A Middle Triassic macroflora from southwestern Gondwana (Mendoza, Argentina) with typical Northern Hemisphere elements: Biostratigraphic, palaeogeographic and palaeoenvironmental implications

Abstract

A Triassic macrofloral assemblage is documented from the Quebrada de los Fósiles Formation, lower unit of the Puesto Viejo Group (Mendoza, Argentina), southwestern Gondwana. The impression-compression flora incorporates fertile and sterile remains of lycophytes, sphenophytes, and the first seed ferns recorded from this unit. Among the novel components, Ptilozamites, a typical Northern Hemisphere genus, is herein described for the first time from Gondwana. Ptilozamites longifolia sp. nov. consists of unipinnate leaves with an often forked rachis, with long, linear to sub-rectangular pinnae attached by their whole bases. The veins are distinct, inserted directly on the rachis, and run almost parallel to the margins, with few dichotomies occurring. The cuticle is thick, stomata constituted by two sunken guard cells surrounded by 4–6 subsidiary cells fused to form a ring-like structure. The association of Ptilozamites, Pleuromeia, Lepacyclotes, Equisetites and Neocalamites was common in many Middle and Late Triassic assemblages in Laurasia, whereas the Southern Hemisphere Triassic assemblages were characterized by the Dicroidium flora. A Middle Triassic age inferred from various elements constituting this macroflora is also concordant with the recent radiometric dates assigned to the unit. The lithofacies associations indicate that this flora grew and was deposited in the floodplains of a high-sinuosity fluvial system where volcanic ashes were deposited episodically. This could have affected the development of the flora by favoring only those plants able to cope with stressful environmental conditions, as exemplified by the fleshy stems and leaves with thick cuticles present in Pleuromeia and Ptilozamites.