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Drewria potomacensis gen. et sp. nov., an Early Cretaceous Member of Gnetales from the Potomac Group of Virginia

Peter R. Crane and Garland R. Upchurch, Jr.
American Journal of Botany
Vol. 74, No. 11 (Nov., 1987), pp. 1722-1736
Stable URL: http://www.jstor.org/stable/2444143
Page Count: 15
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Drewria potomacensis gen. et sp. nov., an Early Cretaceous Member of Gnetales from the Potomac Group of Virginia
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Abstract

Drewria potomacensis 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.

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