Reconstructing a giant isoetalean lycopsid from the Early Permian of Argentina (Rio Genoa Formation, Patagonia).

TOMESCU, MIHAI; CÚNEO, RUBÉN; ESCAPA, IGNACIO

Boise

Congreso; BOTANY 2014; 2014

Late Carboniferous - Early Permian lycopsids are known throughout South America (Argentina, Brazil, Uruguay, Peru, Bolivia) from stem adpressions and casts. Many of these fossils are assigned to morphotaxa whose taxonomy is obscured by the fragmentary character of specimens and small size of collections. The Rio Genoa Formation comprises deltaic deposits hosting abundant Early Permian plant assemblages. Extensive sampling has yielded lycopsid collections from ten localities (corresponding to several stratigraphic levels) allowing for assembly of a whole-plant concept for an isoetalean lycopsid. The reconstruction is based on (1) physical connection between specimens; (2) continuous series of leaf base morphologies (within- and between specimens and localities); (3) patterns of association of similar plant parts (stems, leaves, spores); (4) previous reports on plant size and spores. The plant has stems up to 60cm in diameter, with persistent ligulate leaves. Rhomboidal leaf bases lack well-defined leaf scars and have long, narrow infrafoliar bladders. Leaves are long (>30cm) and linear; the lower part of the lamina, appressed to the stem, is ca. 1 cm wide, 3-4cm long, parenchymatous; distally, the lamina is up to 4 mm wide, coriaceous. Characteristic teeth line the leaf margin; they are denser and longer (1.5mm) close to the base of the lamina, and widely spaced, shorter (0.5mm) distally. Micro- and megasporangia produced along bisporangiate fertile zones are ovoid-elongate, have longitudinal dehiscence slits, similar sizes (20-27 x 6-8.5mm), and contain trilete spores. Megaspores are circular, ca. 1 mm diameter, subgulate, and densely baculate-spinose on the distal surface and around the contact area. Microspores are circular-subtriangular, zonate, 60-70µm in diameter. The Rio Genoa lycopsid is an arborescent plant with a cormose rhizomorph and no documented branching. Although anatomy is unknown, the size of the plant is consistent with secondary growth. This plant displays all the characters of the isoetalean family Chaloneriaceae. The Rio Genoa plant is similar to some incompletely characterized lycopsids, such as Angaran taxa and plants from the Rio Bonito Formation (Permian, Brazil) which combine features of Brasilodendron and Lycopodiopsis, two morphogenera based on vegetative stem adpressions and casts. Because such morphogenera are notoriously artificial taxa and since the Rio Genoa lycopsid is a whole-plant concept (the first among Late Paleozoic Gondwanan lycopsids), erection of a new taxon may be necessary. Additional whole-plant reconstructions of lycopsid taxa are needed to understand the diversity of the clade in South America, with implications for resolution of Late Paleozoic lycopsid phylogeny worldwide.