MORPHOLOGY AND ANATOMY OF A FERN GROWING UNDER STRESS CONDITIONS FROM THE APTIAN OF SANTA CRUZ, ARGENTINA. The morphology and anatomy of pteridophyte fronds attached to rhizomes with adventitious roots from the Aptian Anfiteatro de Ticó Formation, Santa Cruz province, Argentina are described. The fossils consist of impressions and compressions preserved in lithofacies dominated mainly by volcanic ash-fall. Observations made with scanning electron microscope on rhizomes and rachides revealed the presence of primary xylem tissues; protoxylem, metaxylem fibers and parenchyma. According to the general morphology, this fern is included in the morphogenus Cladophlebis Brongniart. Comparisons made with known Cladophlebis species of Argentina show that the material here studied has different morphological features. Furthermore, rachides in organic connection to rhizomes having a well preserved anatomy was not reported for Cladophlebis species from Argentina. Therefore, a new name, Cladophlebis cinerium, is assigned to this fern. Finally, it is suggested that this fern probably grew under humid conditions given by the fluvio-lacustrine environment of the Anfiteatro de Ticó Formation, but also with continuous stress caused by the abundant ash fall produced by the recurrent explosive andean vulcanism that took place during the Lower Cretaceous in Patagonia.