Fossil Cycadales of Argentina

A survey of Cycadalean taxa of Argentina (including Antarctica) is presented. The record of leaves represented by Nilssonia, Pseudoctenis, Ctenis, Mesodescolea, Ticoa, Almargemia, Kurtziana, and Zamia genera are summarized. Recent investigations made of cuticles with transmission and scanning electron microscopy are included. In stems, a preliminary study of two forms found in the Upper Cretaceous of Rio Negro Province are incorporated into Michelilloa, Bororoa, and Menucoa. The fossil record shows some characters of leaf morphology, presence of inverse xylem and medullary bundles, two kinds of leaf traces, and monoxylic and polyxylic steles of systematic importance when compared with both extant and fossil cycads. Affinities of fossil taxa related to extant families are suggested through analysis of the above-mentioned characters. The presence of medullary vascular rings related to the emission of terminal reproductive strobili are recognized in Bororoa and Menucoa. This character associated with polyxyly makes it possible to assign these genera to the Zamiaceae-Encephalartoideae sensu Stevenson, widening the paleogeographical distribution of this subfamily.