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FIRST OCCURENCE OF THE FOSSIL HEMICHORDATE RHABDOPLEURA (GRAPTOLITHOIDEA) IN
NORTH AMERICA
PIOTR MIERZEJEWSKI, CYPRIAN KULICKI and STEFANO SCHIAPARELLI
North American Paleontology Convention 2005, Abstracts, pp.125-126
The remains of a pterobranch hemichordate, recognized as numerous fragments of zooidal tubes and remnants of the
stolon system (
i.e. stolons and capsules or cysts of dormant buds), were obtained by chemical isolation from the early
Llandovery limestone (Beescie Formation, member 4) from the well known locality named A5C on the Island of Anticosti
(Quebec, Canada). These fossils, referred to as a new species of the genus
Rhabdopleura, are first fossil
representatives of the order
Rhabdopleuroidea in North America. Previously, similar forms were described from the
Ordovician and Silurian of Baltoscandia as the thecate hydroid cnidarian
Kystodendron longicarpus (Eisenack) Kozłowski
or the rhabdopleurid pterobranch
Kystodendron longicarpus (Eisenack) sensu Kozłowski. The skeletal material of the
form under study was investigated using SEM techniques. Fragments of zooidal tubes are built of typical erect and
creeping parts, usually strongly flattened, sometimes distorted, and revealing distinct fusellar structure similar as in
other extinct and extant species of
Rhabdopleura. The periderm looks membraneous, amber brown, more or less
translucent, provided with distinct fusellar collars. The outer surface of zooidal tubes is devoid of pellicle or the
pellicle is so delicate that it reflects roughly the shape and orientation of underlying fusellar fibrils. Internal secondary
deposits, composed of cortical fibrils, arranged in parallel, similar to that in Recent
Rhabdopleura compacta, are
recognized on the inner surface of zooidal tubes. This is the first finding of inner fibrillar secondary deposits in the
fossil
Rhabdopleuroidea and these deposits are equivalent to true graptolite endocortex. The cortical fibrils, closely
packed or even cementated, were found also in fusellar collars. Cysts of dormant buds are subcylindrical, variable in
length (usually 0.6-0.8 mm long), closed or ending with terminal stolon. Peduncular stolon of cysts is usually short, with
constrictions and swellings, rapidly widening and passing into cysts.
GRAPTOLITE NET
is edited and periodically updated by
Dr Piotr Mierzejewski, Count of Calmont
since 2002