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A NEW RHABDOPLEURAN PTEROBRANCH FROM THE MIDDLE CAMBRIAN OF NÄRKE,
CENTRAL SWEDEN
Adam URBANEK & Stefan BENGTSON
In: Bjerreskov, M. & Stouge, S. (Eds.) Third International Graptolite Conference Abstracts, 1985, 97.
A rich and well-preserved material of a rhabdopleuran pterobranch was collected from Middle Cambrian (Eccaparadoxides
oelandicus
Zone) clay shales at Kvarntop, Närke, by Jan Johansson, Sköllersta. Most of the colonies encrusted dead shells
of
Acrothele granulata Linnarson, and a few were found attached to fragments of paradoxidid trilobites. Part of the
material was isolated from the shale matrix through HF treatment.

The colonies are composed of irregularly branching repent tubes and fairly long erect tube portions extending radially
above the attachment surface. The slender repent tubes consist of a basal membrane, adnate to the encrusted substrate
(usually a brachiopod periostracum, the ornamentation of which it closely replicates), and of a convex upper wall with a
fairly regullar fusellar structure. The repent tubes comprise both stolothecal tubes and creeping, proximal parts of
lateral zooidal tubes.

The lack of sclerotized stolon may be inferred from the absence of stolonal vestiges within the bleached stolothecal
tubes as from the absence of stolonal fragments in the residues.

The free, erect portions of the tubes display a distinct fusellar structure, in places fairly capricious but otherwise
remarkably regular with no zigzag sutures. The erect tubes widen steadily and terminate with an oblique apertural
margin, thus forming a well-pronounced apertural process. Lateral branches may occasionally be formed on the erect
tubes, their traces being sometimes preserved in the form of "knots". No evidence for a regular dimorphism of the tubes
was recognized, although some erect tubes very much shorter and narrower than others, their aperture, however, been
elaborated in the usual way. This oldest known representative of the Rhabdopleurida combines featres of the tubarium in
extant species with the appearance of an encrusting tuboid colony.
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