THE GNETALES (GYMNOSPERMOPHYTA) IN THE FOSSIL RECORD OF THE CRETACEOUS SAN LUIS BASIN, ARGENTINA: SYSTEMATIC ASPECTS

GÓMEZ M.A.; GABRIELA G. PUEBLA; PRAMPARO M.B.; IGLESIAS ARI

Mendoza

Congreso; 4th International Paleontological Congress; 2014

CONICET MENDOZA

The Gnetales currently include three genera of seeds plants: Ephedra, Gnetum and Welwitschia, with an evolutionary history still poorly known. Extant species are considered a remnant of a wider biodiversity due to their divergent morphology and wide ecological range of life forms, and by the palaeontological record (diverse and abundant polyplicate pollen grains). The phylogenetic relationships between Gnetales and other spermatophytes have been debated as an important evolutionary problem until recently. The present debate includes several different phylogenetic hypotheses, alternatively relating them to different groups of gymnosperms or basal angiosperms, which has led to a growing interest in their study. In the La Cantera Formation (Late Aptian), San Luis Basin, central-western Argentina, a palynological assemblage, including abundant pollen grains, as well as macroremains assigned to this group, were recovered. Polyplicate pollen grains constitute more than 40% of the total of palynomorphs recorded in some levels of the type sectionand are associated also with well-preserved macroremains. Among the pollen grains there are morphological varieties with over 20 different morphological types assigned to different fossil genera with gnetalean affinity: Ephedripites, Steevesipollenites, Gnetaceaepollenites. Pollen grains are elliptical, inaperturate and polyplicate with a wide range of variation in pollen size and twisted spiral or longitudinal plicae (similar to Ephedra). Some polyplicate grains show a longitudinal aperture, similar to Welwitschia pollen grains. The macroremains include leaves, stems, cones and seeds, and are comparable to Ephedra. Striated stems and branches with whorled leaves have been identified among the studied material. Specimens with two leaf types were recognized: one type with two linear leaves in opposite arrangement and the second type represented by scale-like leaves, with threemerous whorled arrangement, with fused bases and triangular apex. Female cones are ovate in shape, with thin bracts in three whorls: three to five small bracts at the first whorl, three broad bracts at the second whorl, and three larger bracts at the third whorl were preserved. The margins of bracts have lateral flanks extension (wings). Two ovoid seeds per cone with outer papillate envelope were distinguished. Some specimens have a good preservation and exhibit cuticles and organic remains that have enabled us to visualize in detailed anatomical features typical of these structures. A detailed study on these fossils will provide new relevant information to the understanding of homologies in reproductive structures of the Gnetales and its relationship with gymnosperms and angiosperms.