Diversity of Mesozoic gnetophytes and the first angiosperms
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- Krassilov, V.A. Paleontol. J. (2009) 43: 1272. doi:10.1134/S0031030109100098
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A provisionary system of gnetophytes is proposed, including recently discovered Mesozoic members. The following principal characters are used, in descending order of significance: strobilar structure of ovulate organs (eugnetophytes) as opposed to flowerlike cupules and their aggregates (anthognetophytes), radial vs. bilateral symmetry of seed cupules, and morphological distinctions in the pollen organs, pollen micromorphology and exine ultrastructure. The fossil members show mosaic associations of morphological traits, which attests to different evolution rates of their ovulate and pollinate structures. The system includes two superorders and eight orders, several of which are new. A dichotomous key is provided to differentiate between genera of fossil gnetophytes based on ovulate structures. Two stages in evolution of gnetophyte morphological diversity are recognized: first in the Triassic and then in the Early Cretaceous, the latter was associated with the origin of new type herbaceous wetland communities. Angiosperms or some of them might have originated as elements of gnetophyte diversity in such communities.