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Fulltext.Pdf Amer. J. Bot. 74(11): 1722-1736. 1987. DREWRIA POTOMACENSIS GEN. ET SP. NOV., AN EARLY CRETACEOUS MEMBER OF GNETALES FROM THE POTOMAC GROUP OF VIRGINIA1 PETERR. CRANEAND GARLANDR. UPCHURCH,JR.~,~ Department of Geology, Field Museum of Natural History, Roosevelt Road at Lake Shore Dr., Chicago, Illinois 60605, and U.S. Geological Survey, Box 25046, Mail Stop 919, Denver, Colorado 80225 ABSTRACT Drewriapotomacensis gen. et sp. nov. from the Lower Cretaceous Potomac Group of Virginia (Zone I, probably Aptian) provides the first definite Mesozoic megafossil record of Gnetales. The stems are slender, display no evidence of secondary growth, and show axillary monopodial branching. Attached leaves are opposite and decussate, borne at swollen nodes, and have clasping sheathing bases. Each leaf is oblong, up to 20 mm long, and has a dense network of longitudinally aligned subepidermal fibers. Leaf venation consists of two, or possibly three pairs of longitudinal parallel veins that form a reticulum at the apex and higher-order crossveins that form apically oriented chevrons. Reproductive structures consist of short, loose spikes that are borne in dichasially arranged groups of three. The dichasia are either terminal or lateral in the axil of a leaf. Reproductive units of the lateral spikes in a dichasium contain seeds, each surrounded by at least one pair of opposite, broadly elliptical or ovate bracts. Characters of D. potomacensis that suggest a gnetalean relationship include the opposite bracts surrounding the seeds, the network of subepidermal foliar fibers, and the distinctive leaf venation, which is very similar to that seen in the cotyledons of extant Welwitschia. Other features consistent with a gnetalean relationship include opposite and decussate leaves, swollen nodes, and the dichasial arrangement of the reproductive spikes. Masses of polyplicate gnetalean pollen comparable to that of extant Welwitschia occur in the same bed as the megafossils. The morphology of D. potomacensis indicates that it was an herb or possibly a shrub. The growth habit of D. potomacensis and associated plants, combined with the sedimentary occurrence of the fossils, indicate that this species and perhaps related taxa were important components of early successional vegetation during the mid-Cretaceous. EXTANTSEED PLANTS comprise five major Gnetum, and the monotypic Welwitschia. N6- clades: Coniferales, Cycadales, Ginkgoales, mejc (1967) suggested that Gnetales were never Gnetales, and angiosperms. All ofthese groups, diverse, but as early as 1908, Arber and Parkin with the exception of Gnetales, have an exten- (p. 507) concluded, "Although the Gnetales are sive megafossil record that extends back into unknown to us in the fossil state, we are by no the Mesozoic (e.g., Taylor, 198 1; Stewart, means precluded ipso facto from the conclu- 1983). The Gnetales, in contrast, lack any well- sion that the group may have once flourished documented Mesozoic megafossil record and more vigorously than at the present day." comprise only three extant genera: Ephedra, Arber and Parkin's suggestion has subsequent- ly been corroborated by the recognition of a I Received for vublication 23 October 1986: revision widespread and diverse group of Mesozoic dis- accepted 2 arch 1987. persed pollen comparable to grains of extant We thank C. B. Beck. J. A. Doyle, M. J. Donoghue, L. Ephedra and Welwitschia in shape, aperture J. Hickey, C. Hotton, and J. A. ~olfefor their assistance configuration, surface sculpture, and exine ul- and discussion of issues raised in this paper. Research was supported in part by National Science Foundation Grant trastructure (e.g., Trevisan, 1980), which are BSR-83 14592 to PRC and a National Research Council especially abundant and diverse in the Early Postdoctoral Research Associateship, U.S. Geological Sur- Cretaceous of low paleolatitudes (e.g., Africa, vey, to GRU. Field work was supported by a Scott Turner South America) (Brenner, 1976). Only one pos- Award in the Earth Sciences, University of Michigan, to GRU. Ms. P. Quasthoff drew the reconstructions, and R. sible kind of fossil gnetalean pollen (Equise- McBride and D. Pocknoll assisted with photography. tosporites chinleanus Daugherty) has been Any use of trade names is for descriptive purposes only found within a pollen-bearing organ (Mascu- and does not imply endorsement by the U.S. Geological lostrobus clathratus Ash, 1972), but the sys- Survey. tematic position of the M. clathratus plant is The listing of authors is alphabetical. Current address: Museum, University of Colorado, uncertain (Ash, 1972) and pollen wall ultra- Campus Box 3 15, Boulder, CO 80309-03 15. structure differs from that of extant Gnetales November 198 71 CRANE AND UPCHURCH -CRETACEOUS GNETALES 1723 (Zavada, 1984). In this report we describe gne- volved removing the cuticle from the rock with talean megafossils, including stems with at- a needle, dissolving the acrylic plastic in ace- tached leaves and reproductive structures, from tone, demineralizing overnight in HF, mac- the Lower Cretaceous Potomac Group of Vir- erating in bleach, staining in Safranin 0 dis- ginia. These remains, found in association with solved in a 5050 mixture of ethanol and masses of Welwitschia- type pollen, provide toluene, and mounting in Histoclad. A centri- the first fully documented report of Mesozoic fuge was used for the bleaching, staining, and gnetalean megafossils and have important im- dehydration stages of processing due to the plications for understanding the ecology and extremely fragmentary nature of the cuticle. evolution of this enigmatic group of seed plants. Dispersed cuticles and associated masses of pollen, prepared in the course of an earlier MATERIALSAND METHODS-Thefossils de- study (Upchurch, 1984a, b), also were exam- , scribed in this paper were collected from a ined to assess variation in cuticular structure 4-cm-thick bed of micaceous gray clay exposed and to determine the possible pollen of the at the northern end of Drewrys Bluff, approx- megafossils. Cuticles were photographed with imately 9 m above high tide level. This bed is Kodak Technical Pan Film 24 15 using a Zeiss dated palynologically as upper Zone I of Bren- Photomicroscope I. Pollen grains were pho- ner (1963), or probably Aptian (J. A. Doyle, tographed with Kodak Technical Pan Film oral communication, 1986) and yields the early 24 15 on an Olympus Vanox Photomicroscope angiosperm leaves with cuticles reported by using Nomarski optics. Upchurch (1984a, b). Although fossil vegeta- tive shoots were collected on the outcrop, al- DIAGNOSIS-Generic diagnosis- Drewria most all of the reproductive material was re- gen. nov.: Stems bearing opposite and decus- covered by splitting blocks of matrix in the sate leaves. Branching monopodial, branches laboratory and examining them under a dis- arising in the axils of leaves. Leaves simple, secting microscope. Organically preserved oblong, margin entire. Leaf venation of two specimens were coated with Krylon acrylic distinct vein orders. First-order venation of plastic spray to prevent further fragmentation. two (possibly three) pairs of longitudinal par- Over 100 specimens on approximately 80 allel veins; the innermost pair running to the pieces of matrix were examined. Leaf cuticles lamina apex and branching to form a reticu- were prepared from one organically-preserved lum. Second-order venation of crossveins that specimen. All specimens are deposited in the typically form apically oriented chevrons. University of Michigan Museum of Paleon- Lamina with abundant, longitudinally orient- tology (UMMP). Systematiccomparisons were ed, subepidermal fibers. Reproductive struc- made using the published literature (e.g., Pear- tures either terminal or in the axil of a leaf, son, 1929; Martens, 197 l; Sporne, 197 l), the consisting of short, loose spikes borne in di- U.S. Geological Survey reference collections of chasial groups of three. Reproductive units of cleared leaves and modern pollen, and speci- the lateral spikes in a dichasium containing mens in the herbarium of the Field Museum seeds, each surrounded by at least one pair of of Natural History. broadly elliptical to broadly ovate bracts. Photographic techniques were critical to de- Holotype: UMMP 654 14b (Fig. 22). termining venation and morphological fea- Etymology: From Drewrys Bluff, Virginia, tures in many specimens. Organically pre- the type locality. served remains and reddish-brown impressions Specijic diagnosis: Drewria potomacensis sp. of vegetative parts were photographed with nov.-As for genus. Kodak Technical Pan 241 5 Film developed Syntypes: UMMP 65409-654 14a, 654 15- with HC-110 developer to medium or high 65419 (Fig. 1-16, 19-21, 23-27). contrast levels. Light gray impressions of veg- Etymology: From Potomac Group, the fos- etative parts were photographed either with sil-bearing rock unit. Polaroid Type 5 1 high contrast film or Kodak Locality: 37"25'25"N, 77"25'30"W, Chester- Technical Pan Film 241 5 under fluorescent field County, Virginia, south of Richmond on light, which enhanced many details of venation the James River at the north end of Drewrys not readily visible to the naked eye. Repro- Bluff, approximately 9 m above high tide level. ductive structures were photographed with Po- Stratigraphic position: Potomac Group (un- laroid Type 55 film. Polarizing filters were used differentiated), upper Zone I of Brenner (1 963), to increase contrast and reduce glare from spec- Lower Cretaceous, probably Aptian. imens coated with acrylic
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