First evidence of the Triassic seed fern Ptilozamites in the Southern Hemisphere

CARIGLINO, BÁRBARA; MONTI, MARIANA; ZAVATTIERI, ANA MARÍA

Salvador

Conferencia; X International Organization of Paleobotany Conference and XIV International Palynological Congress; 2016

The genus Ptilozamites Nathorst is a common Middle Triassic to Early Jurassic morphotaxon, found only in Northern Hemisphere localities (i.e., Europe, Greenland, and Asia). However, recent work at the San Rafael Block, in Mendoza province (Argentina) yielded new plant macrofossils, among which abundant pteridosperm foliage attributable to Ptilozamites was found. The fossil plants were recovered from the Quebrada de los Fósiles Formation (lower unit of the Puesto Viejo Group), a continental succession composed of alluvial and fluvial deposits intercalated with volcaniclastic sediments, and radiometrically dated in 245 ? 235 My. The largest pinnate frond measures 22 cm long and 10 cm wide. The leaves are characterized by long, subquadrate pinnae with an obtuse to rounded apex, attached sub-oppositely to the rachis by their entire base at a 35°-70° angle. Veins arise from the rachis and run almost parallel to the apex, with very few dichotomies and anastomoses. The rachis is thick, longitudinally striated, and bifurcated. The epidermis is thick. Epidermal cells are irregular to isodiametric polygonal, with thickened trichomes bases. The stomata are surrounded by 5-6 subsidiary cells, forming a strong ring, a defining character of the genus. Although the pinnae are very similar to those of the cycadophyta, the bifurcated rachis and cuticular characters allow placing it under Ptilozamites, a pteridospermous morphogenus of unknown affinities. Until now, only lycopsids and sphenophytes had been described from the unit. Thus, this pteridosperm foliage represents the first evidence of seed ferns for the formation, suggesting the flora that grew in proximity of these shallow floodplain lake deposits was more heterogeneous than originally thought. The thick epidermis with trichomes in Ptilozamites could have been a way of protecting the plant from the intense volcanic activity that influenced the sedimentary evolution of this succession. Sedimentation was dominated by arid or semiarid climates, and a concomitant seasonality. The finding of Ptilozamites represents the first record in all fossil assemblages in Gondwana, proving a wider paleogeographic distribution for this genus, and further corroborating the floral interchange between the northern and southern hemispheres during the Triassic.