We present a morphological-systematic study of new fossil specimens of equisetaleans (horsetails) from the Cortaderita Formation, Sorocayense Group, Middle Triassic, Cuyana Basin, southwest of the San Juan Province, Argentina. In this unit, nine fossiliferous strata with plant remains have been recognized (EF4–12). The fossils studied come from EF4, located in the lower member of the formation, and correspond to impressions-compressions. The fossils are assigned to the family Equisetaceae and the genus Equicalastrobus, being the oldest record of the genus and the first record of this taxon for Argentina and Gondwana. The samples are closed and open strobili composed of peltate sporangio-phores, with glabrous hexagonal discs and with an umbo on its external face which projects, acquiring a leaf shape. Some specimens are found in organic connection with leaf sheaths and axis fragments, allowing the whole plant to be reconstructed.

Key words: Sphenophytes, Equisetales, Equisetaceae, Triassic, Cuyana Basin, strobili, sporangiophores.

Jano Nehuén Procopio Rodríguez [procopio.jano@gmail.com; ORCID: https://orcid.org/0009-0000-9823-5702 ], Josefina Bodnar [jbodnar@fcnym.unlp.edu.ar; ORCID: https://orcid.org/0000-0001-7707-396X ], División de Paleo-botánica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque s/n, B1900F-WA La Plata, Buenos Aires, Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Marisol Beltrán [meduself@hotmail.com; ORCID: https://orcid.org/0000-0003-3562-7201 ], Instituto de Investigacio-nes en Biodiversidad y Medioambiente (INIBIOMA), CCT CONICET-Patagonia Norte, Av. De Los Pioneros 2350, 8400, San Carlos de Bariloche, Río Negro, Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).