A new member of the zygopteridales from the lower Upper Devonian (Frasnian) of Ellesmere, N.W.T., Arctic Canada
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A newly discovered fern-like fossil plant is described from the Lower Upper Devonian of southern Ellesmere. This plant occurs as an element of an Archaeopteris dominated flora preserved in sediments of the Nordstrand Point Formation (mid-late Frasnian) at Bird Fiord. The plant demonstrates a pinnate vegetative system with three orders of branch and laminate pinnules, of a general sphenopteroid type. Primary pinnae usually diverge from the main axis in distichous pairs (Le., in a quadriseriate manner), but rarely depart singly (i.e., in a biseriate manner). Each primary pinna bears an aphlebia in the catadromic position. Laminate pinnules are broadly lobed with a proximally overtopped vein system. Anatomically, this plant is characterized by an elongate, mesarch, bipolar protostele that is ribbon to clepsydroid in shape. Proximal to each primary pinna node, an initially crescent-shaped, bipolar pinna trace diverges from the main axis stele. This trace appears to become four-ribbed before dividing to yield a pair of bipolar primary pinna traces. A pair of circular, centrarch aphlebia traces depart from the catadromic ribs of a primary pinna trace in its four-ribbed stage. Protoxylem is helically thickened, with metaxylem pitting ranging from scalariform to circular. Secondary xylem is unknown. Both the morphology and anatomy of this plant are non-gymnospermous and suggest affinity instead with zygopterid ferns. Within the Zygopteridales, this plant seems to align most closely with the Rhacopytaceae. The Frasnian dating of this plant suggests that laminated foliage had been achieved by some zygopterid ferns long before previously recognized. The presence of Sphenopteris-like pinnules in this Frasnian plant also shows that one should be careful in attributing such foliage to early gymnosperms.
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