Title page for ETD etd-10062009-020228
|Type of Document
||Hill, Stewart Adams
||A new member of the zygopteridales from the lower Upper Devonian (Frasnian) of Ellesmere, N.W.T., Arctic Canada
||Master of Science
|Scheckler, Stephen E.
|McLean, Dewey M.
|Porter, Duncan M.
|Date of Defense
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
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