Cretaceous tree ferns of western North America:: Rickwoodopteris hirsuta gen. et sp nov (Cyatheaceae s.l.)

A distinctive new permineralized stem from marine deposits of western North America provides additional evidence for the diversity of Upper Cretaceous tree ferns. The fossil occurs in a calcareous concretion from the Late Campanian Spray Formation at Shelter Point, Vancouver Island, British Columbia, Canada. It measures 20 cm long and 7 cm. in maximum diameter, with adventitious roots diverging between persistent, helically arranged fronds. Frond traces are derived from an amphiphloic dictyostele, and the stem produces no medullary or cortical bundles. The pith has a sclerenchymatous center and a parenchymatous outer zone. Sclerenchyma sheaths accompany both cauline and foliar vasculature. Frond traces diverge as six to ten bundles, most often eight. At the stem periphery, the cortex produces a homogeneous sclerenchymatous hypodermis, and is clothed by a dense ramentum of both uniseriate and large multiseriate trichomes. This novel combination of characters reveals the presence of a new genus and species of tree ferns, Rickwoodopteris hirsuta gen. et. sp. nov. Cladistic analysis of stem characters infers that R. hirsuta conforms to the dicksoniaceous grade of Cyatheaceae s.l., and further clarifies our understanding of evolutionary diversification among Mesozoic filicalean tree ferns, (C) 2004 Elsevier B.V. All rights reserved.