ELLESMERIS SPHENOPTEROIDES, GEN ET SP-NOV, A NEW ZYGOPTERID FERN FROMTHE UPPER DEVONIAN (FRASNIAN) OF ELLESMERE, NWT, ARCTIC CANADA

A new fern-like fossil plant is described from the lower Upper Devonia n of southern Ellesmere Island, Canadian Arctic Archipelago. The plant occurs in an Archaeopteris-dominated flora preserved in the Nordstran d Point Formation (Mid-Late Frasnian) near Bird Fiord. The plant has a pinnate vegetative system with three branch orders and laminate sphen opteroid pinnules. Primary pinnae usually diverge from the main axis i n distichous pairs (quadriseriate), but can depart singly (biseriate). Each primary pinna bears a basal catadromic aphlebia. Anatomically, t he plant exhibits a mesarch, bipolar protostele that is ribbon- to cle psydropsoid-shaped in the main axis. Primary pinna traces are also ini tially bipolar and crescent-shaped, but may become four-ribbed before dividing into a pair of bipolar traces. The morphology and anatomy of this plant are nongymnospermous and are most similar to Zygopteridales (particularly Rhacophytaceae and Zygopteridaceae). The Frasnian age o f Ellesmeris shows that laminated foliage had evolved in some zygopter id ferns much earlier than previously recognized. The Sphenopteris-lik e pinnules of Ellesmeris indicate the need for caution when attributin g such a convergent foliar design to other plant groups, such as the D evonian gymnosperms.