Oral Paper

         Paleobotany

A rare anatomically-preserved lycopod macrofossil from the Triassic of Antarctica

Presenting Author
Meg Nibbelink
Description
The rhizomorphic lycopod lineage (Isoetales herein) has a history that extends over 400 million years into the Devonian and persists still today. During the Carboniferous, arborescent lycopods dominated the Earth’s landscape and left an abundant fossil record, placing the Carboniferous Isoetales among the best characterized groups of fossil plants. However, climatic and geologic changes triggered a shift in the ecological role of Isoetales, diminished their diversity in terrestrial ecosystems, and created conditions unfavorable to plant fossilization (e.g., Permian-Triassic coal gap). Together, these events obscure key data on the last 320-million-years of their evolutionary history, including pivotal evidence on the origins of modern Isoetes. Only a small fraction of isoetalean macrofossils are known from the Triassic, and an even smaller portion of these are anatomically preserved. Similarly, a few isoetalean macrofossils are known from Antarctica but none are Triassic permineralizations. Here we report a new lycopod fossil from the Triassic Fremouw Formation of Antarctica. This specimen is permineralized in a silicified peat and was studied using the cellulose acetate peel technique. The fragment is partially compressed and incompletely preserved. Anatomical details of this specimen can only be observed when peels are attached to the slab surface, making many details of their morphology and anatomy challenging to reconstruct. Nevertheless, the anatomy that is preserved indicates the specimen has an isoetalean affinity. The primary vascular tissues are not preserved but the secondary xylem cylinder is present, measuring approximately 15-20 cells (3.2 mm) thick. To the outside of the secondary xylem is a multilayered cortex composed of homogenous, parenchymatous cells. These layers create a rippling pattern, although it is unclear whether this is a taphonomic artifact or true to the specimen, and regularly arranged isoetalean rootlet traces are found on one side of this axis. Owing to the rarity of permineralized Triassic isoetaleans, this specimen provides crucial data on the anatomy and diversity of Mesozoic Isoetales and will advance our understanding of the evolutionary history of rhizomorphic lycopods.