Additional diversity of marattialean ferns in the Early Permian Rio Genoa Formation, Patagonia, Argentina.

LUNDGREN, KOLBY; CÚNEO, RUBÉN; ESCAPA, IGNACIO; TOMESCU, MIHAI

Edmonton

Congreso; BOTANY 2015; 2015

The Rio Genoa Formation hosts the most diverse Early Permian flora in western Gondwana. The fern diversity described from the unit surpasses that of most other Early Permian Gondwanan floras and includes fronds of Asterotheca, Dichotomopteris, Pecopteris, Sphenopteris, Oligocarpia, Cladophlebis, Corynepteris/Alloiopteris, and stem impressions (Caulopteris, Artisophyton). Among these, marattialean ferns are represented by five previously described species assigned to Asterotheca. Paleozoic Marattiales are well documented in the Northern Hemisphere, where their taxonomy is well circumscribed. In contrast, Southern Hemisphere Marattiales have not been fully explored, which excludes key taxa from studies seeking taxonomic resolution in the group. Based on additional collections from the Rio Genoa Formation, we describe a new marattialean fern represented by vegetative and fertile frond segments. The fronds are at least bi-pinnate, with alternate ultimate pinnae displaying strongly decurrent divergence. Vegetative ultimate pinnae maintain a uniform width for most of their length, displaying very gradual tapering toward the tip. Pinnules, 13 or more per pinna, are evenly spaced, alternately arranged, with obovate shape and very shallow marginal lobation. Pinnules are attached to the rachis by a free, slightly cuneate base. Pinule midveins are decurrent and curve sharply away from the rachis before straightening out and extending to the tip of the pinnule. Pinnules have 6-8 lateral veins that bifurcate once distally, with the basalmost lateral vein in a basiscopic position. Fertile ultimate pinnae bear 14-17 evenly spaced alternate pinnules with obovate shape, broadly cuneate free base, and weakly lobed margins; the basalmost pinnule occupies a basiscopic position on the pinna. The pinnule midvein is strongly decurrent and extends into the pinnule tip where it normally bifurcates. Pinnules display 8-9 alternate lateral veins (the basalmost in a basiscopic position). Only the basal three lateral veins of a pinnule bifurcate once close to their base. Fertile pinnules hold 3-8 synangia attached along lateral veins. Developing synangia are large and bulbous; when fully dehisced they become stellate, with six radially arranged fusiform sporangia that radiate from a central point of attachment. This character combination places the Rio Genoa specimens among the Late Paleozoic Marattiales, and represents a new marattialean type for this unit. This discovery marks a first step in renewed efforts to reconcile the taxonomy of Permian ferns from the Rio Genoa Formation with the most recent taxonomic treatments for the group, which up until now have emphasized northern hemisphere taxa.