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A silicified stem from the Ischigualasto Formation (Trias) in northern Argentina, is described and named Michelilloa waltonii nov. gen. et spec. It is compared with fossil stems of the genera Cycadeoidea and Bucklandia, and with the living Cycadales. A combination of characters (in particular the indumentum of long filamentous hairs, multiseriate bordered pitting of the xylem tracheids, and the structure of the leaf gap and leaf trace bundle which are exactly similar to those of the cycad Dioon spinulosum) suggests that Michelilloa is an early representative of the Cycadales.
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Archangelsky et al. (1963) studied this question.
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