Middle Triassic freshwater green algae and fungi of the Puesto Viejo Basin, central-western Argentina: palaeoenvironmental implications

Abstract

Diverse freshwater green algal microfossils and dispersed fungal spores among rich terrestrial palynofloras were recovered from the upper part of the Quebrada de los Fósiles Formation, Puesto Viejo Group, cropping-out in the fault-bounded San Rafael Basin, Mendoza Province, Argentina. Sedimentological data suggest that the vegetation recorded from this unit grew in floodplains of high-sinuosity fluvial systems when intense episodic volcanic activity provided great quantities of ash deposits to the sites. Small and temporary shallow lake deposits and thin peat ponds were recorded at two sections, yielding palynofloras composed of variable contents of spores, pollen, freshwater algae and fungal associations. The taxa described and illustrated herein consist of representatives of colonial Chlorophyta algae (Botryococcus), a diverse group of Zygnematales zygospores represented by the genera Lecaniella, Maculatasporites, Mehlisphaeridium, Ovoidites and the incertae sedis algae Leiosphaeridia and Retirotundia as well as representatives of fungi assigned to Microsporonites and Portalites. Two new species of Ovoidites are described herein, Ovoidites tripartitus and Ovoidites circumplicatus, and the new combination Ovoidites scissus is also presented. The terrestrial and aquatic vegetation grew associated with shallow water bodies in floodplain environments developed under warm temperate, strongly seasonal climatic conditions. The environment was stressed by volcanism that probably restricted the colonization and development of the ecosystem. The occurrence of algal microfossils in the Quebrada de los Fósiles Formation suggest shallow and episodic locally restricted aquatic environments inhabited by autochthonous freshwater green algae consistent with the sedimentological evidence. A Middle Triassic age is inferred from recent radiometric dating obtained from the basal and upper ignimbrite beds of the Quebrada de los Fósiles Formation indicate an age of 243.9 ± 2.0 Ma and 235.8 ± 2.0 Ma, respectively, thus mostly constrained to the Ladinian age. The composition of the fossil flora (palynoflora and macroflora) also indicates Middle Triassic age.

Ana M. Zavattieri [[email protected]], Unidad de Paleopalinología, Departamento de Paleontología, Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), CCT-CONICET-Mendoza, Av. A. Ruíz Leal s/n, Parque General San Martín, M5502IRA Mendoza, Argentina; Pedro Raúl Gutiérrez [[email protected]], Museo Argentino de Ciencias Naturales “B. Rivadavia”, CONICET, Sección Paleopalinología, Área Paleontología. Buenos Aires, Argentina, Av. Ángel Gallardo 470, C1405DJR Buenos Aires, Argentina; Mariana Monti [[email protected]], YPF Tecnología S.A. (Y-TEC), Avenida del Petróleo s/n, entre 129 y 143, B1924CKU Berisso, Argentina.

Keywords:

• Palynology
• freshwater microalgae
• fungi sporae dispersae
• palaeoecology
• Ladinian
• Argentina
• south-western Gondwana

Acknowledgments

Thanks to A. Moschetti (IANIGLA-CCT-CONICET-Mendoza) for palynological laboratory preparations of samples for light optical microscopy studies, as well as to F. Tricárico who assisted with the scanning electron microscopy. Thanks, are given also to Prof. Dr E.G. Ottone, an anonymous reviewer and particularly to the Associate Editor, Dr Chris Mays, for the suggestions and corrections that substantially improved the final version of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research has been partially funded by the Agencia Nacional de Promoción Científica y Tecnológica, Argentina [research grants ANPCYT-PICT 2016-0637 and ANPCYT-PICT 2016-0431]. Parts of the research that led to the revision of this material were financially supported by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina [PIP N° 0705 PRG and PIP N° 11220090100605 AMZ].