A Fertile Spike Moss ( Selaginellites argentinensis sp. nov.) with In Situ Spores from the Triassic of Argentina: First Fossil Record of a Selaginellaceae Lycophyte for South America

Abstract

Premise of research. Fossils of herbaceous lycophytes are scarce overall, most possibly due to biases against their preservation. In particular, the record of selaginellalean macrofossils is almost non-existent for the Southern Hemisphere, whereas its northern counterpart presents a more complete register extending from the Carboniferous onwards. Here, we present a new fossil record of an herbaceous lycophyte attributed to a selaginellalean based on a fertile specimen with in situ sporangia and spore content, which was recovered from the uppermost section of the Upper Triassic Potrerillos Formation (c. 232 Ma) at the Quebrada del Durazno locality in Mendoza, Argentina. Methodology. The single specimen was recovered as a clean impression and did not require any particular treatment for its study, since neither cuticle nor other organic matter were preserved. The fossil was photographed in detail to register its original condition and features. Subsequently, the fossil was coated in graphite to obtain SEM images with enough contrast. Description and measurements were performed from the images. Pivotal results. The specimen is characterized by a short, dichotomous shoot bearing isophyllous leaves and in situ megaspores and microspores, but in which the sporangia are not clearly organized in terminal cones; the latter condition is considered unusual in the Selaginellaceae. We discuss the differential preservation state of the sporangia based on the observation of isolated spores, spores in tetrads, and the molds left by their corrosion inside the microsporangia. Based on developmental studies on living herbaceous lycophytes, we consider the possibility that some microsporangia were in a stage previous to meiosis, while others almost ready to release their spore content, but got deposited before being able to do so.Conclusions. The recognition of in situ mega- and microspores allowed for its attribution to the Selaginellaceae. This represents the first record of a fossil Selaginellaceae lycophyte for South America, and only the second known in the entire Southern Hemisphere, making it a remarkable finding.