PALEOECOLOGY AND OTHER FEATURES OF PLUMALINA INCERTAE SEDIS

Plumalina is a distinctive and sometimes common problematic feather-like fossil, known for more than a century from the Devonian of New York. Numerous previous hypotheses of its taxonomic affinities include graptolites, hydroids, ferns, and lycopods. These studies have focused only on morphology, overlooking paleoecology, and largely on animals for modern comparisons. Recent discovery of apparently in situ Plumalina in the Upper Devonian Ithaca Shale, as well as detailed reexamination of several hundred specimens collected throughout New York and comparison with modern green alga provide new insights into the morphology, ecology, and phylogenetic position of this distinctive form.

Plumalina deposits in the Ithaca Shale occur in siltstone and shale, often occurring as repeated layers and associated with numerous rhynchonellid brachiopods (frequently attached to fronds), as well as crinoids and bivalves. P. plumaria is the dominant organism in three beds. Patterns of orientation and clustering suggest oscillating currents, consistent with previous interpretations of the Ithaca Shale as deposited above storm wave-base. Measurements of 5 characters on over 200 fronds reveal more variation within P. plumaria than previously recorded.

Most noteworthy in this deposit is the occurrence of apparently vertically oriented Plumalina fronds, occurring above horizontally oriented fronds. These are interpreted as epifaunally attached. The fronds are associated with (but not yet found connected to) horizontal elongate cylindrical structures, possibly interpretable as stems or stolons.

Plumalina has not previously been seriously considered as a possible alga. Yet, similar “feather-like” morphology occurs in a number of modern green algae (e.g., Caulerpa, Myriophyllum). Preliminary results of taphonomic experiments on Myriophyllum spicatum are consistent with Plumalina having had a similar original structure. Although interpretations of Plumalina as a hydrozoan cannot yet be completely discounted, the data currently available suggest that its affinities may lie elsewhere.