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First occurrence of Plumalina incertae sedis in the Silurian


, : First occurrence of Plumalina incertae sedis in the Silurian. Abstracts with Programs - Geological Society of America 28(7): 429

The first occurrence of the ?Cnidarian fossil Plumalina n. sp. is documented from mid Silurian strata of central and western New York State. This new discovery extends the lower limit of this group's range approximately 35 million years, from Eifelian (Middle Devonian) to Wenlockian (mid Silurian).First described by James Hall in 1858 and best known from the Upper Devonian of New York, Plumalina is also noted from Middle Devonian strata in New York and Germany. More recently, a single poorly preserved specimen of Plumalina n. sp. was collected from calcareous shales of the Silurian Rochester Shale of western New York. Comparison of the new specimen with Middle and Upper Devonian forms reveals a morphologic transition not previously recognized. Trends include: 1) an overall increase in the number of side branches or pinnae along the main axis (rachis), and 2) an apparent increase in thickness of pinnae. Most Upper Devonian specimens possess encrusting rhynchonellid brachiopods attached to the main axis and embryonic individuals attached to robust pinnae. The presence of these epibionts supports the notion that Upper Devonian forms had a more rigid morphology. The angle of attachment of pinnae along the rachis has been used by previous authors as an important criterion for designating the three known species. We feel this criterion is unreliable, since several nearly complete specimens of Plumalina plumaria and P. densa display a consistent change in attachment angle, from 80-82 degrees in distal portions to 50-55 degrees near the apex. This change in attachment angle is a result of normal colony growth or astogeny. Plumalina has been classified as a graptolite, a lycopod, a gorgonian and a hydrozoan by previous authors. Although the new specimen provides no definitive evidence of its systematic position, its general morphology and taphonomy strongly suggests affinities with modern plumularian hydrozoans.

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