PETRIFIED CYCAD STEMS FROM ARGENTINA 121 Issued 00 August 2004 © 2004 The New York Botanical Garden Copies of this issue [70(2)] may be purchased from the NYBG Press, The New York Botanical Garden, Bronx, NY 10458-5126, U.S.A.; nybgpress@nybg.org. Please inquire as to prices. 121 The Botanical Review 70(2): 121–133 Two New Petrified Cycad Stems, Brunoa gen. nov. and Worsdellia gen. nov., from the Cretaceous of Patagonia (Bajo de Santa Rosa, Río Negro Province), Argentina ANALÍA E. ARTABE, 1 ALBA B. ZAMUNER, 1 AND DENNIS WM. STEVENSON 2 1 División Paleobotánica Facultad de Ciencias Naturales y Museo de La Plata 1900 La Plata, Argentina 2 New York Botanical Garden, Bronx, NY 10458, U.S.A. I. Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 II. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 III. Locality, Materials, and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 IV. Systematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 V. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 VII. Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 I. Abstract Polyxylic columnar stems covered by persistent leaf bases and found in sediments assign- able to the Upper Cretaceous of Bajo de Santa Rosa, Río Negro Province, Argentina, are de- scribed as two new generic entities in the Cycadales. Anatomical characters are the basis for their being assigned to the Encephalartoideae of the Zamiaceae. Brunoa santarrosensis gen. et sp. nov. is characterized by the presence of polyxyly, cone domes, mucilage cavities, and uniseriate to triseriate araucaroid, scalariform, or bordered intervascular pitting. Worsdellia bonettiae gen. et sp. nov. has polyxyly, anastomosing medullary vascular bundles, centripetal xylem, mucilage canals, and concentric extraxylary bundles. Some characters (polyxyly, med- ullary vascular bundles, and cone domes) were used to determine the systematic position, while other characters (mucilage reservoirs and centripetal xylem) were used to establish the rela- tionship between polyxylic and monoxylic forms. II. Introduction Cycadales are the most primitive order within gymnosperms with extant representatives. The evolutionary history as known from fossils begins in the Upper Paleozoic (Mamay, 1969, 1976; Taylor, 1969; Zhu & Du, 1981) and reaches maximum distribution during the Mesozoic, with decline beginning toward the end of the same period. Presently, Cycadales are a very well