several interesting fruits of uncertain affinities (Pigg and Manchestel: 1998), and additional typical Miocene elements including members of Taxodiaceae, Vitaceae. Rosaceae, and Cornaceae (Pigg and Tcherepova, 2000).

Fern fossils were obtained from the area locally known as the "County Line Holes" and from the area within the locality designated the "Ho Ho" by the original collectors. Fractured surfaces show laminar vegetative pinnules and small fragments of fertile pinnules. while numerous rhizomes with at- tached stipes are matted together throughout the matrix. These specimens occur with abundant taxodiaceous and pinaceous conifer remains, dicot fo- liage, and woody axes and occasional seed and fruit remains. These fossils are preserved below pillows of basalt at the base of the Museum Flow Pack- age within the interbeds of the Sentinel Bluffs Unit of the central Columbia Plateau NI Grande Ronde Basalt of the Columbia River Basalt Group (Upper Tertiary. middle Miocene) and are dated 15.6 + 0.2 million years old by Arl Ar dating technique (Borgardt and Pigg. 1999).

Weathered surfaces showing laminar foliage were dCgaged and photographed. Small fragments of indusiate sori from surfaces were photographed with reflected light and mounted on stubs for study with the scanning electron microscope (SEMI. Rhizomes and frond bases were studied serially from wa- fered sections cut on a Buehler Isolnet slow speed saw (Lake Bluff, Illinois, USA) or an intermediate-sized trim saw, with some specimens ground thin enough for study with transmitted light. Sections were mounted on microscope slides with UV-adhesive mounting medium from T.H.E. Company (Lakewood. Colorado. USA) and studied with both reflected and transmitted light. Spec- imens were photographed with reflected light, which proved most successful in revealing anatomical detail. Living material of Woodwc~i.ilia virgirzicn was collected in August 1998 during a field trip to the New Jersey Pine Barrens associated with the Botanical Society of America meeting in Baltimore (Figs.

I. I 1. 14). Material was collected along a roadside on the edge of a red maple swamp on Coullty Road 613. Waterford Township, Burlington County. New Jersey. USA. Additional fertile material of extant I.V. vir.ginicii was supplied by Kathleen M. Pryer from the Field Museum (Specimen F 2090338; col- lected in Hillsborough Co., Florida. USA: Fig. 5) and Donald J. Pinkava, ASU (Sheet 39569; collected in Jasper Co., South Carolina. USA: Fig. 16). Extant material was prepared by standard techniques for light microscopy and SEM.

Specimens are housed as part of the TuggleIFoisy Collectioll at the Burke Museum of Natural History and Culture, University of Washington, Seattle. (UWBM), USA, and in the Plant Fossil Collections. Arizona State University Herbarium. Tempe (ASU). Arizona, USA, as indicated in figure captions and systematics section

RESULTS

Yakirna fern remains-Two types of silicified fern remains can be recognized as distinct from one another (Fig. 2-4, 610, 12, 13, 15, 17-24) and from the previously described Os-m~trzda welzrii (Miller, 1982). Of the three ferns known from this locality, surface morphological views of vegetative and fertile pinnules are presently known only for Woodwardia vir- ginica, with Osnzuizda wehrii and the small fern Wessiea represented by only rhizomes and frond bases. Vegetative and fertile pinnules of W. virginica are found on the highly weath- ered surfaces of chert blocks that tend to fracture along the plane of deposition of the fronds (Figs. 2-4, 6, 15, 17, 18). Because most of the organic material has been leached out of the matrix, it was not possible to recover spores or cuticle from macerations. However, surface fragments of indusiate soral casts have yielded taxonomically informative details of the fertile organs (Figs. 15, 17, 18). Blocks in which fern fossils are preserved typically contain masses of ramifying rhizome and frond base remains, with all three ferns intertangled with taxodiaceous conifer needles and other plant organs, primarily roots (Fig. 7). Anatomical structure of the rhizome and frond base is distinctive for both newly discovered ferns and both are considerably smaller than the Osnzurzdcr plants, with which they are intermixed.

Woodwardia virginica (L.) J. E. Smith-Woodwardirr virgiizica is represented by vegetative, pinnatifid pinnules (Figs. 2-4, 6) and fragments of fertile pinnules with indusiate sori of annulate sporangia (Figs. 15, 17, 18) that are preserved on fractured surfaces and by branching rhizomes with diverging frond bases and adventitious roots that reveal internal cellular anatomy (Figs. 7-10, 12, 13).

Foliage-The most extensively preserved pinnae are incom- plete, but consist of opposite-subopposite pinnatifid segments up to 7.9 cm long and 2.4 cm wide (Fig. 2) that are remaskably similar to those of modern W. virgirzica (Fig. I). Individual lobes are 3-14 mm long (mean = 9.92 mm) and 3-6 mm wide (mean = 4.63 mm) with the shortest lobes occurring toward the apex. The venation pattern includes a pinnule mid- vein 0.3-1.1 mm wide, a pinnule lobe midvein, and higher order veins (Figs. 2-4, 6). There is a secondary vein that ex- tends between successive pinnule lobe midveins and that par- allels the pinnule midvein in both the living and fossil speci- mens (Figs. 4, 5). Secondary veins in each pinnule lobe anas- tomose to form primary areoles that parallel the pinnule lobe midvein, and then extend to the margin (Figs. 4-6). Some veins are unbranched distal to the areoles. whereas others fork one or two times. Although most pinnull lobes are relatively smooth, teeth can occasionally be found along the margin of

May 200 11 PIGGAND ROTHWELL-MIOCENE FERNS 783

and fossil specimens (Figs. 15 and 16 at right, and 18 at top), and the sorus is somewhat sunken into the pinnule surface.

Although nearly all of the organic material has been leached away from most Yakima Canyon fossils and specimens often have little contrasting mineral stain, features of the sporangia are preserved as molds and casts on the rock surface (Figs. 15, 17, 18). Under SEM, these features are represented by the rock surface (Fig. 17), and in light microscopy, the annulus is revealed by differential patterns of light refraction (Fig. 18). Sometimes both light and SEM preparations of the same spec- imen are required to see overall features of the sorus as well as details of the sporangia (cf. Figs. 17 and 18). Sori are -1 mm wide and 1.5-5 mm long. The smaller sizes are coinua-

"

rable to sori of living W. virgirzica before sporangial dehis- cence (e.g., Figs. 16-18; Small, 1964; R. Cranfill, personal communication), while the largest fossil representatives (Fig. 15) are comparable to the much larger sizes of sori that we measured in living specimens that were pressed after the spo- rangia had dehisced.

As in the living specimens (Fig. 16) each sorus consists of numerous sporangia with narrow stalks (Fig. 17 at arrow) and a capsule with a uniseriate vertical annulus (Figs. 16-18). Also as in living specimens, the capsule is round-oval when viewed parallel to the plane of the annulus (Fig. 17 at arrow) and flattened at right angles to the plane of the annulus (Figs. 16- 18). The capsule of a sporangium in Fig. 17 (at a-row) mea- sures 220 ym long and 155 p,m wide, and this falls within the size range of sporangia that we measured from living speci- mens. Cells of the annulus are -65 ym in periclinal direction for both the living and fossil specimens, as measured by the raised thickenings that extend at right angles to the row of annulus cells (Figs. 16-18). We have observed some oval structures within sporangia of the fossils, but otherwise pres- ervation is not conducive to preservation of spores.

Wessiea yakimaensis Pigg et Rothwell gen. et sp. nov- Wessiea is represented by elongated, narrow rhizomes and he- lically arranged frond bases (Figs. 19-22). Rhizomes measure 1.5-3.0 mm~in diameter and typically are surrounded by sev- eral stipes (Fig. 19). In cross section the rhizome displays a simple dictyostele with 4-5 cauline meristeles and paired, di- verging frond traces (Figs. 19-24). Cauline meristeles are rep- resented primarily by xylem, the other vascular tissues not being evident in most specimens. However, in a few sections there is a narrow zone of small cells surrounding each mer- istele, and this suggests the position of phloem and possibly a thin bundle sheath like that of W. virginica. Cauline meristeles are ovoid to tangentially elongated in cross section, measuring 0.4-1.2 mm long and 0.2-0.4 mm thick. Metaxylem tracheids are angular in cross sections and 28-69 X 42-97 ym in di- ameter. Small tracheids reveal the positions of protoxylem strands in some bundles (Fig. 24 at px and at arrow). In other bundles protoxylem could not be identified.

In most specimens the ground tissue consists of uniformly thin-walled parenchyma cells in both the pith region and cor- tex (Fig. 23). Thicker walled hypodermal cells like those of

W. virginica are typically absent, but a lighter colored zone near the periphery of the cortex in some Wessiea specimens suggests that hypodermis may be weakly developed (Fig. 19). The outer margin of the rhizome is relatively smooth in most specimens, suggesting that the tissues are complete. However, an outer layer of differentiated epidermal cells usually can be identified. Leaf trace divergence is helical, as indicated by the series of cross sections in Figs. 19-22.

There are typically 4-6 vascular bundles plus two diverging frond trace bundles in each cross section (Figs. 19-24). Four or five of these are cauline meristeles and the sixth (when present) represents the xylein of a diverging adventitious root. When followed through an acropetal series of transverse sec- tions (from Fig. 22 to Fig. 19), the vascular architecture of Wessiea can be identified as conforming to the general pattern exhibited by asplenioid ferns (sensu Ogura, 1972; White and Weidlich, 1995; Stockey, Nishida, and Rothwell, 1999). Pro- gressing distally from a level where there are four cauline mer- isteles (Fig. 22), a large meristele with a more-or-less centrally located protoxylem strand (as in Fig. 24, at arrow) divides into three (Fig. 23, at bottom). The central strand diverges as a root trace (as in Fig. 23, at bottom), and this produces a root gap between the other two resulting bundles (Fig. 23, at rg). At this level there are five cauline meristeles. Distally, the two resulting cauline meristeles each divides tangentially to pro- duce a pair of frond traces that extend into a stipe from the gap formed by the diverging root (e.g., Figs. 19-22 at 3 and at 4). This conforms to the pattern of trace divergence recently recognized as characteristic of many dryopterid ferns (White and Weidlich, 1995; Stockey, Nishida, and Rothwell, 1999). Periodically, two cauline meristeles fuse distally to close a gap from which frond traces have diverged at a lower level, and this returns the number of cauline meristeles to four (e.g., Figs. 20-22, 24).

Stipes of Wessiea are roughly D-shaped with an irregular margin in cross sections (Figs. 19-22). Each consists of two oval frond traces embedded in ground tissue like that of the rhizome. The frond traces are typically incompletely preserved (Fig. 22), but well-preserved examples in the rhizome cortex and stipe display hippocampiform xylem strands (Figs. 19, 20, 24). Small diarch roots occur in the rock matrix. They have a parenchymatous cortex with the same brown coloration as the ground tissues of Wessiea, but have not been found in organic attachment to the rhizomes.

SYSTEMATICS

Order-Filicales.

Family-Dryopteridaceae sensu Kramer (1990).

Genus-Wessiea Pigg et Rothwell gen. nov. (Figs. 19-24).

Species-Wessiea yukitnaensis Pigg et Rothwell gen. et sp. nov. (Figs. 19-24). Combined generic and specifzc diagnosis-W. yakinzaensis gen. et sp. nov.

Dictyostelic filicalean fern rhizomes with diverging stipes displaying two hippocampiform-shaped leaf traces. Gaps in stele formed by diverging root; more distal leaf traces diverg- ing tangentially from amphicribral meristeles, and extending through common leaf gap before entering stipe base.

Holotype-Specimen on 3Al #1-3 and 3 E, #1-3, UWBM 56441, Figs. 19-22. Paratypes-Specimen on 3Fl #2 top, UWBM 56441, (Fig. 23), 2A, #1 bot, UWBM (Fig. 24).

Type locality-The "Ho Ho" known locally as one of the "County Line Holes" is -7.3 km north of the Interstate 82 Firing Center Exit, Yakima County, on Yakima Canyon Road (T14N, R19E, NE 114 of NW 114 of Sec. 3).

Age and stratigraphy-Upper Tertiary, lniddle Miocene,

15.6 2 0.2 Ma. In the Museum Flow Package within the in- terbeds of the Sentinel Bluffs Unit of the central Columbia Plateau N, Grande Ronde Basalt of the Columbia River Basalt Group (Borgardt and Pigg, 1999).

Etymology-The generic name, Wessiea, honors Wesley C. Wehr for his numerous contributions to Tertiary paleobotany of western North America. The specific epithet, yakirnaensis, refers to the locality in Yakima Canyon.

DISCUSSION

Systematics of Woodwardia-Woodwardia (Blechnaceae) is a genus of 11 temperate to subtropical species and two pos- sible hybrids of terrestrial habitat that occur today in disjunct areas of the northern hemisphere with centers of diversity in eastern Asia and North America (Chiu, 1974; Lucansky, 1981; Tryon and Tryon, 1982; Cranfill, 1998; R. Cranfill, personal communication). The main center of diversity is in eastern Asia, especially within mountainous regions of central China and Taiwan. Asian species include W. kernpii, W. japonica, and W. lzarlandii, while Eurasian species are W. prolifera, W. unigemnzata, W, orientalis, and W. radicans. In North Amer- ica, five species occur naturally and one is introduced. Woodwardia virginica, the Virginia chain fern, and the dimorphic

W. aerolata are native to eastern coastal North America (McVaugh and Pyron, 195 1; Shaver, 1954; Small, 1964; Lu- cansky, 1981 ; Cranfill, 1993). Woodwardia Jimbriata occurs along the west coast of North America, from British Columbia to Baja California and Sonora, Mexico, while W. spinzilosa appears from Sonora, Mexico to Costa Rica. Woodwardia martinezii is narrowly endemic in the highlands of central Mexico (Tryon and Tryon, 1982). Possible hybrids include W. apogama and a new form of probable hybrid origin between

W. rnartinezii and W. spinzilosa also occurs in Mexico and Central America (R. Cranfill, personal communication). The southern European native W, radicans is rarely also an escaped cultivar in eastern North America.

Features of fossil and living W virginica-The silicified Woodwardia fossils from Yakima Canyon are identical to ex- tant W. virginica in all features of pinnule morphology, ve- nation, rhizome and stipe anatomy, sorus configuration and sporangial details and can thus be assigned to the extant spe- cies with confidence. This determination is possible in part

of these numerous differences between W, virginica and other extant Woodwardia species and because many of these distinguishing characters are clearly seen in the fossils.

Foliage of W. virginica is pinnate pinnatifid, with pinnules that are broadest at or near the middle, tapered slightly at the base and with broad, blunt segments with little or no sinus. Pinnule size and shape of fossil and extant W. virgini.cn are strikingly similar (Figs. 1, 2), with elongate pinnatifid pinnae with opposite to subopposite segments to each pinna. Whereas foliage of fossil W. virginica from Yakima is represented by isolated and incomplete pinnae, the general form is quite com- parable to ultimate segments of extant fronds (Figs. 1, 2). Moreover, extant W. virginica pinnae abscise from the rachis in the fall (R. Cranfill, personal communication, 2000). Given the many similarities of the Yakima material and extant W. virginica, this phenomenon most likely also occurred in the Miocene forms, explaining in part why we find exclusively dispersed pinnae rather than larger segments of complete fronds. Venation in the Yakima pinnules is also identical to that of extant W. virginica (Figs. 4, 5). Pinnules have a mid- vein, as well as a conspicuous secondary vein that parallels the long axis of the pinna. Other secondaries fork once or twice with occasional, but relatively few primary anastomoses, when compared to some other extant species such as W. ar- eolata. Of modern species of Woodwardia, W. virginica is distinguished by its relatively small number of anastomoses, partly as a result of its relatively narrow lamina. Scales, which are usually present in modern species (Fig. 11), but not par- ticularly diagnostic (Lucansky, 198 1 ; Cranfill, 1998), are not preserved in the fossils.

Rhizome morphology is a taxonomically valuable character in extant Woodwardia. Rhizomes of modern W, virginica are described as being long-creeping, with the external appearance of a stretched rope with very few twists, and having relatively few adventitious roots in a current year's growth (Cranfill, 1993). Extant W. virginica rhizomes range in size from 0.6 to

1.5 cm in diameter. Those of the Yakima Canyon plants are smaller, at 2-5 mm in diameter and bear relatively few ad- ventitious roots. Rhizomes of the Yakima fossils are elongate, branch infrequently and dichotomously, and are characterized by inflated stipe bases like those of extant W, virginica, that may be up to two-thirds the diameter of the axis (Figs. 9, 10).

Stelar architecture and anatomy are also like that of extant W, virginica, with rhizomes having a simple dictyostele con- taining 3-5 round to oval meristeles and numerous leaf traces (compare Figs. 9-1 1). Although details of rhizome histology are somewhat limited by preservation, characteristic bundle sheaths of the meristeles and a narrow hypodermis at the mar- gin of the rhizome are noted (compare Figs. 10, 11, 13, 14). While Lucansky (1981) demonstrated that histological features of rhizomes were fairly similar in various species of Woodwardia, he observed that W. virginica tends to have more aerenchymatous ground tissue and suggested this was because of its frequently submersed plant habit in swa~npy environ- ments. As is common in extant Woodwardia virginica, the e~idermisamears to be sloughed off in the fossil forms.

L L .4

Another characteristic definitive for W. virginica is the num- ber and of traces to stipes, While most wood- wardias typically have two large adaxial bundles surrounding a variable number of smaller strands, in W. virginica the pat- tern is two large, adaxial bundles occurring on either end of a semicircle of 4-7 smaller abaxial strands (Lucansky, 1981). We see this organization in the Yakima fossil material as well

,-.

*A\

(rig. 1~).

In addition to vegetative features, the Yakima fossils show details of indusiate sori and sporangia that are characteristic of W. virginica. While indusia in some species of Woodwardia are thick and persistent, those of W, virginica tend to be thin- ner and more ephemeral. As sporangia mature and dehisce, they expand to displace the delicate, flap-like indusia and in- dividual sori can no longer be clearly delimited. Specimens of Yakima plants show a size range comparable from younger undehisced sporangia to older fronds in which mature sporan- gia have dehisced, and both show remnants of indusia (Figs. 15-18). Sporangia are typical leptosporangia with a small di- ameter and a long stalk (Fig. 17). Sporangia show a conspic- uous vertical annulus that characterizes the genus (Figs. 15- 18). Because little to no organic material remains and we were unable to macerate for spores, we cannot report on spore shape or organization, which is typically monoiete with diffuse, pa- pillate surface in extant W. virginica (Tryon and Lugardon, 1990).

PEant community ecology-From what is known of the flo- ristics and depositional setting of the Yakiina Canyon flora, fern and other floral associatio~ls of the Yakinla plants are re- markably similar to those of extant Woohvardia virgiizica. Living W. virginica is a plant of the Atlantic Coastal Plain that occurs from Canada through Florida and into Texas. Its most common habitats are in open swampy pine woods, wet swampy woods, acid bogs: and along streams and roadside ditches (McVaugh and Pyron. 1951; Shaver. 1954; Lucansky, 1981). In some cases it occurs directly as a submerged plant and even produces floating mats of rhizomes (Power, 1914; Gams; 1038).

In the Yakima flora Iliootlwnrdia occurs with O.sr77w~cin tvellrii and the small o~locleoid fern Wes~ien.This type of as- sociation is similar to some of W. virgirzic~t'smodern associ- ations. Throughout its range, extant W. liilgir7ica co-occurs with Osrrruurda cinrlanzorrrea and 0. ragalis. In the northern, glaciated portion of its range, additional co-occurring ferns include Tlzely1)teri,s palustri.~, T. sintulat~i, Dl-j;opferis cristata, and D. carthu,siar~a. Onoelea serzsihilis grows nearby but is seldom intermixed, as Woodn~ardil~

thrives in inore acid soils. To the south, ferns found in associatiori with W. virginicn may include other species of Theljpteris, particularly T. gongylo- ides, W. aerolata, and various subtropical and tropical ferns in Florida (R. Crailfill, personal cornmunicatioii. 2000). Oiie of us (KBP) has seen W. ~.irgirzica growing along with $1.'. ncrolata, Orzoclea sensibilis, Osnzirnrfn cirznntnomea, and The-Ipreris sirnulrrt~~ along margins of a sed maple swainp in the New Jersey Pirie Barrens and in Ta.xodi~ur~zswamps and road- side ditches in coastal North Carolina along with 0. regalis,

0. cinrzannonzea. Woodtt-ardici aerolatn, Pteridiurn aquilinz~m.

and a varicty of other filicalean ferns. The species is also known in a Taxodi~~i?~

swamp in Vinton County, Ohiol where it grows along with Osi~rurzd~~,

Lygodiuin, and Oiloclca. These last two sites are interesting because they contain a number of floristic elements common to the Yakirna swamp including taxodiaceous conifers, pines, Liqi~idambar.white-oak Querczrs, L'itis, Nyssa, Plat~znzls, Cornzts, and rosaceous plants (F. Bryan, Cape Fear Botanical Garden. Fayetteville, North Car- olina, personal communication; K. Pigg, persolla1 observa- tion). In summary, although we do not imply that all the as- sociated floral elements share an idetltical evolutionary or phy- togeographic history, it appears that W. virginica had estab- lished modern habitat tolerances and had similar floristic associatiorls in the Miocene as it has today.

Fossil record of Woodwardia-Woo~lwanlin is a fairly commoll leaf compression form found throughout the Tertiary of Europe, North America, and Asia. Over 50 species have been listed under Woodwardiu or the varient name Woodwardites in the Fossilium Catalogus (Jonginans and Dijkstra, 1965; Dijkstra and Van Amerom, 1988): one author suggests that perhaps around six are valid (Hurmiick, 1976). Fossil chain ferns are typically recognized as resembling the following ex- tant species: W, virgirzica, W. spi~zulosa, W. apmlata, W. rad- icrxtzs, W. i~iartirzezii, and W. japowica (Pabst. 1968; Hurnick, 1976; Coliiiison, in press).

Previously described fossil remains inost similar to W. vir- giuica are co~npressed crosiers. vegetative and fertile fronds, and sporangia from the Miocene Succor Creek flora named 1V. de$esipitr.na by I-Telen Smith (Smith, 1938; Graham, 1965). These ferns are found in abundance at a single locality where they dominate and otherwise are relatively rare in the flora (Graham, 1965; P. Fields, Michigan State Univessity, personal

communication). Vegetative fronds of 1V. dqp~

',YE>'~rtnaare cs- sentially identical to extant 5%'. virginica and the Yakima ma- terial in details of pinnule morphology and venation (K. Pigg. personal observation). Fertile rnaterial has not been available for further study. 111addition to this species, several other forms froin western North America and Europe have also been coinpared to W. virginica. These include W. muristeriatza, W roessixrriarzn, and W. ma.xoni (Hurnick, 1976). Of these, Hur- nick figures some specimens that clearly resernble W.virgiizica (Plate 2a, b, 4a of Hurnick, 1976) and others that may be more similar to other soecies. Other fossils attributed to Woodu~urdin from ~eitern North America. Europe. and Asia, many needing further study (Collinson, in press), have been dis- cussed elsewhere (Knowlton, 1926; Berry, 1929; Pabst, 1968; Boureau, 1970; Chandrasekharam, 1974; MCIL cr and Basinger, 1993; KvaEek, 1993).

Relationships of Wessiea-In the most comprehensive sur- vey of fern anatomy yet attempted, Ogma (1972) refers to ferns with paired hippocampiforrn racheal traces like those of Wessiea as belonging to the "Oizoclea stelar type" but similar traces are also produced by some other athyrioid fenls such as living species of Diplaiictrll (Ogura, 1972) and the recently described fossil genus i2;akotopteris (Stockey, Nishida, and Rothwell. 1999). Moreover. the production of gaps in the rhi- zome stele that result from root divergence? rather than leaf trace divergence. is concordant with the stelar architecture of both athyrioid and blechlloid ferns (White and Weidlich. 1995).

The combination of a simple dictyostele with arnphicribral cauline meristeles and paired hippocampiform rachis traces in Wessiea is characteristic of a wide range of highly derived filicalean ferns assignable to the Dryoptcridaceae sensu &a-mer (1990). Roots xe diarch with a parenchyn~atous pith, an- other character that is consistent with this broad systematic assignment (Schneider, 1996). Although this familial concept is not f~illy concordant with the more widely used classifica- tion of Pichi Sermolli (1977), it encompasses a broad range of derived filicalean ferns includillg those with cauline and stipe vasculature like that of Wessiea. Moreover, in the absence of frond and spora~lgial characters, a more precise systematic assignment for Wessiea is extremely difficult. Regardless of its Inore precise systematic relationships, Ct/c,ssiea yaki~~laei?sis clearly represents a highly derived filicalean similar to those found in the living flora.

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