Ultrastructural analysis of selected Cretaceous megaspores from Argentina

Abstract. Detailed ultrastructural analysis of an assemblage of Lower Cretaceous megaspores from the Baqueró Formation of Argentina allows a complete description of two new taxa as well as additional information on the structure of several previously described types. Erlansonisporites sparassis displays a complex wall stratification, a separable surface reticulum, and attached microspores. Horstisporites iridodea sp. nov. possesses an elaborately patterned wall like that seen in several extant species of the modern lycopod genus Selaginella. On the basis of developmental information from modern heterosporous lycopod megaspores, the mesospore bearing Bacutriletes triangulatus sp. nov. is recognised as an aborted product of a meiotic tetrad. Also reported from this locality are Hughesisporites patagonicus, Bacutriletes sp. A, and an additional loosely defined megaspore type with characteristic clavae. Transmission electron microscopy provides the opportunity to infer previously unattainable biological information from dispersed spores. Such studies provide new insights as a foundation for the further elucidation of the developmental and evolutionary significance of wall construction in heterosporous pteridophytes.